Download Connections between ecology, biogeography, and paleobiology

Evolutionary Ecology, 1995, 9, 586-604
Connections between ecology, biogeography, and
paleobiology: relationship between local abundance
and geographic distribution in fossil and recent
molluscs
B R I A N J. E N Q U I S T * , M A R K A . J O R D A N , and J A M E S H . B R O W N
Department of Biology, Universityof New Mexico, Albuquerque, NM 81731, USA
Summary
We used data on Contemporary and Pleistocene molluscs at one site in the Gulf of California to evaluate
and extend earlier ideas about the relationship between local abundance and geographic distribution. For
each species whose shells occurred in one Recent and two Pleistocene deposits, we measured its abundance
in the sample and relative latitudinal position within its contemporary geographic range. Species near the
edges of their ranges showed uniformly low abundances, whereas those near the centres exhibited a wide
range of abundances. Species near the edges of their ranges also appear to have exhibited greater changes in
abundance, including more colonization and extinction events, between the Pleistocene interglacial sample
and the Recent one. The constraint of location in the geographic range on maximal local and regional
abundance appears to offer an example of a connection between patterns and processes on local, regional,
and geographical scales. Characteristics of community structure, such as relative abundance of individual
species and frequency of local co-existence of multiple species, may be influenced by the location of the
sample site with respect to the geographic ranges of the constituent species. These results demonstrate
emergent, statistical features of population ecology and community organization that are manifest over
geographic space and evolutionary time.
Keywords: abundance; geographic range; turnover; mollusc assemblages; Pleistocene; macroecology;
biogeography; fossils
Introduction
This paper addresses questions at the interface of three disciplines; ecology, biogeography and
paleontology. Historically there has been little communication between these disciplines.
Ecologists have been concerned primarily with the regulation of abundance and distribution of
species at local spatial and short time scales. Biogeographers have focused on the influence of
contemporary and historic factors on species distribution at regional to global spatial scales.
Paleontologists have studied changes in the composition of assemblages of species over geological
time scales. Recently, however, several authors have attempted to make connections between
the patterns and processes that occur on vastly different spatial and temporal scales (e.g. ecology
and biogeography (Hengeveld and Haeck, 1982; Rapoport, 1982; Bock and Ricklefs, 1983;
Brown, 1984; Brown and Maurer, 1987; Schoener, 1987; Brown, 1995, Brown et al., in press;
biogeography and paleontology (Jablonski, 1986, 1987; Ricklefs and Latham, 1992) and ecology
and paleontology (Davis, 1986; Graham, 1986; Betancourt et al., 1991; Kauffman and Fagerstrom,
* To whom correspondence should be addressed.
0269-7653
1995 Chapman & Hall
Macroecology of geographic range position
587
1993). This interaction is important because the perspectives common to one discipline can
introduce original insights to related disciplines.
Here we are concerned with the questions of how local abundance and population fluctuations
are related to position within the geographic range and how abundance and composition of
species in a local assemblage change over geological time. We use data from exceptionally well
preserved Pleistocene fossil and Recent molluscs from one site in the Gulf of California to ask the
following
(1) What is the relationship between local, within-community abundance and the larger, more
regional scale of abundance often sampled by fossil deposits?
(2) How is interspecific variation in abundance related to position in the geographic range?
(3) How is change in abundance between time periods separated by 100 000 years, related to
the position in the range and to initial abundance?
(4) How is turnover of species owing to local colonization and extinction events related to
position in the range?
Brown's model
Why would one expect a relationship between position in the geographic range, local abundance
and changes in abundance over geological time? Brown (1984) suggested a specific mechanism
that links a species' abundance and distribution on different spatial and temporal scales. This
mechanism is based on Hutchinson's (1957) idea that both abundance and distribution are limited
by abiotic and biotic environmental factors that comprise the dimensions of the multidimensional
ecological niche. Brown (1984) extended Hutchinson's concept of the niche to develop a model of
how abundance varies over the landscape. The model is based on two assumptions.
(1) Local abundance is determined primarily by the extent to which different resources and
conditions of the local environment meets the requirements of a given species. These requirements can be defined so as to be independent of each other, thereby constituting orthogonal axes
of the species-specific niche. Thus, the niche is a characterization of the environmental
requirements of a species.
(2) Local environments vary in their capacities to meet these niche requirements. Spatial
variation in the relevant environmental parameters exhibits substantial autocorrelation, so that
nearby sites are likely to provide more similar combinations of variables than those that are more
separated.
Abundance across geographic space
From these assumptions it follows that the abundance of a species should be highest where the
combination of environmental variables best fits its niche requirements and should decline with
increasing distance from this site. If the pattern of variation is both spatially autocorrelated and
relatively heterogeneous, then abundance should be highest near the centre of the geographic
range and should decline relatively gradually and symmetrically towards the edges of the range.
This predicts a pattern of abundance along a transect through the geographic range that
resembles a normal curve (see also Whittaker, 1956, 1967; Hengeveld and Haeck, 1982;
Hengeveld, 1990).
