Download Habitat Fragmentation – In Theory

Kelly Marie Henry
NRS 534 – Ecology of Fragmented Landscapes
May 18, 2005
Habitat fragmentation: the theories which provide the framework for the study of
habitat fragmentation
The common occurrence of habitat destruction results not only in habitat loss and
habitat degradation, but also in fragmentation of the remaining habitat. A focal point for
the research of many landscape ecologists is on the effects of habitat fragmentation to
the individuals, populations, and ecosystems experiencing this phenomenon. Habitat
fragmentation can be defined as a process resulting in the transformation of a large
section of relatively homogeneous habitat into smaller habitat patches of a
heterogeneous composition (Fahrig 2003; Reed et al. 1995). Of particular importance is
that the resulting smaller habitats are typically isolated from each other by a matrix of
habitats unlike the original. The concepts many landscape ecologists use when
formulating habitat fragmentation hypotheses are not new ideas to the discipline of
ecology. Two of the main theories providing the backbone to habitat fragmentation
studies, the island biogeography theory and the metapopulation theory, were originally
developed to explain observations made in community and population ecology (Collinge
1996). Some common parameters examined when studying habitat fragmentation
include the size, the degree of isolation, the context (matrix), and the degree of
heterogeneity of the fragment as well as the impact of edge on the fragment.
Island Biogeography Theory
The island biogeography theory, originally proposed by MacArthur and Wilson in
1967, states that the size of an oceanic island and its distance from a mainland source
of colonizing species influences the number of species present on that island. While it is
evident that the island biogeography theory was originally developed to explain species
composition on oceanic islands, landscape ecologists studying habitat fragmentation
apply the theory to the terrestrial islands created by habitat fragmentation (Fahrig 2003;
Whittaker 1998; Collinge 1996). Habitat fragmentation research structured around the
island biogeography theory typically focuses on the size of the fragment and the degree
of isolation of the fragment.
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The theory of island biogeography formed the basis of a study conducted by
Walters et al. (1999) who examined the adverse effects of habitat fragmentation on
brown treecreepers (Climacteris picumnus). Upon completing their research, Walters et
al. (1999) found evidence that brown treecreepers experience lower male to female
pairing success due to disrupted dispersal patterns in fragmented habitats versus
unfragmented habitats. Another study with framework centered around the island
biogeography theory was that of Davies and Margules (1998) who studied populations of
carabid beetles. Based on previous studies, these scientists hypothesized that carabid
beetle species richness would decrease in fragmented habitats, and carabid beetle
abundance would decrease with the occurrence of fragmentation, with decreasing size
of the fragment and with proximity to the edge of the fragment. Upon completing the
study, Davies and Margules found that habitat fragmentation did, in fact, impact carabid
beetle species richness and it appeared to alter species composition as well.
Furthermore, habitat fragmentation did appear to decrease the abundance of the two
species experiencing complete isolation due to habitat fragmentation (the other six
species examined were not completely isolated).
Metapopulation Theory
The second theory supporting habitat fragmentation research is the
metapopulation theory. Collinge (1996) defines a metapopulation as “a set of spatially
separated groups of conspecific individuals.” Originally developed by Levins (1969) in
the late 1960’s, the metapopulation theory states that while local populations of
organisms experience periods of colonization and extinction events, the metapopulation
as a whole continues to thrive. In order to be considered a metapopulation, the
subpopulations must remain interconnected by gene flow, extinction, and recolonization
(Whittaker 1998). There are two types of metapopulation models, the classic model and
the source-sink model. The classic model assumes all subpopulations are of the same
size, while the source-sink model assumes a large core population that persists
indefinitely during the periods of extinction and recolonization experienced by the smaller
sink populations. Habitat fragmentation research incorporating the metapopulation
theory typically focuses on the connectivity and the exchange of individuals between
habitat fragments.
