Download Mixed effects of habitat fragmentation on species richness and

Ecological Entomology (2005) 30, 684–691
Mixed effects of habitat fragmentation on species
richness and community structure in a microarthropod
microecosystem
M A R T I N H O Y L E 1 , 2 and A L A S T A I R R . H A R B O R N E 2
1
School of Biology, University of
Nottingham, Nottingham, U.K. and School of Biological and Chemical Sciences, University of Exeter, Exeter, U.K.
2
Abstract. 1. Theory is unclear about the optimal degree of isolation of habitat
fragments where the aim is to maximise species richness. In a field-based microecosystem of Collembola and predatory and non-predatory mites, moss patches
of the same total area were fragmented to varying degrees. The habitat was left
for several months to allow the communities to approach a new state of
equilibrium.
2. The species richness (in particular of predatory mites) of a given area of
habitat was greater when it was part of a large mainland area than part of an
island, in agreement with theory.
3. Conversely, species richness and abundance were largely unaffected by fragmentation of a fixed area of island habitat. In this case, it is suggested here that
the advantages of several small patches (e.g. reduced impact of environmental
stochasticity, wider range of habitats overall) were equally balanced by the
advantages of a single large patch (e.g. reduced effect of demographic stochasticity, wider range of habitats within a single patch, reduced edge effect), or that
both effects were small.
4. The shapes of rank–abundance curves were similar among the levels of
fragmentation of a fixed area of island habitat, implying that fragmentation
had little impact on community structure. Conversely, the species composition
of non-predatory mites varied weakly, but significantly, by fragmentation.
Key words. Beta diversity, conservation, habitat heterogeneity, metapopulation, SLOSS.
Introduction
After fragmentation, habitats undergo a process of community disassembly, or ‘relaxation’ (Diamond, 1972). The
number of species that will be lost in the future is the
‘extinction debt’ (Tilman et al., 1994), which is partly due
to demographic stochasticity. The island species richness is
expected to decrease over time to its new equilibrium value.
The degree of habitat fragmentation that maximises species
richness may depend on the stage in the relaxation process.
Correspondence: M. Hoyle, School of Biological and Chemical
Sciences, Hatherly Laboratories, University of Exeter, Prince of
Wales Road, Exeter EX4 4PS, Devon, U.K. E-mail:
[email protected]
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Immediately after habitat fragmentation, the number of
habitats and the proportion of species in common between
the sub-populations will be critical (Higgs, 1981).
Fragmented habitat dispersed over a large area is likely to
capture a greater range of habitat types than a single large
habitat patch of the same total area. As many species are
restricted to particular habitats, several small patches may
contain more species than a single large patch. The further
apart the habitat patches, the greater the habitat diversity
likely to be encompassed, and the greater the species richness (Simberloff, 1986). Some species require more than
one habitat type (e.g. used by different life stages), and if
movement is restricted among habitat patches, a single
large patch may contain more species than several small
patches.
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2005 The Royal Entomological Society
Microarthropod habitat fragmentation
After relaxation, on the other hand, in addition to the
number of habitats and the proportion of species in common between the sub-populations, metapopulation processes such as migration rates between patches and
environmental and demographic stochasticity will influence
the optimal number of patches. The species richness of
several small habitat patches depends in part on the rate
of accumulation of species extinctions. To estimate the time
to extinction for a population, it is necessary to know the
distribution of extinction times of sub-populations and the
correlation of disturbances among sub-populations. The
effect of demographic stochasticity is more pronounced
for smaller populations, thus species should be more
prone to extinction in a fragmented landscape than in a
large remnant patch of the same total area (Burkey, 1989),
suggesting that nature reserves should be designed to be as
continuous as possible. On the other hand, environmental
stochasticity such as fire, disease, invasions by non-native
species, and drought may reduce the persistence time of
populations in a single large patch (Mangel & Tier, 1993)
and may counteract the effect of demographic stochasticity,
making populations in fragmented systems less vulnerable
to extinction than populations in continuous habitats. The
magnitude of these effects will depend on the organism in
question, the scale of the system and the degree of environmental correlation.
