Distribution of the native earthworm fauna of the Perth metropolitan sector of the Swan Coastal Plain

Abstract

Assessment of areas suitable for inclusion in a comprehensive, adequate and representative (CAR) reserve system has been based primarily on distribution of original native vegetation and occurrence of vertebrates, particularly birds and mammals. However, reliable predictors of vertebrate and floristic diversity are not necessarily adequate predictors of invertebrate diversity. We sampled the earthworm fauna of the Perth metropolitan Swan Coastal Plain (SCP) to examine whether vegetation-based criteria are sufficient for identifying a conservation estate for native earthworms. Twenty-one native species were collected from 136 sample localities. All five previously described native species from the region and three native species previously collected but not formally described were again collected, while 13 previously uncollected species were found. Species abundances of native earthworms were uneven, in common with species-abundance relationships for many other invertebrate assemblages, with 10 singleton occurrences of species and few common species. Species diversity increased away from the coast across the sandy geomorphic units Quindalup, Spearwood and Bassendean. Our study did not resolve whether dlifferences in earthworm faunas reflect the gradient in soil qualities across these units, gradients in species-area effects, habitat diversity effects or a combination of these. Blocks of remnant vegetation identified in the Western Australian Government's Bush Forever plan as containing natural areas of regional conservation value are also likely to support at least one native earthworm species. However, many of the blocks of remnant vegetation so identified are not within the formal conservation estate. Two species identified in this survey fortuitously persist only in remnant vegetation patches not considered regionally significant. Actual regional diversity was estimated to be 38 native species, indicating many uncollected relatively rare species. Although earthworms are a low diversity group compared with other invertebrates, the localized distributions of most species indicate that the formal conservation estate does not provide adequate protection. Ongoing degradation of unprotected remnant vegetation will result in extinctions of localized invertebrate species.