Download Interspecific Abundance-Range Size Relationships

Journalof Animal
Ecology1997,
66, 579-601
Interspecificabundance-rangesize relationships:an
appraisalof mechanisms
KEVIN J. GASTON*t,
TIM M. BLACKBURN:
and JOHN H. LAWTON:
tDepartmentof AnimalandPlantSciences,Universityof Sheffield,SheffieldSIO2TN, UK;andtNERC Centre
for PopulationBiology,ImperialCollege,SilwoodPark,Ascot,BerkshireSL5 7PY, UK
Summary
1. Positive relationships between the local abundance and the range size of the species
in a taxonomic assemblage are very general.
2. Explanations for these relationships typically focus on two mechanisms, based on
differencesin the niche breadths of species, or metapopulation dynamics. Others have,
however, also been suggested.
3. Here we identify and clarify all the principal mechanisms proposed to explain
positive interspecific abundance-range size relationships. We critically assess the
assumptions and predictions that they make, and the evidence in support of them.
4. A number of predictions are common to all of the biological (as opposed to
artefactual) mechanisms, but the combination of predictions and assumptions made
by each is unique, suggesting that, in principle, conclusive tests of all of the mechanisms are possible.
5. On present evidence, no single mechanism has unequivocal support. We discuss
reasons why this might be the case.
Key-words:abundance, distribution, metapopulation, niche breadth, range size.
Journal of Animal Ecology (1997) 66, 579-601
'Who can explain why one species ranges widely and is very numerous, and why
another allied species has a narrow range and is rare?'
C. Darwin (1859)
Introduction
Arguably,in the absenceof strictempiricallaws the
foundationsof much of populationand community
ecologywill have to be laid on broadstatisticalgen& McCoy 1993).One
eralizations(Shrader-Frechette
such generalizationis that locally abundantspecies
tend to be widespreadand locallyrarespeciestendto
be narrowlydistributed.Thatis, fora giventaxonomic
assemblage,there is a positive interspecificabundance-rangesize relationship(Fig. 1). Such relationshipshavebeenwidelydocumented(Gaston1996collates publ. studies;for additionssee Taylor,Winston
& Matthews1993;Solonen 1994;Waltho & Kolasa
1994; Durrer & Schmid-Hempel1995). They have
been reportedfor a diversityof taxa, from a variety
of habitatsand geographicalregions,and at a spectrum of spatial scales (for reviewssee Brown 1984;
? 1997British
EcologicalSociety
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Fig.1. Therelationshipbetweenlocaldensity(log0iterritories
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on farmlandCommonBirdCensusplots in Britainin 1975.
For furtherdetailssee Blackburnet al. (unpublished)
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580
Abundance-range
size relationships
Gaston & Lawton 1990a; Lawton 1993; Gaston
1994a, 1996, 1997). In addition to contemporary
assemblages they have also been reported for palaeontological ones (Buzas et al. 1982; McKinney 1997).
Indeed, positive interspecific abundance-range size
relationships have come to be described as 'almost
without exception' (Hanski, Kouki & Halkka 1993).
Although there is considerable unexplained variance about positive interspecific abundance-range
size relationships, the existence of the pattern has
motivated a search for a general explanation that transcends the idiosyncrasies of particular assemblages. A
number of mechanisms have been proposed, embracing sampling artefacts, species attributes and population dynamics. A few explicit tests of their validity
have been performed (e.g. Burgman 1989; Hanski et
al. 1993), but with no clear resolution as to whether
any one explanation is appropriate in all cases;
whether more than one mechanism is operating; and,
if so, the circumstances in which different mechanisms
might apply or predominate.
In this paper we do four things: we identify and
clarify the principal mechanisms postulated to determine positive interspecific abundance-range size
relationships (summarized in Table 1); we assess the
evidence for their assumptions; we identify the predictions which the different mechanisms make; and,
where it is available, we assess the evidence in support
of these predictions. Throughout, we concentrate on
assemblages of closely related, ecologically similar
species. It is for these that abundance-range size
relationships tend to be strongest (Brown 1984; Gaston 1994a). We concentrate on studies in which both
the local abundances and distributions of species are
assessed across the same study area, rather than the
circumstance in which local abundances are assessed
across study sites in a very confined area relative to
that over which distributions are determined. We also
deal, briefly, with the few exceptions to the general
positive relationship. For convenience, following Gaston & Blackburn (1996a), we distinguish between two
kinds of analyses. First, there are those performed
over areas that embrace all, or a very large proportion,
of the extents of the geographical ranges of the species
concerned (e.g. most studies performed at continental
or oceanic scales). Second, there are those performed
over areas that embrace the entire geographical ranges
of none, or only a small proportion, of the species
concerned (e.g. most studies performed at national,
provincial and smaller scales). We term these 'comprehensive' and 'partial' analyses, respectively, (Gaston & Blackburn 1996a). Most studies of abundancerange size relationships have been partial.
At the outset, it is desirable to clarify some terminology. By 'local abundance' we mean the number
of individuals of a species at a site, and where averaged
across space we assume that (unless otherwise specified) only those sites are included at which individuals
of the species are present (this avoids artefactual positive abundance-range size relationships resulting
from mean abundances being a direct function of the
number of sites at which species did not occur; Lacy
& Bock 1986). We use 'range size' in a generic sense
to refer to the distributional extent of a species in a
study area; we do not restrict its application to the
entire geographical ranges of species.
Eight principal mechanisms have been proposed to,
or might conceivably, generate positive relationships
between the local abundance and regional distribution
of species. The first two mechanisms we regard as
essentially artefactual and the other six as essentially
biological. We consider the form, assumptions and
predictions of each of these in turn. Predictions that
are common to all of the biological mechanisms, albeit
for different reasons, are addressed in a later section.
1. Sampling artefact
The first question to ask of the positive interspecific
abundance-range size relationship is whether or not
it is real. Of most concern is that the pattern may
simply represent a sampling artefact. If levels of sam-
sizerelationships
Table1. Summaryof the principalmechanismsproposedto explainpositiveinterspecific
abundance-range
Mechanism
Relationshipresultsfrom:
1. Samplingartefact
of the rangesizesof specieswithlowerlocal
Systematicunderestimation
abundances
of speciesas datapointsfor statisticalanalysis
Non-independence
Speciescloserto the edgesof theirgeographicalrangeshavelowerabundancesin,
and occupya smallerproportionof, studyarea
Attainmentof higherlocal abundancesandwiderdistributionsby specieswith
greaterresourcebreadths
Attainmentof higherlocal abundancesandwiderdistributionsby specieswith
greaterresourceavailability
habitatselection
Density-dependent
Metapopulationstructures
intrinsicgrowthratesacrosssitesfor
Similarspatialpatternsof density-dependent
deathratesableto
differentspecies,but specieswith lowerdensity-independent
attain higherabundancesand occupymoresites
2. Phylogeneticnon-independence
3. Rangeposition
4. Resourcebreadth
5. Resourceavailability
? 1997British
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Journalof Animal
Ecology,66, 579-601
6. Habitatselection
7. Metapopulationdynamics
8. Vitalrates
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581
K.J. Gaston,
T.M. Blackburn&
J.H. Lawton
pling effort are insufficient, then species that occur
at low densities will tend to be recorded from fewer
localities than species that occur at high densities, even
if they are actually equally widely distributed. Indeed,
it is not unknown for locally rare species, originally
thought to have very restricted ranges, to be found to
be considerably more widely distributed after greater
sampling effort. A spurious, but potentially strong,
positive interspecific abundance-range size relationship may result from sampling artefacts. This possibility was apparently first explicitly noted by Brown
(1984), and has subsequently been discussed by a number of other authors (Gaston & Lawton 1990a;Wright
1991; Hanski et al. 1993; Gaston 1994a; see also
McArdle 1990; Reed 1996).
EVALUATION
Determining empirically the effect of sampling on
observed abundance-range size relationships is not
straightforward. Only Hanski et al. (1993) have
explicitly attempted to do so. They assume a distribution of local abundances that is negative
binomial, and thence that the fraction of unoccupied
sites (p0) would be approximately related to average
density (x) across all sites (occupied and unoccupied)
by the expression:
ln(-lnpo)
? 1997British
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Journalof Animal
Ecology,66, 579-601
ln(lnx)-2
n CV
eqnl
where CV is the coefficient of spatial variation of
abundances (also calculated across all sites). The distribution of a species decreases with decreasing density
and increasing spatial variation in densities. A significant positive relationship between CV and po was
observed when controlling for the effect of density,
using data sets for several assemblages. Hanski et al.
(1993) argue that this suggests that sampling contributes substantially to many observed interspecific
abundance-range size relationships.
The difficulty with this approach is that it remains
unclear whether this result is a product of sampling
per se, or whether it simply reflects genuine underlying
changes in the spatial distribution of individuals of
species of differing regional distribution and density.
Without perfect knowledge of the real detailed distribution of local abundances it is not possible to
distinguish between the effects of sampling and real
changes in the patterns of abundance of individual
species.
Brown (1984) has argued that a sampling effect is
not sufficientto account for positive abundance-range
size relationships for two reasons. First, the effect is
too small, in at least some cases, to account for the
observed magnitude of change in geographical range
size with increasing density. Second, the distributions
of species in some assemblages are very well-known,
and unlikely to be seriously underestimated. Neither
argument denies the possibility that there could be a
sampling component to observed abundance-range
size relationships, rather they suggest that it is not the
sole explanation for the patterns. A sampling explanation seems most likely to apply at small spatial
scales, especially when the distribution of a species is
determined from the proportion of samples in which
it occurs (Hanski 1991a; Hanski et al. 1993), and when
sampling intensity is moderate to low.
