A Review on the Effect of Habitat Fragmentation on Ecosystem

Abstract

Habitat fragmentation is considered a primary issue in conservation biology. This concern centers around the disruption of once large continuous blocks of habitat into less continuous habitat, primarily by human disturbances such as land clearing and conversion of vegetation from one type to another. Habitat loss and fragmentation are the primary causes of species extinction worldwide. The largest single threat to biological diversity worldwide is the outright destruction of habitat, along with habitat alteration and fragmentation of large habitats into smaller patches. Fragmentation is caused by both natural forces and human activities, each acting over various time frames and spatial scales. Physical Features of the landscape, associated with very slow geomorphic processes (e.g., erosion) may also cause some patches to remain isolated over evolutionary timescales. The effects of fragmentation also vary depending on the cause of fragmentation (for example, fragmentation of agriculture versus for logging). As a result, there is necessity to take effective actions to maintain biodiversity in fragmented landscapes.

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A Review on the Effect of Habitat Fragmentation on Ecosystem
Dagnachew Mullu
Department of Biology, College of Natural Sciences, Arba Minch, University
PO Box 21, Arba Minch Ethiopia
Abstract
Habitat fragmentation is considered a primary issue in conservation biology. This concern centers around the
disruption of once large continuous blocks of habitat into less continuous habitat, primarily by human
disturbances such as land clearing and conversion of vegetation from one type to another. Habitat loss and
fragmentation are the primary causes of species extinction worldwide. The largest single threat to biological
diversity worldwide is the outright destruction of habitat, along with habitat alteration and fragmentation of large
habitats into smaller patches. Fragmentation is caused by both natural forces and human activities, each acting
over various time frames and spatial scales. Physical Features of the landscape, associated with very slow
geomorphic processes (e.g., erosion) may also cause some patches to remain isolated over evolutionary time-
scales. The effects of fragmentation also vary depending on the cause of fragmentation (for example,
fragmentation of agriculture versus for logging). As a result, there is necessity to take effective actions to
maintain biodiversity in fragmented landscapes.
Keywords: corridors, fragmentation, island, habitat, metapopulation.
1. Introduction
Landscapes all over the world are being fragmented at an unprecedented rate due to fragmentation of the natural
habitats of wildlife. These influenced the process of species extinction in several populations. Whether it is wise
to connect habitat remnants together with dispersal corridors or leave such isolated populations to evolve and
fluctuate on their own and whether it is better to protect a single large area or a set of smaller isolated areas in
biological reserves, are questions that require immediate attention before the possibility of taking actions to
reduce the effect of habitat fragmentation (Reed, 2003). Habitat loss and fragmentation are the primary causes of
species extinction worldwide (Weidong et al., 2002).
Habitat fragmentation is considered a primary issue in conservation biology (Meffe and Carroll, 1997).
This concern centers around the disruption of once large continuous blocks of habitat into less continuous habitat,
primarily by human disturbances such as land clearing and conversion of vegetation from one type to another.
The classic view of habitat fragmentation is the breaking up of a large intact area of a single vegetation type into
smaller units (Lord and Norton, 1990).
Habitat fragmentation involves the splitting of natural habitats and ecosystems into smaller and more
isolated patches. It reduces the availability and the suitability of adjacent areas for wildlife. This process leads to
conditions whereby individual animal and plant species, as well as their wider populations, to become
endangered leading to extinction. Fragmentation is a complex process, in which the loss and isolation of natural
habitats are the most important factors. It is a process in which one continuous habitat is transformed into a
larger number of smaller patches, of smaller total area, isolated from each other by a matrix, which is usually
compositional or structurally different from the original habitat (Wilcove et al., 1986). This process implies
habitat loss, but also a change in habitat configuration (Fahrig, 2003), that implies different number of fragments
or relative isolation among them. Classical theories in community and population ecology, such as the Theory of
Island Biogeography (MacArthur and Wilson, 1967) and the Theory of Metapopulation Dynamics, predict that
smaller and more isolated fragments support an impoverished fauna, compared to larger and closer fragments.
Many empirical data have supported these predictions, suggesting that habitat fragmentation negatively affects
the abundance and species richness of organisms (Levins, 1969). But, as most researchers have not separated the
effects of habitat loss and habitat configuration, the negative effects attributed to habitat fragmentation may be
representing only the negative effects of habitat loss (McGarigal and Cushman, 2002). Fahrig (2003) pointed out
that while habitat loss has large, consistent negative effects on biodiversity, a change in habitat configuration has
a much weaker effect, and may be negative but also often positive. The positive effects recorded involves
(Collins and Barret, 1997), crabs (Caley et al., 2001) and insects (Collinge and Forman, 1998). On the one hand,
habitat fragmentation may increase population density through a crowding effect, where surviving individuals
move from the removed habitat to the remaining fragments (Collinge and Forman, 1998). Nevertheless, this may
be a short-term effect (Debinski and Holt, 2000). On the other hand, habitat fragmentation may increase species
richness, both in the fragment (i.e., local, species diversity) as well as in the landscape (i.e., regional, species
diversity) level, not only because of the crowding effect, but also due to a series of small or medium-sized
fragments distributed in a larger area and relatively far apart from each other (Collinge and Forman, 1998).
By far, the largest single threat to biological diversity worldwide is the outright destruction of habitat,

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along with habitat alteration and fragmentation of large habitats into smaller patches (Meffe and Carroll, 1997).
The two components of habitat fragmentation are: 1) the reduction of the total amount of a habitat type in a
landscape; and 2) the reapportionment of the remaining habitat into smaller, more isolated patches of habitat.
The magnitude of habitat fragmentation reflects the influence of humans on the environment from local through
regional, national and global. While the direct effects of habitat loss are typically considered to pose the greatest
current threat to biodiversity, the size and spatial arrangement of remnant fragments is recognized to have a
major effect on population dynamics and species persistence (Lord and Norton, 1990) , with impacts that are
more dangerous than habitat loss alone (With, 1997). As a consequence, habitat fragmentation has become a
central issue in conservation biology (Meffe and Carroll, 1997).
