Dispersal rates, extinction risk, and metapopulation dynamics of Bobolinks
In the Midwest, the widespread conversion of land to agriculture has virtually eliminated native grassland habitat, while intensification of farming practices has altered the quality of agriculturally derived grasslands, such as hayfields and pastures. The Bobolink (Dolichonyx oryzivorus) is a grassland-dependent bird that has experienced population declines throughout its range because of decreases in habitat quantity and quality. Restriction of grassland-dependent birds to discrete habitat patches in a largely agricultural matrix creates a collection of spatially divided populations. The current paradigm for describing the dynamics of species occupying spatially discrete habitat patches is the metapopulation model. To classify population dynamics and determine whether a metapopulation model applies requires estimates of both dispersal and extinction probabilities. The purpose of this study was to quantify Bobolink breeding dispersal patterns and extinction probabilities, and to use these estimates to determine whether populations occupying a fragmented agricultural landscape behaved as a metapopulation. I banded 201 adult Bobolinks and used a multistate model in program MARK to obtain parameter estimates and to test hypotheses about sources of variation in movement probabilities. I performed a population viability analysis using program RAMAS Metapop to estimate local and metapopulation extinction risk, assess the sensitivity of the model to changes in parameter values, and determine the risk of extinction due to mid-season haying. Ten males dispersed up to 14.2 km. The most parsimonious model was one in which dispersal probability (0.24) was constant among populations. Local turnover of banded individuals with unbanded individuals indicated that populations were connected to other populations beyond the study area. Under the baseline scenario, metapopulation quasi-extinction risk was 23%. Local extinction probability was 4–6%, indicating that populations were “midlands” and vulnerable to extinction. Medium-sized, well-connected populations constitute a patchy metapopulation. Mid-season haying increased metapopulation extinction risk to 100%. Increasing fecundity by ensuring that populations are not disturbed during peak breeding season may ensure long-term persistence. Future studies and management practices should occur at the scale of the county or larger.
Dunning, Purdue University.
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