PREDICTIONS
The sampling explanation makes one simple prediction: adequate sampling will find no real interspecific abundance-range size relationship. Anything
less admits that there must be a role for some aspect
of species biology or some other artefact in generating
a positive relationship, and hence that the sampling
explanation alone is not sufficient. Those assemblages
where independently of sampling a positive abundance-range size relationship exists beyond all reasonable doubt (e.g. British birds; Fuller 1982; O'Connor
& Shrubb 1986; O'Connor 1987; Sutherland & Baillie
1993; Lawton 1996a; Gaston et al. 1997) falsify this
prediction. Therefore, other explanations for the positive relationship must be sought.
2. Phylogenetic non-independence
An artefactual positive interspecific relationship
between abundance and range size could also result
from the shared common ancestries of species in an
assemblage. Because of their phylogenetic relatedness,
species do not constitute independent data points for
analysis, inflating the degrees of freedom available for
testing statistical significance (Harvey & Pagel 1991;
Harvey 1996). If sufficient, this inflation may falsely
imply that relationships exist which in reality do not.
Some of the clearest examples of such difficulties
derive from analyses of morphological and life history
traits. However, the same arguments apply to traits
that affect ecological relationships among species
(Harvey 1996). For example, Nee et al. (1991a)
showed that the negative relationship between abundance and body mass in British birds resulted from a
difference between passerines, which tend to be smallbodied and common, and non-passerines, which tend
to be large-bodied and rare. Within each group there
was no evidence for any association between abundance and body mass. Therefore, this relationship
results from a single evolutionary difference between
passerines and non-passerines, rather than any general
tendency for abundance and body mass to be negatively related. The positive abundance-range size
relationship could likewise represent a simple difference between taxonomic groups (say, one set of related
taxa that are geographically restricted and rare, and
another set of related taxa that are geographically
widespread and abundant), rather than any general
tendency for more abundant species to have larger
range sizes.
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582
EVALUATION
Abundance-range
size relationships
In common with studies of most purported patterns in
macroecology, the phylogenetic relatedness of species
has seldom been controlled for in analyses of abundance-range size relationships. Those studies that
have been performed suggest, however, that significant positive correlations do not result from the
non-independence of data points. First, Gaston et al.
(1997, in press) consistently recover significant positive abundance-range size relationships for British
birds when controlling for phylogeny, when abundance and range size are calculated at a variety of
spatial and temporal scales. Secondly, Blackburn et
al. (1997) document positive interspecificrelationships
between total population size and range size for both
bird and mammal assemblages in Britain. Thirdly,
Quinn et al. (in press) find positive relationships when
controlling for phylogeny in analysing abundancerange size relationships for British macrolepidoptera.
PREDICTIONS
The phylogenetic non-independence explanation for
the positive abundance-range size relationship, like
that based on sampling effects, makes one simple prediction: there will be no interspecificabundance-range
size relationship once the effects of phylogenetic
relatedness among species have been controlled for.
Those assemblages where a positive relationship has
been demonstrated after accounting for phylogenetic
effects (see above) falsify this prediction. For British
birds in particular, neither of the first two, essentially
artefactual, hypotheses for the abundance-range size
relationship apply. As it is unlikely that this assemblage is unique in these respects, general biological
explanations for the pattern must be sought.
CONCLUSION
? 1997British
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Journalof Animal
Ecology,66, 579-601
It is important to control for the phylogenetic relatedness of species in comparative analyses (Harvey &
Pagel 1991; Harvey 1996). The failure of most studies
of abundance-range size relationships to do so means
that the reported statistical significance of positive
correlations may potentially result from the non-independence of data points. We suggest, however, that
on present evidence it seems highly unlikely that most
such results are indeed artefacts of this kind. A biological (as opposed to artefactual) explanation for
these relationships is required. However, of greater
concern is that much of the evidence available with
which to test possible biological determinants of abundance-range size relationships derives from analyses
of interspecific relationships between various other
variables, or between other variables and abundance
or range size. In the vast majority of cases these analyses have not controlled for phylogenetic effects. In
subsequent sections of this paper, in the absence of
anything better (we cannot repeat the large numbers
of analyses done by others), we will make use of the
available evidence, taking it at its face value and
largely ignoring problems of phylogeny. Nonetheless,
we caution that this is an optimistic approach, and
that the results of some of the studies which we cite
may ultimately prove to have been misleading.
3. Range position
If, on average, abundances decline towards the edges
of the geographical ranges of species (Grinnell 1922),
and species occupy a smaller proportion of a study
area when they are closer to the edges of their ranges,
then positive interspecific abundance-range size
relationships could arise as a result (Bock & Ricklefs
1983; Bock 1984). Species closer to the edges of their
ranges might have a smaller range size in the study
area in two ways. First, they might only penetrate a
relatively small way into the study area (e.g. they
might be constrained to its northern parts). Second, if
not only do abundances decline towards range limits
but occurrence also becomes more patchy, then a species closer to the edge of its range might be widely
dispersed through a study area but occupy a relatively
small proportion of it. Whichever situation applied,
the species for which the centres of their geographical
ranges overlapped the study area would occur at relatively high abundance and be widely distributed,
whilst those for which only their range edges overlapped the study area would occur at relatively low
abundances and would be restricted in occurrence.
EVALUATION
The first step in evaluating this hypothesis is obviously
to ask how good is the evidence that levels of abundance and occupancy indeed change across the ranges
of species in the fashion postulated. Maps of abundance surfaces across parts or all of the geographical
ranges of particular species illustrate that these are
often very complex (e.g. Root 1988; Gibbons, Reid &
Chapman 1993; Price, Droege & Price 1995), and may
vary dramatically through time (e.g. Taylor & Taylor
1979; Taylor 1986; Cammell, Tatchell & Woiwod
1989) although this may not always be the case
(Brown, Mehlman & Stevens 1995). Nonetheless, a
number of studies have demonstrated declines in average abundances or occupancy towards range limits
(e.g. McClure & Price 1976; Bock, Bock & Lepthien
1977; Hengeveld & Haeck 1981, 1982; Brown 1984;
Bart & Klosiewski 1989; Roberts, Dawson Shepherd
& Ormond 1992; Svensson 1992; Telleria & Santos
1993; Hengeveld 1994; Maurer 1994; Whitcomb et al.
1994; Brown et al. 1995; Carey, Watkinson & Gerard
1995). For example, Telleria & Santos (1993) observe
such a pattern for species of insectivorous passerines
in Iberian forests. Simple models of decline in average
abundances seem inadequate, however, and a more
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583
K.J. Gaston,
T.M. Blackburn&
J.H. Lawton
appropriate model is one in which species near the
edges of their ranges show uniformly low abundances,
whilst those near the centres exhibit a wide range of
abundances (Brown et al. 1995; Enquist, Jordan &
Brown 1995). Hengeveld (1989) suggests that similar
distributions of densities occur at local scales as at
regional ones. Thus the abundance structures of the
geographical ranges of species can be visualized as a
series of abundance peaks, with the magnitudes of
these peaks declining towards the range periphery.
There are case studies that find conflicting patterns
or less convincing evidence for declines in average
abundance or occupancy (e.g. Rapoport 1982; Brussard 1984; Carter & Prince 1985; Woods & Davis
1989), and concerns have been expressed that results
may, in part, rest on the spatial resolution of analysis
(Wiens 1989). Moreover, it tends to be assumed that
if abundance declines towards range limits so does
occupancy, and vice versa, but there is no necessary
reason that this should be so, and in some examples
it plainly is not. Indeed, Carter & Prince (1988) argue
that whilst, in general, there is evidence for a decline
in the frequency of populations (occupancy) towards
range limits, there is little evidence to support the view
that populations become smaller. There have been
very few studies which, for the same species, have
sought to document spatial variation both in local
abundances and occupancy towards range limits.
PREDICTIONS
? 1997British
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Journalof Animal
Ecology,66, 579-601
Despite these uncertainties and difficulties, if we
accept that assumptions regarding the abundance and
occupancy structure of the edge vs. the centre of species' ranges are reasonable, what predictions does this
particular model make?
1. A positive interspecific abundance-range size
relationshipwill not exist whenbased on measuresof the
entire geographical ranges of species and their average
abundancesacross those ranges. There is evidence that
this is not so. Analyses of such data document positive
abundance-range size relationships. For example, in
determining abundance-range size relationships for
some North American winter landbirds, Bock (1984)
selected only species whose New World winter ranges
appeared to be largely (> 75% by area) confined to
the area for which good abundance data were available. A positive abundance-range size relationship
was still recovered.
2. Thosespecies in an assemblage whichare locally rare
or occupy a small range size will tend, on average, to
be nearer the edge of their geographical range. Hengeveld & Haeck (1982) have argued that this is indeed
so (see also Hodgson 1986). They have shown, for
several assemblages, that there is an increase in the
numbers of individuals or numbers of grid squares
occupied by species for which an area is more central
to their geographical range. Unfortunately, the
interpretation of such analyses is complicated by more
general positive correlations between the abundances
or range sizes of species at different spatial scales (see
Gaston 1994a for a review). If species which are closer
to the edge of their geographical range in a study area
are also those which have smaller geographical range
sizes or lower range-wide local abundances, it is not
always clear to what extent the lower abundances and
more restricted distributions in the study area are a
product of proximity to range edge and to what extent
they are a product of self-similarityin abundances and
range sizes.
3. Interspecificabundance-rangesize relationshipsmay
tend to be lower triangular,such that widely distributed
species may have either high or low densities, while
geographically restrictedspecies can only have low densities. This prediction follows from the observation
that some species which are close to the edge of their
geographical ranges, and hence have low local abundances, may nonetheless be quite widespread in a
study area. Triangularrelationships appear to be more
typical of comprehensive analyses than partial ones,
for assemblages of greater taxonomic and ecological
diversity, and when range sizes are measured as
extents of occurrence (the area within the outermost
limits to a species' occurrence) rather than areas of
occupancy (the area within the outermost limits that
the species actually occupies; Gaston 1991). However,
there are exceptions, and the pattern might be
expected for a variety of other reasons (see 'Predictions in common' for discussion and references).