The metapopulation theory was applied in a study by Dunham et al. (2003)
examining the short and long-term impacts of fire on native fish populations. The study
found that the persistence of local native fish populations in larger habitat patches could
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be explained by the larger population size able to inhabit the patch or by the increased
habitat heterogeneity frequently existing in these larger habitat patches. However, the
persistence of populations in smaller habitat patches was likely due to the dispersal of
individuals from nearby populations.
It is the combination of both the island biogeography theory and the
metapopulation theory, which together support habitat fragmentation studies. Although
the island biogeography theory focuses on the size and degree of isolation of habitat
fragments and the metapopulation theory focuses on connectivity and exchange
between habitat fragments, many researchers study different combinations of these
effects on individuals, populations, and ecosystems. Both theories have a strong spatial
component, which causes the two theories to become tightly interwoven in studies of
habitat fragmentation. A strong emphasis is placed when studying habitat islands on the
degree of isolation or the connectivity of the habitat with surrounding patches. The
spatial location of individuals, populations, and communities influences the
metapopulation dynamic. Hanski (1998) states that the spatial structure of the
metapopulation is as important in determining the metapopulation dynamic as birth and
death rates since the spatial structure allows for immigration.
Following an event of habitat destruction resulting in habitat fragmentation, a
patch typically experiences faunal relaxation (Viveiros de Castro and Fernandez 2004;
Whittaker 1998). Once fragmentation occurs, the newly created patch is supersaturated
with species. After the “extinction debt” or time lag following the fragmentation is over,
the number of species slowly decreases until a new equilibrium level is achieved (Hanski
1998). However, immigration and extinction both continue to occur during the relaxation
process and after equilibrium has been established. In general, the response to
fragmentation by a metapopulation is non-linear due to the manner in which habitat
connectivity is lost (Hanski 1998). However, evidence does suggest that relaxation and
the sequence of species lost are highly structured and should be predictable (Viveiros de
Castro and Fernandez 2004). The idea of a metapopulation allows for satellite
populations to come and go, with the core population remaining indefinitely. However,
due to the stochastic nature of faunal relaxation, species can become extinct throughout
entire metapopulations.
Common parameters used in habitat fragmentation studies
When studying the effects of habitat fragmentation on an ecosystem, landscape
ecologists focus on a wide variety of habitat parameters. Some recurring themes in the
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study of habitat fragmentation include the size, the degree of isolation, the context
(matrix), and the degree of heterogeneity of the fragment, as well as the impact of edge
on the fragment. The size of the fragment influences the ecological process able to
occur within the fragment (Collinge 1996). As habitat fragments become more isolated,
the dynamics within the fragment become increasingly important. It is therefore
important to maintain a minimum dynamic area, defined by Pickett and Thompson in
1978 as the “smallest area with a natural disturbance regime, which maintains internal
recolonization sources, and hence minimizes extinction” (Dunham et al. 2003). Many
studies have found a positive correlation of decreasing species richness and individual
abundance with decreasing fragment size due to low reproduction and survival rates in
the smaller fragments (Smith and Hellmann 2002; Collinge 1996). It should be noted
that not all studies examining the size of a fragment relative to its impact on a population
find negative effects associated with decreasing patch size (Eggleston et al. 1999;
Davies and Margules 1998).
The degree of connectivity and the composition of the surrounding matrix
influence the species interactions between habitat fragments. The persistence of a
population in the face of increasing habitat fragmentation can be explained by the
metapopulation theory only if some degree of connectivity is maintained between the
fragments. As connectivity decreases, population persistence decreases due to
isolation from the supporting metapopulation (Dunham et al. 2003). The context of
surrounding habitat will influence the degree and type of interaction between the
fragment and the surrounding area. The degree of dissimilarity strongly influences the
flow of nutrients and materials, as well as the persistence of plant and animal species
between the fragment and the surrounding matrix (Mesquita et al. 1999; Collinge 1995).
When the components of the matrix are structurally similar to the components of the
fragment, the impacts of fragmentation are less as species in the fragment are able to
use areas outside of their fragment for habitat or interfragment movement (Viveiros de
Castro and Fernandez 2004).