1.1. Definition
Habitat fragmentation can be described as the splitting of natural habitats and ecosystems into smaller, more
isolated patches (Fig. 1, C). It is the process of subdividing a continuous habitat into smaller pieces (Fig. 1, A). It
implies a loss of habitat, reduced patch size and an increasing distance between patches, but also an increase of
new habitat It is defined as the breaking apart of the connectivity of habitat (Fig. 1, B) (Andren, 1996). The
term fragmentation has been used to encompass a broad variety of changes in landscapes, changes that include
reduction in habitat area, increased isolation of habitat patches, extension of the length of edges between habitats,
and amplified contrast between habitat and the surrounding matrix (Fahrig, 2003).
Fragmentation is the dissection of landscapes into spatially isolated parts, is a major driver of
environmental change worldwide (Fischer and Lindenmayer, 2007). Usually, the process of fragmentation is
caused by human activities (roads, agriculture and logging). It also reduces the value of the landscape as habitat
for many species. It alters natural habitat in many ways, including reduction of patche sizes, increase of distances
between similar patches and increase of edges (Robinson et al., 1995). Landscape fragmentation customarily
refers to a reduction in connectivity between parts of a landscape or the conversion of the landscape into a
mosaic of cover types, some of which differ from the original habitat. Ecological implications of these changes
remain unknown (Southworth et al., 2004).
Habitat fragmentation is often defined as a process during which a large expanse of habitat is
transformed into a number of smaller patches of smaller total area, isolated from each other by a matrix of
habitats unlike the original (Wilcove et al., 1986). By this definition, a landscape can be qualitatively categorized
as either continuous (containing continuous habitat) or fragmented, where the fragmented landscape represents
the endpoint of the process of fragmentation. This definition of habitat fragmentation implies three effects: (a)
reduction in the extent of habitat, (b) increase in number of habitat patches, and (c) increase in isolation of
patches. These effects form the basis of most quantitative measures of habitat fragmentation. However,
fragmentation measures vary widely; some include only one effect (e.g., reduced habitat or reduced patch size),
whereas others include more.
1.2. Theory of habitat fragmentation
The two key theoretical developments in community and population ecology to study fragmentation are: the
Theory of Island Biogeography (MacArthur and Wilson, 1967) and the Theory of Metapopulation Dynamics
(Levins, 1969).
1.2.1. Theory of Island Biogeography
In the context of habitat fragmentation, the Theory of Island Biogeography has focused primarily on the
influences on habitat fragment size and isolation of species composition ( Robinson et al., 1992).
The theory of island biogeography (MacArthur and Wilson, 1967), which accounts for the observation
that the number of species on islands is lower than in the mainland areas of comparable size. It proposes that
species richness on islands represents a balance between the rate of colonization of new species and of extinction
of species already present (Fig. 3). The rate of colonization is determined primarily by isolation from the
mainland, the rate of extinction mainly by island size. A small island will have fewer species than a larger island
of comparable isolation and for islands of similar size, those that are distant will have fewer species than those
close to a mainland source (Fig. 2). This theory extends to mainland isolates and habitat fragments and a
framework for studying the effects of habitat fragmentation (Diamond, 1975). The theory that species richness
and individual abundance will decrease with reduced patch size (Fahrig, 2003). The concept of isolation has also
changes somewhat from distance to a mainland to distance between neighboring patches (Haila, 2002).
It proposes to explain species composition of animal communities on oceanic islands. In particular, this
theory postulates that the size of an oceanic island and its distance from a continental source of colonizing
species will determine the number of species present on the island. Islands close to a mainland will have higher
immigration rates than more distant islands, and large islands will have lower extinction rates than small islands.
Thus, large islands close to continents are predicted to have a higher number of species than small islands which
are more distant from continents. Species on island represent a dynamic equilibrium between the immigration of

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new colonizing species and the extinction of previously established ones. This theory focuses on species
composition on oceanic islands, the predictions may be consistent for plant and animal communities inhabiting
terrestrial islands (MacArthur and Wilson, 1967).
1.2.2. Theory of Metapopulation Dynamics
The Theory of Metapopulation Dynamics concept has focused attention on connectivity and interchange between
spatially distributed populations (Hanski et al., 1995). Metapopulation theory was originally conceived to
describe and predict the population dynamics of species occupying naturally patchy habitats (Levins, 1969). A
metapopulation is a set of spatially separated groups of conspecific individuals. In this model, local populations
of organisms undergo periodic colonization and extinction, while the metapopulation as a whole persists
indefinitely (Fig. 4). Ecologists have directly applied the understanding of the oscillations of such naturally
transient populations to predicting the persistence of species which occur in human-induced habitat fragments
(Hanski et al., 1995).
Like Island Biogeography Theory, this theory also focuses on local extinctions and colonization in the
context of heterogeneous spatial pattern of habitat patches. It differs from island biogeography by the following
points. It assumes a network of small patches with no mainland habitat and it considers population dynamics of
only one species at a time (Hanski, 2002).
1.2.2.1. Landscape Dynamics
The higher the rate of landscape change, the lower the probability of regional population survival. The rate of
change in landscape structure may be more important than the degree of patch isolation for determining
population survival and abundance. This body of spatial theory suggests that habitat configuration is important,
above and beyond the effects of a loss in habitat area associated with fragmentation processes. Thus, spatial
management strategies for increasing landscape connectivity, and increasing area/edge ratios, should help to
alleviate habitat fragmentation effects (Kindvall, 1996).
2. Spatial and ecological attributes of habitat fragments
Numerous ecological studies have investigated the consequences of habitat fragmentation for plant and animal
population persistence, community composition, and ecosystem processes. For example, many studies relate the
number of animal or plant species observed in fragments to some designated fragment characteristics, usually
area, shape, degree of isolation, context, or some measure of habitat quality or heterogeneity. While each of
these attributes plays an individual role in determining ecological function, they may also interact to influence
ecological processes. Each of these fragments characteristics separately, recognizing that there are correlations
among them (Fahrig, 2003).
