Dynamics and impacts of fragmentation

The first thing to realize about fragmentation is that there are some important differences between distributions that are naturally patchy and those that become patchy because of habitat fragmentation. Groom et al. [3, p. 219] mention four in particular:

1. Fragmentation reduces the extent and connectivity of remaining habitats, and species may or may not be able to persist as a result of those changes.

2. Natural patches have a rich internal patch structure versus relatively a simple, uniform patch structure in patches associated with human alteration of the environment.

3. Natural patches typically have a smaller contrast between interior and exterior of patch in natural patches (largely because of 2).

4. The matrix in which anthropogenically produced fragments are embedded may harbor activities and produce byproducts that are a direct threat to the patches that remain, while the threats from patch exteriors to natural fragments is less severe.

Another, less obvious, consequence of fragmentation is that typically the size of fragments becomes increasingly skewed as habitat destruction proceeds. Even if the total area of relatively undisturbed habitat remains fairly large, the size of remaining fragments may be quite small.

And remember what we said in one of the earliest lectures in this course. The number of species found in any place is related to its geographical area. Smaller fragments will contain a smaller number of species. Moreover, there are some ``area-sensitive'' species that require large areas to persist. They will be lost from all fragments. Consider the red-eyed vireo or the scarlet tanager, for example (Figure 2.) For either species fragments must be on the order of 10ha for a breeding pair to be found, and fragments must be on the order of 30-100ha to have a 90% chance of being occupied.

Figure 2: Relationship between size of forest fragment and probability of occupancy in red-eyed vireo and scarlet tanager (Figure 7.6 in [3]). Notice that the x-axis is on a logarithmic scale.

Wilcove et al. [10] use observations like these to predict how species diversity changes as a function of habitat loss and fragmentation(Figure 3). There are two very important facts to note in this figure:

Figure 3: Loss of species diversity for species with large area requirements and low vagility (closed circles) and for species with less stringent area requirements and greater vagility (open circles).

1. Loss of species doesn't occur until a significant fraction of the habitat has been lost.

2. Once species begin to be lost from the system, they are lost very rapidly (see 3).

Halting fragmentation is always worthwhile if it is feasible. Each additional increment of habitat fragmentation leads to loss of more species than did the last. Moreover, Seabloom et al. [8] illustrate that realistic scenarios of habitat loss, i.e., those in which habitat is aggregated into certain regions rather than spread randomly across a landscape, lead to even faster rates of species loss.