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Effects of forest fragmentation on populations of the marbled murrelet



Effects of forest fragmentation on populations of the marbled murrelet



Studies in Avian Biology 25: 221-235



The Marbled Murrelet (Brachyramphus marmoratus) is a threatened seabird that nests on branches of large trees within older coniferous forest in coastal areas of the Pacific Northwest. Surveys suggest that murrelets often nest in continuous stands of mature, complexly structured forest but they also nest in younger forest and in stands varying in size from several to thousands of hectares. We examined how murrelet abundance and reproduction are related to the amount and pattern of nesting habitat at regional, watershed, landscape, and nest site scales. At the regional scale, abundance of murrelets, estimated from offshore surveys, was found to be correlated with amount of nesting habitat in some areas and to a lesser extent with fragmentation of that habitat. We found a similar pattern at the watershed scale. At the scale of nest sites and surrounding landscapes, fragmentation may have greater effect on likelihood of nesting and nest success. Observations of active nests from other studies indicated high failure rates (47 of 71 nests with known outcomes), mostly due to predation (33 of 47 nests). Corvids have been implicated as primary predators. Forest fragmentation can affect the abundance and distribution of corvids, and thus it is possible that fragmentation might lead to higher rates of predation on murrelet nests. Over the past 5 years we have tested this assumption in Washington using artificial nests located in stands of varying structural complexity, levels of fragmentation, and proximity to human activity. Results indicate, first, that a broad suite of predators, including at least 10 mammalian and avian species, prey on simulated eggs and chicks. Second, rates of predation are higher within 50 m of forest edge, but this relationship varies with proximity to human activity and with the structure of the adjacent regenerating forest. Predation increased with proximity to forest edges when the matrix contained human settlements and recreation areas, but not when it was dominated by regenerating forests. Abundance of some predators (e.g., Steller's Jays, Cyanocitta stelleri) was greater in more fragmented landscapes, but abundance of other potential predators (e.g., Gray Jays, Perisoreus canadensis) was greater in continuous forests, making generalizations about the effects of fragmentation difficult. Research is needed to understand how fragmentation affects both murrelet nest site selection and the risk of nest predation so that managers can provide landscapes able to support large populations of successfully breeding murrelets.

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Related references

Marbled Murrelet Nest Predation Risk in Managed Forest Landscapes: Dynamic Fragmentation Effects at Multiple Scales. Ecological Applications 19(5): 1274-1287, 2009

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