Degree of heterogeneity and quantity of edge habitat both influence the diversity,
abundance, and composition of species within a fragment. Studies suggest that the
more heterogeneous a fragment the greater number and variety of species the fragment
will support. The mosaic of a habitat fragment may directly or indirectly impact species
distribution, abundance, or behavior (Eggleston et al. 1999; Whittaker 1998). An
important occurrence associated with the fragmentation of a habitat is the increase in the
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length of the boarder between the habitat fragment and surrounding habitat. Increasing
the edge along an area leads to increases in light, temperature, and wind, and
decreases in relative humidity (Whittaker 1998; Collinge 1996). These changes in
microclimate impact the plant and animal communities at the edge of the habitat
fragment. In particular, forest edges usually contain more pioneer species (Collinge
1996).
Landscape ecologists have developed the parameters discussed above as well
as others for measuring the impacts of habitat fragmentation on individuals, populations,
and ecosystems. In addition, their studies are based on two well researched theories of
community and population ecology. However, throughout habitat fragmentation
literature there is a thread of disbelief that the study of habitat fragmentation is different
from the study of habitat loss. Many authors emphasize the difficulty in separating the
negative effects of habitat loss from the positive and negative effects of habitat
fragmentation (Fahrig 2003; Walters et al. 1999). This has proven a trying task to many
landscape ecologists, and brings up the question, “is fragmentation a useful term?”
(Fahrig 2003).
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Annotated Bibliography:
Collinge, S.K. 1996. Ecological consequences of habitat fragmentation: implications for
landscape architecture and planning. Landscape and Urban Planning. 36:59-77.
This review addresses the importance of studying the impacts of habitat fragmentation
on the ecology of various systems. While the fragmentation of landscapes resulting in
land transformation, habitat loss, and patch isolation can occur naturally, it appears
humans are increasing the rate of land conversion and subsequent habitat
fragmentation. Collinge provides an overview of two theories behind the study of habitat
fragmentation, the island biogeography theory and the metapopulation theory. The
island biogeography theory examines the influence of habitat fragment size and degree
of isolation on the composition of species within the fragment. The metapopulation
theory uses the concept of a metapopulation (“a set of spatially separated groups of
conspecific individuals”) to describe the local extinction and subsequent recolonization of
species in patchy or fragmented habitats. While neither of these theories were
developed directly to explain the impacts of habitat fragmentation, they have been
applied to the development of experimental hypotheses and designs studying the
impacts of habitat fragmentation. Collinge then reviews existing literature summarizing
the various aspects of habitat fragmentation including the size, connectivity, shape,
context, and heterogeneity of the fragment as well as the effect of edge on the fragment.
In conclusion, the importance of maintaining fragment connectivity and heterogeneity as
well as decreasing the amount of edge is emphasized in order to mitigate the negative
impacts of habitat fragmentation. This review article provided a comprehensive
explanation of habitat fragmentation, the theories supporting habitat fragmentation
studies, and the parameters used to examine the effects of habitat fragmentation.
Coulson, R.N., B.A. McFadden, P.E. Pulley, C.N. Lovelady, J.W. Fitzgerald, and S.B.
Jack. 1999. Heterogeneity of forest landscapes and the distribution and
abundance
of the southern pine beetle. Forest Ecology and Management. 114:471-485.
Coulson et al. analyzed how the southern pine beetle, Dendroctonus frontalis, perceives
and responds to habitat heterogeneity. In light of human activities altering both the
content and the context of the habitat of the southern pine beetle (as well as the habitat
of other organisms), it is necessary to determine if these activities are enhancing or
inhibiting the beetle’s habitat. By creating a functional heterogeneity map, Coulson et al.
were able to analyze how the arrangement of landscape elements influenced the
distribution and abundance of the southern pine beetle. Acceptable tree species, the
susceptibility of forest stands, and lightening-struck trees are all important elements of
the landscape essential to the persistence of metapopulations of southern pine beetles.