More recently, Brown et al. (in press; see also Rapoport, 1982; Tellerfa and Santos, 1993;
Brown, 1995) pointed out that, while the tendency for mean abundance to decrease from centre
to the edge of the range still holds, there is a great deal of spatial variation that causes the pattern
to be far from smooth and unimodal. Thus, the spatial distribution of abundance for a given
species should exhibit the kind of pattern shown in Fig. la. Sampling this species on a transect
Enquist, Jordan and Brown
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Cross-Section of Geographic Range
# Individuals
(a)
Edge
Center
Edge
# of Individuals
Center
(b)
Edge
Distance from Center
Figure 1. (a) Representation of spatial variation in abundance across a transect of the geographic range of a
single hypothetical species. Note that maximum abundance, mean abundance and variation in abundance is
greatest near the centre. At the edge, both abundance and variation in abundance is low. Also, the number
of sites where the species is absent increases with increasing proximity to the edge. (b) The relationship
between local abundance and distance from geographic range centre is produced by taking a series of
samples along the above transect across the range (from (a)). For a single species, the points tend to fall
within a triangular space.
across its geograpl,:c range would give the pattern shown in Fig. lb. A plot of local abundance as
a function of distance from the centre of the range would have the points falling within a
triangular constraint space: near the centre of the range, abundance can take on a wide range of
values (at least two or three orders of magnitude in some species), whereas near the edges of the
range, not only does abundance tend to be uniformly low, but also the proportion of the area that
is uninhabited (i.e. number of zero values) tends to increase.
Extending Brown's model to assemblages: assumptions
In this study we extend this model for a single species to make predictions about how the
abundance of many species within a local community should be related to the relative position of
each species relative within its geographic range (Fig.2). We assume the following.
(1) Within each species, abundance tends to be highly variable near the centre of the range but
predictably low at the edges (from above).
(2) Between species there can be a substantial variation in the maximum (or mean) abundance,
owing in part to factors such as body size, trophic status and ecological specialization (e.g.
Rabinowitz, 1981; Rabinowitz et al., 1986;)
(3) Geographic ranges of species are distributed relatively independently of other species (e.g.
Whittaker, 1956, 1967; Fowells, 1965; Brown and Kodric-Brown, 1979).
589
Macroecology of geographic rangeposition
GEOGRAPHIC TRANSECT
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Figure 2. (a) Distribution of multiple species across an environmental gradient (data from Whittaker, 1967).
This pattern characterizes the way that we would expect the maximum or mean abundances of species to be
distributed in a transect across geographical space. Whittaker's (1967) data suggested that each species is
distributed relatively independently of other species and that abundance is greatest near the centre of the range.
Note, the normal-shaped curves for any given species smooths the much more spiked pattern of the local
community actual abundance shown in Fig. la. For a sample of the local community at one site on this transect
(a), the abundances of co-existing species should all fall within a triangular constraint space (b). Four points
were randomly chosen along this transect (a). The abundance and relative distance of the site from the centre of
the range was measured for each species occurring at that site. The relative abundance was plotted as a function
of the relative position from the geographic centre (using Equation 1) giving the pattern shown. The pattern
expressed in (b) can be characterized by a triangular constraint space (c). The abundances of multiple species at
a given site should fall within this space with no points falling in the upper right region of this graph.
590
Enquist, Jordan and Brown
Predictions of assemblage abundance
The above assumptions imply that the normal-shaped curves drawn by, for example, Whittaker
(1956, 1967) and illustrated in Fig. 2a characterize the way that maximum or mean abundances of
multiple species would be distributed in a transect across geographic space. Then it follows that
for any single sample taken at any site along this transect, the abundances of the co-existing
species should all fall within an approximately triangular constraint space (as shown in Fig. 2(b)):
species near the centres of their ranges should exhibit a wide range of abundances, whereas those
near the edges of their ranges should have predictably low abundances.
Thus, when relative abundance is graphed as a function of relative position in the range, as in
Fig. 2(c), the points are predicted to fall largely within the triangular region and to be absent from
the upper, right-hand corner.
Predictions of assemblage temporal variability
The same conceptual framework also enables us to make predictions about the dynamics of
populations with respect to position in their ranges. Whenever environmental conditions remain
similar (or return to a similar geographic configuration, e.g. in glacial-interglacial climatic cycles
(see Davis, (1986)), the range boundaries and the regional pattern of abundance of an individual
species should be relatively unchanged. Local abundance, however, should exhibit more
variation at the edges rather than near the centre. When environmental conditions are already
low in quality, slight changes are likely to cause large changes in environmental suitability and
these are more likely to occur near the edges than near the centre of the range. By extending this
intraspecific pattern to multiple species, we predict the following.
(1) In such regional samples, those species that are near the centres of their ranges should
exhibit less temporal change in abundance (and fewer local colonization and extinction events)
than those that are near the edges of their ranges.
(2) Since we would also expect from random sampling that large populations would exhibit less
variation in relative abundance than small ones, support for the above prediction would require
that the location within the geographic range be a better predictor of temporal change in
abundance than initial abundance.
