Landscape connectivity and population distributions in heterogeneous environments

Landscape connectivity refers to the functional relationship among habitat patches, owing to the spatial contagion of habitat and the movement responses of organisms to landscape structure. Heterogeneous landscapes provide a particular challenge for modelling population-level responses to habitat fragmentation, because individuals may be utilizing multiple habitats to varying degrees across the landscape. We apply neutral landscape models to understand how species' habitat affinities interacted with landscape structure (i.e., habitat abundance, distribution, and quality as measured by carrying capacity) to affect the redistribution of individuals. Two types of neutral models are presented: random maps, in which the distribution of habitat is spatially independent and fractal maps, in which habitat exhibits an intermediate level of spatial dependence. The neutral landscapes comprised varying proportions of three habitat types, for which species exhibited a preference gradient (high, medium, low). We performed a series of simulation experiments as a factorial design of parameter states to tease apart the underlying factors responsible for population distributional patterns (random vs clumped) in spatially complex mosaics. Landscape connectivity is a threshold phenomenon, in which even a minimal loss of habitat near the critical threshold (p(c)) is likely to disconnect the landscape, and which may have consequences for population distributions. The exact value of p(c) depends upon the spatial arrangement of habitat; fractal landscapes exhibited connectivity across a greater range of habitat abundance (p) than random maps (fractal p(c) = 0.29-0.50, random p(c) = 0.59). Although the spatial arrangement of habitat (random vs fractal) was the most important determinant of population distributional patterns, different landscape factors were important in structuring populations in the two types of maps. The relative abundance of habitat had the greatest effect on populations in random landscapes, whereas scale-dependent patterns were evident in fractal landscapes. At fine scales, population dispersion was determined by habitat abundance in both random and fractal maps, although populations were more aggregated (as measured by Morisita's Index, I-m) at this scale in random landscapes. But at coarse scales on fractal maps, population distribution was primarily influenced by species' habitat affinities. Assessment of the independent effects of habitat affinity and habitat carrying capacity on population distributions revealed that the differential interaction of species with landscape structure (i.e., different residence probabilities in each habitat type) was the primary determinant of distributional patterns. Neutral landscape models thus provide a useful tool for determining the relative importance of various components of landscape structure that affect population distributions.