PARENT SESSION
Oral Session #64: Avian Ecology. Presiding: P. Warren.
Thursday, August 9, 2001. 8:00 AM to 12:00 PM. Hall of Ideas I.
Population trends in North American songbirds: Interactions between habitat fragmentation and landscape occupancy patterns.
Donovan, Therese1, Flather, Curtis2, 1 2
ABSTRACT- Fragmentation of breeding habitat has been hypothesized as a cause of population declines in forest-nesting migratory birds. However, in spite of reduced fecundity and densities in fragmented systems, many species of forest-nesting passerines have increased in numbers over time. We hypothesized that range-wide population change in species whose reproductive success is negatively affected by habitat fragmentation should depend on the proportion of the population that actually occupies fragmented landscapes. We predicted that fragmentation-sensitive species should increase globally in numbers at a greater rate than species that readily occupy fragmented landscapes because fragmentation-sensitive distributions place a large proportion of the global population in contiguous landscapes that are superior for breeding. We used Breeding Bird Survey data (BBS) and associated landscape metrics to test this prediction for 10 species of forest-nesting passerines in the United States that experience reproductive dysfunction associated with habitat fragmentation. Our approach was to: (1) quantify landscape features associated with BBS routes across the eastern United States, (2) classify the landscapes around BBS routes as "fragmented" or "contiguous", (3) estimate the proportion of detected individuals that occurred in fragmented landscapes on a species-by-species basis, and then (4) associate 10-year trends for each species with the proportion of breeding individuals occupying fragmented landscapes. Regression analysis indicated that there was a significant, negative relationship between proportion of the breeding population occupying fragmented landscapes and population trend from 1970-1980. Although this result links habitat fragmentation to population change, and hence provides support for the fragmentation hypothesis, other factors (e.g., land-use change, weather, varying life history traits, varying winter survivorship, habitat amount thresholds) could generate similar results. More work is needed to partition the relative influence of these factors on regional bird population dynamics if conservationists are to more clearly understand the effects of fragmentation on the distribution and abundance of species across their range.
KEY WORDS: habitat fragmentation, Breeding Bird Survey, habitat selection, area sensitivity