Habitat Fragmentation

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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
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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.
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Habitat loss can be distinguished from
fragmentation
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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
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A Survey of Habitat Fragmentation
Experiments Before looking at a number of
specific examples, Ill first present some
results from a meta-analysis of fragmentation
(Debinski and Holt 2000). Fragments varied in
size from lt1m2 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.
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• 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?

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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.
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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
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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.
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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.
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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
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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
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Now lets 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.
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An aerial photo of two of the isolated patches
at Manaus, Brazil
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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
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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 army-ant followers, which
feed on insects fleeing swarming army ants, and
mixed species flocks of insectivores.
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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 un-altered 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.
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Mammal species are also affected. Primate
diversity went almost immediately to near 0 in
isolates. Of pre-isolation estimates of 20 mammal
species present, only 7 persisted in the isolated
fragments. Heres 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
? - -
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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
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Oryzomys rice rat
Didelphis another opossum
Cebus - brown capuchin
Agouti paca
Dasypus - armadillo
tapir
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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.
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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
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• 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
• Home range. Fragments may be too small to provide
  minimum home range requirements for particular
  species. Ivory-billed woodpeckers require from
  6.5-7.6 km2 of bottom forest. European goshawks
  need 30-50 km2 fragments. And mountain lions have
  a home range gt400 km2.

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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.
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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.
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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.
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• 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.

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Now lets consider Schmiegelows (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
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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 didnt 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.
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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 .4
6 .70 1994 .42 .75 1995 .44 .74 Connected
1993 .40 .87 1994 .44 .78 1995 .25 1.02

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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.
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Anticipating upcoming lectures, there is one more
consideration in evaluating fragmentation
effects how fragmentation affects the genetic
diversity of remnant populations. Well 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 thats the a priori idea.
Does it usually happen?
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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
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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
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For mammals Undisturbed Fragmented Allozyme
A 2.75 ? 0.46 1.43 ? 0.13 He 0.34 ? 0.16 0.11
? 0.051 Microsatellite A 8.18 ? 0.69 6.17 ?
0.54 He 0.65 ? 0.026 0.59 ? 0.029 The data for
plants is qualitatively similar, but the
differences (losses) as a fraction of undisturbed
levels are smaller.
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• The second review is specific, produced by
  Aguilar et al. (2008) to plants. Some of the
  expected (hypothesized) results
• 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

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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 short- lived species compared
to long-lived species.
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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 populations 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.
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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?
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Whatever the source or explanation, the genetic
losses that result from habitat fragmentation
will prove to be an important consideration in
designing conservation strategies.
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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
175177-5188. 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
42859-866. Brum, H.D., et al. 2008. Rainforest
fragmentation and the demography of the
economically important palm Oenocarpus bacaba in
central Amazonia. Plant Ecology
199209-215. DiBattista, J.D. 2008. Patterns of
genetic variation in anthropogenically impacted
populations. Conservation Genetics
9141-156. 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, ed.
Sinauer, Sunderland, MA. Wilcove, D.S., C.H.
McLellan and A.P. Dobson. 1986. Habitat
fragmentation in the temperate zone. in
Conservation Biology - The Science of Scarcity
and Diversity. M.E. Soulé, ed. Sinauer,
Sunderland, MA.
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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 221513-1522.