Using the angular moment of inertia index, functional heterogeneity, and connectivity of
southern pine beetle habitat were quantitatively measured and then mapped. This
article was not particularly useful for explaining the theories behind habitat
fragmentation, however, it did provide a good example of a method for assessing how
habitat fragmentation effects the distribution and abundance of the southern pine beetle.
Davies, K.F. and Margules C.R. 1998. Effects of habitat fragmentation on carabid
beetles: experimental evidence. Journal of Animal Ecology. 67:460-471.
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This study examines the effects of habitat fragmentation on the carabid beetle species
richness and abundance in fragmented forest habitats relative to non fragmented forest
habitats. Based on previous studies, Davies and Margules hypothesized that carabid
beetle species richness would decrease in fragmented habitats, and carabid beetle
abundance would decrease with the occurrence of fragmentation, with decreasing size
of the fragment, and with proximity to the edge of the fragment. The experiment was
conducted in south-eastern Australia using three experimental fragment sizes each
replicated six times. Four of the experimental eucalyptus forest plots were surrounded
by pine forest, while the other 2 experimental eucalyptus forest plots were retained
within a surrounding eucalyptus forest matrix. Poisson and logistic regressions were
used to analyze the data as the resulting experimental data was asymmetrical versus
normally distributed. Upon completing the study, Davies and Margules found that
habitat fragmentation did impact carabid beetle species richness, although it did appear
to alter species composition. Habitat fragmentation did appear to decrease the
abundance of the two the two species experiencing complete isolation due to habitat
fragmentation (the other 6 species examined were not completely isolated). No clear
trend in carabid beetle abundance was found relative to habitat size or proximity to edge
of fragment. Davies and Margules provide a classic example of a study examining the
effects of habitat fragmentation on a particular species incorporating ideas from both the
island biogeography theory as well as the metapopulation theory.
Dunham, J.B., M.K. Young, R.E. Gresswell, and B.E. Rieman. 2003. Effects of fire on
fish populations: landscape perspectives on persistence of native fishes and
nonnative fish invasions. Forest Ecology and Management. 178:183-196.
In this article, fire is identified as an agent of disturbance, and Dunham et al. explore
short and long-term impacts of fire on native fish populations in order to aid in the
development of a successful fire management technique. Fire is recognized as a natural
disturbance that may be necessary to the persistence of many native fish populations.
However, fire may decrease the connectivity of a landscape, and subsequently decrease
the persistence of a population by causing isolation from the metapopulation. Dunham
et al. review a study by Detenbeck et al. (1992), which examined the impacts of fish
populations to “pulse” (short-term) versus “press” disturbances (long-term). Detenbeck
et al. found that recovery was dependent upon the length of the disturbance and the
location of the populations from the source population as well as specific lifecycle
requirements for the population at hand. In conclusion, several fire management
approaches are addressed by Dunham et al. including pre-fire management of the
system, managing the fire once it has begun, managing the system after the fire, and
monitoring for adaptive management. The importance of maintaining connectivity
throughout a metapopulation is strongly emphasized throughout this article, which
provides an excellent example of the metapopulation theory applied to fragmented
habitats.
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Eggleston, D.B., W.E. Elis, L.L. Etherington, C.P. Dahlgren, and M.H. Posey. 1999.
Organism responses to habitat fragmentation and diversity: Habitat colonization
by estuarine macrofauna. Journal of Experimental Marine Biology and Ecology.
236:107-132.