Overall, the prediction finds moderate support.
CONCLUSION
In summary, while the predictions of the range position hypothesis are often upheld, controlling for its
effects reveals that it cannot explain some documented
interspecific abundance-range size relationships. This
is not to say that the effect does not contribute to
many documented patterns, especially in partial
analyses. The extent to which it does will depend upon
the proportion of between-species variation in abundances which can be accounted for simply by the
position of the area in which those abundances were
measured with respect to the centres of the geographical ranges of the different species.
In discussing the range position hypothesis, we have
not given reasons why population abundances and
proportion of sites occupied might decline towards
the edges of species ranges. Such a pattern might be
generated by several of the processes discussed below
(e.g. hypothesis 4; see Brown 1984). In other words,
this hypothesis, and the hypotheses that follow are
neither completely distinct nor independent. We will
encounter this problem again at several points later in
this paper.
4. Breadth of resource usage
Brown (1984) took the possibility of a link between
the abundance structure of species' geographical
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ranges and the interspecific abundance-range size
relationship a step further than the range position
hypothesis. He argued that the decrease in the abundance of a species towards the periphery of its range
and the interspecific abundance-range size relationship were united by a common theoretical explanation, based on three assumptions: (i) the abundance
and distribution of each species is determined by combinations of many physical and biotic variables, and
that spatial variation in population density reflects
the probability density distribution of the required
combinations of these variables; (ii) some sets of
environmental variables are distributed independently
of each other, and environmental variation is spatially
autocorrelated; (iii) closely related, ecologically similar species differ substantially in only one or a very
small number of niche dimensions. From the first two
assumptions it follows that density should be highest
at the centre of the range of a species and should
decline towards the boundary. Other models of the
abundance structure of ranges exist, although most
seem closely related to Brown's model (e.g. Huffaker
& Messenger 1964; Williams 1988; Hengeveld 1989,
1993; Hall, Stanford & Hauer 1992). If niches are
multidimensional and variation in the environment
tends to be spatially autocorrelated, there should also
be a positive correlation between abundance and
range size. Species that have broad environmental
tolerances and are able to use a wide range of resources
will in so doing achieve high local densities and will
be able to survive in more places and hence over a
larger area; the 'jack-of-all-trades' is master of all.
Those that have a narrow environmental tolerance
and are able to use only a narrow range of resources
will be unable to attain either high local densities
or extensive distributions; the specialist is never very
successful. This has subsequently become known as
'Brown's hypothesis'. For present purposes we will,
however, be more explicit and refer to it as the
'resource breadth' hypothesis. In so doing, and in
subsequent discussion, we mean 'resource breadth' to
embrace the sets of environmental conditions which a
species exploits, as well as resources which are appropriated; it will thus be used interchangeably with
'niche breadth'.
Brown's explanation of the interspecific relationship between local abundances and range sizes
assumes that more abundant and widespread species
have an ability to use a broader range of resources.
There are both theoretical and empirical difficulties
with this assumption. Theoretically, whilst it seems
reasonable to presuppose that range size might
increase with resource breadth, it is more difficult to
see why resource breadth should affect local abundance in a similar fashion (Kouki & Hayrinen 1991;
Hanski et al. 1993). There is no obvious reason why
? 1997British
an ability to exploit a range of resources and hence
EcologicalSociety
occur more widely should enable species to attain a
Journalof Animal
579-601
Ecology,66,
greater local abundance (unless species with greater
584
Abundance-range
size relationships
niche breadths are able to exploit more kinds of
resources locally and hence become more abundant;
see below). Nonetheless, a positive relationship
between niche breadth and abundance is assumed by
many models of species-abundance distributions (e.g.
Sugihara 1980; Kolasa 1989; Tokeshi 1990).
EVALUATION: ABUNDANCE AND NICHE
BREADTH
Empirical tests seem to substantiate the theoretical
difficulty of relationships between density and
resource breadth. We will concentrate principally on
the small number of studies carried out within the
framework of 'niche pattern', a plot in three-dimensional space of the densities, niche breadths and niche
positions of the species in an assemblage (Shugart &
Patten 1972). Here, niche breadth measures the range
of physical conditions or habitat used by a species,
and niche position measures the availability or typicality of these conditions. We will also be concerned
solely with more recent studies, which have accounted
for many of the statistical inadequacies of earlierwork
in this area (see Carnes & Slade 1982; Van Horne &
Ford 1982).
In virtually all published cases of which we are
aware, studies of niche pattern fail to document a
positive interspecific relationship between niche
breadth and density (Table 2); many correlations are
negative. There are several reasons why this result
might be obtained. First, it could arise simply because
of the small numbers of species typically involved.
However, a meta-analysis reveals no significant overall relationship (population effect size [weighted mean
r]= -0.048, NS, total n = 158, number of studies = 8) and no evidence that studies do not share a
common effect size (%2 = 6.747, d.f. = 7, NS).
Second, it has been suggested that the failure to
find positive correlations between niche breadth and
abundance could arise because analyses have been
performed at an inappropriate spatial scale. Undoubtedly the habitat occupancy of a species changes with
spatial scale (e.g. Wiens, Rotenberry & Van Horne
1987). Seagle & McCracken (1986) argue that the
results of niche pattern analyses and explanations of
positive abundance-range size relationships based on
differences in niche breadth are not incompatible,
because in the former niche breadth and abundance
are measured at the same spatial scale, while in the
latter this is not the case. However, their argument
is based on the assumption that differences in niche
breadth only directly determine differences in range
size, not differencesin local abundance (because if they
did determinedifferencesin local abundance, we would
expect to see positive relationships between niche
breadth and local density). If the resource breadth
hypothesis is correct, niche breadth should determine both range size and abundance. This serves to
highlight two additional implicit assumptions of
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585
K.J. Gaston,
T.M. Blackburn&
J.H. Lawton
Table 2. Summary of published studies of niche pattern (plots in three-dimensional space of the densities, niche breadths and
niche positions of the species in an assemblage) and the documented interspecific relationships between density and niche
breadth(seetext for details)
P
n
-0.38
-0.35
-0.12
-0.31
-0.10
NS
NS
NS
NS
NS
7
8
7
13
7
MDS
DFA
DFA
DFA
0.08
-0.51
-ve
NS
NS
?
92
11
13
14
DFA
DFA
0.13
-0.85
NS
<0.01
13
8
Taxon
Technique
Slugs
Smallmammals
Salamanders
Birds
Rodents
PCA
FA
DFA
DFA
PCA
Birds
Rodents
Lizards
Lizards
Rodents
[Rodents*
r
Comments
Source
Abundancein
optimalhabitat
Meandensity
Maximaldensity
[Bukhara]
A 'weak'relationship
[Mapimi]
Maximaldensity
Maximaldensity
Seagle& McCracken(1986)
Seagle& McCracken(1986)
Seagle& McCracken(1986)
Seagle& McCracken(1986)
Robey et al. (1987)
Mac Nally (1989)
Rogovinet al. (1991)
Shenbrotet al. (1991)
Shenbrotet al. (1991)
Shenbrot(1992)
Shenbrot(1992)]
f Statisticsnot givenin originalpublication.
* Common& abundantspeciesonly.
'Technique' is the ordination method used: (PCA, principal components analysis; DFA, discriminant function analysis; FA,
factor analysis; MDS, multidimensional scaling); r is the correlation coefficient, P the probability and n the number of species.
Brown's hypothesis: the niche breadths exhibited by
species at local and regional scales should be positively
correlated, and there should be little spatial variation
in the use of resources made by individual species (i.e.
there should be little local adaptation). These two
points are closely related, and, put simplistically, concern where species tend to lie in the four cell matrix
of local niche breadth (specialist or generalist) vs.
regional niche breadth (specialist or generalist; Table
3). If a species shows local adaptation, so that individuals use a limited range of resources locally, but
the species as a whole uses a wide range of resources
across a region, then there is no reason why the local
abundance of the species should be greater than that
of a second species that uses an equally limited range
of resources locally, but shows no variation across the
region. The findings of studies of herbivorous insects
on particular food plants confirm this point, with no
evidence that species specializing on one or a small
number of host species are less abundant than those
which feed on a variety of hosts (Gaston & Lawton
1988; Root & Cappuccino 1992).
Mac Nally (1995) notes a dearth of studies of
relationships between local and regional ecological
versatility, and the absence of a simple relationship in
those studies which have been performed (e.g. Ford
1990; Mac Nally 1995). It has even been suggested
that, entirely contrary to the requirements of the
resource breadth hypothesis, relationships are likely,
if anything, to be negative (Cody 1974); species which
tend to have broad local niches will tend to have
narrow regional ones, and vice versa. This concords
with the large body of evidence that species do exhibit
local adaptation, exploiting different resources and
exhibiting different environmental tolerances in
different areas (e.g. Machado-Allison & Craig 1972;
Fox & Morrow 1981; Davidson 1988; Svensson 1992;
Bertness & Gaines 1993; Ayres & Scriber 1994; Krebs
& Loeschcke 1995); there is some evidence that niche
breadth changes towards range limits (Svensson
1992). In fact, Brown (1984, p.275) admits the possibility of spatial variation in the niche of a species
but assumes that niches are constant over space, an
assumption he considers justified so long as ecological
variation within species is small relative to that across
species.