2.1. The edge phenomenon
Edges are often sites of intense interactions that strongly influence the biotic units so bounded (Wallace et al.,
1997). The term edge effect is a collection of different phenomena (Lidicker and Peterson, 1999). It is the
portions of a fragment that are altered by external conditions, while unaffected portions are called core habitat.
The proportion of a fragment that is core habitat is a complex function of fragment size and shape and the nature
of the surrounding landscape matrix (Laurance and Yensen, 1991).
Conversion of continuous habitat into disjunct habitat remnants usually increases the length of the
border between fragments and their surrounding habitats. Particularly in forests, because of their dominant
vertical structure, removal of vegetation from an area results in dramatic changes in the structural characteristics
of the habitat (Murcia, 1995). Isolated forest remnants, which were once embedded in continuous forest, are
exposed to the altered physical environment of the adjacent cleared area. The amount of light reaching plants is
obviously higher at the edge of a forest fragment than in the forest interior. Consequently, temperature increases
and relative humidity decreases at the forest edge. Moreover, wind velocities are higher at the edge than in the
interior of the forest. The changes in light, moisture, temperature, and wind, most pronounced at the fragment
edge, may significantly alter the plant and animal communities which occur there. Additionally, the extent to
which the edge experiences these environmental changes may be significantly influenced by the aspect or
orientation of the edge. In the Northern Hemisphere, south-facing edges are generally warmer, drier and wider
than north-facing edges; the opposite is true in the southern hemisphere. Similarly, windward edges of forest
patches tend to be warmer, drier and wider than leeward edges (Harris, 1984).
Associated with the changes in light, temperature, moisture and wind conditions at forest edges are
changes in the structure and composition of the existing plant communities. For example, in the deciduous forest
patches of southeastern Wisconsin, forest edges typically contained more pioneer and xeric plant species than the
interior, higher densities of shrubs and herbaceous ground layer vegetation for several meters into the forest, and
higher species richness than the interior. Higher species richness in forest edges may often be due to the invasion
of exotic plant species (Ranney et al., 1981).
Some animals appear to select or prefer edges as suitable breeding habitat, despite the fact that mortality

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rates at edges can be much higher than in fragment interiors. This phenomenon has been termed as ecological
trap (Ries and Fagan, 2003). Edges support higher β-diversity than fragment interiors (Didham et al., 1998). It
also increases variability in trophic interaction strengths which results hyper-dynamism in a range of ecosystem
process rates, where the frequency and/or amplitude of ecosystem dynamics is increased. Hyper-dynamism can
result in the destabilization of animal populations (Laurance, 2002).
2.2. Fragment size/area
The size of a particular habitat fragment markedly influences the ecological processes occurring therein, partly
due to the changes induced by the creation of habitat edges discussed above. Because edge effects in a particular
habitat there is a constant distance from the border to the center of a habitat fragment, smaller fragments will
contain a higher proportion of edge habitat than will larger fragments. The decline in species richness in small
habitat remnants results from decrease in population size of a particular species, and eventually, local extinction
of those populations. Population decline due to direct effects of habitat loss, or due to indirect effects, for
example, modified interspecific interactions associated with habitat isolation and edge effects. As fragment size
increases, the relative proportion of edge habitat decreases, and interior habitat increases (Groom and Schumaker,
1993).
2.3. Fragment connectivity
Landscape connections play an important role in ecological dynamics within and between habitats. The
conservation of vegetated corridors among otherwise isolated habitat remnants is predicted to moderate the
negative effects of habitat fragmentation by maintaining landscape connectivity. In the context of ecological
studies of habitat fragmentation, the term corridor generally refers to a linear landscape element composed of
native vegetation which links patches of similar, native vegetation. The integrity of riparian corridors, in
particular, is of critical importance in preventing soil erosion and maintaining high water quality. It is well
documented that vegetation in riparian zones provides bank stability and control of water flow regulates light
and temperature characteristics of the adjacent water bodies, and provides habitat for aquatic life in the form of
coarse and fine woody debris (Naiman et al., 1993).
Vegetated corridors are predicted to facilitate the movement of plants and animals among habitat
fragments, which may allow more species to exist and/or populations to persist longer than would be expected
based solely on fragment size. For example, computer simulations have suggested that populations of the white-
footed mouse, Peromyscus leucopus, have higher growth rates and thus lower probabilities of local extinction in
woodlots connected by fencerows than in isolated woodlots (Fahrig et al., 1983). A recent experimental field
study in an agricultural landscape showed that vole dispersal was greater between old-field fragments connected
by vegetated corridors than in completely isolated fragments (Harris and Scheck, 1991).
Animal use of corridors may vary depending upon their foraging patterns, body size, home range size,
degree of dietary specialization, mobility and social behavior. Lindenmayer and Nix (1993) noted that linear
remnants of Montana forest harbored several species of large, arboreal marsupials, while smaller species were
absent. These authors suggested that species occurrence in these corridors was largely determined by foraging
behavior and social behavior rather than body size. Large animals foraged singly and fed on readily available
leaves, while smaller species foraged in social groups and fed on more widely dispersed arthropods. The
perception and use of corridors by animals may also differ according to the physical dimensions and landscape
context of the corridor. For interior habitat specialists, the typically long, narrow dimensions of a corridor may
be perceived as largely edge habitat and avoided. Because it does not facilitate movements of larger sized
animals through it. And the animals which can pass through it will be exposed to danger due to its longer size.
Moreover, how such movements via vegetated corridors might translate into population persistence and
community composition of native habitats is not well understood. The existence of vegetated corridors between
otherwise isolated habitat fragments may modify patterns of species richness and composition by increasing the
effective size of the fragments. Thus, connected remnants would be predicted to maintain the attributes of
continuous habitat, and support a greater biological diversity than completely isolated remnants (Groom and
Schumaker, 1993).
