Document: LEN-3-8-1

Effect of habitat fragmentation on the extinction threshold: a synthesis.

FAHRIG, L.*

Ottawa-Carleton Institute of Biology, Carleton University, Ottawa, ON K1S 5B6, Canada ¹

Abstract:
I review and reconcile 4 models that predict the effect of habitat fragmentation on the population extinction threshold, and I compare these predictions to empirical studies. All 4 models predict that habitat fragmentation can increase the extinction threshold such that more habitat is required for persistence in more fragmented landscapes. However, empirical studies predict positive and negative effects of habitat fragmentation, with about equal frequency, indicating that the models lack some important process(es). Two of the models ("CE" models) predict that fragmentation can increase the extinction threshold by up to 60-80%. The other 2 models ("BIDE" models) predict much smaller potential effects of fragmentation on the extinction threshold, of no more than a 10-20% shift. This difference in the model predictions has important implications for conservation. If fragmentation has a large effect on the extinction threshold then alteration of habitat pattern (independent of habitat amount) is an effective tool for conservation. This is particularly significant in forestry where many hope that judicious planning of the spatial pattern of harvests can compensate for loss of forest. On the other hand, if the effects of fragmentation on the extinction threshold are small, then this is a limited option. I suggest that the difference in model predictions results from differences in the mechanisms by which the models assume the extinction threshold occurs. In the CE models the threshold occurs by a reduction in colonization rate with decreasing habitat amount. In the BIDE models, the threshold occurs by an increase in dispersal mortality with decreasing habitat amount, which indirectly reduces colonization rate. When dispersal mortality is included in a model, it imposes a constraint on the potential for habitat fragmentation to mitigate effects of habitat loss. Empirical studies to date of the independent effects of habitat loss and fragmentation suggest that habitat loss has a much larger effect than habitat fragmentation on distribution and abundance of birds, supporting the BIDE model prediction, at least for these species.

Keywords: extinction, population persistence, threshold, habitat loss, habitat fragmentation, spatial models, dispersal mortality

This abstract is being presented at: 1:15 PM in session:
Symposium # 18: The Role of Theoretical Ecology in Biodiversity Conservation and Management.