Methods
Study site
We tested the above predictions by sampling the fossil and recently dead molluscs near Punta
Chueca (28.5 ° N, 112 ° W) approximately 35 km north of Bahia Kino, Sonora on the Gulf of
California (Mar de Cortez). Just north of Punta Chueca are Pleistocene fossil beds containing
exquisitely preserved hard parts of intertidal and subtidal marine invertebrates (Ortlieb, 1982,
1984; Beckvar, 1986; Beckvar and Kidwell, 1988). The fossil terrace extends for 1.5 km along the
shoreline above the current intertidal zone. We collected all shells from three strata:
(1) Paleo In Situ (Tagelus bed of Beckvar (1986, 1988; Beckvar and Kidwell, 1988)). This
deposit, 25-30 cm thick, has a relatively low density of shells. The stratum represents an intact
Pleistocene intertidal community that was buried by a single massive deposit of sediment. The
layer is categorized by articulated bivalve shells with natural spacing patterns and living positions
preserved between individuals. This deposit has been dated to the last interglacial period,
approximately 100 000 years ago (Beckvar, 1986).
(2) Paleo Storm (Felaniella bed of Beckvar (1986, 1988; Beckvar and Kidwell, 1988)). This
Macroecology of geographic range position
591
deposit, 15-25 cm thick, lies immediately below the Tagelus bed and contains an extremely high
density of shells, representing the accumulation resulting from a single, very large storm event.
Many of the bivalves were still articulated, but they had been washed out of their living positions
and collected in one very dense layer on the shore. This deposit is very similar in age to the
preceding layer, but reflects a much larger area of sampling - probably at least 10 km 2. This is
indicated by the co-occurence in this storm-collected sample of species from both rocky and softbottom substrates and from both intertidal and subtidal locations.
(3) Recent Storm. - Storm-deposited shells of recently dead molluscs were collected at Punta
Santa Rosa, at the mouth of an estuary 9 km south of the fossil terrace (see Marquet, 1993; TuU
and Brhning-Gaese, 1993). This locality was chosen because the abundance of shells approached
that of the Paleo Storm sample and the distance from the fossil terrace precluded any possibility
of collecting redeposited Pleistocene shells (but see Flessa et al., 1993; Flessa and Kowalewski,
1994). As in the case of the Paleo Storm sample, this sample reflects the accumulation of shells
from a much larger region than a local community, again probably at least 10 km 2 in area.
Several subsamples from each stratum were collected and combined for the present analyses.
Each subsample consisted of approximately 9460 cm 3 of shell-bearing substratum. Because of the
lower density of shells in the Paleo In Situ stratum, more subsamples of this layer were taken.
These samples, collected without bias with respect to observable species, were sieved through a
commercial window screen (1.6 mm mesh) to remove fine sediment. The samples were washed
with seawater to facilitate separation of shells from sediment. Samples were further sorted by
hand and all intact and only slightly damaged shells (so that unequivocal identification was still
possible) were saved.
These samples of shells were returned to the laboratory, identified to species (using Morris,
1969; Keen, 1971; Morris et al., 1980; Flessa, 1987) and counted. Two characteristics of the shells
facilitated identification. First, the shells were uniformly well preserved, so that even fine details
of sculpturing were still present. Second, a few hard to identify species of bivalves could be
separated because they exhibited unique allometric ratios among shell length, width and depth.
Calculation of relative range position
Two kinds of data from each species represented in these three assemblages are used in tile
following analyses. First, the abundance of each species was calculated as the relative abundance
in the sample, assigning a value of 1.0 to the most abundant species. This was done to permit
quantitative comparisons among the different-sized samples (actual numbers of shells are given in
the Appendix). Second, a range centre index (RCI) was calculated from published data on
latitudinal limits of the contemporary geographic range along the Pacific coast of North America
(Keen, 1971; Bernard, 1983; Skoglund, 1992; see the Appendix). The distributions of molluscs in
the region between British Columbia and southern Mexico are probably better known than
anywhere in the world (Jablonski and Valentine, 1990). Nevertheless, there were often
differences between the two references in the reported latitudinal limits and when this occurred
we used the widest range of latitudes. The range centre index was computed as
RCI = 2(D - S)/R
(1)
where D equals the location of Punta Chueca in degrees North latitude (28.5°), S equals the
midpoint of the range and R equals the total latitudinal breadth of the range, all in degrees
latitude. Appropriate corrections were made for ranges extending into the Southern Hemisphere.
Measuring the location of the study site in this way provides a quantitative measure of a species
relative position within the range with respect to one variable - latitude - while ignoring other
592
Enquist, Jordan and Brown
potentially interesting variables - such as temperature, depth or water volume occupied. For
example, a species with a northern range limit of 32 ° latitude would have its northernmost
populations in the upper Gulf of California or on the outer (Pacific) coast of Baja California, two
areas which have quite different environmental conditions. In some cases, for both Pleistocene
(Paleo In Situ, Paleo Storm) and Recent (Recent Storm) samples, we found species present
beyond the limits of their known ranges. We assumed that these species were at the edges of their
ranges and assigned them a centre index value of 1.0.
