Island biogeography is based on the idea that the larger an island and the lower its isolation the higher the biodiversity it contains. The premise of island size contributing to diversity is based on the fundamental ecological concept - the species area relationship (SAR). Wetlands and in particular arid-zone spring wetlands can be considered islands as they are wetted habitat surrounded by a harsh and dry landscape – “girt by dirt” – forming an ideal system to study island biogeography. While studies in spring wetlands have identified SARs that conform to the concept, others identify conflicting information. The aim of our study was to assess the relationship between wetted area of spring wetlands and the taxonomic richness of aquatic invertebrates.
Wetted areas were surveyed and aquatic invertebrates sampled across three Great Artesian Basin spring complexes and assessed using a combination of quantile regression and multivariate analyses. No significant relationship between wetted area and taxonomic richness was identified across all springs, but taxonomic richness and spring complex was significantly related to the ordination of the invertebrate community. The results for similar analyses within one complex, “Edgbaston Springs”, showed a negative relationship, indicating wetland size constrains invertebrate taxonomic richness. These results contrast with the basic SAR concept but can be explained by a “small island effect”, an exception to the SAR rule, which is associated with habitat diversity and disturbance regime.