RESEARCH:  Habitat Fragmentation

The effects of fragmentation on wildlife have been well described for many taxa. However, despite the large body of work on fragmentation, the mechanisms underlying species loss post-fragmentation are still largely unknown. For example, what role does the urban or agricultural matrix that the habitat remnants are nested in influence species composition in the remaining habitat? How does fragmentation facilitate the penetration of exotic species? How do stochastic and deterministic processes interact to influence overall extinction dynamics?

My own work thus far has focused on bird and ant communities because they tend to be relatively species rich and easily censused. Avian communities, for example, provide an ideal system to examine the dynamic responses of species to habitat fragmentation. Because of their mobility, birds can be used to assess turnover rates and extinction/colonization dynamics in fragmented systems. In the late 1980s, the loss of scrub specialist birds was investigated in scrub habitats nested in the urban matrix of San Diego. In collaboration with Kevin Crooks (University of Wisconsin, Madison) and Doug Bolger (Dartmouth University), I have recently revisited this system in an attempt to examine the dynamics of local extinction. Rather than taking a static approach consisting of a single snapshot of presence/absence, we are repeatedly censusing these same fragments over time to estimate species-specific extinction and colonization rates in this system. Our preliminary results suggest that bird communities in urban landscapes are shaped by the differential responses of individual species to development and habitat fragmentation. I am particularly interested in determining why some species persist in disturbed systems whereas other, close-related species do not.

Fragmentation may promote the success of some species, particularly invasive species that do well in disturbed areas and native species that use edge habitats. Our work with ant communities in scrub fragments in southern California suggest that the penetration of exotic species from the surrounding urban matrix is deterministically causing reduction in native ant species diversity and abundance more than stochastic processes typically invoked for species loss post fragmentation. In contrast, Indigo Buntings in southern Illinois forest fragments provide an example of how native species attracted to fragmented landscapes may be negatively affected. Indigo Buntings nest densely along abrupt edges of forest fragments in agricultural landscapes in the Shawnee National Forest. However, nesting success along these edges can be much lower (due to an increase in nest predation rates and parasitism by Brown-headed Cowbirds) than along gradual edges associated with streams, tree fall gaps and smaller wildlife openings. Abrupt edges may be "ecological traps" for disturbance-dependent species, because they are attracted to them, yet suffer higher mortality due to increased predator abundance.

• Mechanisms of success in biological invasions
• Impacts of biological invasions
• Patterns and mechanisms of nestmate recognition in social insects