Randomization tests
Randomization tests were used to evaluate the extent to which the data points for each
assemblage fell outside the predicted triangular shape of the relationship between abundance and
distance from the centre of the range (Fig. 2b). We constructed the triangular space by drawing a
line between values of 1.0 for both the relative abundance and centre index. We then tested the
null hypothesis that there were no more points outside this space (above the line) than expected
from random combinations of the two variables.
Using the raw data, we randomly drew a value of relative abundance and a second value for the
centre index, plotted the resulting point in the bivariate space and repeated this process, drawing
values without replacement until all values were drawn. This produced a randomized assemblage
containing the same number of species and the same distribution of values for the relative
abundance and centre index as the real one. For each such random assemblage, we measured the
sum of the squared deviations from the constraint line for those points falling above the line. This
procedure was repeated 1000 times, the summed squared deviations were plotted and the
probability of obtaining this result under the null hypothesis was estimated by determining the
number of simulated values that were less than or equal to the observed value. Such
randomization tests were performed for each assemblage separately and for all three assemblages
combined.
Results
While studies of data from fossilized individuals, populations and communities permit incorporation
of a temporal scale not possible in neontological research, data from fossils must be treated with
caution because of possible evolutionary changes and biases in preservation (see Dodd and
Stanton, 1981; Kidwell, 1985; Kerr, 1991). All of the fossil shells in our samples were readily
identifiable with contemporary species. This suggests considerable stasis in the evolution of
molluscan hard parts (see also Stanley and Yang, 1987; Tull and BOhning-Gaese, 1993). It also
supports the findings of Valentine (1989) and Kidwell and Bosence (1991; see also Kerr, 1991)
that nearly all of the marine mollusc species that presently occur along the Pacific Coast of North
America also occur virtually unchanged in Pleistocene deposits in the same region.
Fidelity of storm deposits
The excellent state of preservation reduced the possible kinds of taphonomic bias, but there
remains a problem of how accurately the storm-accumulated deposits of shells, both Pleistocene
and Recent, reflect the composition of living communities. We evaluated this by comparing the
relative abundances of all species in our Paleo Storm sample with the Paleo In Situ sample, the
former representing a regional assemblage and the latter a contemporaneous intact community
(Fig. 3). The significant positive correlation indicates that the storm deposit fairly accurately
reflected the abundances of living molluscs (see also Marquet, 1993). Note that an extremely high
correlation would not be expected, because the in situ sample represents a very local community
Macroecology of geographic range position
593
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Figure 3. Correlation between the relative abundances of each species that occurred in the Pleistocene in
situ layer that also occurred in the Pleistocene storm layer. The significant positive correlation (n = 22,
r = 0.683, y = 2.2841e-2 + 0.51582x, p = 0.0005) suggests that the storm layer is a more regional-scale
sample that still reflects the composition of local communities.
whereas the storm sample reflects material collected from a much larger area and a wider variety
of substrates and depths.
Using the same sampling protocol and recent mollusc deposits used in our study, Marquet
(1993) found that the Recent Storm deposits resemble local living mollusc communities in both
species composition and relative abundance (see also Tull and B6hning-Gaese, 1993). Therefore,
we conclude that the fossil and Recent shells in our samples, especially the storm deposits,
provide reasonably accurate measures of regional abundance.
Abundance and RCI
Graphs of relative abundance as a function of the range centre index for each of the samples and
for all three samples combined showed the predicted triangular pattern (Fig. 4). Species near the
centres of their ranges exhibited a wide range of abundances, while those near the edges tended
to show consistently low abundances. Randomization tests of the observed data against the null
hypothesis of no association between the two variables were rejected for each assemblage
individually and for all three assemblages combined (all p values <0.05; Table 1, Fig. 5). Since
the randomization tests evaluate the probability of points falling outside the predicted triangular
constraint space, the range of variation was consistent with the predicted triangular pattern. We
conclude that local abundances of co-existing species within local communities are substantially
influenced by the location of the sample site within their geographic ranges.
Temporal change in abundance
There were substantial changes in the relative abundances of most species between the
Pleistocene and Recent samples (see the Appendix). We calculated the change in abundance for
those species that were present in both the Paleo and Recent Storm samples and plotted the
resulting values as a function of the centre index. The result (Fig. 6a) is a positive correlation
(r 2 = 0.52, p < 0.003), indicating greater changes in abundance in species that are closer to the
edges of their ranges. A factor that potentially confounds this relationship is the consistently low'
Enquist, Jordan and Brown
594
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Figure 4. The relationship between the relative abundance and range centre index (RCI) for (a) Recent
Deposit, (b) the Pleistocene in situ sample, (c) Pleistocene Storm Layer and (d) all layers combined. Species
of RCI values of zero have the centres of their geographic ranges at the study sitewhereas RCI values of one
indicate species that are at the edge of their geographic range at our study site.