A third reason for the failure to find positive correlations between niche breadth and abundance could
be that these analyses are failing to measure the relevant niche axes (the appropriate axes have not been
measured at all, or the wrong combinations of axes
have been considered). This is difficult to refute, and
Table3. Combinationsof the nichebreadthsof speciesat local andregionalscales,and theirimplications
Niche breadthat local scale
Narrow
? 1997British
EcologicalSociety
Journalof Animal
Ecology,66, 579-601
Niche breadth
at regionalscale
Narrow [Use samenarrow
resourcesin differentlocalities]
Broad [Use differentresources
in differentlocalities]
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Broad
[Makebroaduse of a smallrange
of resources-an impossiblecombination?]
[Usewiderangeof resourcesin all localities]
586
Abundance-range
size relationships
reflects a more general difficulty with the resource
breadth hypothesis. The concept of an n-dimensional
niche on which it is based, while a powerful heuristic
tool, is impossible to make fully operational (Colwell
& Futuyma 1971). This essentially makes Brown's
hypothesis impossible to test. Failure to find appropriate relations between niche breadths and local
abundances can always be explained away, and where
such relations are documented it remains unclear how
robust they would be if niche breadths were measured
using alternative axes. One circumstance in which this
problem would be reduced, although still not avoided,
would be if it were to be demonstrated that, for the
assemblage of concern, niche breadths on different
axes were positively correlated. We are not aware of
evidence that this is so. Indeed, it would be surprising
if it were.
EVALUATION:
RANGE
SIZE AND
NICHE
BREADTH
Thus far, we have restricted discussion to the validity
of assuming that locally more abundant species have
greater niche breadths. Equally, however, the resource
breadth hypothesis requires that more widespread
species also have greater niche breadths. Unfortunately, many (perhaps most) empirical studies of
interspecificrelationships between breadth of resource
use and range size are confounded by sample size
effects (Gaston 1994a). If environmental or habitat
breadth is determined for more individuals or at a
greater number of sites for abundant and widespread
species than for locally scarce and restricted species,
then a positive correlation would be expected. This
effect particularly frustrates attempts to draw some
general conclusion in the present context from studies
of the relationship between the size of a species' geographical range and the number of habitats or breadth
of environmental conditions which lie within that
range (e.g. Jackson 1974; Thomas & Mallorie 1985;
Stevens 1989;Pomeroy & Ssekabiira 1990;Pagel, May
& Collie 1991; Shkedy & Safriel 1992; Mawdsley &
Stork 1995), and of the relationship between the number or diversity of host species that a consumer
exploits and the size of its geographical range (e.g.
Hodgson 1993; Kitahara & Fujii 1994).
Where sampling effects have clearly been accounted
for, significant positive relationships between breadth
of resource use and range size have not in the main
been documented. Thus, Burgman (1989) found that
the habitat volumes (environmental tolerances) of a
suite of plants from southern Western Australia were
not significantly different for regionally scarce and
ubiquitous species once sample bias was taken into
account, although they were if this bias was ignored.
Likewise, the relationships between the number of
? 1997British
sites inhabited by species and their habitat volumes
EcologicalSociety
5% of variance when sample bias was
Journalof Animal
explained
Ecology,66, 579-601 taken into account, but 40% when it was ignored. A
similar pattern was observed for separate guilds of
plants, negating any explanation of the results as a
consequence of species being ecologically too dissimilar. Similarly, Kouki & Hayrinen (1991) found no
relationship between local abundance and habitat specialization after standardizing for sample size.
PREDICTIONS
We conclude that the empirical evidence for the
assumptions of the resource breadth hypothesis is
weak. Nevertheless, following the logic adopted in
considering the previous hypotheses, we can still ask
what predictions it makes. Explicit predictions have
not to date been derived from the resource breadth
hypothesis. However, three pieces of information have
been argued to be consistent with it. First, it has been
found that when the local abundances of species are
measured in habitats which are atypical of the spectrum of habitats in the geographical region of interest
(i.e. that over which range sizes are determined), a
negative abundance-range size relationship results
(Ford 1990; Gaston & Lawton 1990a). This would
follow from the resource breadth hypothesis if there
were no spatial autocorrelation in environmental variables between the typical and atypical habitats. However, this argument implies that there are certain limits
within which broad-niched species are 'master-of-all'.
It is not clear whether those limits are set by features
of the species or features of the habitat, and if it were
the latter, the pattern would be more consistent with
the resource availability hypothesis (hypothesis 5,
below; Gaston 1994a).
Second, Lawton (1993) suggests that the resource
breadth hypothesis may also gain some support from
the observation that introduced species are, in some
studies at least, more likely to establish the more widespread and abundant they are in their native environments (e.g. Forcella & Wood 1984; Forcella, Wood &
Dillon 1986; Moulton & Pimm 1986; Crawley 1987;
Hanski & Cambefort 1991; Roy, Navas & Sonie
1991). This observation would result if ecological
flexibility improves the likelihood of successful
invasion. It might, however, equally be expected from
several other hypotheses for the abundance-range size
relationship.
Third, it has been argued that some kind of hierarchical organization of the ecologies of species is a
necessary condition for the existence of nested patterns of species composition, in which smaller assemblages contain successive subsets of species in larger
ones, and that the best candidate may sometimes be a
pattern of included niches (Patterson & Brown 1991).
Here, species occupying fewer and fewer sites have
narrower and narrower niche breadths. However, we
are not aware of any rigorous tests of this version of
the resource breadth hypothesis, and can see difficulties in designing unbiased field tests of its assump-
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587
K.J. Gaston,
T.M. Blackburn&
J.H. Lawton
tions for the reasons already outlined, for example
sampling effects.
CONCLUSION
The resource breadth hypothesis is elegant and, in
many respects, makes good intuitive sense. It has proven important and influential, providing a stimulus for
many studies in community and population ecology.
However, accepting the impossibility of a full test
(embracing entire multidimensional niches), the evidence for the hypothesis as an explanation for positive
interspecific abundance-range size relationships
appears to us to be unconvincing. Others have reached
similar conclusions (Burgman 1989; Kouki & Hayrinen 1991).
5. Resource availability
Although this and the previous hypothesis are easily
conflated, a second distinct explanation for a positive
interspecific abundance-range size relationship based
on resource usage has been suggested (we continue to
maintain the broad application of 'resource' defined
above). This is that those species that are locally abundant and widespread utilize resources which are themselves locally abundant and widespread, whilst those
species that are locally rare and restricted in occurrence utilize resources with similar relative levels of
abundance and distribution (Hanski et al. 1993; Gaston 1994a). The two hypotheses based on resource use
may, of course, not be independent, especially if those
species with broader resource use tended also to
exploit the more abundant and widespread resources.
However, there is no necessary link.
EVALUATION
? 1997British
EcologicalSociety
Journalof Animal
Ecology,66, 579-601
This mechanism escapes the difficulty of explaining
why environmental generalists should attain higher
local abundances, but necessitates that locally abundant resources are also widely distributed. For host
specialist consumers this is obviously the case, because
positive interspecific abundance-range size relationships are found for groups of organisms at different
trophic levels. Thus, the distribution of the host plant
of a specialist herbivore is likely to be widespread if the
plant is locally abundant, and, on average, specialist
consumers feeding on widespread and locally abundant hosts will themselves tend to be widespread and
abundant. Given that the local abundance and distribution of the host plant set the upper bounds to
the local abundance and range size of the consumer,
positive relationships between the abundances or
range sizes of host and consumer may be a more
appropriate null model than no relationship. Indeed,
several such relationships have been documented. At a
local scale, Root's 'resource concentration hypothesis'
(Root 1973) predicts that phytophagous insects will
be more abundant on more abundant species of host
plants, although the supporting evidence is weak
(Karieva 1983; Strong, Lawton & Southwood 1984).
On larger scales, Dixon & Kindlmann (1990) find
a significant positive relationship between the rank
abundances of 12 species of callaphid aphids and the
rank abundances of their host plants. Similarly, Gilbert (1991) documents a positive correlation between
the rank adult abundance of species of Heliconius
butterflies and the rank of their host availability. Such
cases serve, however, merely to shift the level of explanation for abundance-range size relationships from
one species assemblage (that of the consumers) to
another (that of the hosts).
What about the resources used by other groups of
species? Are those resources which attain high levels
of local availability also widely distributed?Certainly
some authors have claimed that they are. For example,
Fuller (1982) argues that the most abundant and widespread species of breeding birds on saltmarshes in
Britain are those which utilize the typical and common
features of the marshes, while the least abundant and
poorly distributed species are those which are restricted by their preference for special habitats. Likewise,
there are examples demonstrating that locally more
abundant or regionally less restricted species utilize
resources which are themselves either locally more
abundant or regionally more widespread (e.g. Hodgson 1986).
Further indication of a relationship between
resource availability and local abundance is provided
from studies of niche pattern which document the
correlation between niche position and local abundance. A large niche position means that a species
occurs in habitats characterized by extreme values
compared with the mean value of all habitats in the
sample, and whilst some studies find no, or a weak
positive, relationship with local abundance (Mac
Nally 1989; Rogovin, Shenbrot & Surov 1991; Shenbrot, Rogovin & Surov 1991), others find negative
relationships (Seagle & McCracken 1986; Robey,
Smith & Belk 1987; Urban & Smith 1989; Shenbrot
1992).
Blackburn, Lawton & Gregory (1996) found that
British bird species with fast development, either
absolutely or for their body size, generally have higher
abundances. They suggested that this relationship
may be related to resource availability. For a species
to be able to rear offspring quickly relative to other
species, adults will have to be able to achieve a high
rate of resource provisioning. This, in turn, requires a
reliable, abundant food resource. If taxa that can rear
offspring quickly can do so because they have abundant resources available, these resources may also be
reflected in higher population abundances. Some taxa
have access to a larger resource base, and this is
reflected both in the abundance a species can attain,
and the rate at which it can provision offspring. Blackburn et al. (1996) noted that this idea is also consistent
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588
Abundance-range
size relationships
with Brown's hypothesis, because species occupying
broad niches may be able to rear offspring more
quickly if niche breadth relates to the amount of
resource available to a species. However, the resource
availability hypothesis can explain the same patterns
more parsimoniously, because it need make no
assumptions about variation in niche breadths.
