Resources, data resolution and small mammal range dynamics.

Abstract

Extensive range shift and mass extinctions resulting from climate change are predicted to impact all biodiversity on the basis of species distribution models of wide-spread and data-rich taxa (i.e. vascular plants, terrestrial invertebrates, birds). Cases that both support and contradict these predictions have been observed in empirical and modelling investigations that continue to under-represent small mammal species (Introduction). Given small mammals are primary or higher order consumers and often dispersal limited, incorporating resource gradients that define the fundamental niche may be vital for generating accurate estimates of range shift. This idea was investigated through the influence of coarse to fine resolution, landscape- and quadrat-scale data on the range dynamics of four temperate and five arid-zone small mammals. This investigation determined: i. Landscape-scale edaphic and biotic factors improved model fit, robustness and transferability for five species, especially arid species, via improved discrimination of unsuitable habitat (specificity) or suitable habitat for a wet-heath specialist (Rattus lutreolus lutreolus; Chapters 1 & 4). ii. Quadrat-scale biotic factors improved model fit for three species; a dense understorey preferring species (Isoodon obesulus obesulus), granivore (Notomys cervinus) and insectivore (Sminthopsis macroura; Chapters 2 & 4), iii. Coarse or fine resolution environmental data were more strongly correlated with the occurrence of different species across variables, reflecting the known ecology of these species (Chapters 2 & 4), and iv. Fine resolution environmental data directly affected the spatial representation of available habitat in a coupled niche-population model, resulting in smaller shifts being detected for a greater number of species (Chapter 3). Biotic interactions can drive adaptations that can lead to species becoming dependent on resource availability for survival or reproduction (Chapter 5). Complex ecosystem dynamics can make it difficult to distinguish resource partitioning caused by specialist adaptations (fundamental niche) from contemporary interactions (realised niche). In this investigation, evidence of biotic environmental variables defining the fundamental niche was provided by improved model transferability: representing direct (e.g. suitable habitat - R. l. lutreolus) or indirect influences on species’ occurrence (e.g. rainfall via food availability on I. o. obesulus, N. cervinus and Dasyuroides byrnei). In addition to better representing resources, fine-scale environmental data affected the spatial configuration of available habitat, leading to smaller estimates of range shift. Hence, it is vital to consider species-environment relationships and conceptualise direct or proximal variables in order to construct robust SDMs. Improving this practice will also identify key relationships that influence community dynamics and require further empirical research (Chapter 5).