2.4. Fragment shape
The geometric shape of a discrete habitat fragment influences the extent to which edge effects permeate (spread
through) the habitat interior. Size and shape thus interact to influence the amount of interior area remaining in a
particular habitat fragment. Shape can be described most simply by calculation of the perimeter/area ratio of a
habitat fragment. A square habitat fragment maintains a greater proportion of interior habitat than does a
rectangular fragment of equal area (Groom and Schumaker, 1993).

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2.5. Fragment context
The context in which a remnant of native habitat is situated will undoubtedly influence the degree and type of
interaction between the fragment and the surrounding landscape. The assertion of Janzen (1983) that no park is
an island emphasized the influence of surrounding habitat types and human activities on the ecological integrity
of areas reserved for conservation.
The type, intensity, and degree of dissimilarity of habitat types, land uses and human activities adjacent
to habitat fragments may markedly influence the flow of nutrients and materials, and the persistence of plant and
animal species in the fragments. The relative importance of these adjacent land-uses on ecological processes will
depend upon other spatial characteristics such as fragment size and shape (Janzen, 1983).
The boundary between a habitat fragment and its surrounding matrix may be relatively impervious, with
a low tendency for the exchange of materials and organisms between the remnant and the matrix. Alternatively,
the boundary may be highly porous, in which case there are frequent and abundant boundary crossings.
Boundary permeability and the perimeter/area ratio may both influence the rate and extent of flows across the
boundary. The supply and flow of nutrients, materials, and energy within habitat fragments, as well as between
fragments and the surrounding landscape, will likely differ depending on the adjacent land-use or activity (Wiens
et al., 1985).
2.6. Fragment heterogeneity
A factor shown to be partially responsible for the relationships found between species composition and fragment
spatial characteristics is the degree of habitat heterogeneity within isolated fragments. Large fragments are more
likely to contain a greater variety of soil types, greater topographic variation, greater microclimatic variation, and
a greater number of habitat types than small fragments (Boecklen, 1986).
Fragments of approximately equal size, which are relatively heterogeneous tend to support a greater
number and variety of species than those which are more homogeneous (Maehr and Cox, 1995). Populations of
plants or animals in heterogeneous habitat fragments may be less susceptible to local extinction than those in
more homogeneous habitats. For example, populations of bush crickets in Sweden were more likely to persist if
they occurred in an area which contained several vegetation types than in an area which contained only a single
or small number of vegetation types (Kindvall, 1996). This result was largely due to the existence of greater
microclimatic variation in the more heterogeneous habitats, which allowed some individuals to persist even
under severe weather conditions. In contrast, areas with little vegetational diversity exhibited little microclimatic
variation; thus in severe weather, the cricket populations went extinct. This result suggests that maintenance or
restoration of a high diversity of vegetation types within habitat remnants may be essential for long term
population persistence. An analysis of temperate zone, forest birds demonstrated that the structural diversity of
forest vegetation significantly influenced bird species composition, in addition to forest fragment size (Kindvall,
1996).
3. Causes of habitat fragmentation
Fragmentation is caused by both natural forces and human activities, each acting over various time frames and
spatial scales. Physical Features of the landscape, associated with very slow geomorphic processes (e.g., erosion)
may also cause some patches to remain isolated over evolutionary time-scales (Schule, 1990).
3.1. Fragmentation Due to Natural Causes
1. Over long time frames (thousands or millions of years), landscapes are fragmented by geological forces (e.g.,
continental drift) and climate change (e.g., glaciations, changes in rainfall, sea level rise).
2. Over short periods (decades or months), natural disturbances, such as forest fires, volcanoes, floods, landslides,
windstorms, tornadoes, hurricanes and earthquake modify and fragment landscapes.
In addition, landscapes are naturally fragmented by mountain ridges, canyons, rivers and lakes. Some
ecosystems also commonly occur in discrete patches and are thus naturally fragmented. Natural processes create
the habitat heterogeneity and landscape diversity upon which many species depend.
3.2. Fragmentation due to Human Activity
The most important and largest-scale cause of changes in the degree of fragmentation is anthropogenic habitat
modification, with nearly all fragmentation indices being strongly correlated with the proportion of habitat loss
in the landscape (Fahrig, 2003).
Humans have modified landscapes for thousands of years. Early hunters influenced the landscape by
burning areas to favor certain game species, as currently ranchers burn grasslands. Many human activities such
as agriculture, settlement, resource extraction (e.g., mining, timber), industrial development (e.g. the construction
of hydroelectric dams) alter and fragment landscapes. Of these activities, agriculture is the leading cause of
ecosystem loss and fragmentation throughout the world today (Tilman et al., 2001).

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3.3. Natural Vs. Human Fragmentation
Several differences exist between human-caused and naturally fragmented landscapes (Fahrig, 2003):
1. A naturally patchy landscape often has a complex structure with many different types of patches. A human-
fragmented landscape tends to have a simplified patch structure with more distinct edges, often with a few small
patches of natural habitats in a large area of developed land.
2. Patch types in human-modified landscapes are often unsuitable to many species, while in a heterogeneous
natural landscape most patch types are suitable to a more diverse group of species.
3. The borders (or edges) of patches in naturally patchy landscapes tend to be less abrupt than in those created by
humans.
4. Adverse effects of habitat fragmentation
Habitat fragmentation results in both a quantititative and qualitative loss of habitat for species originally
dependent on that habitat type (Temple, 1986). As a consequence, the abundance and diversity of species
originally present often declines, and losses are most noticeable in smallest fragments. Most importantly,
fragmentation affects movement and dispersal and modifying behavior (Haila, 2002). The process of habitat
fragmentation involves three factors, which have important repercussions on plant and animal species that
originally occupied large continuous areas of wild habitat (Schimiegelow and Monkkonen, 2002).
