Patterns in the biodiversity of terrestrial environments in the Western Australian wheatbelt

Three hundred and four quadrats each of ca. 1 ha were chosen to represent the geographical extent and diversity of uncleared terrestrial environments in a 205000 km2 area knovvn as the Western Australian agricultural zone. This study area occupies 75°/) of south-western Australia, one of the world's 25 bodiversity hotspots under threat. Seventy-four percent of the study area has been cleared for agriculture and 4% is now salinised. Its subdued landscape comprises isolated remnants of bushland embedded in a mosaic of wheatfields. Six phylogenetIc groups were sampled, and a total of 2071 plant and 807 small ground-dwelling animal species were recorded from the quad rats. An average of 3.1 species per quad rat were recorded from only a single quadrat, but they were not randomly arrayed. The richest assemblages of singletons were around the periphery of the study area, where biogeographical edgeeffects would be expected. A total of 1922 species remained after the singletons were removed from the data set, an average of 71.1 per quadrat (s.d. = 22.8). They comprised 1335 472 66 reptIles, 21 frogs, 11 mammals and 17 scorpions. Seventeen assemblages were distmgUlshed when the 1922 species were classified according to similarities in their presence or absence at the 304 quadrats. Each assemblage could be characterised in terms of the Australiawide habitat preferences (where known) of its component species. Also, separate quadrat similarity matrices were generated for the four main types of organism sampled (plants, spiders, scorpions and herpetofauna) and for 100 random matrices. These were output as linear similarity vectors so that the differences in their biodiversity patterns could be quantified as a single matrix of correlatIon coefficients. Analvses revealed that: 1. Ce~graphical patterns in assemblage species composition were correlated with processes operating at both biogeographical and local (ecological) scales: the compositional structure of each assemblage was related to a different set of climatic plus soil and/or landform attributes. Since these attribute-sets were consistent with the assemblages' extrinsic characterisatIons, they are unlikely to be artefacts of quad rat positIoning or studv area extent and were treated as realised environmental niches. 2. Broad 'biogeographical patterns in biodiversity showed strong relatIonships with temperature and rainfall gradients, especially 'warmest period mean temperature' and 'precipitatIon seasonality'. 3. The centres of diversity of the 17 assemblages revealed by our analysis were found to cross various biogeographical boundaries currently recognised in the study area, both regional and sub-regional. Because they provide a view of geography from the perspectIve of a wide range of organisms, these' centres' provide a tactical framework for managing the effect of salinitv on biodiversltv. 4. The ecologic;lly different 'clades (plants, spiders, scorpions and herpetofauna) each had a distInct influence on the biodiversity model; cross-taxon congruence levels were low. If a bio-regionalisation based on only one or two types of organism was used as a framework for conservation activities in the study area it would produce distorted outcomes. 5 There was an inverse relationship between richness and soil salinity for the eight assemblages that occur on, or extend onto, landforms affected rismg salme ground-water. Six assemblages are dependent on surface-types low in the landscape, and their uncleared remnants need to be assessed against hydrological models so that priority land parcels can be identified. CollectIvely, these six assemblages encompass populations of 50.1% of the 1922 included in our analysis. The compositIonal loss associated WIth secondary salll1isation affects all biodiversity c()mponents surveyed: plants, vertebrates, spiders and scorpions. DOI: 10.18195/issn.0313-122x.67. 2004.293-335