If migration between reserve fragments is possible, the
‘rescue effect’ (Brown & Kodric-Brown, 1977) may significantly increase species richness and abundance (Gilbert
et al., 1998; Gonzalez et al., 1998). Single-species metapopulation modelling (Pelletier, 2000; Ovaskainen, 2002) suggests that the optimal degree of fragmentation of a given
total area of habitat depends on the rate of migration
among patches and the rates of mortality in habitat and
non-habitat patches. With low or no migration, many small
patches may be preferable to a single large reserve of the
same total area (Pelletier, 2000; Hubbell, 2001). With
migration, some studies suggest that an intermediate number of sub-populations maximises metapopulation persistence (Stacey et al., 1997; Pelletier, 2000 when the survival
rate in the non-habitat patch is low; Ovaskainen, 2002), but
others favour a single large reserve (Etienne & Heesterbeek,
2000). In a recent review of empirical studies, Debinski and
Holt (2000) found a lack of consistency in the effects of
habitat fragmentation on species richness and abundance.
The evidence amongst microecosystem studies is also equivocal; extinction rates may be lower in unfragmented habitats (Forney & Gilpin, 1989; Burkey, 1997) or in
fragmented habitat (Holyoak & Lawler, 1996) of the same
total area/volume.
Experimental tests of population dynamic processes have
often used laboratory microcosms because of their tractability and short timescales of change, e.g. Forney and
Gilpin (1989) and Burkey (1997). Here a moss–microarthropod microecosystem is used. This has the advantage
of being a completely natural system occurring in the field
in both continuous and fragmented states. Thus experimental fragmentation merely reproduces patterns that occur
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naturally. Even though the generation times of the microarthropods are typically several months (Christiansen
et al., 1992; Norton, 1994), the effects of habitat fragmentation on the fauna are measurable after just 6 months
(Gilbert et al., 1998; Gonzalez et al., 1998; Gonzalez &
Chaneton, 2002). Furthermore, predators suffer greater
rates of extinction than non-predators (Gilbert et al.,
1998), and relaxation continues for at least 1 year
(Gonzalez, 2000). Although the moss–microarthropod
microecosystem is based on a far smaller spatial scale
than a nature reserve, it has provided insights into the
efficacy of wildlife corridors and the causes of the species–
area relationship. Connecting patches of moss habitat by
moss ‘corridors’ slows the rate of species extinction (Gilbert
et al., 1998; Gonzalez et al., 1998), possibly by the rescue
effect. The contributions of the metapopulation effect
(higher colonisation and lower extinction rates on local
mainland than on local island areas) to the species–area
relationship, relative to that of the combined effects of
habitat heterogeneity and sampling are approximately
equal after 6 months (Hoyle, 2004). With further relaxation
of the fragmented moss–microarthropod community, the
metapopulation effect is expected to increase.
Using the moss–microarthropod microecosystem, a fieldbased study designed to test the effects of habitat fragmentation on species richness and abundance and community
composition is presented. Fragmented and unfragmented
patches were left for sufficient time to allow the communities to relax. It was found that mainland patches were
more species rich than island patches. Species richness was
unaffected and community structure only weakly affected
by the degree of fragmentation.
Materials and methods
Eight replicate sets (blocks) of four treatments (Fig. 1) were
cut from continuous moss [Isothecium myosuroides (Brid.)
var. myosuroides] growing on eight large rocks in
Snowdonia (U.K.), leaving bare rock in-between. The
bare rock was considered to be an inhospitable environment for the majority of the moss taxa, restricting (but not
eliminating) movement among patches. Treatments were
arranged in a random order in June 2001, keeping a minimum distance of 10 cm of bare rock between ‘islands’ (as
Gilbert et al., 1998; Gonzalez et al., 1998) to minimise
migration across the rock. Six months later the moss was
removed in concentric rings of one unit of area (Fig. 1).
