Analysis of landscape sources and sinks: the effect of spatial pattern on avian demography

Abstract

To develop theoretical insights into the relationship between spatial pattern and demography, we coupled a spatially structured demographic model with neutral landscape models to investigate how landscape structure affected population persistence and the source–sink potential of landscapes for a generalized, territorial migratory songbird. Four species-types, with different sensitivities to habitat area and edge effects, were simulated on replicated landscapes across a range of habitat abundance (1–90%) and fragmentation or spatial contagion (random, fractal with minimal contagion, and fractal with maximum contagion). For each species-type in each landscape, the expected number of female offspring produced per female (fecundity, b) was modeled as an explicit function of habitat area and spatial structure (patch edge-to-area ratio). Fecundity estimates (b) were combined with survivorship in a life-table analysis to estimate the net lifetime reproductive output (R0) for the population of each landscape. Landscapes for which R0<1 were identified as population sinks, and as potential population sources when R0>1. As expected, reproductive output (R0) was generally highest on fractal landscapes with maximum clumping (minimum fragmentation) and lowest on random landscapes (maximum fragmentation), especially for species with high edge sensitivity. For species with low edge sensitivity, population persistence was unlikely when the landscape had <40% habitat (i.e. R0<1.0 in these landscapes and were population sinks). Thus, thresholds in population persistence could be identified for different species in these landscapes, but the level of habitat required for persistence ranged widely from 5 to 90% depending upon the species’ response to habitat fragmentation (edge sensitivity) and the specific landscape pattern. Our results caution against the adoption of general rules or ‘cookbook prescriptions’ in conservation regarding the amount of habitat required for population persistence (e.g. the ‘20% rule’). Landscape configuration (the ‘details of how habitat is arranged’) can mitigate the effects of habitat loss and enhance population persistence in fragmented landscapes.