Transcript Habitat Fragmentation

Habitat Fragmentation
We have mostly made a mess of things. We have
repeatedly destroyed natural environments, and even
when we haven't destroyed them, we have fragmented
them in ways which have had serious consequences
for the survival of ecosystems and species.
What is Habitat Fragmentation?
Modern views (e.g. Fahrig 2003) take in two
components; one is called fragmentation. They are:
1) habitat loss (destruction or conversion) and
2) habitat fragmentation, in which the habitat is
broken apart after controlling for any habitat loss
Habitat loss almost always has strong negative effects
on biodiversity, whereas habitat fragmentation
usually produces weaker effects.
Fahrig (2003) suggests that the term fragmentation
should be used to describe only the breaking apart of
habitat.
This diagram represents
habitat loss, even though
it occurs through
‘fragmentation’.
Habitat loss can be distinguished from fragmentation…
In addition to habitat loss, fragmentation can result in:
1) increase in the number of patches;
2) decrease in patch sizes;
3) increase (or decrease, in resulting patches) in
isolation of patches.
One of the important concerns in addressing
fragmentation is the importance of scale.
Fragmentation may have differing impacts at the
landscape scale when compared to effects within
habitat patches. Fahrig, in her review, gave us some
diagrammatic patterns to work from…
A Survey of Habitat Fragmentation Experiments
Before looking at a number of specific examples, I’ll
first present some results from a meta-analysis of
fragmentation (Debinski and Holt 2000).
Fragments varied in size from <1m2 to 1000 ha, and
replication from single fragments to 160 in a
category.
Most experimental studies were performed in North
America and Europe. Others came from Brazilian
tropical rain forest, African Serengeti plains,
Australian rain forest, northern Japanese forest, etc.
The questions generally fell into 6 categories:
• Was species richness related to area or to patch
shape, as island biogeography/metapopulation
biology suggest?
• Did species density or abundance increase with
fragment area (or aspects of shape)?
• Were species interactions affected by fragmentation?
• How did edge effects affect what are termed
"ecosystem services"?
• How did corridors influence movement between
fragments? And how did connectivity (in whatever
form) influence species richness of connected
fragments?
1) Species richness only increased as a function of
fragment area in 6 of 14 studies that could be
evaluated.
What happened in all those other studies?
There were clear explanations in most. For a short
time, at most 1-2 years after fragment formation,
species driven out by the change from habitat to
matrix invade the remaining fragments and
temporarily increase diversity.
In other cases edge species are able to survive in the
increased edge area caused by fragmentation, and
add to the species counts.
2) Abundance also declined in fragments in
comparison to continuous habitat. Decreased
abundance was seen in species from weevils to trees.
3) One of the problems in rejected studies was
difference in sampling effort with patch size (see
Yamaura et al. 2008). Some apparently significant
effects of size were negated when correction for effort
was incorporated.
An analogy…
3. When considering interaction effects, predator-prey
interactions were more successfully studied.
Fragmentation affects the predators' ability to rapidly
find and concentrate in regions of prey abundance.
Different predator-prey systems function at different
spatial scales.
One example: Aphids have more frequent outbreaks,
escaping control by coccinelid beetle predators, in
more fragmented landscapes.
4) Edge effects are commonly found, both in abiotic
conditions and within the biological system. Abiotic
effects include altered nutrient cycling and changes in
the temperature, light, and relative humidity regimes
within the fragments. Biotic effects include altered
invasibility, recruitment, and the presence of species
in edge habitats not likely to persist in continuous
habitat.
Edge effects affect different species differently…
Yamaura et al.’s (2008) study separated birds,
butterflies and forest trees into edge, neutral and core
area species to determine which species were affected
by area and patch shape in naturally occurring patches
on Hokaido. Both area and shape affected interior
species of birds and butterflies. Only area affected
interior forest trees.
Forest interior birds
woodland butterflies
Open ground butterflies (edge species) were affected
significantly by shape – the more circular the patch the
less edge per unit area, and fewer edge butterflies.
5) Corridors should enhance inter-fragment
movement. In 4 of 5 studies that examined corridors,
the movement of at least some species was enhanced.
The results were very species specific.
Corridors are controversial because they are rarely so
wide as to incorporate a continuous path of core
habitat. Therefore, they become prime places for
predators and edge species to occupy
Now let’s consider a few examples of experimental
fragmentation. Most of the results are remarkably
similar though they come from very different
communities.