Each patch was dismembered and left in a separate
Tullgren funnel for 48 h. Emerging microarthropods were
collected in an alcohol–glycerol–water (7:2:1) mixture,
sorted into morphospecies (Table 1) and identified with
help from relevant experts (see Acknowledgements). In
the rest of this paper, the term ‘species’ refers to these
morphospecies, although note (Table 1) that most were
single species. Although the choice of the duration of the
experiment is partly arbitrary, it was set at 6 months
because previous work (Gilbert et al., 1998; Gonzalez
2005 The Royal Entomological Society, Ecological Entomology, 30, 684–691
686 Martin Hoyle and Alastair R. Harborne
Rock 1
Moss
Mainland
Bare rock
> 10 cm
Total area (units) 4
4
4
2
Fig. 1. The four experimental treatments were circular moss ‘islands’ of one, two, and four units of area (1 unit ¼ 39 cm2) and a circular
patch of ‘mainland’ moss of two units of area growing naturally on rock. Each rock was approximately 5 m from the next nearest rock. The
order of treatments was random (here shown as progressing from more to less fragmented). After 6 months, moss was removed in concentric
rings of one unit of area (indicated by white dotted lines). Only one (1) of the eight rocks (1 to 8) is displayed.
et al., 1998) showed that this is sufficient to allow the
effects of fragmentation to occur. Note that it is not possible to sample the fauna at the start of the experiment.
Nevertheless, any initial differences in community composition are dealt with by replication in the statistical analysis.
Moss samples were weighed before and after extraction to
give the moss wet and dry weights.
Fragmentation of fixed area
Species richness and abundance in constituent patches in
each treatment were summed within a given rock. The
groups tested were: total microarthropod species, non-predatory mite species, and predatory mite species, and total
microarthropod individuals, non-predatory mite individuals,
Mesostigmata mite individuals, Prostigmata mite individuals,
Collembola individuals, the six most common
Cryptostigmata mite species (morphospecies 7, 8, 11, 12, 15,
and 16: Table 1), a Prostigmata mite species (morphospecies
4), and two Collembola species (morphospecies 1 and 2).
Generalised linear mixed-effects models (‘glmmPQL’
Venables & Ripley, 2002) with Poisson errors (suitable for
count data) were implemented in ‘R’ (Ihaka & Gentleman,
1996; Crawley, 2002), with moss dry weight as a covariate,
and block as a random factor. Model simplification was
performed by backward elimination from the maximal
model to the minimum adequate model, and factor significance was gauged by a w2 test of the increase in deviance
after factor deletion.
We might expect the smaller patches to be drier than the
larger patches, due to their greater edge to area ratio,
especially if it is a long time since the last rain.
Furthermore, the moisture content of moss is likely to
influence species richness and abundance. Therefore the
above test was repeated on the moss wet weight using
normal errors, with moss dry weight and patch fragmentation as covariates in a mixed-effects model (routine ‘lme’:
Ihaka & Gentleman, 1996). Furthermore, to test for an
edge effect in the moss patches, species richness and abundance were compared between the concentric rings of the
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two- and four-unit area patches using Poisson errors, with
ring number and ring dry weight as covariates. To test
whether the moss was drier towards the edge of the patch
of area four units, a possible cause of any edge effect, the
wet weight of the moss rings was analysed using normal
errors with ring number and ring dry weight as covariates.
Whilst comparison of species richness and abundance by
fragmentation is important, this gives little information on
community composition and no information on species
turnover between habitats of differing fragmentation. For
example, the species richness of two habitats could be
equal, but the species composition might not overlap.
Community composition was investigated graphically and
statistically. Firstly, rank–abundance graphs were plotted
for each of the fragmentation levels. Rank–abundance
plots are preferable to diversity indices because no information is lost. Secondly, multidimensional scaling was performed on the abundance of all microarthropod species
between the three levels of fragmentation, based on the
Bray–Curtis similarity coefficient (Bray & Curtis, 1957).
The measure takes a maximum value of 1 when the species
composition in two habitats is identical and a minimum
value of 0 when the habitats have no species in common.
The similarity (a measure of decreasing b diversity) between
fragmentation levels j and k is defined as
p P
Xij Xik 1 i¼1
p P
Xij þ Xik i¼1
where Xij is the abundance of the ith species in the jth
fragmentation level, and where there are p species overall.