Eggleston et al. studied the interactive impacts of patch size, habitat diversity, and
experimental site on the colonization of benthic marine macrofauna. They tested their
hypotheses by placing trays filled with either, seagrass, oyster shell, or a mixture in a
subtidal marsh located in within Back Sound, NC. Previous studies have determined
that the mosaics of habitat types may directly or indirectly impact predator distribution,
abundance, and behavior. In addition, the special arrangement of habitat fragments
may also impact water flow which is responsible for the distribution, settlement, and
accumulation of drift algae or detritus as well as animal behavior. Before beginning the
study, Eggleston et al. predicted colonization would be greater in smaller patches as the
probability of encounter with the patch would be increased. Also, they predicted that
smaller macrofauna would show a stronger response to habitat patchiness than large
organisms, and that colonization would be higher in mixed habitat plots versus
monotypic plots. They found that organism response to the spatial arrangement of
habitats was dependent upon the spatial scale, the habitat type, and the organism’s
body size. The study showed one very important implication for management;
biodiversity experiences greater negative impacts with the fragmentation of oyster shell
habitats than with the fragmentation of either seagrass or a mixture of both habitats.
Unfortunately, many of the western Atlantic subtidal oyster habitats have been severely
fragmented.
Fahrig, L. 2003. Effects of habitat fragmentation on biodiversity. The Annual Review of
Ecology, Evolution, and Systematics. 34:487-515.
In this review article, Fahrig identifies many of the inconsistencies in habitat
fragmentation studies. While there is no shortage of literature exploring the effects of
habitat fragmentation on ecosystem biodiversity, the results of all these studies can be
difficult to interpret. Authors tend to measure habitat fragmentation using different
experimental designs, which has leads to varying conclusions in regard to the impacts of
habitat fragmentation on biodiversity. Habitat fragmentation can be measured as a
process or as a pattern. When measuring habitat fragmentation as a process, the study
will typically only examine one continuous landscape and one fragmented landscape
causing the results to be largely comparative and qualitative. When measuring habitat
fragmentation as a process, the scientist can quantitatively measure the effects of (a)
habitat loss, (b) change in habitat configuration, (c) decrease in patch size, or (d)
increase in patch isolation. Which variable(s) measured in the study, can influence the
conclusions the scientist draw. While the effects of habitat loss are frequently negative,
the effects of habitat fragmentation can be both positive and negative. Negative effects
of habitat fragmentation result from patches that are too small to support a population as
well as from negative edge effects. The positive effects of habitat fragmentation result
from (1) higher immigration rates when there are many small patches close together. In
addition, (2) when the amount of habitat is held constant as the degree of fragmentation
increases, then the dispersal distance between patches will decrease. Furthermore, (3)
increasing fragmentation, increases land complementation, resulting in increased
biodiversity. Increasing fragmentation has positive effects for (4) species which require
more than one habitat, and (5) the effects of edge may be positive for some species.
Fahrig emphasizes the importance of determining the effects of habitat fragmentation on
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biodiversity, separate from the effects of habitat loss on biodiversity. This review is a
must read for anyone studying habitat fragmentation. Throughout the article, Fahrig
brings up numerous controversial points causing the reader to questions whether the
study of habitat fragmentation measures effects separate from those of habitat loss. By
the end of the article it is impossible not to ask the question, “Is fragmentation a useful
term?”
Hanski, I. 1998. Metapopulation Dynamics. Nature. 396:41-49.
In this review article, Hanski emphasizes the importance of studying the spatial structure
of populations. It is the spatial location of individuals, populations, and communities,
which influence the metapopulation dynamic. Hanski believes the spatial structure
amongst populations is as important as the birth and death of individuals within
populations when determining local extinction. Throughout the review, Hanski
addresses aspects of theoretical ecology, metapopulation ecology, and landscape
ecology, focusing on the theory of metapopulation ecology. Extinction-colonization and
regional stochasticies and their influence on the spatial structure and persistence of
metapopulations are discussed. Of particular interest was Hanski’s description of the
response of metapopulations to habitat destruction. Metapopulations respond to habitat
destruction and the subsequent loss, decrease in quality, and fragmentation of the
habitat (1) in a non linear manner, (2) with a time lag or “extinction debt.” The third
conclusion in regards to the response of metapopulations to habitat destruction is the
number of empty habitats before the destruction equals the extinction threshold of the
metapopulation, this is also known as “Levin’s Rule.” In conclusion, spatial ecology is
useful because it allows for complex spatial patterns in seemingly uniform environments,
and it allows for species to be absent where conditions are favorable and for species to
be present in locations where the environment is unfavorable. As a final caution, Hanski
warns ecologists and conservationist not to assume all species exist as
metapopulations.