One, Schmiegelow et al. (1997) comes from the boreal
forest. The other, in an extensive series of papers, is
about manipulated fragmentation in Brazilian tropical
rainforest (Lovejoy et al 1984 is the first review).
The tropics first: Lovejoy used Brazilian requirements
that 50% of areas of tropical forest intended for
development had to be left forest.
An aerial photo of two of the isolated patches at
Manaus, Brazil
They made arrangements with developers to leave
behind pre-marked tracts of different area and
isolation to be followed after the clearing of
surroundings. Replicate isolates over a size range from
1 to 1000 ha were studied prior to isolation, then
followed afterward. A single 10,000 ha 'mainland‘ was
also retained.
Bird density immediately after isolation showed a
transient increase in density, lasting up to 200 days.
The authors describe this as a 'crowding effect'. The
degree of crowding depended on the area cut and prior
population density
Thereafter, density, measured as mist net captures per
hour, declined to levels below pre-isolation numbers,
but still relatively similar to equivalent areas not
isolated.
Interestingly, on average residents prior to isolation do
worse than those who move in. Diversity only
declines. Within 5 months after the post-isolation
sampling, the curve of species accumulation is
asymptotic at a lower diversity.
Effects are not uniform over bird species. Two guilds
are particularly suppressed following isolation: armyant followers, which feed on insects fleeing swarming
army ants, and mixed species flocks of insectivores.
The former disappeared rapidly following isolation of
1 and 10 hectare fragments. The latter disappeared
more slowly, over 1-2 years, from these smaller
fragments.
The distance the fragments were separated from unaltered forest made a difference, which varied among
taxa. For important pollinating insects (e.g. euglossine
bees), pollination results indicated that 15 species
would not cross 100m cleared strips. The population
biology of at least 30 plant families will be/has been
affected by reduced or lack of pollination. Dung and
carrion feeding beetles responded similarly to a 100m
barrier. Decomposition will be slowed.
Mammal species are also affected. Primate diversity
went almost immediately to near 0 in isolates. Of preisolation estimates of 20 mammal species present,
only 7 persisted in the isolated fragments. Here’s a
table of size effects:
Species
Intact forest 10 ha
1 ha
Marmosa parvidens
+
Didelphis marsupialis ++
+
Alouatta seniculus
+++
+++
Cebus apella
+
Dasypus novemcinctus +++
+?
Sciurus gilvigularis
+
+
Oryzomys capito
+
+
+
Agouti paca
++
Tapirus terrestris
+?
-
Common names for some of these species are:
Alouatta – red howler monkey, Cebus - capuchin
monkey, Sciurus - squirrel, Agouti- agouti, Dasypus armadillo, Tapirus- tapir.
red howler monkey
Marmosa - opossum
capuchin monkey
Didelphis – another opossum
Oryzomys – rice rat
Cebus - brown capuchin
Agouti paca
Dasypus - armadillo
tapir
Other lessons learned: Size of protected area cannot
be the sole criterion. The Amazon forest project has,
for example, discovered frogs with very critical
breeding habitat. Without ecological knowledge and
planning, reserve areas in Amazonia could protect
large areas, but not incorporate this breeding habitat.
This was (and is) a single study site in the tropics
(there have been many papers published about it). Are
the results identical in other tropical studies of
fragmentation? Not exactly!
Avifaunal extinctions as a proportion of guilds in 5
neotropical rain forest sites.
Site
Raptors
Insectivores Frugivores
Brazil
54
74
57
Panama
22
22
16
Ecuador
56
18
33
Colombia
33
31
36
Puerto Rico
14
7
22
What are the mechanisms explaining extinctions
following fragmentation in the temperate zone? The
reasons cover an essentially identical spectrum of
ideas. Here are the explanations:
1)Home range. Fragments may be too small to provide
minimum home range requirements for particular
species. Ivory-billed woodpeckers require from 6.57.6 km2 of bottom forest. European goshawks need
30-50 km2 fragments. And mountain lions have a
home range >400 km2.