Table 1. The observed sum of squared deviations for all points lying
above the constraint line (see text and Fig. 2c) and the probability of
accepting the null hypothesis of no association between the RCI and
relative abundance for each assemblage
Pleistocene Storm
Recent Storm
Pleistocene In Situ
Observed SS distance
Probability
0.61653
0.35354
0.13758
p = 0.003
p = 0.047
p = 0.042
Macroecology of geographic range position
595
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Figure 5. Results of the randomization test of the null hypothesis of no association between the RCI and
relative abundance for all layers combined (see text for method). The arrow denotes the location of the
value for real assemblages (sum of squared distance = 1.113) and shows that they deviate less from the
expected triangular space (see Fig. 2c) than 1000 randomized assemblages (p < 0.001).
abundances of species near the edges of their ranges. Stochastic demographic processes and
sampling errors might produce a negative correlation between the change in abundance and the
mean abundance. We tested for such a relationship, but it was not significant (r 2 = 0.003, p > 0.8;
Fig. 6b). Thus, we conclude that changes in abundance over the 100 000 years between the Paleo
and Recent samples were more influenced by the position within the geographic range than by
abundance per se.
Assemblage turnover and RCI
Unlike the above species that were present in both the Paleo and Recent samples, however, the
majority of species were present only in one or the other (see the Appendix). The turnover
between Pleistocene and Recent was fairly equally balanced between those present in the
Pleistocene that went at least locally extinct and those absent in the Pleistocene that had
colonized by the Recent (25 and 26 species in each category respectively; see Table 2.). Persistent
species are those species present in both the Pleistocene storm and Recent Storm deposits, while
turn-over species are present in only one sample.
We tested the hypothesis that such turnover (as measured by the number of species showing
local colonization/extinction events versus the number of species who were present in both
Pleistocene and Recent Storm samples) was more likely to have occurred in species near
the edges of their ranges, by comparing species near the centres of their geographic ranges
(RCI ~< 0.70, n = 12) to those species near the edges of their geographic distribution (RCI >
0.70, n = 55). Species near the edges of their geographic distribution exhibited more local
extinctions/colonization events than those near geographic centres (X 2 = 5.486, p < 0.025).
Again we evaluated the possible confounding effect of abundance, but found no differences in
596
Enquist, Jordan and Brown
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Figure 6. (a) The change in abundance as a function of the RCI for the 16 species that occur in both the
Pleistocene storm and Recent Storm deposit. We calculated the change in abundance as maximum
abundance minus minimum abundance divided by maximum abundance. The positive correlation is highly
significant (n = 16, r = 0.800, y = 0.37724 + 0.54946x, p < 0.0001) indicating that the species near the
edge of their geographic ranges experienced greater temporal variation in population size. (b) The change in
abundance as a function of In (mean abundance) for the same species as (a). The lack of a relationship
between these variables (n = 16, y = 0.67742 - 1.6701e - 2x, r = 0.1260 p = 0.6417) suggests that
temporal changes in abundance may have been more influenced by the distance from the centre of the range
than by abundance per se.
the a b u n d a n c e s b e t w e e n those species that t u r n e d o v e r ( m e d i a n a b u n d a n c e = 12, n = 51) and
those that persisted ( m e d i a n a b u n d a n c e = 28.25, n = 16; M a n n - W h i t n e y W = 649.5,
p = 0.1223). T h e a b u n d a n c e for persistent species was calculated as the m e a n a b u n d a n c e
b e t w e e n P a l e • S t o r m a n d R e c e n t Storm. It a p p e a r s as if species n e a r the edges o f their
g e o g r a p h i c r a n g e w e r e m o r e likely to s h o w local colonization o r extinction a n d that such t u r n o v e r
was m o r e influenced by position in the g e o g r a p h i c r a n g e t h a n b y local a b u n d a n c e per se.
Macroecology of geographic range position
597
Table 2. Relating local extinction/colonization events
observed between the Pleistocene Storm and Recent
Storm deposits with the position in the current range
Direction of geographic centre
Extinction
Colonization
South
North
3
3
21
23
Southern and Northern species are defined as those species
whose present geographic centroids lie to the north and
south of our study site, respectively. The table indicates
that there has been an equal number of local colonization
and extinction events between southern and northern
species. We would expect unequal colonization and extinction events if there had been consistent north or south
movements in species associatedwith climaticchange. One
species was excluded from the analysis because its geographic centre lies at our study site.
Discussion
Inferring Pleistocene ranges from contemporary distributions
W e first address two potential problems in our use of data on geographic ranges for contemporary
molluscs to make inferences about abundance and distribution during and since the Pleistocene.
First, range boundaries at the time that the Paleo samples were deposited might have been very
different from those at present. Second, apparent colonization and extinction events between the
Paleo and Recent samples might have reflected major shifts in ranges rather than local turnover
of species.
Several kinds of evidence suggest that range boundaries of most of the species might have been
similar at the times when both the Paleo and Recent samples were deposited. First, of the 25
species in the Pleistocene sample that have gone locally extinct, only six have modern
distributions that do not overlap with our study site, implying a different Pleistocene range.
Furthermore, of these six species, only one (Corbula tenuis) has a contemporary range limit more
than 1° latitude from our site. Furthermore, this species exhibited a low abundance that then may
have been near the Pleistocene range boundary (see the Appendix). Only one of the 25 species
that went locally extinct at our study site appeared to have experienced large-range shifts.
