Describe differences between core habitat and edge habitat and list some species that might occur?

Question

Describe differences between core habitat and edge habitat and list some species that might occur?

Answer 1

What is Habitat Fragmentation?

The Process of Habitat Fragmentation

Fragmentation occurs when a large region of habitat has been broken down, or fragmented, into a collection of smaller patches of habitat. Fragmentation typically occurs when land is converted from one type of habitat to another. For example, a forest habitat may become fragmented when a highway is built across the forest. The highway would split a single, large, continuous patch of forest into two smaller patches.

History diagram

Stage 1: the clearing of forest to make fields. Patches of cleared land appear within the matrix of forest.

Stage 2: as farms expand and a region develops, more land is cleared of forest. The patches expand until there is as much forest as there is open agricultural fields.

Stage 3: Development continues; the farm fields coalesce into large open areas of land. Patches of forest now exist within a matrix of agricultural fields.

Figure 2. Series of aerial photographs showing the stages of the fragmentation process. The process proceeds from development of small patches within a large area of continuous forest (a) to expansion of the developed patches (b), and ultimately to conversion of the dominant land cover type from forest to human land use (c).

Effects on wildlife population

Forest fragmentation affects plant and animal populations at several scales. Over the larger landscape of the Chesapeake Bay watershed, once continuous populations of forest dwelling species were broken up into smaller subpopulation occupying the remaining forest fragments. Scientists believe these subpopulations may act as metapopulations. A metapopulation is a collection of small populations occupying a number of habitat patches (Figure 3).

Figure 3. A collection of forest patches occupied by populations of a forest dwelling species. The weight of arrows indicates the rate of occasional exchange of individuals among populations (heavier arrows indicate more exchange). Note that the rate of exchange is correlated with how close patches are in space.

Individuals occasionally move between patches, and populations can go extinct in individual patches as a result of chance events. For example, the two males in a population of six females may be eaten by a predator. If no other males immigrate into the patch from other patches, the population in that patch will go extinct. Small populations are particularly susceptible to this type of chance extinction. However, because individuals occasional move between patches, the vacant patch will eventually be colonized and occupied again in the future (Figure 4). If colonization rates of vacant patches are higher than extinction rates, the metapopulation will persist. This is because while some patches are experiencing extinction others are being colonized. In effect, the movement of individuals among populations ties all of the populations into a metapopulation capable of persisting in the altered landscape.

Figure 4. Occupancy of collection of forest patches supporting a metapopulation of a forest dwelling species at two different time. Solid patches are occupied and open patches are not. Note that the occupancy of individuals patches changes through time (i.e. patches are colonized and populations in patches go extinct), but the number of occupied patches remains the same through time.

The problem for land planners is that the rates of movement of individuals among patches is determined by how close or far apart they are. Patches that are far from other patches will not exchange individuals with other patches, and the small population remaining in the patch will eventually go extinct. What this tells us is that as patches become more and more isolated from each other the colonization rates go down to the point that extinction rates are higher than colonization rates, and the whole metapopulation will go extinct. This will occur before all of the patches in a landscape are gone. The bottom line is that patches that are close to each other provide better habitat than isolated patches of the same size.

At the scale of the individual forest patch, several factors affect its value as plant and wildlife habitat. In general, larger patches support more species. This is because larger forest patches have more different kinds of habitats and support larger populations that are less vulnerable to chance extinction. Additionally, only larger patches are likely to contain enough habitat to support species like larger mammals that requiring larger areas.

Individual forest patches are also affected by their surrounding. At the forest edge, wind and sun light result in dryer conditions than are found in the interior of the forest patch. Forest edges are also more accessible to predators and parasites that may occur in adjacent fields or developed areas. For example, house cats that kill small birds are often more common in forest edges adjacent to residential developments. Cowbirds, which are nest parasites, are also more common in forests adjacent to the open fields where they feed. Cowbirds lay their eggs in the nests of other birds. The host birds will care for the cow bird eggs. When the eggs hatch, the larger cow bird nestlings will out compete the host nestlings for food, and may even push the host nestlings out of the nest.

Some species of birds, known as forest interior species, are not tolerant of the dryer conditions or the predators and parasites that occur at the forest edge. These species only occur in the core habitat of forest patches (Figure 5).

Figure 5. Graph showing the relationship between soil moisture and edge and core habitat in a forest patch. Many species of birds cannot tolerate conditions in the edge habitat and are only found in core habitat.

Summary-Why is fragmentation a problem?

The fragmentation of a species' habitat can threaten that species survival for a variety of reasons. These include:

Reduction of total habitat area. As a patch of habitat is cut into smaller and smaller pieces, there is less habitat available for a species. This reduced amount of habitat will support a correspondingly smaller population.

Vulnerability during dispersal to other patches. As a habitat becomes fragmented into many smaller patches, these patches become separated from one another by relatively inhospitable terrain. Any individual that attempts to cross between patches of habitat becomes temporarily vulnerable to predators, harsh environmental conditions, or simply to starvation. For example, salamanders may respond to a drying pond or to overpopulated environment by moving on to a new pond. As the amphibian moves across an open field, pavement, or a forest floor in search of a new patch of habitat, it runs the risk of being eaten, starvation, or dying in an unsuitable environment. These migrations become more difficult as fragmentation progresses.

Isolation of a population. Populations can become isolated within their patch when all of their surrounding patches of habitat are destroyed. This makes migration into different patches difficult and hazardous. Isolated populations are prone to decline due to inbreeding, chaotic swings in numbers due to overexploitation of their habitat, and sudden removal from the patch due to small disturbances.

Edge effects: As habitat becomes fragmented into smaller patches, more of the habitat will end up adjacent to a different type of habitat. Land that is deep in the middle of a forest is different than land that is at the edge of a forest- more light penetrates, and there is a different collection of species that prefer the edge of a forest to the core of a forest.

Edge effects- changes in microclimate. Many habitats have such an impact on the physical environment that they can create their own microclimate. For example, dense forests tend to be shadier, more humid, and less windy than adjacent unforested land. This pattern becomes more pronounced the farther you go away from the edge of the forest. If a patch of forest becomes small enough, then you will always be near the forest's edge. Some species require the solitude, deep shade and protection from wind that you can only find in the middle of a dense old-growth forest.

Edge effects-vulnerability to external competition and predation. If a prey species wanders too close to the edge of their protective habitat, they can be quickly captured and eaten. Other species may be excellent competitors deep within their own specialized habitat, but are less successful against species found at the edge of their habitat that take advantage of the wide variety of conditions found between two dissimilar habitats.

  

© CGIS at Towson University

This is some of what I found and I hope it helps you somewhat.