First, fragmentation leads to the breaking of large patches into numerous smaller, resulting in a net
habitat loss. This results in a decrease in the amount of resources and shelter areas available to wild species and,
therefore, leads to a general reduction in the number of individuals that can be hosted. The most obvious effect
of fragmentation is an outright quantitative loss of habitat for species dependent on the original habitat type in a
region groups of species directly impacted by habitat loss through fragmentation including those with large home
range requirements, very specific microhabitat requirements, and poor disposal abilities.
Second, by opening core areas, fragmentation of continuous habitat patches, leads to a dramatic
increase in edges. Edges present distinct micro-climatic conditions from the core and therefore might become
less suitable for species. These edges also lead to higher predation rates by favoring generalist predator influx,
which in turn greatly impacts the population of resident species.
Third, habitat fragmentation results in the geographic isolation of islands of habitat among a matrix of
urban or agricultural land-uses. This might greatly restrict the mobility of certain organisms, and thereby isolate
some populations. However, small isolated populations can be threatened by inbreeding, which represents a
serious problem for their survival and could lead, in case of severe inbreeding to population extinction.
Moreover, small populations are more sensitive to stochastic events, such as fires or epidemic outbreaks that
could drive local population to extinction.
The isolation of habitat patches increases and the probability to be recolonized decreases. Therefore,
long-term survival of isolated populations cannot be assumed in any case. Nonetheless, not all species have the
same sensitivity to habitat fragmentation. Naturally rare, sedentary species, with specialized in habitat
requirement show an important decline whereas abundant mobile generalist species are less affected or even
favored, in the case of edge specialists. Also the degree of isolation of habitat patches might depend on the
migration capacity of each species living within them. As a consequence, habitat fragmentation cannot be
generally described, and should be specified for every individual species (Parker and Nally, 2002).
Habitat fragmentation creates landscapes made of altered habitats or developed areas fundamentally
different from those shaped by natural disturbances that species have adapted to over evolutionary time (Meffe
and Carroll, 1997). Generally, according to (Parker and Nally, 2002), adverse effects of habitat fragmentation to
both wildlife populations and species include:
• Increased isolation of populations or species, which leads to:
- Adverse genetic effects; i.e. inbreeding depression (depressed fertility and fecundity, increased natal mortality)
and decreased genetic diversity from genetic drift and bottlenecks;
- Increased potential for extirpation of localized populations or extinction of narrowly distributed species from
catastrophic events such as hurricanes, wildfires or disease outbreaks;
• Changes habitat plant composition, often to weedy and invasive species;
• Changes the type and quality of the food base;
• Changes microclimates by altering temperature and moisture regimes,
• Changes flows of energy and nutrients;
• Changes availability of cover and increases edge effect, bringing together species that might otherwise are not
interact, potentially increasing rates of predation, competition and nest parasitism;
• Increases opportunities for exploitation by humans, such as poaching or illegal collection for the pet trade;
Habitat fragmentation diminishes the landscapes capacity to sustain healthy populations or metapopulations in
five primary ways: loss of original habitat, reduced habitat patch size, increased edge, increased isolation of
patches and modification of natural disturbance regimes (Forman, 1999).

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4.1. FRAGMENTATION AS HABITAT LOSS
The most obvious effect of the process of fragmentation is the removal of habitat. This has led many researchers
to measure the degree of habitat fragmentation as simply the amount of habitat remaining on the landscape
(Robinson et al., 1995). When ecologists think of fragmentation, the word invokes more than habitat removal:
fragmentation; not only causes loss of the amount of habitat, but by creating small, isolated patches it also
changes the properties of the remaining habitat (van den Berg et al., 2001).
Habitats can be removed from a landscape in many different ways, resulting in many different spatial
patterns. The assertion that habitat fragmentation means something more than habitat loss depends on the
existence of effects of fragmentation on biodiversity that can be attributed to changes in the pattern of habitat
that are independent of habitat loss. In addition to loss of habitat, the process of habitat fragmentation results in
three other effects: increase in number of patches, decrease in patch sizes, and increase isolation of patches.
Measures of fragmentation that go beyond simply habitat amount are generally derived from these or other
strongly related measures (e.g., amount of edge). There are at least 40 such measures of fragmentation
(McGarigal et al., 2002), many of which typically have strong relationships with the amount of habitat as well as
with each other (Robinson et al., 1995).
The interrelationships among measures of fragmentation are not widely recognized. Most researchers
do not separate the effects of habitat loss from the configurationally effects of fragmentation. This leads to
ambiguous conclusions regarding the effects of habitat configuration on biodiversity. It is also common for
fragmentation studies to report individual effects of fragmentation measures without reporting the relationships
among them, which again makes the results difficult to interpret (Robinson et al., 1995).
4.2. REDUCED HABITAT PATCH SIZE
Reduction in habitat patch size is a principal consequence of fragmentation. MacArthur and Wilson have
suggested that the rate of species extinction in an isolated patch of habitat is inversely related to its size. As
habitats become smaller, they are less likely to provide food, cover and other resources necessary to support the
native communities. Small patches are also more susceptible to catastrophic disturbance events such as fire or
severe weather that can decimate local populations. Fragmentation also decreases the area of interior habitat.
Interior habitat is the area far enough from the edge to maintain communities of the original larger habitat
(Parker and Nally, 2002).
Once a landscape has been fragmented, the size of the remaining patches is a critical factor in
determining the number and type of species that can survive within them. For all species that cannot cross a
forest edge or leave a patch, all requirements to complete their life cycle must be met within the patch itself. This
is especially important for species with complex life cycles, each with distinct habitat requirements. For example,
many amphibian species have aquatic larval stages and a terrestrial adult phase. Also, some species require large
areas of continuous habitat and cannot survive in small patches. These are referred to as area-sensitive species.
Furthermore, large patches typically support larger populations of a given species and thereby buffer them
against extinction, inbreeding depression, and genetic drift (Schimiegelow and Monkkonen, 2002).
