PARENT SESSION
Oral Session 18: Ecological Theory I: Modeling.
Presiding: R Zabel
Monday, August 2, 8:00 AM to 11:30 AM, Meeting Room F 152.

Habitat fragmentation reduces herbivore abundance by restricting access to spatial heterogeneity in resources.

Hobbs, N. Thompson ^*,1, Tavener, Simon¹, Adair, Carol¹, ¹ Colorado State University, Fort Collins, CO, USA

ABSTRACT- Habitat loss occurs when the area of a landscape suitable for a species becomes unsuitable as a result of natural disturbance, community succession, or human-caused shifts in land use and land cover. By contrast, the modern view of fragmentation refers to changes in relative isolation of habitats distinct from changes in their area. One of the most important, unresolved questions in contemporary studies of fragmentation asks: "How does habitat fragmentation influence the abundance of organisms apart from the effects of habitat loss?" There is evidence that effect of fragmentation can amplify the effect of habitat loss; that is, isolated patches of habitat may support smaller populations than patches of the same size that are not isolated. The amplifying effect of fragmentation on habitat loss can be portrayed mathematically as follows. Imagine an intact landscape with a given area of habitat (= A₀), a landscape capable of supporting a population of N_0* individual organisms at equilibrium. We now divide that habitat into j=1...J isolated fragments, each with area = A_j. We assume fragmentation occurs with no change in total habitat area (i.e., A₀ = A₁ + A₂ +...+ A_J ). At equilibrium, each fragment supports N_j* individuals. If fragmentation adds to the effect of habitat loss then it must be true that N₀ > _j=1...J N_j. (1) We develop a simple model of interactions between herbivores and their food resources in spatially heterogeneous environments and use the model to show analytically that equation (1) will be true under a broad range of conditions. We use simulation to illustrate that fragmentation amplifies the effect of density dependence on growth rates of herbivore populations by limiting their access to heterogeneity in resources, resulting in lower equilibrium densities in fragmented habitats relative to intact habitats of the same area. Our model also provides a mechanism explaining non-linear relationships between per capita population growth rate and population size.

Key words: herbivore, fragmentation, population dynamics