PARENT SESSION
Organized Oral Session 10: Reducing habitat fragmentation by roads: A comparison of measures and scales
Organizer(s): JAG Jaeger, L Fahrig, and W Haber
Monday, August 8, 1:30 PM - 5:00 PM, Meeting Room 511 B, Level 5, Palais des congrès de Montréal

Patch-based monitoring to assess the effect of wildlife passages on the viability of metapopulations.

van der Grift, Edgar^*,1, Verboom, Jana¹, ¹ Alterra, Wageningen University and Research Center, Wageningen, Netherlands

ABSTRACT- It has been proven that wildlife crossing structures, such as badger pipes, amphibian tunnels or wildlife overpasses, are frequently used by a variety of species. Unclear yet, however, is if these defragmentation measures affect population viability. Transport corridors as well as accompanying mitigation measures affect populations in a complex way. Wildlife passages may improve reproductivity, reduce mortality and increase both immigration and emigration. Wildlife fences prevent mortality but increase, at the same time, the barrier effect of transport corridors, resulting in a decline in gene flow or a reduced recolonization probability. Considering these complex relations between mitigation measures and population dynamics, monitoring the effectiveness of defragmentation measures is not an easy task. Based on metapopulation theory we suggest a so-called patch-based monitoring to measure the effects of wildlife crossing structures at transport corridors on the survival of populations. In this method the presence or absence of a species is assessed in all spatially distinct habitat patches suitable for the species. Presence in a habitat patch is as important as absence, based on the characteristics of metapopulations that not all suitable patches are inhabited simultaneously at a certain moment in time and that over time populations become locally extinct and habitat patches become recolonized again. Survey results can be statistically compared with model predictions of the probability that a species occurs in each habitat patch, based on differences in patch size, isolation, and patch quality, as well as characteristics of the species itself such as dispersal capacity. In such predictive models the barrier effect of infrastructure as well as the defragmentation effect of wildlife crossing structures can be included. To prove an effect of defragmentation measures on population viability both study species and study sites should be carefully selected. Study species should, among others, be sensitive to both fragmentation impacts by transport corridors and defragmentation impacts by mitigation measures. Study sites can be best chosen at locations where defragmentation measures will result in a considerable shift in population viability. Surveys should preferably be conducted over many years.

Key words: population viability, wildlife passages, defragmentation, patch-based monitoring