4.3. INCREASED ISOLATION
Fragmentation leads to increased isolation of patches. Wildlife populations in isolated patches can be sustained
by immigration of species from surrounding patches. However, as fragmentation continues, distances between
patches get longer and dispersal and immigration rates decrease. The diversity of species moving between
patches also decreases; small species with limited mobility are particularly sensitive. As immigration rates
decrease, factors like inbreeding and catastrophic disturbances can cause the number of species in a patch to
decline to zero over a long period of time (Robinson et al., 1995).
4.4. INCREASED EDGE
Although an increase in edge (the boundary between two plant communities) due to fragmentation may benefit
some species, some researchers believe that increasing edge may be detrimental to the protection of native
biodiversity. Edges act as barriers causing some predators to travel along them. High predator densities along
edges can result in higher mortality for edge dwelling prey species or species moving through narrow corridors.
Nest parasitism by brown headed cowbirds (Molothrus ater) also appears to be higher in species nesting in edge
habitat. Least bells vireo (Vireo bellii pusillus) is an endangered species that inhabits the edges of riparian
corridors in southern California. Parasitism by cowbirds appears to be as significant as the loss of riparian habitat
in the decline of the least bell's vireo on Camp Pendleton, California (Schule, 1990).
One of the most obvious changes to a fragmented landscape is the increase in edge environment. Edge
environments or ecotones mark the transition between neighboring habitats. In a naturally forested landscape,
edge is usually limited to a small area, (Laurance and Bierregaard, 1997). Natural edges are usually less abrupt
than human-formed edges and show a gradual transition from one habitat type to another. Along agricultural

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frontiers, the original landscape may be fragmented into long narrow strips or shreds, interspersed with areas of
agriculture. These strips may separate different crops, thus serving as windbreaks, or the boundary between two
landowners. As a result, this remaining fragment is entirely made up of edge environment. Residual trees along
rivers provide another example of narrow, edge-dominated environments (Debinski and Holt, 2000).
The extent of edge environment in a fragment patch is determined in part by its shape. The ratio of the
perimeter to area (or the amount of edge environment to the amount of interior) is one measure of patch shape. A
circular patch has the maximum area per unit edge and will have less edge environment and fewer edge effects
than a rectangular patch of the same size. Because edge effects may extend 200 meters (and sometimes more),
small patches may be entirely composed of edge environment (Laurance and Bierregaard, 1997).
Edge effect is a general term used to describe a number of different impacts, and can be categorized into
several types: physical (e.g., microclimatic changes), direct biological impacts (changes in species composition,
abundance, and distribution), and indirect biological impacts (changes in species interactions such as predation,
competition, pollination, and seed dispersal). Moreover, many of the effects of fragmentation are synergistic; for
example, fragmentation can lead to increased fire risk, increased vulnerability to invasive species, or increased
hunting pressure (Hobbs, 2001).
4.5. Effects of Different Types of Fragmentation
The effects of fragmentation also vary depending on the cause of fragmentation (for example, fragmentation of
agriculture versus for logging). It is difficult to make generalizations about the effects of a specific type of
fragmentation on a particular landscape, as the consequences may be very different in a temperate vs tropical
region or in a grassland vs a forest, largely because the plants and animals present have different sensitivities to
fragmentation (Harris and Scheck, 1991).
The potential effects of a particular type of fragmentation based on how the new environment is
perceived by the original species present and whether the change to the landscape is permanent or temporary.
The matrix that surrounds fragments has a large effect on what species remain it and their dispersal ability
between fragments (Hansski et al., 1995).
4.5.1. Effects on Species Abundance, Richness, and Density
Impacts of fragmentation on species abundance, richness, and density are complex. Studies of the effects of
fragmentation on species abundance, richness and or density relative to fragment size have inconsistent results,
some indicating an increase in species and in others, a decline. However, it is important to keep in mind that
simply counting the number of species does not measure impacts of fragmentation on behavior, dispersal ability,
or genetic diversity (Debinski and Holt, 2000).
Some species respond positively to fragmentation. Fragmentation may increase species richness by
allowing generalist species to invade. In a study of the impact of fragmentation on frogs in a lowland Amazonian
forest, species richness was strongly and positively related to fragment area. After fragmentation, species
richness increased largely as a result of invasion by frog species from the surrounding matrix into the remaining
forest fragments. It is unclear if this increase will be sustained over time. For example, if this same spot was re-
surveyed in 50 years, total species richness might decline as interior forest species disappear (Laurance and
Bierregaard, 1997).
Immediately following fragmentation, the density of individuals may increase as animals crowd into
the remaining forest. This inflation of density will ultimately prove short-lived because patches are rarely
adequate to support the same population density for long as more extensive habitats (Schmiegelow et al, 1997).
4.5.2. Interactions Among Species and Ecological Processes
Fragmentation causes the loss of animal populations by a process termed faunal relaxation, the selective
disappearance of species and replacement by more common species. Large-bodied vertebrates, especially those
at high trophic levels, are particularly susceptible to habitat loss and fragmentation, and are among the first
species to disappear. Thus, predators are often lost before their prey, and those species that do survive on small
fragments (usually herbivores) tend to become far more abundant than populations of the same species on larger
species-rich fragments. There are two principal explanations for this increased abundance. The first is ecological
release from competition: when competing species are removed, the resources they utilized become available to
the persisting species. The second is that prey escape to small fragments from predators that normally limit their
abundance on larger fragments. Lack of predators in small fragments can also lead to an overabundance of
herbivores that tend to weed out palatable plant species and convert the landscape into a forest of herbivore-
proof plants. Furthermore, as large predators disappear, smaller predators often increase; this is known as
mesopredator release (Soulé et al., 1988). For example, in California, as coyotes disappear from fragments, there
is an overabundance of smaller predators, such as skunks, raccoons, grey fox, and cats. These smaller predators
then prey on scrub-breeding birds. Fragmentation thus triggers distortions in ecological interactions that drive a
process of species loss, the end point of which is a simplified ecological system lacking much of the initial
diversity (Chalfoun et al., 2002).