The null hypothesis of no significant difference in b-similarity among the three levels of fragmentation was tested
using analysis of similarities (ANOSIM; Clarke, 1993). ANOSIM
contrasts observed similarities among groups of samples
with those within groups. Permutation is then used to
evaluate the significance of the statistic. Tests were applied
to the standardised abundances so that the means and
standard deviations for all levels of fragmentation were
equal.
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Microarthropod habitat fragmentation
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Table 1. Mite and Collembola morphospecies.
Morpho-species
Cryptostigmata mites
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Plus five unidentified morphospecies.
Mesostigmata mites
1
2
3
4
5
6
7
8
9
Prostigmata mites
1
2
3
4
Plus four unidentified morphospecies.
Collembola
1
2
3
4
5
6
Species identified
Ceratoppia bipilis (Peloppiidae ¼ Ceratoppiidae) (Hermann)
Chamobates borealis (Chamobatidae) (Trägårdh)
Chamobates cuspidatus (Chamobatidae) (Michael, 1884)
Minunthozetes pseudofusiger (Mycobatidae) (Schweizer, 1922)
Phthiracarus longulus (Phthiracaridae) (Koch)
Sphaerozetes piriformis (Ceratozetidae) (Nicolet)
Tectocepheus sarekensis (Tectocepheidae) (Trägårdh, 1910)
Eueremaeus (possibly oblongus) (Eremaeidae) (Koch)
Quadroppia quadricarinata virginalis (Oppiidae) (Lions, 1982)
Achipteria nitens (Achipteriidae) (Nicolet, 1855)
Carabodes marginatus (Carabodidae) (Michael, 1884)
Carabodes labyrinthicus (Carabodidae) (Michael)
Oribatula tibialis (Oribatulidae) (Nicolet, 1855)
Dissorhinaornata (Oppiidae) (Oudemans)
RamusellaRamusella cf. Assimilis (Oppiidae) (Mihelcic, 1950)
Suctobelba trigona (Suctobelbidae) (Michael)
Porobelba spinosa (Damaeidae) (Sellnick, 1920)
Caleremaeus monilipes (Caleremaeidae) (Michael)
Euzetes globulus (Euzetidae) (Nicolet)
Chamobates schuetzi (Chamobatidae) (Oudemans)
Carabodes willmanni (Carabodidae) (Bernini, 1975)
Odontocepheus elongatus (Carabodidae) (Michael, 1879)
Trichoribates trimaculatus (Ceratozetidae) (Koch)
Phthiracarus nitens (Phthiracaridae) (Koch)
Phthiracarus montanus (Phthiracaridae) (Perez-Iñigo)
Paragamasus integer (Parasitidae) (Bhattacharryya, 1963)
Paragamasus schweizeri (Parasitidae) (Bhattacharryya, 1963)
Zercon zelawaiensis (Zerconidae) (Sellnick, 1944)
Geholaspis longispinosus (Macrochelidae) (Kramer, 1876)
Geholaspis mandibularis (Macrochelidae) (Berlese, 1904)
Paragamasus robustus (Parasitidae) (Oudemans, 1902)
Pergamasus crassipes (Parasitidae) (Linnaeus, 1758)
Pergamasus longicornis (Parasitidae) (Berlese, 1906)
Pergamasus septentrionalis (Parasitidae) (Bhattacharyya, 1963)
Cosmolaelaps claviger (Laelapidae) (Berlese, 1883)
Uropoda (Cilliba subgenus) sp. (Uropodidae)
Holoparasitus calcaratus (Parasitidae) (Koch, 1839)
Holoparasitus inornatus (Parasitidae)
Uropoda misella (Uropodidae)
Bdellidae (species unknown)
Eupodidae (species unknown)
Eupodidae (species unknown)
Cryptognathidae (species unknown)
Pseudoisotoma sensibilis (Isotomidae) (Tullberg)
Xenylla boerneri (Hypogastruridae) (Axelson)
Orchesella villosa (Entomobryidae) (Geoffroy)
Tomocerus minor (Entomobryidae) (Lubbock)
Neanura muscorum (Hypogastruridae) (Templeton)
Entomobrya nivalis (Entomobryidae) (Linnaeus)
Dicyrtomina minuta (Sminthuridae) (Fabricius)
Lepidocyrtus curvicollis (Sminthuridae) (Bourlet)
Plus three unidentified morphospecies.