Second, there was an almost equal n u m b e r of colonization and extinction events between the
Paleo and Recent samples (Table 2). This, coupled with the fact that the vast majority of species
are nearer the present northern than southern limits of their ranges (44 and six, respectively),
indicates that there have not been large, consistent shifts in the ranges of many species. If the
Paleo and Recent climates had been different, we would expect an unequal distribution of
colonization and extinction events, reflecting coordinated northward or southward shifts in the
ranges of multiple species. Our data suggest that the differences in species composition between
the Pleistocene interglacial and recent samples reflect colonization and extinction responses of
species in reflection of their individualistic requirements. Valentine and Jablonski (1993) suggest
a similar explanation for changes in the composition of fossil mollusc assemblages during the
Pleistocene and Holocene along the Pacific Coast of North America. The fact that the majority of
species are nearer their northern than southern range boundaries reflects the p r o n o u n c e d
598
Enquist, Jordan and Brown
latitudinal gradient in shallow-water mollusc species diversity in the Eastern Pacific as a whole
(e.g. Jablonski and Valentine 1990) as well as within the Gulf of California.
Third, if our predictions are sound but there had been major shifts in ranges between Paleo and
Recent samples, we would expect only the Recent assemblage to show the predicted triangular
relationship between the abundance and centre index clearly. The fact that both Paleo samples
exhibit constraint patterns virtually indistinguishable from the Recent one suggests that the Paleo
communities were subjected to the very similar environmental conditions.
We do not want to imply that the Pleistocene interglacial and contemporary range boundaries
were necessarily coincident. The large number of local extinctions and colonizations suggests that
the environments were somewhat different when the Pleistocene and Recent samples were
deposited. Thus, it would not be surprising that there had been some shifts in geographic range
boundaries. Other studies have found associations of Pleistocene interglacial fossil molluscs from
the Pacific Coast of North America that contain species whose contemporary geographic ranges
do not overlap. These changes in community composition and geographic ranges have been also
interpreted as reflecting the individualistic responses of species to somewhat different past and
present environments (e.g. Valentine and Jablonski, 1993). Nevertheless, the fact that position
within the present geographic range seems to predict the constraints on past as well as present
abundance, suggests that proximity to the edge of the range indexes the existence of
environmental conditions that preclude high abundance.
Species extending beyond panamic province
We used the range centre index values as a simplifying means by which to estimate the relative
latitudinal range position of each species within our study site. The utility of this measure, as
mentioned earlier, ignores other potentially influential measures of range size (total length or
width of range, volume of water mass occupied). Due to the linear nature of the North and South
American Pacific coastline and narrow continental shelf, the ranges of molluscs can be simplified
as linear (Jablonski and Valentine, 1990). However, our sample site lies within the Gulf of
California, approximately 40% of the species found at our site extend beyond the Gulf of
California and thus inhabit both the Gulf of California and Baja coasts (see the Appendix). This
raises a potential problem. A species whose range includes the Gulf of California and also
extends north beyond the tip of Baja occupies more coastline and potential geographic area than
a species whose range is restricted to the Gulf. Even though they may both have the same
northern most range terminus of the head of the Gulf, species extending beyond the Gulf may
potentially confound our analysis by exhibiting different abundance patterns.
We evaluated the robustness of our initial patterns to this effect by comparing the relationship
between the relative abundance and RCI for species extending beyond and those whose range is
restricted to the Gulf of California and the south (see the Appendix). Abundances and RC!
values for each species were replotted and the randomization analysis was repeated for both
groups. Species extending beyond the Gulf of California (SEB) yielded abundance/per cent
relative abundance patterns indistinguishable from species restricted to the Gulf and the south
(SRT) and to Figs 2b and 4a, (SEB, observed SS distance from constraint = 0.42402, p < 0.001;
SRT, observed SS distance from constraint = 0.66915, p = 0.005) This indicates that the relative
latitudinal position as measured by the RCI is not affected by this biogeographical feature and
seems to index conditions dictating local abundance accurately.
Implications of relative position in the geographic range
We conclude that the location of a population within the geographic range of a species influences
both the relative abundance of that species within local and regional assemblages and the
Macroecology of geographic range position
599
magnitude of temporal change in abundance between long-separated periods of similar
environmental conditions. In both cases, location within the range does not permit accurate
prediction of abundance, but it does constrain the range of possible variation. Interestingly,
location within the range appears to be a better predictor of long-term variation in abundance
than initial abundance.
The triangular relationship between relative position in the range and abundance appears to be
yet another case demonstrating the utility of a 'macroecological' approach that attempts to draw
constraint envelopes to characterize ranges of possible or probable variation (Fig. 2c; for other
examples, see Brown and Maurer (1987, 1989)). The constraint of location in the geographic
range on maximal local and regional abundance appears to offer an example of a connection
between patterns and processes on local, regional and geographical scales (see Ricklefs, 1987;
Ricklefs and Schluter, 1993) and, thus, between ecology and biogeography and between
contemporary and historical phenomena.