Smith, J.N.M. and J.J. Hellmann. 2002. Population persistence in fragmented
landscapes.
Trends in Ecology and Evolution. 17:(9)397-399.
Smith and Hellmann review a study by Lesley and Michael Brooker examining the
impact of habitat fragmentation on the reproduction, survivorship, and movement of the
blue-breasted fairy-wren, Malurus pulcherrimus. While many conservation biologists
believe dividing habitats decreases habitat area, which intern decreases reproduction as
well as survival, many studies of habitat fragmentation do not exhibit these results.
Smith and Hellmann review what they believe is one of the few studies demonstrative
the negative effects of habitat fragmentation on a specific population. After carefully
studying M. pulcherrimus populations in western Australia for five years, the Brookers
found evidence of similar demographic behavior throughout different size habitat
fragments. Wren reproduction was higher in smaller habitats, but wren survival was
lower in these smaller fragments. Conversely, the reproductive success of wrens was
poor in larger fragments due to brood parasitism. This study also demonstrated that
poor habitat connectivity can leads to greater dispersal loss and a subsequent decrease
in population.
Van Dyck, H. and E. Matthysen. 1999. Habitat fragmentation and insect flight: a
changing ‘design’ in a changing landscape? Trends in Ecology and Evolution.
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14:(5)172-174.
Van Dyck and Matthysen review a study conducted by Jane Hill, Chris Thomas, and
Owen Lewis examining the impact of habitat fragmentation on the flight morphology of
insects. Previous studies have shown that while in the presence of habitat
fragmentation different species of insects do not experience the same morphological
evolution with increasing isolation, many species of insects do show some type of
morphological evolution. Evidence demonstrates that butterflies exhibit evolutionary
response in flight morphology to changes in landscape structure. Hill et al. examined the
butterfly, Hesperia comma, and found the thoraxes of individuals in metapopulations
where habitat patches were further apart to be heavier than those of individuals in
metapopulations where habitat patches were closer together. (The thorax of a butterfly
contains the flight muscles.) Van Dyck and Matthysen also explore other explanations
for changes in insect morphology with habitat fragmentation including changes in
behavior with increasing isolation due to decreased population size within fragments. In
addition, morphological evolution may occur with the change in habitat microclimate
associated with habitat fragmentation. The changes in morphology associated with
habitat fragmentation may not always favor increased mobility. While there are many
implications for managing populations with short generations times and rapid
evolutionary responses, one of the major threats for managing insect populations were
evolution may favor a decrease in mobility is the threat of localized extinctions. The
structure of this article is not particularly well organized, however the authors do bring an
important point. All species do not react to the effects of increased isolation in the same
manner. This is an important concept to keep in mind when studying the effects of
habitat fragmentation on different species.
Viveiros de Castro, E.B. and F.A.S. Fernandez. 2004. Determinants of differential
extinction vulnerabilities of small mammals in Atlantic forest fragments in Brazil.
Biological Conservation. 119:73-80.
The authors initially identified that different species should have varying levels of
vulnerability to fragmentation and that the ability to predict species vulnerability to
fragmentation would be extremely beneficial to the management of fragmented forests.
After fragmentation, forests experience a faunal relaxation, which leads to a decrease in
species richness until a new levels of diversity able to be sustained by the current habitat
fragments is achieved. After evaluating whether vulnerability could be predicted based
on body size, trophic level, longevity or fecundity of the mammal, the pre-fragment
population density of the mammal, or the surrounding matrix tolerance of the mammal,
the authors found the only beneficial predictor of small mammals to habitat
fragmentation was the mammal’s vulnerability to the surrounding matrix. The ability to
live in the matrix and/or to travel through the matrix allows for a metapopulation to
persist even after fragmentation. In conclusion, connectivity between forest fragments
due to tolerance of the surrounding matrix allows for recolonization or immigration
opportunities. This was another article which did not prove particularly useful for
explaining the theories behind habitat fragmentation, however, it did provide a good
example of the importance of the surrounding matrix in maintaining population
persistence in the event of habitat fragmentation.