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2005 The Royal Entomological Society, Ecological Entomology, 30, 684–691
688 Martin Hoyle and Alastair R. Harborne
unit, and also between the island treatment of area two
units and the mainland patch of area two units, using
generalised linear mixed-effects models with Poisson errors.
Any difference in species richness or abundance could have
been caused partially by a difference in moss moisture
content. Therefore moss wet weight was also compared
between the mainland and island patches, with moss dry
weight as a covariate, assuming normal errors.
(a)
4
3
2
1
Results
0
Forty-two thousand microarthropod individuals of 69 morphospecies were recorded, mostly Acari (44%, only adults
counted because juveniles are not extracted efficiently by
the Tullgren method) and Collembola (55%, non-predatory) (Table 1). The Acari were Cryptostigmata (55%,
non-predatory), Mesostigmata (2%, predatory), and
Prostigmata (43%, predatory). On average, over all treatments and rocks, per unit area of moss, there were 94 adult
Cryptostigmata individuals of eight species, four adult
Mesostigmata individuals of two species, 74 Prostigmata
individuals of two species, 211 Collembola individuals of
three species, and two other microarthropod individuals of
two species.
(b)
In(species richness)
4
3
2
1
0
(c)
4
Fragmentation of fixed area
3
2
1
0
Fragmentation
Fig. 2. Species richness did not vary significantly by fragmentation for (a) all microarthropods combined, (b) predatory mites, and
(c) non-predatory mites. The total number of species was counted
from constituent patches within the same experimental rock. See
Fig. 1 for clarification of the icons representing the degree of
fragmentation of the patches. Error bars represent 1 SEM.
‘Mainland’ vs. ‘Island’
A lower species richness and abundance in the ‘island’
patches compared with the ‘mainland’ patch might be
anticipated, because the migration rate of microarthropods
is presumably lower across bare rock than across moss.
Hence, the total number of microarthropod species and
individuals and the number of predatory and non-predatory mite species were compared between the island treatment of area one unit and the mainland core of area one
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The species richness of predatory mites, non-predatory
mites, and all microarthropods combined were unaffected
by fragmentation of the fixed 156 cm2 area of habitat
(Fig. 2). Numbers of one of the Collembola species (morphospecies 1, Table 1) decreased significantly with decreasing habitat fragmentation (w21 ¼ 18.3, P < 0.0001),
whereas numbers of one of the Cryptostigmata mite species
(morphospecies 11, Table 1) and the number of
Mesostigmata mites increased significantly with decreasing
habitat fragmentation (w21 ¼ 15.5, P < 0.0001 and
w21 ¼ 8.6, P ¼ 0.003 respectively). The abundances of the
following taxonomic groups were independent of habitat
fragmentation: total microarthropods, total non-predatory
mites, total Prostigmata mites, total Collembola, five of the
six Cryptostigmata mite species tested, the Prostigmata
species, and one of the two Collembola species tested. For
the tests of species abundance, experimental block was
usually highly significant, indicating that the microhabitat
varied significantly among rocks. Community structure, as
measured by rank–abundance, varied little among the three
levels of fragmentation (Fig. 3). Furthermore, tests of bdiversity by fragmentation were non-significant for all
microarthropods (ANOSIM measure of community dissimilarity based on Bray–Curtis measure R ¼ 0.089,
P ¼ 0.235) and predatory mites (R ¼ 0.018, P ¼ 0.364),
but weakly significant for the non-predatory mites
(R ¼ 0.357, P ¼ 0.024).
Moss wetness increased with patch size (w21 ¼ 5.5,
P ¼ 0.019) (water content for patches of size 1, 2, and 4
2005 The Royal Entomological Society, Ecological Entomology, 30, 684–691
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Microarthropod habitat fragmentation
2
–2
0
ln(abundance)
4
Most fragmented
Intermediate
Least fragmented
Fig. 3. loge(species rank abundance) of
the three fragmentation treatments, each
averaged over all rocks.
0
units of area were 103%, 107%, and 137% of dry weight
respectively). There was no significant trend in moss wetness by concentric ring (w21 ¼ 1.4, NS) in the largest patch.