PREDICTIONS
The resource availability hypothesis makes the following predictions.
1. Species that share a common resource base will not
exhibit a positive interspecific abundance-range size
relationship. For example, monophagous species utilizing the same host plant should not exhibit a positive
relationship because they should be able to attain the
same range size (assuming some equivalence of plants
in different regions). This is problematic to test,
because of the paucity of taxonomically reasonably
closely related species sharing a common resource
base.
2. Resource specialists can be abundant and widespread. Indeed, specialists on widely distributed, abundant resources should be more abundant and widely
distributed than either specialists or generalists that
utilize limited resources. Evidence for this is provided
by the failure of studies to document positive relationships between abundance or range size and niche
breadth (see above).
CONCLUSION
In summary, although scattered, there is evidence,
direct and indirect, that assumptions of the resource
availability hypothesis are met, at least in some assemblages. This begs the obvious question of why widely
distributed resources should also be locally more
abundant.
6. Habitat selection
? 1997British
EcologicalSociety
Journalof Animal
Ecology,66, 579-601
Brown's hypothesis is completely deterministic and
contains no dynamics (Brown 1995, p.65). The local
abundance of a species does not directly affect the
size of its range. Rather, greater local abundance is
associated with larger range size because the two are
both determined by breadth of resource usage.
Plainly, however, this is simplistic. It has long been
known that some species exhibit density-dependent
habitat selection (driven through intraspecific competition), occupying more habitats when densities are
high and less when they are low (for a review see
Rosenzweig 1991). Assuming some broad commonality between species in this dynamic, then locally
more abundant species will tend to occupy more habitats and to be more widespread, without the necessity
of local abundance being determined by niche
breadth. Whatever the precise mechanism, the out-
come will be a positive interspecific abundance-range
size relationship (O'Connor 1987).
EVALUATION
The principal assumptions of this model are that density-dependent habitat selection is widespread, and
that within a taxonomic assemblage the local abundance-habitat diversity relationships exhibited by
different species are broadly similar. Both are difficult
to assess. Whilst density-dependent habitat selection
is known to be exhibited by several species in particular taxa (e.g. passerine birds, fish; O'Connor 1987;
Wiens 1989; MacCall 1990; Marshall & Frank 1995),
in other groups for which abundance-range size correlations are well established (butterflies, higher
plants) there is no evidence for density-dependent
habitat selection. Also, examples are known of assemblages for which species do not show broader habitat
use at higher densities (Rogovin & Shenbrot 1995).
Some of these problems can be circumvented if
number of habitats is replaced by number of patches,
so that increasing density is associated with an
increase in patch occupancy, rather than in the types
of patches occupied. A model in this form has been
suggested by Maurer (1990), who examined the number of individuals of different species that can occupy
a series of habitat patches. The critical parameter in
his model is the probability that an individual can
obtain sufficient resources to remain in a patch, which
depends on attributes both of the species and of the
patch. It predicts a non-linear increase in the average
number of individuals of a species per patch as the
proportion of patches occupied increases. Assuming
that the distribution of resources was spatially autocorrelated, that the resources available in each patch
were drawn from the same n-dimensional resource
availability distribution, and that species resource use
distributions were drawn uniformly from the overall
resource distribution, this translated into a positive
interspecific abundance-range size relationship, with
slope proportional to resource number (n). Therefore,
this model combines density-dependent habitat selection with aspects of the resource breadth hypothesis.
PREDICTIONS
The habitat selection hypothesis makes the following
predictions.
1. Species that have undergonemajorreductionsin total
population abundance (driven by extreme density-independent events-heavy overwinter mortality for
instance) should not only have reduced ranges after
the event, but also occupy fewer habitats Indeed, the
resulting reduction in range size should be driven primarily by withdrawal from particular habitats.
2. Further to thefirst prediction, the first habitat to be
abandonedwill be that in whichpopulationperformance
is lowest, followed by the habitat with the next lowest
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K.J. Gaston,
T.M. Blackburn&
J.H. Lawton
populationperformance, and so on. The problem with
testing this prediction is, of course, that if individuals
are distributed at random across a landscape made up
of different types of habitats in scattered patches,
small populations may, by chance, occupy fewer habitats than large populations. Critical tests therefore
require both the development of a suitable null model,
and an assessment of population performance in
different habitats. 'Population performance' also
requires careful measurement. If density-dependent
habitat selection generates an Ideal-Free Distribution
(Fretwell & Lucas 1970) of individuals across habitats,
then at equilibrium, and on average, all individuals
have the same fitness; a more appropriate (but still
not perfect) measure is the intrinsic rate of increase of
the population in each habitat when rare.
3. This leads to an additional prediction (which is an
inherent assumption of the model) that as populations
decline, and habitats are abandoned, survivingpopulations in core habitats should show density-dependent
increases in populationperformance.
4. Maurer's model predicts that the slope of the interspecific abundance-range size relationship should be
proportional to number of resources (Maurer 1990).
Maurer tested this prediction by comparing the slope
of two bird communities, one each in low and high
productivity boreal forests, on the basis that the high
productivity region should have higher values of n.
The community in the high productivity region did
indeed have the higher slope. His interpretation is
that unproductive environments show less variation in
resources available in a patch, so that narrow-niched
species can use most patches, and broad-niched species gain no advantage in higher densities by being
able to use a wide range of resources. In contrast,
productive environments show more variation in
patch resources, so that only broad-niched species can
occupy many patches, and their ability to exploit a
range of resources leads to higher densities in the
patches they do occupy. Clearly, the aspects of this
model in common with the niche breadth hypothesis
will suffer from identical problems. Exactly how the
model performs with the niche breadth assumptions
removed is unclear at present.
CONCLUSION
Although plainly postulated as a mechanism determining interspecific abundance-range size relationships (O'Connor 1987), the habitat selection hypothesis has subsequently largely been ignored or
overlooked. It is unlikely to be of importance in determining these relationships for many taxa, but may
perhaps play a role for some.
? 1997British
EcologicalSociety
Journalof Animal
Ecology,66, 579-601
7. Metapopulation dynamics
A positive abundance-range size relationship is an
assumption of some metapopulation models (e.g.
Hanski 1982), but is a prediction of other, often closely
related, metapopulation models. Following Hanski
(1991b), we can divide the latter models into two
groups, respectively, termed the carrying capacity
hypothesis and the rescue effect hypothesis.
CARRYING CAPACITY HYPOTHESIS
The carrying capacity hypothesis (Nee, Gregory &
May 1991) assumes that different species in an assemblage have different local carrying capacities, and that
those which attain higher local population sizes have
a lower extinction rate and/or a higher colonization
rate than those which attain small local population
sizes. It follows that the locally more abundant species
will occupy more patches at equilibrium. Hanski
(1991b) argues that if we assume that the local carrying capacity of a species is a reflection of its ecological specialism, then the carrying capacity hypothesis is Brown's (1984) resource breadth hypothesis (see
above) in another guise. There are, however, some
important distinctions. First, the carrying capacity
hypothesis embodies a dynamic link between local
abundance and regional distribution. In contrast,
Brown's model has no such requirement, the local
abundance and regional distribution of a species are
both determined independently by its breadth of
resource use. Second, this genre of metapopulation
models assume that all patches are equal, and hence
that all species have the capacity to occupy all patches.
Brown's (1984) hypothesis explicitly assumes that patches are very different and hence that not all species
have the capacity to occupy all patches. Third, the
carrying capacity hypothesis does not require that
the interspecific variation in carrying capacities be
determined by differences in breadth of resource use.
It could equally be generated by differencesin resource
availability alone.
RESCUE EFFECT HYPOTHESIS
The rescue effect hypothesis (Hanski 1991a, b; Gyllenberg & Hanski 1992; Hanski & Gyllenberg 1993;
Hanski et al. 1993) does not assume interspecific
differences in carrying capacities. Rather it assumes
that immigration decreases the probability of a local
population becoming extinct (the rescue effect), and
that the rate of immigration per patch increases as the
proportion of patches which are occupied increases.
Here, for many parameter values, a positive relationship between local abundance and number of occupied patches can result.
EVALUATION
A significant difference between the carrying capacity
and rescue effect hypotheses is that in the former local
abundance affects distribution, but not vice versa,
whilst in the latter local abundance affects distribution
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590
Abundance-range
size relationships
and distribution affects local abundance; Hanski
(1991b) suggests the two mechanisms may, in practice,
complement one another.
Whatever the details, foremost, metapopulation
models predicting interspecific abundance-range size
relationships require that most species in a given taxonomic assemblage exhibit metapopulation dynamics.
They must exist as a series of discrete local populations
within which most individuals are born and die, with
the populations linked by the dispersal of individuals,
and exhibiting extinctions and recolonizations (Hanski 1991a). Examples have been documented both of
species that do (Harrison, Murphy & Ehrlich 1988;
Kindvall & Ahlen 1992; Hanski 1994; Hanski,
Kuusaari & Nieminen 1994; Nave et al. 1996) and
do not (Gotelli & Kelley 1993; Harrison, Thomas &
Lewinsohn 1995; Murdoch et al. 1996) appear to meet
the assumptions of metapopulation structure. The
balance of evidence at present is, however, probably
against most plant and vertebrate species exhibiting
metapopulation dynamics, although the models may
be applicable to a number of insect species and
although it remains unclear how good a caricature
of the dynamics of species metapopulation dynamic
models may provide even when all of their assumptions are not met. It has been suggested that the classical picture of a metapopulation needs to be broadened
considerably if it is to become more realistic (Harrison
1994).