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While predator-prey relationships are often altered in fragmented landscapes, it is not always possible to
predict what the change will be. Studies in Central Canada, for example, found that nests in forest patches
adjacent to agricultural land had increased predation, while those next to logged areas did not. It appeared that
the predator community did not change in the logged areas, while forest patches next to agricultural land had
increased densities of red squirrels that preyed on the nests (Bayne and Hobson, 1997). Other studies have shown
that songbirds are subject to increased predation along edges, particularly in deforested areas. In other words, the
type of fragmentation and the habitat adjoining the fragment influences predator-prey relations. Nest predation is
less affected by a single road bisecting an area, but is greatly affected along edges of areas that have been
deforested (Tocher et al., 1997).
Overall a combination of landscape type and structure, predator community, and level of parasitism are
important in anticipating the outcome of fragmentation. For example, unlike studies in the Midwest and
Northeast of the United States, where the landscape has historically been patchy, it was found that predation
rates actually decreased as human-caused fragmentation increased. This study indicated that the type of predators
in an area, as well as the habitat structure were key inputs to anticipate the impact of fragmentation on bird nest
predation rates (Robinson et al., 1995) .
Not all groups of species experience an increase in predation due to fragmentation. It is found that avian
predators were more likely to benefit from fragmentation than mammalian predators (Chalfoun et al., 2002).
Another study found that turtle nests located along roads had lower predation rates than those located in edges or
in forests (Hamilton et al., 2002). Fragmentation can also take an indirect toll on plants whose pollinators or seed
dispersers are forced to navigate an increasingly fragmented landscape in search of their host plants. In western
Australia, only small, isolated populations of the cone-bearing shrub, Good’s banksia (Banksia goodi), remain,
and many of these no longer reproduce because their pollinators have disappeared. Fragmentation often alters
animal behavior, due to changes in the environment or predator activity (Davies and Margules, 1998).
4.6. Fragmentation vs. connectivity
As fragmentation has an obvious negative influence on habitats and their species, it is necessary to avoid
fragmentation, and provide more connections in the environment. Undamaged habitats are likely to control and
maintain a microclimate. However, sunlight, logging and other types of disturbances penetrate such habitats and
change their microclimate. As a result, species in these habitats will be affected. Efforts must be directed to
identify crucial species or group of species as well as the processes that affect them in the habitats if
management plans are to be more effective (Noss, 2001).
5. Response of organisms to habitat fragmentation
The effects of fragmentation range from the obvious losses to the more subtle and indirect. Some effects can be
repaired. However, most are not being restored or regenerated.
Species vulnerable to fragmentation: Recognizing factors that might make a certain species
vulnerable to extinction in habitat fragmentation is one of the most challenging issues in Conservation Biology.
Although species vary in terms of vulnerability to predators in fragmented habitats, causes of vulnerability are
poorly understood (Webb et al., 2002). Studies of Ranius and Hedin (2001) state that a low dispersal rate and
range are the reasons that lead to vulnerability of species to fragmentation. Webb et al. (2002) explain that
habitat specific species are less able to withstand the rapid changes and modifications in their habitats than those
who are denoted as generalists species. Therefore, vulnerable species are likely to go extinct. Species differ from
each other in respect to vulnerability. Several kinds of species are predicted to be most vulnerable to habitat
fragmentation. These may include, the following (Laurance and Bierregaard, 1997): Rare species, wide-ranging
species, species with poor dispersal abilities, species with low fecundity, ground nesters and persecuted species.
Rare species: There are two main categories of rare species, restricted geographic distribution (endemic species)
and low population densities.
Wide-ranging species: Large species often require large areas for their daily and seasonal movements.
They need to travel through fragmented landscapes, facing hazards.
Species with poor dispersal abilities: This refers to species that have low dispersal abilities. Usually,
they do not travel far away from where they were born. Roads and clear cuts act as barriers in this situation.
Species with low fecundity: Due to their low reproduction rates, some species. e.g., Neotropical migrant
birds cannot quickly rebuild their population when they encounter harsh environmental conditions such as
fragmented landscapes.
Ground nesters: Species that nest on or near the ground are more vulnerable to predators than those who
nest on or near top of trees.
Persecuted species: Human activities, such as hunting are obvious and higher fragmented landscapes
than in forested landscapes because they are easily seen by people.
Initial exclusion: Elimination of species is one of the threats of habitat fragmentation in an ecosystem.

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Species that are endemic to a specific habitat type in the landscape are likely to be excluded from their habitat
range. For instance, if patches of a certain type are destroyed by fragmentation, species that require these patches
are likely to extinct. Local extinctions are expected to be negatively correlated with the patch size as well as the
quality of habitat in the landscape (Meffe and Carroll, 1997).
Barriers and isolation: Populations can become isolated within their patches when all of the
surrounding habitats are destroyed. Removing these habitats make species migration into different adjacent
patches difficult and dangerous. While some species require a single type of habitat to carry out their activities,
there are many species that need multiple seral stages to do their activities. Generally, species require a mix of
different habitats for various activities e.g., food patches, roost sites, and breeding sites (Meffe and Carroll ,
1997). An important factor acting as barriers in fragmented landscape is road construction, where roads divide
population into small fragments, exposing them to extinction. Roads contribute directly to the mortality of
species when they disperse (Davies and Margules, 1998).