Nevertheless, there was a strong edge effect in the largest
moss patch: average total microarthropod species (19 in
central core vs. 12 in outer ring, w21 ¼ 8.8, P ¼ 0.003),
total microarthropod individuals (450 vs. 292, w21 ¼ 8.7,
P ¼ 0.003), non-predatory mite species (9.6 vs. 6.6,
w21 ¼ 13.7, P < 0.001), and predatory mite species (4.6 vs.
2.1, w21 ¼ 7.7, P ¼ 0.006).
‘Mainland’ vs. ‘Island
The average number of microarthropod species was significantly greater in the mainland compared with the island
moss for one unit of area (17.9 vs. 15.4 species, w21 ¼ 4.9,
P < 0.027), and two units of area (22.9 vs. 18.5 species,
w21 ¼ 7.8, P ¼ 0.005). The number of predatory mite species was significantly greater in the mainland for one unit of
area (5.4 vs. 3.4 species, w21 ¼ 10.3, P ¼ 0.001), and two
units of area (4.6 vs. 3.6 species, w21 ¼ 6.7, P ¼ 0.010).
These differences were not due to variable moisture content, as this did not vary significantly between the mainland
and island (although wetness increased with patch size, see
above). In contrast, the number of microarthropod individuals and non-predatory mite species did not differ significantly for either area. There was little evidence of an edge
effect for the island patch of area two units. Total microarthropod species, non-predatory mite species and predatory mite species did not vary by ring, but there were fewer
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10
20
30
40
50
60
Rank
microarthropod individuals towards the edge (w21 ¼ 8.0,
P ¼ 0.005).
Discussion
The most important findings of this study are as follows:
(1) species richness, abundance, and community composition were largely unaffected by fragmentation of a fixed
area (156 cm2) of moss habitat (even though the most
fragmented patches were drier), and conversely (2) the
species richness (in particular of the predatory mites) of a
given area of habitat was greater when it was part of a large
mainland area (>> 156 cm2) than part of an island.
The first result suggests either that the advantages of
several small patches (e.g. reduced impact of environmental
stochasticity, wider range of habitats overall) were equally
balanced by the advantages of a single large patch (e.g.
reduced effect of demographic stochasticity, wider range
of habitats within a single patch, reduced edge effect), or
that both effects were small. In particular, it would be
instructive to design a further experiment with a greater
range of patch sizes, and to run the experiment for longer
to allow relaxation to continue to a greater extent. Then
species richness might be found to depend on habitat fragmentation. Surprisingly, there was an edge effect even
though moss wetness did not vary within a patch (although
any moisture gradient is likely to depend on the time of last
rain). There may have been some other environmental
influence at the edge of the patch that was unfavourable
to the fauna. The shapes of the rank-abundance curves
2005 The Royal Entomological Society, Ecological Entomology, 30, 684–691
690 Martin Hoyle and Alastair R. Harborne
were similar among the levels of fragmentation, implying
that fragmentation had little impact on community structure. Conversely, species composition did vary weakly, but
significantly, by fragmentation for the non-predatory mites.
The results of the mainland vs. island comparison agree
with the findings of Gilbert et al. (1998), Gonzalez et al.
(1998) and Gonzalez and Chaneton (2002) that mainland
moss contains more microarthropod species than island
moss, possibly due to the rescue effect (Brown & KodricBrown, 1977) (and not due to moss wetness, as this covariate was non-significant). The results also agree with empirical (Gilbert et al., 1998) and theoretical (Diamond, 1984;
Schoener, 1989) studies, that habitat fragmentation
adversely affects predators more than non-predators.
However, there was only weak non-significant evidence
that the mainland moss contained more microarthropod
individuals, possibly because there were many more nonpredatory than predatory mite individuals, and the nonpredatory mites were less affected by fragmentation.
The conclusions of the metapopulation modelling studies
mentioned in the Introduction depend on the level of
migration between fragmented patches. It was assumed
that the minimum inter-patch distance of 10 cm, created
at the beginning of the experiment, was sufficient to restrict
or prevent dispersal between moss fragments, as a gap of
5 cm apparently represents a substantial dispersal barrier
for these organisms (Gilbert et al., 1998; Gonzalez et al.,
1998). However dispersal is one of the least known aspects
of the biology of soil microarthropods (Norton, 1994).