The ability to demonstrate this constraint and to make this connection seems to depend on
species-specific niche requirements of the mollusc species that have not changed significantly in
approximately 100 000 years. There is much paleontological evidence of morphological stasis for
periods of thousands to millions of years (e.g. Eldredge and Gould, 1972; Stanley and Yang,
1987). We observed such stasis in these molluscs. All of the Pleistocene fossils were indistinguishable from shells of contemporary species. Tull and B6hning-Gaese (1993) have documented
virtual stasis between the Paleo and Recent deposits at this same site in the predatory behaviour
of naticid and muricid gastropods as evidenced by the patterns of drilling of shells of prey. Our
results suggest that many ecological relationships of these molluscs have changed very little in
approximately 100 000 years (see also Marquet, 1993).
Connections between ecology, biogeography and paleontology
While the results of the present study cannot be taken as unequivocally validating Brown's (1984;
modified in Brown et al. (in press)) niche-based model for the regulation of abundance and
distribution, they are consistent with that model. All of the a priori predictions made on the basis
of the model were supported empirically. A reformation of Hutchinson's (1957) concept of a
multidimensional niche appears to have promise not only in understanding the individualistic
pattern of distribution and abundance exhibited by each species, but also in obtaining more
general insights. Thus, although it is almost tautological to say that each species has a unique set
of niche requirements, nevertheless the majority of species appear to show similar patterns of
variation in abundance over space (Brown et al., in press) and probably also in time.
Furthermore, the ways that the niche requirements and location within the geographic range
interact to constrain the local abundance of each species also constrain the relative abundances of
different species within local and regional assemblages. It is well known that communities contain
a few common and many rare species (e.g. Preston, 1948, 1962a,b; MacArthur, 1957; Williams,
1964). The extensions of Brown's (1984) model and the empirical results presented here suggest
that the few common species that dominate local communities are populations near the centres of
their geographic ranges. The converse, however, is not true: locally rare species can represent
populations from anywhere in their species ranges.
Hanski (1982) has suggested that communities comprise what he calls core species, which are
abundant within and tend not to turn over between local assemblages - and what he calls satellite
species, which are rare and frequently turn over between even nearby sample sites. While our
results do not show the bimodal segregation into core and satellite species recognized by Hanski
(1982), they do suggest that those species which frequently co-exist in both space and time tend to
be not only the abundant species but also those near the centres of their geographic ranges. Thus,
600
Enquist, Jordan and Brown
fundamental characteristics of community structure, such as the relative abundance of individual
species and the frequency of co-existence of multiple species, are influenced by the location of the
sample site with respect to the geographic ranges of the constituent species. This has important
ecological and evolutionary implications. For example, the strength and predictability of
interspecific interactions and the opportunities for co-evolution should depend in part on the
location of the species within their geographic ranges.
Contemporary ecology is still struggling to reconcile the different perspectives on community
organization debated by Gleason (1917, 1926) and Clements (1916, 1949). On the one hand,
there is abundant evidence that many assemblages consist of species whose abundances and
distributions exhibit highly individualistic patterns of spatial and temporal variation (e.g.
Whittaker, 1967). These appear to reflect the relationships between the unique niche requirements of each species and the special features of local environments. On the other hand,
assemblages appear to exhibit emergent patterns of structure and dynamics that are sometimes
very general (e.g. Odum, 1969; Morse et al., 1988; Brown and Maurer, 1989; Brown and
Nicoletto, 1991; see also DiMichele, 1994). Many of these emergent properties are exhibited in
statistical distributions that reflect fundamental constraints on the pattern of variation.
Both the Gleasonian individualism and the Clementsian emergent properties are exhibited by
these molluscan assemblages. The individualism is exemplified by the wide variation in
abundances among contemporaneously co-existing species, even those near the centres of their
ranges. As above, we attribute much of this variation to differences in the niches of these species,
i.e. differences in trophic status, habitat specialization and biotic interactions.
The Clementsian emergence is exhibited in the characteristic relationship between abundance
and position within the geographic range for the communities as a whole (Fig. 2c) and in the
stability of this relationship over time. One way to sort through the Gleasonian individualism in
search of the general, emergent properties is to increase the scale of ecological studies to include
large numbers of species, geographic spatial scales and long time periods. That is the approach
that we have taken here. We have studied a large, ecologically heterogeneous assemblage of
molluscs, rather than a single species or guild. We have analysed the abundances and
distributions with respect to geographic space and evolutionary time. We have discovered
seemingly robust statistical patterns.
Acknowledgements
This paper was greatly improved via discussions and suggestions from George C. Stevens, David
Jablonski and one anonymous reviewer. Furthermore, assistance from Pablo Marquet and Dave
Mehlman is greatly appreciated. We are also grateful to B. Weir, Astrid Kodric-Brown, Lee
Fitzgerald, Ann S. Evans, Justin Olson, Deb Tull, Carolyn A. Finnance and the 1993 UNM
Advanced Field Biology Class. This research was supported through National Science Foundation
Grant DEB-9318096 to J.H.B. and G.C.S.