2) Loss of habitat heterogeneity. The remaining
mosaic of habitats may not include all types present
before fragmentation. Small ponds may be necessary
for some birds to nest and reproduce. Some species
use different habitats during different seasons or at
different points in their life cycles. Both (or more)
habitat types must be present in a fragment for these
species to persist. Again, open water is an obvious
example for amphibian species.
3.Effects of habitat between fragments. The landscape
between terrestrial fragments may be inhospitable,
but may also be survivable. This could reduce
effective isolation of fragments. However, the
landscape can also be a source of species damaging
to native populations of the fragments. Land
development or second growth can affect the
population sizes and migration among fragments for
many species.
4) Edge effects. In addition to effects on songbirds
from nest predators like cowbirds that come from
edge areas, there are effects on plant communities as
well. Seed rain in small fragments may, even at the
core of the fragment, largely be constituted of edge
species propagules.
Long-term study of one economically important
palm, an edge species, found that edge effects even
differ between established and young plants (Brum,
et al. 2008). Large Oenocarpus (a date palm)
increased in edge areas over 22 years, but not in
interior forest. Seedlings and saplings did not
respond in the same way.
Demographic patterns may, if this is not a unique
result, not be simple in documenting edge effects.
5) Secondary extinctions. These are species losses
traceable to modifications in the structure of
interactions (competition, predation, mutualism)
among species components of the unfragmented
area that are determined by the fragmentation
process. Omnivores controlled by top predators
prior to fragmentation can become important nest
predators in fragments, for example.
Now let’s consider Schmiegelow’s (1997) study of
boreal forest. The experimental design is quite similar
to the study of tropical forest fragmentation.
There were control areas demarcated in uncut boreal
forest, isolated patches of boreal forest, and patches
connected by 100m wide corridors to remaining
forest. Each condition was represented by replicate
patches of 1, 10, 40 and 100 ha.
Bird species were identified from point sampling
stations in each patch both before forest cutting and
for two years afterward.
What follows are the general results. Much of it
parallels what was observed in tropical forest…
In most patches there was no significant change in
bird species richness from before to the end of the 2
year period. There was crowding initially after cutting,
as in tropical fragments. Bird species diversity
depended on area (remember island biogeography)
after cutting, decreasing in smaller patches. Since
richness didn’t change, but diversity did, community
structure was altered.
Abundance of species was related to migratory
strategy. Neotropical migrants declined in both
connected and isolated fragments. These and some
resident species depend on ‘core’ forest, which is lost
when patches are isolated and the central area is too
close to the edge.
Fitting the basic species-area equation S = cAz
Species-area relationships (all species):
Controls Year
z
c
1993
.42
.76
1994
.39
.76
1995
.38
.84
Isolated 1993
.46
.70
1994
.42
.75
1995
.44
.74
Connected 1993
.40
.87
1994
.44
.78
1995
.25
1.02
However, in the smallest patches (1 ha) connected by
corridors, diversity increased, apparently due to the
movement in of transients displaced by harvesting,
using the corridors to accumulate in the small patches.
Turnover rates (replacement of one species by another,
even though richness remains unchanged) were higher
in isolated than in connected fragments.
Changes in response to fragmentation were less
dramatic here than in the tropics, though parallel in
direction. Why? Probably because effects were
followed for only 2 years and because the boreal forest
is subject to more frequent disturbance than the
tropics.
Anticipating upcoming lectures, there is one more
consideration in evaluating fragmentation effects: how
fragmentation affects the genetic diversity of remnant
populations. We’ll consider two reviews:
DiBattista (2008) did a literature review of the genetic
impacts of human-mediated environmental change.
One category was fragmentation.
Habitat fragmentation results in population
subdivision into smaller, more discrete subunits within
which genetic variation is likely to decrease due to
genetic drift and inbreeding within the smaller subunit
populations. At least that’s the a priori idea. Does it
usually happen?
DiBattista used the number of alleles per locus and/or
heterozygosity as measures of genetic diversity; the
diversity was measured using isozyme and
microsatellite data.
Microsatellites and allozyme data differed
quantitatively, but not qualitatively. Microsatellites
were more diverse. Both sources showed decreased
diversity under fragmentation (as the type of
disturbance). Considering microsatellite and allozyme
data together, note the consistent differences in the
table below, and how consistent the differences are…
Undisturbed
Fragmented
Allozyme
A
2.13  0.09
1.56  0.09
He 0.19  0.016
0.14  0.02
Microsatellite A
8.84  0.57
6.83  0.52
He 0.65  0.018
0.59  0.023
A is the mean number of alleles per locus.