One of the spatial consequences of habitat fragmentation is that fragments become isolated in space and
time from other patches of suitable habitat. Isolation disrupts species distribution patterns and forces dispersing
individuals to traverse a matrix habitat that separates suitable habitat fragments from each other. While isolation
is most often defined by the euclidean distance between habitat fragments, it is, in fact, matrix dependent. An
extreme example of this was highlighted by Bhattacharya, et al. (2003), who have found that two species of
Bombus bumble bees (Bombus franklini) would rarely cross roads or railways despite the presence of suitable
habitat that was within easy flying range. Some matrix habitats inhibit dispersal more than others and species
differ in their willingness to disperse through matrix environments, (Ricketts, 2001). For example, genetic
differentiation between invertebrate populations was clearly related to fragment isolation in some studies, but not
in others (Krauss et al., 2004). Similarly, the relationship between invertebrate species richness and isolation can
be positive (Baz and Garcia-Boyero, 1996), negative (Baz and Garcia- Boyero, 1995) or absent (Krauss et al.,
2003). One likely reason for these conflicting results is that species with different traits differ in their
susceptibility to isolation (Laurance and Bierregaard, 1997).
Crowding effects: Crowding occurs usually in the isolated fragments immediately after cutting takes
place in the landscape. Logically, when a population is isolated by fragmentation into smaller patches, its
intensity is likely to be high when time progresses, leading to congestion (overcrowding) in the population in the
isolated patches (Debinski and Holt, 2000). This is an ecological phenomenon known as crowding of the ark. As
a result of this phenomenon, a population collapse follows the crowding because of the limited place and the
high intense competition among members of the same species on nutrient resources (Meffe and Carroll, 1997).
Climate changes: Fragmentation is considered as a factor that changes microclimate. During the day
in a fragmented area, the soil surface absorbs most energy that comes from sun, warming the layer above it. This
energy is radiated back during the night. As a result, evaporation, moisture and wind will be changed, which in
turn will affect vegetation in the fragmented areas, creating their own microclimate (Smith and Smith, 2001).
Changes in species composition: Meffe and Carroll (1997) mentioned that there are several studies
suggesting species composition and abundance will change as fragmentation takes place in landscapes by losing
those species that require large areas.
Disturbance effects: Small fragments are more vulnerable to natural disturbances. Post-disturbance
succession may be influenced by species invading from the matrix, including exotic species. The disturbance
regime itself is altered by fragmentation (Jordan et al., 2003).
Behavioral effects: Organisms may change their behavior and/or social interactions if confined to
small enough fragments (Primack and Gerwein, 2002).
Anthropogenic effects: Hunters may increase pressure in forest remnants, or humans may otherwise
alter ecological processes. Where landscapes are fragmented, species with low dispersal capability may have
difficulty responding to anthropogenic climate change. These effects may not occur immediately (Lag effects)
and may interact with each other (Synergistic effects) ( Laurance, 1991).
Matrix effects: A growing body of evidence suggests that matrix quality is crucially important in
determining the abundance and composition of species within fragments (Laurance, 1991). The traditional Island
Biogeography Theory approach to the study of habitat fragmentation failed to recognize that the penetration of
edge effects from outside a fragment alters habitat characteristics within the fragment and that the matrix may
not be completely inhospitable to the fragment dwelling fauna. In fact, there is often substantial overlap between
species that inhabit fragments and matrix habitat. This species spill-over is most prevalent in small patches and
at the edges of large patches, and may obscure area and isolation effects. Island Biogeography Theory
predictions had a better fit when species that occurred in the matrix were removed from the analysis. In addition,
increasing species mortality rates in the matrix can have the drastic effect of completely reversing the outcome of
competitive interactions within fragments, allowing inferior species to supplant dominant ones within fragments.
Generally, some of the matrix effects on biological species are not crossing to adjacent patches, increased risk of
predation (e.g. mesocarnivores) and increased exotic species invasions (Cook et al., 2002).

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6. Management of habitat fragmentation
Habitat fragmentation has a variety of impact on the environment and its organisms. Fragmentation benefits
some species, and at the same time put other species at a great deal of risk. As a result, there is necessity to take
effective actions to maintain biodiversity in fragmented landscapes.
6.1. Establishment of effective corridors
Inclusion of corridors as a protection strategy in habitat fragmentation is imperative to maintain biological
diversity (Rosenberg and Noon, 1997). Corridors function as connection channels between separated fragmented
patches (Noss, 2001), so they are crucial in managing habitat fragmentation. Corridors serve two purposes:
facilitating movement between various types of patches, including breeding, feeding and birthing; and
facilitating immigration and emigration of individuals among such patches (Meffe and Carroll, 1997). Moreover,
corridors can improve the population viability in both fragmented and isolated landscapes if they are
appropriately designed (Meffe and Carroll, 1997, Noss, 2001).
6.2. Buffer zone
Through well-planned projects, buffer zones provide protection for wilderness from human activities and
developments (Meffe and Carroll, 1997). It can increase the ratio of rare and common population by softening
the edge effect (Martino, 2001).
7. Conclusion and recommendations
Generally, adverse effects of habitat fragmentation to both wildlife populations and species include increased
isolation of populations or species, which leads to: adverse genetic effects; i.e. inbreeding depression (depressed
fertility and fecundity, increased natal mortality) and decreased genetic diversity from genetic drift and
bottlenecks; increased potential for extirpation of localized populations or extinction of narrowly distributed
species from catastrophic events such as hurricanes, wildfires or disease outbreaks; changes habitat plant
composition, often to weedy and invasive species; changes the type and quality of the food base; changes
microclimates by altering temperature and moisture regimes; changes flows of energy and nutrients and Changes
availability of cover and increases edge effect, bringing together species that might otherwise are not interact,
potentially increasing rates of predation, competition and nest parasitism. As a result, there is necessity to take
effective actions to maintain biodiversity in fragmented landscapes.
8. Acknowledgments
I gratefully acknowledge Arba Minch University, College of Natural Sciences for their support and providing
necessary materials for this work. I wish to thank Arba Minch University Natural Science Library workers’ and
for their help by giving reference materials for this paper.
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Figure 1. Different types of definitions of habitat fragmentation. Source (Hobbs, 2001).

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Figure 2. The model of Theory of Island Biogeography (MacArthur and Wilson, 1967).
Figure 3. The equilibrium model of Theory of Island Biogeography (MacArthur and Wilson, 1967).
Figure 4. The classic metapopulation model (Collins and Barrett, 1997).