If it were the sole mechanism, in those circumstances in which metapopulation dynamics were
unlikely to apply a positive abundance-range size
relationship should not exist. Hanski et al. (1993)
argue that some support is provided by the finding
by Gaston & Lawton (1990b) that there is no such
relationship among freshwater fish in Britain.
However, this is a rather weak test, given that the
local abundances of species in that study were drawn
from a single site. Indeed, reversing the argument,
Brown (1995) suggests that his resource breadth model
for the abundance-range size relationship is supported by the probable existence of such a pattern
among islands of archipelagoes, where over-water dispersal is very low, and hence where metapopulation
dynamics are very unlikely.
PREDICTIONS
If we accept that a metapopulation model may be
appropriate under some circumstances, then metapopulation dynamic mechanisms for the positive
abundance-range size relationship make several predictions. The first pertains to the carrying capacity
? 1997British
EcologicalSociety
Journalof Animal
Ecology,66, 579-601
hypothesis.
1. The creation of new patches of suitable habitatfor a
species will not alter its abundancein existing patches,
and likewise the destruction of some existing patches
will not alter its abundancein those that remain. This
follows from the fact that, unlike the rescue effect
hypothesis, under the carryingcapacity hypothesis the
distribution of a species does not have an effect on its
local abundance.
The other predictions pertain to the rescue effect
hypothesis.
1. There will be positive interspecificabundance-range
size relationshipseven whenthereis no differenceamong
species in breadths of resource use (Hanski & Gyllenberg 1993). This is a consequence of the prediction
that frequency distributions of the levels of patch
occupancy achieved by species should be bimodal,
such that most species occupy either a small or a high
proportion of the available sites. At its most simple,
if the rate of colonization of empty patches (m) is
greater than the rate of extinction in occupied patches
(e), then patch occupancy (p) should tend to 100%.
Conversely, if m < e, then p should tend to 0%. If (me) is a random variable with variance much greater
than mean, p should tend generally to close either to
0 or 100%, leading to the bimodal distribution of
occupancy within a species over a long period of time,
or across species at any one point in time (Hanski &
Gyllenberg 1993). Since occupancy affects abundance
under the rescue effect hypothesis, then a positive
interspecific abundance-range size relationship is
expected, even given identical resource usage.
Unfortunately, any evidence that might be presented in support of this prediction potentially suffers
the same problem as evidence in support of the
resource breadth hypothesis. A set of species exhibiting a positive abundance-range size relationship in
the apparent absence of any differences in resource
usage may always be argued to differ on some other,
unstudied, niche axes, for example in their environmental tolerances.
2. Local abundance will decrease with the increasing
isolation of habitat patches (Hanski 1991b). Isolated
patches will incur lower levels of immigration, and
hence benefit less from the rescue effect. They will
therefore have higher extinction rates, and exhibit
lower densities on average. Evidence for this prediction is equivocal. Hanski et al. (1993) cite one field
and one laboratory example where increased isolation
results in lower density in patches. The situation is
confused, however, because populations occupying
one set of patches that are clearly more isolated,
offshore islands, often show density compensation in response to species-poor island faunas (e.g.
MacArthur 1972; Blondel, Chessel & Frochot
1988; Blackburn et al. 1997).
3. Species that have large local populations but restricted distribution will have a high local growth rate in
relation to emigration rate, and low mortality during
dispersal; the opposite attributes will characterizespecies with low abundancebut wide distribution (Hanski
et al. 1993). Hanski (1991a; Hanski et al. 1993) argue
that the findings of Soderstrom (1989) in a study of
epixylic bryophytes in late successional forest in
northern Sweden are supportive of at least the first
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half of this prediction. Three species in Soderstrom's
study had high local abundances but restricted distributions. These were the only species that regularly
reproduced asexually (by producing gemmae), but
were not observed to reproduce sexually (by producing spores). The dispersal rate of gemmae is likely
to be low relative to spores, and Hanski argues that
the three species in question are likely to have low
dispersal rates relative to growth rates.
4. Species with high dispersalrates will show a negative
deviation from abundance-range size relationships.
That is, more dispersive species should have wider
distributions than less dispersive species for the same
local density. The only explicit test of this prediction
of which we are aware is that by Hanski et al. (1993),
who found that it was upheld in British butterflies,
but not in four other assemblages.
More recent comparative analyses of the interspecific abundance-range size relationships of British
birds and mammals also bear on this issue (Blackburn
et al. 1997). Here, the slopes of the positive relationships are approximately the same, but for any given
range-size mammal species have densities w 30 times
those of bird species. While this supports the dispersal
rate prediction (birds have much greater powers of
dispersal than mammals), contrary evidence is provided by British bats, which do not attain significantly
lower population sizes for a given range size than do
non-volant mammals. In summary, evidence for the
dispersal rate prediction is equivocal at best.
5. There will be a positive relationship between the
final' range of invadingspecies and their rate of spread.
Explicit tests of this prediction based on intrinsic rates
of increase are lacking. However, if we assume that
these are correlated with initial rates of distributional
spread (Skellam 1951), then it finds some support.
Forcella (1985) documents a positive relationship
between the initial rate of spread and 'final' size of
distribution amongst 40 alien weed species in northwestern USA, although no abundance data are available to test explicitly for an abundance-range size
relationship in this case.
CONCLUSION
? 1997British
EcologicalSociety
Journalof Animal
Ecology,66, 579-601
The literatureon metapopulation dynamics comprises
an elegant body of theory which has contributed much
to our understanding of the patterns of change in, and
interdependence of, spatial and temporal abundances
(e.g. Gilpin & Hanski 1991; Gotelli & Kelley 1993;
Hanski & Gyllenberg 1993; Hanski 1994; Hanski &
Thomas 1994; Hanski et al. 1996). A metapopulationbased mechanism seems unlikely to be the most frequent explanation for positive abundance-range size
relationships given our present understanding of the
proportions of species exhibiting metapopulations.
However, there is some evidence for some of the predictions of the rescue effect hypothesis, and there may
be certain taxa for which metapopulation structure is
the rule, rather than the exception.
8. Vital rates
One of the simplest biological explanations for abundance-range size relationships is also the most recent.
This is the proposition by Holt et al. (1997) that if a
set of species differ with respect to their responses to
spatially independent, density-independent mortality
factors, then a positive relationship will result.
Consider a set of species distributed at a scale large
enough that regular immigration is sufficientto permit
colonization of suitable sites, whilst not a dominant
determinant of local abundance (an assumption fundamentally different from that made by rescue effect
metapopulation models). They will persist solely at
those sites where their intrinsic population growth
rate r (which equals birth rate minus death rate)
exceeds zero. Assume that at low densities all these
species have the same spatial patterns of birth rates,
that they experience intraspecific density-dependence
in birth rates in the same way, and that each species
has a constant, spatially invariant density-independent death rate. Then, equilibrium local density
varies directly with r, and any factor which tends to
increase r across all sites for a species will simultaneously enlarge the number of sites potentially
occupied and increase abundance at occupied sites
(Fig. 2). More formally, Holt et al. (1997) derive the
expression:
<N> = sR/2u
eqn 2
where <N> is mean abundance, R is range size, s is
the slope of the spatial pattern of birth rates (Fig. 2),
and u is the strength of density dependence.
EVALUATION
Whilst the principal assumptions of this model do not
seem unreasonable, there is little explicit empirical
evidence for or against them. In large part this reflects
the difficulty in directly measuring the relevant quantities. Nonetheless, the following observations provide
some limited support for the model: (i) various dimensions of birth rates do appear in some cases to decline
towards range limits (e.g. Peakall 1970; Caughley et
al. 1988; Carey et al. 1995) although the inverse pattern may also occur (Johansson 1994); and (ii) species
with smaller ranges appear in some assemblages to
have lower relative maximal population growth rates,
or likely correlates thereof, than do species with larger
range sizes (e.g. Glazier 1980; O'Connor 1987; Spitzer
& Leps 1988; Sutherland & Baillie 1993; Blackburn et
al. 1996).
Conversely, if cases in which there is no coincidence
between the areas in which species attain high reproductive rates and high densities (e.g. some source-sink
systems) are preponderant, then the assumptions of
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592
Abundance-range
size relationships
(a)
bo(x)
Environmental
gradient
(b)
<N>
S
2
Range
2.
Birth
rates
at
low
Fig. (a)
(bo)
densityat eachsite(x) along
an environmentalgradientdecline linearly(with slope s)
awayfromthe optimumsite,withthe samegradients for all
species.Note that this figureneed not indicatean actual
spatialstructurein birthand death rates;the x-axis could
denote an abstract orderingof sites that in reality are
arrangedin a complexspatialmosaicof habitatsof varying
quality (Lawton 1996a). Species 1 and 2 have different
death rates (d1and d2),giving rise to
density-independent
(b) a positiverelationshipbetweenmean abundance(<N>)
and rangesize (fromHolt et al. 1997).
highly unlikely that sufficient data could ever be collected to make a test a practical possibility.
3. All else being equal, species with higher densitydependentbirth rates should have higher range sizesfor
a given abundance.This prediction arises directly from
the position of u in eqn 2.
The exact form of the differencebetween two taxa in
their interspecific abundance-range size relationship
will depend on the way in which they vary in u and s.
Relationships for two taxa that vary in s but not in u
would be predicted to have differentregression slopes,
but with the elevation of the slopes dependent on the
exact pattern of difference in s. Relationships for two
taxa that vary in u but not in s would be predicted to
have identical regression slopes, but different slope
elevations (as per prediction [3]). Interestingly, a situation where two taxa do indeed differ in the elevation
of their abundance-range size relationships, but not
in their regression slopes, has recently been demonstrated by Blackburn et al. (1997) for British breeding birds and mammals. The vital rates hypothesis
would be supported if birds could be shown to have
lower density-dependent birth rates than mammals.