He is the expected heterozygosity.
These trends were observed across a variety of
taxonomic groups. There were sufficient data to
analyze mammals and plants as separate statistical
groups. The data were particularly dramatic for
mammals…
For mammals:
Allozyme
Microsatellite
A
He
A
He
Undisturbed
2.75  0.46
0.34  0.16
8.18  0.69
0.65  0.026
Fragmented
1.43  0.13
0.11  0.051
6.17  0.54
0.59  0.029
The data for plants is qualitatively similar, but the
differences (losses) as a fraction of undisturbed levels
are smaller.
The second review is specific, produced by Aguilar et
al. (2008) to plants. Some of the expected
(hypothesized) results:
1.Erosion of genetic variability (a general result)
2. Increased interpopulation genetic divergence due to
increased random genetic drift and inbreeding, and
reduced gene flow
3. a lower proportion of polymorphic loci and a
reduction in the number of alleles per locus are
expected within the fragments
4. If fragmentation conditions persist over successive
generations, decreased heterozygosity due to
random drift and increased inbreeding are expected,
resulting in the accumulation of deleterious
recessive alleles, lowering the fecundity of
individuals, increasing seed/seedling mortality, and
reducing the growth rate of individuals, eventually
driving populations to extinction.
Because genetic erosion in fragmented habitats
should be more pronounced after several
generations, it is expected to find stronger
negative effects on the adult generation of shortlived species compared to long-lived species.
5. The ploidy level of plants may influence the effects
on genetic diversity due to fragmentation.
Polyploids, with ‘extra’ copies of genes, are less
likely to lose locus heterozygosity as rapidly
following isolation.
6. The loss of genetic variation may reduce a
population’s ability to respond to future
environmental change.
7. Sudden decreases in effective population sizes due
to habitat fragmentation would then have stronger
negative effects on within-population genetic
diversity of outcrossing species compared to selfers.
8. Vector-pollinated and/or dispersed species will be
strongly affected by the relative isolation compared
to vector movement distances.
9. Common species are more susceptible to genetic
loss than rare ones, since rare species already have
a reduced genetic diversity.
These are all accepted hypotheses. What was found in
this meta-analysis?
Whatever the source or explanation, the genetic losses
that result from habitat fragmentation will prove to be
an important consideration in designing conservation
strategies.
References
Aguilar, R., M. Quesada, L. Ashworth, Y. Herrerias-Diego and J. Lobo. 2008. Genetic
consequences of habitat fragmentation in plant populations: susceptible signals in plant
traits and methodological approaches. Molecular Ecology 17:5177-5188.
Bierregaard, R.O.Jr., T.E. Lovejoy, V. Kapos, A.A. dos Santos and R.W. Hutchings.
1992. The biological dynamics of tropical rainforest fragments. Bioscience 42:859-866.
Brum, H.D., et al. 2008. Rainforest fragmentation and the demography of the
economically important palm Oenocarpus bacaba in central Amazonia. Plant Ecology
199:209-215.
DiBattista, J.D. 2008. Patterns of genetic variation in anthropogenically impacted
populations. Conservation Genetics 9:141-156.
Lovejoy, T.E., et al. 1984. Ecosystem decay of Amazon forest remnants. In Extinctions.
M.H. Nitecki, ed. Univ. of Chicago Press. pp. 295-325.
Lovejoy, T.E. et al. 1986. Edge and other effects of isolation on Amazon forest
fragments. in Conservation Biology - The Science of Scarcity and Diversity. M.E.Soulé,
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Wilcove, D.S., C.H. McLellan and A.P. Dobson. 1986. Habitat fragmentation in the
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Soulé, ed. Sinauer, Sunderland, MA.
Schmiegelow, F.K.A.; C.S. Machtans; S.J. Hannon. 1997. Are Boreal Birds Resilient to
Forest Fragmentation? An Experimental Study of Short-Term Community Responses.
Ecology, Vol. 78, No. 6. (Sep., 1997), pp. 1914-1932.
Yamaura, Y., T. Kawahara, S. Iida and K. Ozaki. 2008. Relative importance of area and
the shape of patches to the diversity of multiple taxa. Conservation Biology 22:15131522.