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Assessment of Indirect Pesticide Effects on Worm-Eating Warbler Populations in a Managed Forest Ecosystem
Citation:
AWKERMAN, J. A., M. R. MARSHALL, S. RAIMONDO, AND R. J. COOPER. Assessment of Indirect Pesticide Effects on Worm-Eating Warbler Populations in a Managed Forest Ecosystem. ENVIRONMENTAL TOXICOLOGY AND CHEMISTRY. Society of Environmental Toxicology and Chemistry, Pensacola, FL, 30(8):1843-1851, (2011).
Impact/Purpose:
Two-patch matrix model developed from field data to predict population-level changes of songbirds resulting from reduced resource availability in treated patches.
Description:
Insecticides that do not cause direct mortality in wildlife species can still cause indirect effects by reducing prey availability for insectivores. Reduced resources for songbirds can result in a lower reproductive rate or poor nestling condition at fledging. While these effects may appear negligible at the individual level, few studies have examined the consequences at the population level. We use a long-term dataset from a forest ecosystem where Bacillus thuringiensis kurstaki (Btk) was applied to control gypsy moth (Lymantria dispar L.). Altered nestling feeding behavior by adult worm-eating warblers (Helmitheros vermivorus) leading to lower nestling mass at fledgling was documented in Btk treated plots through video surveillance of feeding trips and weighing of nestlings. Observed productivity on Btk plots was also lower following treatment resulting in an intrinsic growth rate < 1, but growth rates over a larger spatial scale and longer term were calculated from vital rates of treated and untreated plots and suggested that overall effects of treatment were minimal. Simulations of reduced juvenile survival expected as a result of lower nestling mass following treatment further reduced intrinsic growth rate. We developed a two-patch matrix model from field data to predict population-level changes resulting from reduced resource availability in treated patches. At the multi-site level, overall population growth was most influenced by dynamics within the control area, which could counteract reductions in reproductive success and juvenile survival on treated plots. We also simulated population dynamics with different proportions of treatment areas, which could inform management strategies in similar systems. Comparison of organismal, local, and multi-site results in this study confirms that changes in vital rates should be examined at the spatial and temporal scale appropriate for risk assessment of pesticide effects on a wildlife population.