Residuals cannot distinguish between ecological effects of habitat amount and fragmentation: implications for the debate |
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Authors: | Nicola Koper Fiona K A Schmiegelow Evelyn H Merrill |
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Institution: | (1) Natural Resources Institute, University of Manitoba, 70, Dysart Road, Winnipeg, MB, Canada, R3T 2N2;(2) Department of Renewable Resources, University of Alberta, Edmonton, AB, Canada, T6G 2H1 |
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Abstract: | Habitat amount and fragmentation usually covary in natural and simulated landscapes. A common way of distinguishing between
their effects is to take the residuals of the fragmentation index or indices regressed on habitat amount, as the index of
habitat fragmentation. We used data on prairie songbird relative abundances from southern Alberta, Canada to compare this
approach with the reverse: taking the residuals of habitat amount regressed on habitat fragmentation as the index of habitat
amount. We used generalized additive models (GAMs) to derive residuals, and modeled relative abundances using linear mixed-effects
models. The modeling approach used strongly influenced the statistical results. Using residuals as an index of fragmentation
resulted in an apparently stronger effect of habitat amount relative to habitat fragmentation. In contrast, habitat fragmentation
appeared more influential than habitat amount when residuals were used as an index of habitat amount. Regression of residuals
may eliminate statistical collinearity, but cannot distinguish between the ecological effects of habitat amount and fragmentation.
Habitat fragmentation may therefore have a larger effect on species than previously studies have shown, but experimental manipulations
of underlying mechanisms are ultimately required to address this debate. |
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Keywords: | Canada Generalized additive models Grasslands Habitat amount Habitat fragmentation Residuals Songbirds |
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