Habitat Fragmentation

Many times, natural habitats show a “patchy” distribution.

This affects the organisms that live there.

However, in today’s world the effect of anthropogenic habitat fragmentation is probably much more significant.

Equally significant is the fact that many of the organisms in these habitats are not “adapted” for such fragmentation.

Activities such as “clearcutting” have created a mosaic of forested and unforested areas in many regions that were once completely covered with forests.

With the growth of human population and the increasing removal of natural habitat, the remaining wild areas begin to take the form of “habitat islands” surrounded by relatively uninhabitable areas.

This habitat fragmentation is an increasingly serious problem in biological conservation.

The most noteworthy effects of the fragmentation of natural habitats are:

1. The formation of isolated patches of habitat.

2. The increasing significance of edge effects.

Since the remaining habitat begins to resemble an island, the ideas of island biogeography theory are applied to them.

On small islands, the number of species results primarily from the interaction of two processes:

Colonization

Extinction

The point at which these two rates are in equilibrium will determine the number of species found on the island.

In theory, the colonization rate will start out high and decline, since there the initial number of species is low (or none).

The extinction rate will start out low (for the same reason) and rise.

Eventually, the two will reach an equilibrium.

Colonization and extinction rates vary with a number of factors.

Among the most important are the size of the island and its distance from the species pool (the “mainland”).

So, what happens if we form an “island” from an area that was once part of a larger habitat.

It will, initially, probably contain more species than the equilibrium of colonization and extinction can support.

This, in theory, would lead to biotic relaxation.

Biotic relaxation is simply a decline in the number of species when a formerly “connected” region becomes isolated as it approaches a new equilibrium.

The effect of area and isolation on the number of species can be examined using species-area curves. These curves plot the number of species versus the size of the area examined. Typically, both are plotted on a log scale. The result is usually a straight line, the slope of which is an indication of how fast the number of species changes with increasing area.

Typically, species-area curves for islands have steeper slopes than those for non-isolated areas.

What does this mean?

It means that, when we reduce the size of a habitat we can expect to lose species.

In habitat islands, the relatively high rate of colonization and extinction result in a rapid “turnover” of species.

In other words, although the number of species may remain relatively constant, the actual species that are present may change.

National Parks are habitat islands, and often show biotic relaxation. This is often most pronounced in the smaller parks. Mount Rainier National Park in Washington has seen a reduction in the number of mammals found there from 68 to 37 species.

Some species seem to be “area-sensitive”, meaning that they are at a high risk of extinction if the habitat reaches some critical minimum level.

A study by Robbins et. Al (1989) identified a number of such species of birds, including pileated woodpeckers, white-breasted nuthatches, and scarlet tanagers.

Habitat fragmentation may create a metapopulation, a group of subpopulations varying in size.

Some of the smaller of these will tend to go extinct often (sink populations), and are only repopulated by individuals dispersing from larger subpopulations (source populations)

In such a situation, the survival of the species in a region depends on the dynamics of the subpopulations.

If too many populations are small, and become sinks, then the whole population will move toward extinction.

The study of these dynamics has introduced modeling into conservation biology.

Spatially explicity models combine population models with landscape maps to make mathematical predictions.

The creation of habitat patches from formerly continuous woodlands modifies the microclimate of the forest, resulting in increased edge effects.

Environmental conditions around edges are different from those in the interior. This results in differences in the types of trees, the understory, and even the animals.

One effect is the ability of predators from open habitats to penetrate forest edges and prey on the animals there.

Striped skunk

Common crow

Blue jay

The brown-headed cowbird is a nest parasite that frequents edge habitats.

A study in California looked at the number of chaparral bird species in isolated canyons in an urban setting. The number of bird species declined as the size of the canyon decreased.

Interesting, canyons visited by coyotes had more bird species than those that were not. The coyotes apparently helped control the abundance of bird predators, like skunks and domestic cats.

How strong is the island effect for different types of organisms?

Study in Western Australia:

Slope for birds: 0.18

Slope for lizards: 0.25

Slope for non-flying mammals: 0.39

Many mammals were extinct on the smaller preserves.

Bandicoot

Estimates are that the Mkomazi Game Reserve in Tanzania would lose 17 of its 39 species of large mammals in the next 300 years if it is separated from surrounding reserves.

Species-area curves can be used to predict how large a reserve must be to preserve its biological diversity.

For the Australian wheatbelt region, estimates are that a reserve of 43,000 hectares would be necessary to preserve all 25 species, and a reserve ¾ that large to preserve 90% of them.

The largest current preserve is 5119 hectares.

What are the problems with estimates like these?

Based on work initiated by Terborgh, 5000 square kilometers has been adopted as a rough minimum size for major tropical forest preserve in the Amazon Basin. Based on very speculative reasoning, it is thought that this might reduce extinction rates to less than 1%.

Biological Dynamics of Forest Fragments Project

Near Manaus, Brazil, forest patches have been established:

1, 10, 100, 1000, and 10 000 hectares.

Censused prior to isolation, and will be studied for at least 20 years.

So, what does it all mean?

Expect to have to address that, in your own thoughts and words.