Habitat heterogeneity : how it is generated and drives spatial patterns in mammals and birds

Abstract

Heterogeneity (variation) in vegetation structure is an inherent feature of all terrestrial ecosystems, characterises their structure, and influences the diversity and distribution of biota. Understanding how both the quantity and heterogeneity of a resource shapes the structure and dynamics of ecosystems is of fundamental importance in ecology. The concept of heterogeneity underpins some of the major theories in ecology, including species coexistence theory, source-sink dynamics, fractal theory, and invasion theory. However, our understanding of the spatial distributions of plants and animals is typically underpinned by measures of abundance of the physical environmental (or habitat structural attributes), such as percentage cover, rather than measures of habitat heterogeneity. Here, I investigated how heterogeneity (variation) and the amount (cover) of habitat features drive spatial distributions in biota, and how spatial heterogeneity in habitat features is generated. First, I tested how heterogeneity of multiple habitat features at different spatial scales is generated by topography and fire components. Second, I tested the effects of both absolute cover and habitat heterogeneity on the spatial distribution of different taxa (birds and mammals). Last, I tested the relationship between heterogeneity of vegetation cover and the absolute amount of vegetation cover for binomial data. I used a combination of landscape mapping, vegetation surveys, bird point counts and camera trapping within a naturally highly heterogeneous landscape to investigate how cover and heterogeneity of habitat feature influenced biota and how it is generated. I found: 1) that heterogeneity of different habitat features is influenced by a variety of different fire and terrain attributes and their interactions, and that their effects can differ depending on the vegetation type and the scale at which variation is measured, 2) that fine-scale heterogeneity can have quite varied effects on biota, depending on the species, life-history traits and community of interest, 3) that both vegetation cover and vegetation heterogeneity contributed to the observed spatial distribution of mammals and birds, and 4) empirical evidence to support our hypothesis that vegetation heterogeneity peaks at intermediate levels of cover, which is consistent with the mean-variance relationship for binomial data My findings highlighted that landscape ecologists should use a combination of environmental factors as surrogate measures of habitat heterogeneity if they are to develop robust predictive models that accurately describe multiple aspects of faunal habitat. Further, my results suggest that small-scale heterogeneity in terrestrial environments is as important as broad-scale heterogeneity in generating spatial patterns in biota. Different communities and species may be adapted to tolerate different degrees of fine-scale heterogeneity - some may prefer highly heterogeneous environments while others would prefer highly homogenous environments. I found strong quantitative support for the generality of a relationship between vegetation cover and heterogeneity which is potentially transferrable to other studies of ecosystem structure, and the distribution of associated biota. This relationship is likely to have important ramifications for understanding the mechanisms driving both quantity and spatial variation of vegetation and habitat, and the theoretical conceptualisation of ecosystem structure and function, as well as how we measure ecosystems to guide their management.