Biogeographic patterns in small ground-dwelling vertebrates of the Western Australian wheatbelt

Abstract

Cround-dwelling frogs, reptiles and small mammals were sampled at 252 quad rats chosen to represent the geographical extent and diversity of uncleared terrestrial environments across the WestE'rn Australian wheatbelt. These sitE's were not overtlv affected by secondarv salinisation, but did include sites that were 'natu~ally' saline. We recorde~i a total of 144 species from 74 genera and 15 families. There was an average of 10.4 species per quadrat with a range from one to 19. Vertebrate species richness was highest on dissection valley floors and sandy depositional surfaces of the 'old plateau' but lowest on saltflats. Total species richness was positively correlated with high levels of sand, with low levels of soil nutrients and with good soil drainage. When frogs, reptiles and mammals were considered separately, temperature and rainfall attributes were also shown to exhibit correlations with species richness. Patterns in species composition could be explained in terms of climatic and substrate variables, including salinity. Two distinct faunas were identified-one concentrated in the semi-arid northern and inland parts of the study area, and one concentrated in the more mesic south and southeast. Further patterning could be discerned within this dichotomy, including the existence of a small group of species that arc associated with saline areas. Correlations between climatic and substrate variables could be discerned even at very low levels in the classification analysis, suggesting strong deterministic patterns in vertebrate species composition across the study area. Integrated management programs over entire catchments will be necessary in order to maintain conservation values, but there is doubt that the impacts ~)f salinisation and fragmentation can be mitigated quickly enough to allow the small, ground-dwelling vertebrate fauna to withstand the effects of these processes in the Western Australian wheatbelt.