We know of no data to test this prediction.
4. Within a taxon showing positive interspecificabundance-range size relationships, all species have essentially similar values ofu (the strength of density-dependent changes in birth rate as afunction of density is the
same in all species), but vary in r (their intrinsicgrowth
rates) (Holt et al. 1997). If u varies among species,
but density-independent factors do not, species will
have the same range, but differentaverage abundances
within their ranges. It is currently unclear to what
degree these assumptions could be violated and still
retain a positive abundance-range size correlation.
The extent to which density-dependence is similar
among populations within a species or across groups
of related species is an important and untested question in comparative population ecology.
CONCLUSION
the model may not be generally valid (e.g. Van Horne
1983; Vickery, Hunter & Wells 1992).
PREDICTIONS
? 1997British
EcologicalSociety
Journalof Animal
Ecology,66, 579-601
1. If invadingspecies are established at a randompoint
within their feasible geographical range, then initial
population growth rates will be a goodpredictor of their
ultimate range size. This prediction is also made by
the rescue effect hypothesis, and the evidence, albeit
limited, is discussed above.
2. The rate of change of birth rates across environmental space will be proportional to the slope of the
abundance-range size relationship. This prediction
arises directly from the position of s in eqn 2. It seems
In summary, the vital rates hypothesis currently lacks
significant evidence in its support. However, that may
be more a function of its relative novelty than an
indication of the extent to which its assumptions and
predictions are realistic. No doubt, further investigation of this mechanism will serve to clarify which
is the case.
Predictions in common
In addition to the predictions given separately for each
hypothesis for the positive interspecific abundancerange size relationship, there are three predictions
which are common to all six of the biological ones
(and additionally to the sampling artefact mechanism). Evidence in support of these predictions alone
obviously provides no basis for discriminating
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593
K.J. Gaston,
T.M. Blackburn&
J.H. Lawton
? 1997British
EcologicalSociety
Journalof Animal
Ecology,66, 579-601
between hypotheses, but allied with other information
may help to do so.
1. Positive interspecificabundance-rangesize relationships will become weaker over progressively larger
areas, and wherean assemblage embracesgreater taxonomic and ecological diversity. In all cases, the greater
the heterogeneity of the environments and species
embraced by a study, the weaker abundance-range
size relationships should become. For example, at
large spatial scales many of the assumptions of metapopulation dynamic models are unlikely to hold true
(e.g. Lawton 1993; Gaston 1994a). There is evidence
that this weakening of relationships indeed does occur
(Gaston 1994a; Brown 1995), although positive abundance-range size relationships have been documented
at large spatial scales for several assemblages (e.g.
Bock 1984; Brown & Maurer 1997; Gaston 1994a),
and across multiple spatial scales for a few (Bock 1987;
Collins & Glenn 1990;Niemela & Spence 1994; Brown
1995). This prediction implies that positive abundance-range size relationships should also be recovered within taxa (using phylogenetic comparative
methods), where variation is probably least. There is
some evidence that this is the case (Gaston & Blackburn 1996b; Blackburn et al. 1997;Gaston et al. 1997).
2. Positive interspecificabundance-rangesize relationships will be weaker when range sizes are measured as
extents of occurrence than when measured as areas of
occupancy (sensu Gaston 1991, 1994b). This is because
areas of occupancy contain more information on
where individual species actually occur (Gaston 1991).
Although there are suggestions that the prediction is
supported (Gaston 1996), critical tests are wanting.
These cannot readily be carried out from existing studies, because the two measures of range size have not
been applied consistently across taxa. Comprehensive
analyses tend to employ extent of occurrence
measures, whilst partial analyses tend to employ area
of occupancy measures (Gaston & Blackburn 1996a).
Nevertheless, Gaston et al. (unpublished) demonstrated that abundance-range size relationships in
a set of British birds were stronger when range size
was measured as number of local sites occupied than
when range size was measured as the number of occupied 10 x 10 km squares in the British national grid.
The latter measure is closer to an extent of occurrence
than the former. However, the local sites used for this
study (Marchant et al. 1990) are not scattered evenly
across the entire British Isles, so that the two range
size variables arguably do not represent differentmeasures of the same quantity.
3. There will be positive intraspecificabundance-range
size relationships. Intraspecific abundance-range size
relationships are predicted by all the biological
hypotheses and the sampling effect hypothesis under
at least some circumstances (note that for the metapopulation dynamics mechanism such relationships
are predicted by the rescue effect hypothesis [Hanski
1991b; Hanski & Gyllenberg 1993] and not the car-
rying capacity hypothesis). These circumstances are
listed in Table 6, and the existence of such relationships in the absence of the particular forms of specific
or environmental variation identified may constitute
powerful tests of the hypotheses. For example, positive intraspecific abundance-range size relationships
are predicted by the vital rates hypothesis only when
birth rates and/or death rates are changing. The loss
of areas of the geographical range of a species in ways
which do not affect these rates (e.g. habitat destruction
without degradation of remaining habitat) will not
result in reduction in local density.
Positive intraspecific abundance-range size
relationships may be quite widespread (Fig. 3). There
is a growing body of evidence for synchronous
increases and decreases in range sizes and local densities at sites where species persist (e.g. Gibbons et al.
1993; Marshall & Frank 1994, 1995; Hanski et al.
(a) -1.4,
ia) -1.45ca
a)
0
I-1.5a)
0
0
-1 .55 -
a)
.0
0
0
0
0
S
0
:LIa)
-1.6-
0
0
0
0
:L
0)-
-1-65
(a)
-o
0)
0 -1.7
0
-1.75
0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8
Proportionof sites occupied
(b)
-0-45.
- -0.475a)
o
-0.5.
-
-0.525-
a)
C)
CL
Co
.a)
S
0
6*
I
0
0
-0.550
o -0.575CD
0
0
0
-0.6
S
?, -0-625
a)
-o
-0-65
0
- -0.675
S
0
0)
0
0
S
_.7
0.93 0.94 0.95 0.96
0.97 0.98 0.99
Proportionof sites occupied
Fig.3. The relationshipbetween local density (logo1territoriesha-') and the proportionof sites occupiedfor (a)
long-tailedtit, Aegithaloscaudatus,and (b) chaffinch,Fringilla coelebs,on farmlandCommonBird Censusplots in
Britainin 1968-91.For furtherdetailssee Blackburnet al.
(unpublished).
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1
594
Abundance-range
size relationships
1995; Pollard, Moss & Yates 1995; van Swaay 1995;
Gaston & Curnutt 1997, Blackburn et al., unpublished). For example, amongst farmland birds in
Britain, those species that have undergone the strongest declines in numbers tend to have shown the most
substantial range contractions (Fuller et al. 1995).
Intraspecific abundance-range size relationships
are not all positive. Positive relationships are exhibited
by 71% of 75 species of British birds censused on
farmland sites in the period 1968-91 (Blackburn et al.,
unpublished). However, in only 42% of species are
these relationships positive and statistically significant, and the proportions are even lower for an overlapping set of species censused on woodland sites. Five
percent of species on both farmland and woodland
show significant negative intraspecific relationships. It
is not obvious how these could be generated by some
of the models for positive interspecific relationships.
To give just one example, the resource breadth
(Brown's) hypothesis predicts positive intraspecific
relationships when there are temporal fluctuations in
breadth of resource use (e.g. temporal fluctuation in
climate affecting the size of a species' realized niche).
A negative intraspecific relationship would require
temporal fluctuations that simultaneously increased
local abundance but decreased range size. Yet,
Brown's hypothesis explicitly forges a positive link
between variation in abundance and variation in range
size. The example of a situation where it can produce
a negative interspecific relationship (e.g. Gaston &
Lawton 1990a; see below) does not apply intraspecifically.
Negative interspecificcorrelations
? 1997British
EcologicalSociety
Journalof Animal
Ecology,66, 579-601
Not all interspecific correlations between abundance
and range size are positive. There are a few that are
negative, and some that show no statistically significant relationship (e.g. Gaston & Lawton 1990a;
Gaston 1996). Non-significant correlations are probably most likely to occur with poor data, but negative
correlations can also be generated by some of the
mechanisms giving rise to positive correlations, but
for substantially different circumstances and parameter values. It is useful to review, briefly, which
mechanisms can generate negative correlations, and
under what circumstances. To avoid repetition, we do
not explicitly discuss cases of zero correlation. Logically, these cases are bound to exist as conditions
pass from those generating 'almost universal' positive
correlations to those generating much rarer negative
ones.
Of the eight hypotheses discussed above that give
rise to positive interspecific abundance-range size correlations, three cannot generate negative ones. They
are the sampling artefact hypothesis (hypothesis 1),
the range position hypothesis (3) and the densitydependent habitat selection hypothesis (6). Under
hypothesis 1, sampling would have disproportionately
to miss common species to generate a negative
relationship. Under hypothesis 3, species would need
higher levels of abundance and occupancy nearer to
their range edge. Hypothesis 6 would require that
species at higher densities occupy fewer habitats. It is
difficult to see how any sensible modification of any
of these models could change this situation.
The phylogenetic non-independence hypothesis
(hypothesis 2) could generate negative interspecific
relationships in cases where the species compared
belonged to distinct taxonomic groups, where species
in the more widespread groups happened to be generally rarer. For example, the negative relationship
between breeding density and geographical distribution found for birds breeding on Handa (Gaston
& Lawton 1990a) could arise from a simple difference
between seabirds (Ciconiiformes), which breed at high
density but at a few restrictedlocalities, and passerines
(Passeriformes),which are geographically widespread,
but do not attain the high densities of nesting seabirds.