Cryptostigmata mites have been recorded moving approximately 3 cm per day in soil (Berthet, 1964) and Collembola
1.4 cm per week (Sjogren, 1997). A high rate of microarthropod dispersal across the bare rock would reduce or
abolish differences in individual densities/species richness
among levels of patch fragmentation. However, the observation that the mainland moss contained more species than
the island moss suggests that dispersal was indeed restricted
across the rock. If it is assumed that there was at least some
migration between patches, then no evidence was found to
support Hubbell (2001) (although an examination of his
Unified Theory in relation to fragmentation has not yet
been fully explored), which favoured several small patches
at low migration levels. Additionally, little evidence was
found to support theoretical models (Stacey et al., 1997;
Pelletier, 2000; Ovaskainen, 2002) favouring an intermediate number of sub-populations, or (Etienne & Heesterbeek,
2000) favouring a single large patch. However, such models
generally assume more generations and more habitat
patches than were available in this study. Furthermore, if
the range of patch fragmentation and the statistical power
of the experiment were greater, and if the experimental
treatments were left for longer to allow relaxation to continue to a greater extent, then some supporting evidence
might be found.
For the taxonomic groups significantly affected by fragmentation, the direction of the relationship was species
specific (the number of individuals of one species increased
with decreasing patch fragmentation, whereas the numbers
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of two others decreased). There was strong evidence that
the number of predatory Mesostigmata individuals
decreased with increasing fragmentation, as predicted by
theory (Diamond, 1984; Schoener, 1989). There were fewer
individuals of one Collembola species for the least fragmented treatment, possibly because there were more predatory
Mesostigmata mite individuals in this treatment, and
Collembola are known to be particularly vulnerable to
predation by predatory mites (Mitchell, 1977). The nonpredatory hard-bodied adult Cryptostigmata mites seemed
unaffected by fragmentation, possibly because they are less
mobile than the predatory mites (thus requiring only a
small home range), or because they are less vulnerable to
predation than soft-bodied Collembola.
Despite their small size, generation times of microarthropods are actually quite long relative to the 6-month duration of this experiment (Christiansen et al., 1992; Norton,
1994). This suggests that the effect of demographic stochasticity in this study was probably limited, implying that
species richness is independent of habitat fragmentation
(Hoyle & Gilbert, 2004). However, although habitat
patches at the scale of a microecosystem may experience
less spatial variation in the weather and other environmental influences compared with patches at the scale of the
nature reserve, environmental stochasticity may still be
important, thus favouring several small patches.
It is difficult to quantify the degree of microarthropod
habitat diversity in moss. Despite using just one species of
moss, on the microscale there may be differences in microclimate among the patches due to differences in the contours and aspects of the rocks (Alpert, 1991). Habitat
diversity within a moss patch depends on the degree of
habitat specialisation of the microarthropods, and may
depend on the ratio of the area of the region sampled to
the size of the organism. Species may require a range of
different habitats (e.g. used by different stages, or for oviposition sites for gravid females), and hence will be found
only in moss patches that encompass this range of habitats,
perhaps the cause of the generally highly significant block
effect.
In summary, species richness and abundance and community composition of both predators and non-predators
were mostly unaffected by fragmentation of a fixed area of
habitat. Nonetheless, species richness was greater in an area
of mainland than in an equal area of island habitat. These
findings may generalise to many other communities of
species.
Acknowledgements
We thank R. Norton, P. Martinez, H. Klompen,
M. Skorupski, and W. Welbourn for mite identification,
B. Cave for Collembola identification, A. Smith for moss
identification, A. Sylvester and M. Roberts for help with
microarthropod counting, the National Trust in
Snowdonia for access to their land for this experiment
2005 The Royal Entomological Society, Ecological Entomology, 30, 684–691
Microarthropod habitat fragmentation
and F. Gilbert for comments on an earlier version of the
paper. NERC funded the work.
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Accepted 8 June 2005
2005 The Royal Entomological Society, Ecological Entomology, 30, 684–691