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Appendix
Summary of data for the 67 mollusc species that occurred in the three samples
Species
Acteocina carinata
Anachis sanfelipensis
Anachis scalarina
Barbatia iUota
Barbatia reeveana
Bulla punctulata a
Calliostoma eximium ~
Cantharus macrospira
Cardita affinis
Cardita laticostata
Cerithidea mazatlanica a
Cerithium stercusmuscarum
Chione californiensis b
Chione pulicaria
Conus reguiaris a
Corbula bicarinata
Corbula biradiata
Corbula marmorata
Corbula tenuis
Crepidula striolata
Crucibulum scutellatum a
Crucibulum spinosum
Cyclinella singleyi a
Diodora alta
Diodora digueti a
Diodora inaequalis
Donax navicula
Felaniella sericata
Fusinus fredbakeri
Glycymeris multicostata
Graptacme semipolitum a
Hipponix panamensis
Laevicardium elatum
Laevicardium elenense a
Liocerithium judithae a
Lottia mesoleuca d
Max N
(o)
Max S
(o)
32
32
23
29
34
32
32
32
23.5
29
7
4 S
4 S
3 S
2
28
29
32
32
32.5
37.5
32
32
32
28
32
4 S
4 S
7
3 S
17
2.5
3 S
1 S
2 S
2 S
8
3 S
33
32
42
32
32
32
32
33
37
31
31
32
32
34
32
32
24.5
0
0
36.5 S
3 S
5 S
3 S
0
3 S
6 S
26
2 S
8
3 S
7
4 S
23.5
0
PS
(n)
PI
(n)
25
56
-
1
1
7
3
4
14
198
1
15
1
3
8
1
17
1
4
186
7
10
20
2
23
-
2
32
27
-
17
1
3
17
1
1
3
3
-
RS
(n)
2
8
21
20
142
25
25
190
367
15
2
8
18
110
15
3
7'
4
28
16
8;
58
1
604
Enquist, J o r d a n a n d B r o w n
Species
Max N
(o)
Max S
(o)
Lottia strigatella c
Lucina fenestrata
Mytella guyanensis a
Nassarius angulicostis
Nassarius iodes
Nassarius versicolor~
Nerita funiculata a
Niso lomana ~
Nucula declivis
Nuculana impar
Oliva spicata
Olivella dama
Pitar paytensis
Polinices uber ~
Protothaca grata ~
Pyrene major
Pyrene strombiformis
Sanguinolaria tellinoides
Tagelus affinis
Tegula mariana
Tegula rugosa
TeUidora burneti ~
Tellina ochracea e
Tellina simulans a
Theodoxus luteofasciatus
Trachycardium consors
Trachycardium panamense a
Turbo fluctuosus a
Turritella gonostoma
Turritella leucostoma a
Vermicularia frisbeyae
30
28
31.5
31
32
32
32
32
31.5
31.5
32
32
32
32
32
26
31
32
35
32
32
32
32
32
32
31
32
32
32
32
28.5
5
5 S
6 S
7
23.5
5 S
3 S
28
4 S
10.5
3
23.5
5 S
5 S
20 S
3 S
3 S
2 S
2 S
4 S
28
2
23
4 S
3 S
1 S
8.5
5 S
2 S
2
13
Number
of individuals per layer
PS
(n)
PI
(n)
19
3
7
44
46
24
28
8
5
138
8
172
88
93
2
54
12
1
42
74
16
1
1
11
34
2
4
15
6
51
103
2
1
53
16
81
1
193
9
15
24
6
-
1364
253
1720
41
23
42
6
-
RS
(n)
1
6
1
1
1
-
Total = 3337
Number
of species encountered per layer
T h e c o l u m n s are, from left to right (1) n o r t h e r n latitudinal limit of range (in degrees), (2) s o u t h e r n latitudinal
limit of range (S indicates south latitude), (3) n u m b e r of individuals in the Paleo Storm (PS) sample, (4)
n u m b e r of individuals in the Paleo In Situ (PI) sample and (5) n u m b e r of individuals in the Recent Storm (RS)
sample. See the text for explanations of the range information.
Total n u m b e r of species e n c o u n t e r e d in all layers = 67.
aSpecies whose n o r t h e r n limit of range is given as 32.0 ° N but also extend out of t h e Gulf of California along the
outer coast of B a j a (see species extending b e y o n d p a n a m i c province section).
bjablonski a n d Valentine (1990) extend this range to M u g u L a g o o n n e a r Santa Barbara, California.
e w e u s e the m a x i m u m n o r t h e r n distribution into the Gulf of California as reported by Skoglund, (1992) as the
n o r t h e r n position of range.
din general, t h e m a x i m u m range extent of species f o u n d in our study site were not influenced by Galapagos
island range extensions. This is due to the fact that the m a x i m u m s o u t h e r n range end-points also tend to
coincide with the s o u t h e r n m o s t continental shelf distribution. However, as reported in Skoglund (1992) and
K e e n (1971), this observation m a y not hold for Lottia mesoleuca.
eBernard et al. (1991) have proposed a range extension to the Galapagos. This has not been verified in other publications (D. Jablonski personal communication), we use the m a x i m u m , K e e n (1971) and Bernard (1983) distribution.