Habitat Fragmentation

Finally, a theoretical paper on dispersal gives more insight into fragmentation and dispersal: Doak et al. (1992) find that "very few studies have addressed the mechanisms by which fragmentation will influence population dynamics or, in particular, the ways in which spatial scale mediate these effects." They argue that scaling, by which they mean whether the fragments are random at all scales, clustered on a ³supercell² scale, or clustered at the quadrat scale, has a major impact on how fragmentation and dispersal interact. **fig. 1, Doak etal** Based on a model for the search time required to find a fragment and settle (their paper isn't explicit on how the "searchers" did their searching, but one can imagine the disk equation applying here), the assessed the impact of different patch distributions on the success of the searchers.

Peacock and Smith (1997) used DNA fingerprinting to compare population structures of pika in fragmented and unfragmented populations. Their key finding is that in unfragmented populations mating is non-random, perhaps fitting an optimal outbreeding model while in the fragmented population mating is random with respect to relatedness, an "incidental effect of dispersal in a fragmented habitat." They interpret dispersal patterns in this species as being driven by competition for resources in both types of habitat and found no difference in dispersal patterns between habitats. This means that, interestingly, the effect of fragmentation is to disrupt the system of mate choice. They go on to hypothesize that predation risk (refer back to: risk-aversion) in moving between fragments inhibits such movements and is therefore the disrupting factor.

Doak, D. F., P. C. Marino, and P. M. Kareiva 1992. Spatial scale mediates the influence of habitat fragmentation on dispersal success: Implications for conservation. Theor. Pop. Biol. 41:315-336.This paper gives a useful list of the studies published before 1992 on fragmentation.

Peacock, M. M. and A. T. Smith. 1997. The effect of habitat fragmentation on dispersal patterns, mating behavior, and genetic variation in a pika, (Ochotona princeps) metapopulation. Oecologia 112:324-332.

page 15-*
copyright ©2001 Michael D. Breed, all rights reserved