Implications of vegetation clearing biases for the interpretation of landscape-scale species-area relationships

Abstract

Vegetation clearing results in the loss of species from landscapes. Indeed, the area of remaining native vegetation is an important determinant of species richness in humanmodified mosaics. Of interest to ecologists and landscape managers is the effect of areathat is, how the number of species a landscape supports changes with the amount of native vegetation, as revealed by the shape and functional form of the species-area relationship. Understanding this is vital for guiding conservation interventions such as setting limits to vegetation clearing or establishing revegetation targets.Crucially though, it is not only vegetation area that affects patterns of species richness at the landscape level-so do environmental attributes such as soil properties and topography. Complicating the matter is the fact that these attributes tend to be correlated ii Second, I explored the extent to which the effect of native vegetation area on species richness differed in 100 km 2 landscapes categorised by attributes such as soil fertility, range in elevation or matrix land use. Using a case study of south-east Australian birds, Ifound that the shape of the species-area relationship varied substantially depending on whether landscapes were, for example, more-or less-topographically variable, or had higher or lower soil fertility. While threshold models depicting a point of sudden change in the effect of area emerged consistently, the amount of vegetation corresponding with observed thresholds differed considerably among landscape types. Therefore, aggregating and analysing species-area data from different landscape types is likely to misrepresent how species richness is affected by vegetation area. This will be exacerbated by clearing biases, because heavily cleared landscapes tend to be characterised by very different attributes to high cover landscapes. When data for the entire study extent were analysed, a remarkable degree of scaleinvariance was observed-namely, a threshold relationship with a change-point at approximately 30% vegetation cover. However, when data were analysed for two regional subsets of the overall dataset, the effect of vegetation area, and the factors moderating this relationship, were scale-dependent. Given this finding, observed thresholds can only reliably be used to guide landscape management at the scale and in the region where the relationship was observed.Finally, I evaluated the implications of accounting for clearing biases when using speciesarea relationships to guide conservation, focussing on a region of Australia undergoing rapid landscape transformation. I found that using observed thresholds from species-area models that do and do not account for landscape attributes yielded different outcomes for landscape-scale species richness conservation, given a scenario of future vegetation loss.Specifically, the number and location of landscapes that could be prioritised for conservation actions varied considerably depending on the species-area model used.This research demonstrates that the eff...