The resource breadth hypothesis (hypothesis 4), as
already noted, can generate negative interspecific
relationships when the local abundances of species
are measured in habitats which are atypical of the
spectrum of habitats in the geographical region of
interest (that over which range sizes are determined).
A negative abundance-range size relationship indeed
results under these circumstances (Ford 1990; Gaston
& Lawton 1990a). It is conceivable under hypothesis
5 (resource availability) for widespread resources to
be rare everywhere, and geographically restricted
resources to be locally abundant. We know of no
plausible demonstration of this possibility. Hypotheses 7 (metapopulation dynamics) and 8 (vital rates)
are both capable of generating negative correlations
for appropriate (and relatively restricted) ranges of
parameter values. The necessary conditions are discussed in the relevant sources (Gyllenberg & Hanski
1992; Holt et al. 1997).
Negative correlations deserve more attention.
Despite their rarity, they offer an opportunity to
explore the assumptions and predictions of the eight
hypotheses in more detail than concentrating on positive relationships alone. If one subscribes to the view
that we should seek the most parsimonious number
of explanations for patterns in nature, then the suite
of possible candidates yielding general explanations
for abundance-range size correlation is reduced from
eight to five, because three of them do not predict the
existence of negative correlations.
Synthesis
We are in no doubt that the positive interspecificabundance-range size relationship is a real pattern, in that
it is not simply a product of sampling artefacts or of
phylogenetic non-independence (see also Brown 1984,
1995; Nee et al. 1991b; Lawton 1993; Gaston 1994a).
Its underlying cause(s) are, however, less readily dis-
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595
K.J. Gaston,
T.M. Blackburn&
J.H. Lawton
cerned. None of the six non-artefactual hypotheses
summarized here has unequivocal support.
There are four main reasons why this could be the
case. First, the models have not been adequately
tested, and if this were achieved one model would
become clearly differentiatedas the explanation of the
observed pattern. The relative dearth of evidence for
some hypotheses (habitat selection, vital rates) may
be more a function of the lack of attention they have
so far received than of their intrinsic merits. By far
the most effort has been expended testing the resource
breadth and metapopulation dynamic models, to the
extent that many authors seem not to recognize that
there are several other potential explanations for interspecific abundance-range size relationships. One aim
of this paper is to correct that narrow view. Nevertheless, given that the models concern some key issues
in ecology (population dynamics, metapopulation
dynamics, niche patterns) it is rather depressing how
difficult it is, on present evidence, to discriminate
amongst them or, at least, to determine the likely
validity of their assumptions and predictions.
Second, it is not possible to discriminate amongst
hypotheses. In principle, and for most models, this
should not be true. In Tables 4 and 5 we summarize
the principal assumptions and predictions of the six
biological mechanisms postulated to explain positive
abundance-range size relationships, and provide a
qualitative indication of the extent to which we believe
there is empirical evidence (as discussed above) in
support of them. Most of the hypotheses make either
unique predictions, or unique combinations of predictions.
Part of the problem to date has been the lack of
rigour in defining what the different models really do
predict. For example, the resource breadth hypothesis
has sometimes been assumed not to predict positive
intraspecific abundance-range size relationships
(Lawton 1995, 1996a), because the niche breadth of a
species is considered a fixed quantity. However, it is
actually easy to envisage positive intraspecific
relationships if a fixed fundamental niche translates
into a variable realized niche under the influence of
environmental variation. The conditions under which
different hypotheses predict intraspecificrelationships
(Table 6) are considerably more subtle than had been
appreciated. The lack of rigour has also meant that
predictions that have been tested to differentiate
between competing hypotheses do not do so: the existence of positive intraspecific relationships per se does
not invalidate the resource breadth hypothesis. Note
also from Table 5 that the predictions for which there
is most evidence are usually those that are the least
useful in distinguishing between the different models.
The relative difficulty in defining the predictions made
by differenthypotheses seems, in large part, associated
with whether the hypotheses have been expressed
mathematically (e.g. metapopulation dynamics, vital
rates) or not (e.g. resource breadth, resource availability).
A third reason why no current theory has unequivocal support may be that none of the mechanisms are
size relationTable4. Explicitassumptionsof mechanismspostulatedas determiningpositiveinterspecificabundance-range
ships (see text and Table 1 for details of the hypotheses). 1, sampling; 2, phylogenetic non-independence; 3, range position; 4,
resourcebreadth;5, resourceavailability;6, habitatselection;7, metapopulationdynamics;and 8, vital rates.'Evidence'is a
qualitativeassessment(on a scale of 0-4) of the availableevidencein supportof the generalityof each assumptionin the
context of abundance-range size plots (several assumptions may be true in a wider ecological context). Evidence involving
severalspeciesis givengreaterweightthan evidencefromjust one, or a few, species- a low numberof crossesimpliesthat
there is little evidencethat the assumptionholds widely, althoughthe evidencemay still be good for particularspecies
assemblages. Some assumptions rate 0 (left blank), no evidence. No assumption warrants more than 2 from a maximum of 4
1
2
3
4
5
6
7
8
Evidence
Thereis no real(biological)positiveabundance-range /
size relationship
Populations occur at lower densities near the edge of
their ranges
Localabundanceswill be greaterfor spp. withwider
breadthof resource-use
Distributionswill be greaterfor spp.withwider
breadthof resource-use
+ +
+
+
+
Abundances will be greater for spp. with more
abundant resources
Distributions will be greater for spp. with more
abundant resources
? 1997British
EcologicalSociety
Journalof Animal
Ecology,66, 579-601
habitatselection
Thereis density-dependent
Spp.existas metapopulations
Abundanceswill be greaterfor spp.withhigher
intrinsicgrowthrates
Distributionswill be greaterfor spp. withhigher
intrinsicgrowthrates
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All use subject to JSTOR Terms and Conditions
++
+ +
++
+
/
+
++
596
Table5. Predictions of mechanisms postulated in hypotheses to account for positive interspecific abundance-range size
Abundance-range
size relationships
3, rangeposition;4, resourcebreadth;5, resourceavailability;
relationships.1, sampling;2, phylogeneticnon-independence;
6, habitat selection; 7, metapopulation dynamics; and 8, vital rates. 'Evidence' is a qualitative assessment (on a scale of 0-4)
size relationships./ indicatesthat
of the availableevidencein supportof eachpredictionin the contextof abundance-range
the hypothesis heading the column produces the prediction in that row. ? indicates a prediction that may arise from a
hypothesis,but whetheror not it does is currentlyunclear
1
size relationshipshouldbe stronger
Abundance-range
usingmeasuresof areaof occupancythanextentof
occurrence
Abundancesand distributionswill declinewhen
distancesbetweenpatchesaregreater
Specieswithgreaterdispersalabilitywill be more
widelydistributedfor a givenabundance
sizerelationshipwill be foundat
Abundance-range
scales
verylarge/continental
size relationshipshouldweakenas
Abundance-range
studyareaincreases
Ultimaterangeof invadingspp. shouldbe positively
correlatedwithrateof spread
Spp.with smallrangesshouldhavelowerrelative
maximalpopulationgrowthrates
For invadingspp.,initialpopulationgrowthrates
shouldbe a good predictorof ultimaterangesize
/
Generalistswill not be consistentlymorewidespread
thanspecialists
Localand regionalnichebreadthsarepositively
correlated
Thereis no abundance-range
size relationshipfor
spp. that sharea commonoverallresourcebase
Thereis a positiveabundance-range
size relationship
for spp. that sharea commonoverallresourcebase
Therewill be positiveintraspecific
abundance-range
size relationships
Can also predictnegativeabundance-range
size
relationships
//
2
3
4
5
6
7
8
Evidence
?
/
/
/
/,
/
++
/
/
J
++
/
+
/
/
/
/
/?
++ + +
/
+++
+ see below
J
/
,/
/
/
++
/
/
+ because
arecorrelated
withr
++
/
++++
/
/
/
/
I
\/
/
/
/
I /
/
v/
/
+
Table 6. Determinants of positive intraspecific abundance-range size relationships for the seven different hypotheses which
predict such a pattern (relationships are based on the abundance and range size of the same species at different times)
Mechanism
Relationshipresultsfrom:
1. Samplingartefact
of the rangesize of a specieswhenit occursat lowerlocal
Systematicunderestimation
abundances
Temporalfluctuationsin the proximityof the studyareato the centreof the rangeof the
species
Temporalfluctuationsin breadthof resourceuse
Synchronoustemporalfluctuationsin the localabundanceand distributionof resource
availability
Temporalfluctuationsin local abundancedrivechangesin habitatoccupancy,andhence
distribution
Metapopulationstructuresof the formof the rescueeffecthypothesis
Temporalfluctuationsin birthand/ordeathrates
3. Rangeposition
4. Resourcebreadth
5. Resourceavailability
6. Habitatselection
7. Metapopulationdynamics
8. Vitalrates
? 1997British
EcologicalSociety
Journalof Animal
Ecology,66, 579-601
appropriate. This is a depressing prospect, but impossible ever to dismiss out of hand, as it is the nature of
science that even the most beautiful theory can always
be killed off by one ugly fact. However, at present this
conclusion is premature, at least until more rigorous
tests of each individual hypothesis, or of combinations
of hypotheses, have been performed.
This leads to the fourth possibility, which is that
some or all of the mechanisms are complementary (see
also Collins & Glenn 1991; Holt et al. 1997), and
that some or all may be appropriate somewhere. This
would not be entirely surprising, both because some
of the mechanisms are quite closely related, and
because of the diversity of taxa, habitats and scales
at which positive interspecific abundance-range size
relationships have been documented, and the variety
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All use subject to JSTOR Terms and Conditions
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