RESEARCH ARTICLE Residuals cannot distinguish between ecological effects of habitat amount and fragmentation: implications for the debate Nicola Koper Æ Fiona K. A. Schmiegelow Æ Evelyn H. Merrill Received: 5 September 2006 / Accepted: 7 February 2007 / Published online: 30 March 2007 Ó Springer Science+Business Media B.V. 2007 Abstract Habitat amount and fragmentation usually covary in natural and simulated land- scapes. 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 fragmen- tation. 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 stron- ger 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. Keywords Canada Á Generalized additive models Á Grasslands Á Habitat amount Á Habitat fragmentation Á Residuals Á Songbirds Introduction Recent literature emphasizes the need to distin- guish between the effects of habitat loss and fragmentation (McGarigal and Cushman 2002; Fahrig 2003; Turner 2005). Habitat loss indicates a reduction in habitat amount, while habitat fragmentation describes the configuration of the remaining habitat. Ecological theory suggests that habitat amount and fragmentation have distinct effects on species. Habitat loss has strong and usually negative effects on populations, such as loss of individuals from the population (Schmie- gelow and Mo ¨ nkko ¨ nen 2002) and increasing isolation among populations inhabiting separate patches (Fahrig 2003), whereas habitat fragmen- tation may reduce connectivity (Schmiegelow and N. Koper (&) Natural Resources Institute, University of Manitoba, 70, Dysart Road, Winnipeg, MB, CanadaR3T 2N2 e-mail: koper@cc.umanitoba.ca F. K. A. Schmiegelow Á E. H. Merrill Department of Renewable Resources, University of Alberta, Edmonton, AB, CanadaT6G 2H1 123 Landscape Ecol (2007) 22:811–820 DOI 10.1007/s10980-007-9083-9