Walters, J.R., H.A. Ford, and C.B. Cooper. 1999. The ecological basis of sensitivity of
brown treecreepers to habitat fragmentation: a preliminary assessment.
Biological
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Conservation. 90:13-20.
This study compared the demography and foraging ecology of brown treecreeper,
Climacteris picumnus, populations experiencing habitat fragmentation to those
populations in unfragmented habitats. In order to asses the impacts of habitat
fragmentation on the brown treecreeper, Walters et al. explored the possibilities of
disrupted dispersal patterns, decreased fecundity due to nest predation, and decreased
food availability due to habitat degradation on populations experiencing fragmentation.
Upon completing their research, Walters et al. found that it was not an increase in nest
predation or a decrease in food quality that was causing the decline in the brown
treecreeper populations with the increase in habitat fragmentation. Their evidence
supported that in fragmented habitats brown treecreepers experience lower male/female
pairing success due to disrupted dispersal patterns. Walters et al. believe the decrease
in populations is due to the difficulty females experience when trying to locate vacant
breeding habitats in the isolated fragments. The possibility also exists that if they do find
a habitat, it may be rejected due to degraded conditions. Walters et al. provide another
classic example of a study examining the effects of habitat fragmentation on a particular
species incorporating ideas from the island biogeography theory.
Whittaker, R.J. 1998. Island theory and conservation. Island Biogeography: Ecology,
Evolution, and Conservation. Oxford University Press, New York. 192-227.
In Chapter 9 of Whittaker’s book, he applies the island theory (developed by MacArthur
and Wilson in 1967) to habitat islands within a fragmented landscape. Whittaker
attributes the process of habitat fragmentation to the formation of habitat islands,
however, he realizes that the separation between habitat islands and the subsequent
movement of species may be “radically different” than between oceanic islands. In the
chapter, several key concepts including minimum viable population size and minimum
viable area are discussed. After introducing the concept of a metapopulation, Whittaker
does an excellent job of incorporating both the island theory and the metapopulation
theory when addressing conservation issues. A main emphasis in the study of habitat
islands is on the degree of connectivity between different habitat patches. When
applying the island theory to oceanic islands, the surrounding matrix is unimportant and
it is assumed that the population of the island will eventually reach an equilibrium state.
Conversely, when applying the island theory to habitat islands, it is important to
remember the “flux of nature.” Physical and biological as well as episodic events will
continue to act upon the habitat island. In addition, the connectivity with surrounding
habitat patches will continue to act as a species filter. The combination of all these
factors is likely to prevent a habitat island from ever reaching an equilibrium state. In
conclusion, Whittaker cautions against oversimplifying island effects on fragmented
habitat patches. For purposes of conservation, he recommends identifying the species
most vulnerable to habitat fragmentation and working towards their preservation as the
effects of connectivity versus isolation as well as the effects of increased amounts of
edge are different for all species. This chapter provides an excellent overview of both
the island biogeography theory (MacArthur and Wilson 1967) and the metapopulation
theory (Levins 1969).
Additional References:
Levins, R. 1969. Some demographic and genetic consequences of environmental
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heterogeneity for biological control. Bulletin of the Entomological Society of
America. 15:237-240.
MacArthur R.H. and E.O. Wilson. 1967. The Theory of Island Biogeography. Princeton
University Press. Princeton, NJ.
Mesquia, R.C.G., P. Delamônica, and W.F. Laurance. 1999. Effect of surrounding
vegetation on edge-related tree mortality in Amazonian forest fragments.
Biological Conservation. 91:129-134.
Reed, R.A., J. Johnson-Barnard, and W.L. Baker. 1996. Fragmentation of a forested
Rocky Mountain landscape, 1950-1993. Biological Conservation. 75:267-277.
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