The ecology of narrow-range endemic macro-invertebrates of Great Artesian Basin springs

Springs are unique freshwater ecosystems that are often overlooked in discussions of global freshwater ecology and conservation. Springs that emerge from the Great Artesian Basin (GAB) in Australia provide permanent wetland habitat in the arid and semi-arid zones that support a high diversity of endemic aquatic species. The majority of these species have a high risk of extinction due to their small geographic distributions and severe habitat loss. However, most are poorly understood and unprotected. They are mostly invertebrates for which basic taxonomic and ecological information is lacking. Available data suggests that most species are endemic to GAB springs, have geographic distributions less than 50km2, and do not often occupy all springs within their distribution or all areas within each spring. Extirpations in a single spring appear common, but an ever-shifting network of viable populations maintains species persistence. The majority of published literature concerning the GAB system focuses on the limited abilities of endemic organisms to disperse across the dry landscape between spring wetlands. This has led to a strong focus on determining dispersal distances, population connectivity and applying metapopulation models to springs systems. This focus on dispersal may be overlooking the potential for environmental heterogeneity within and across springs to also influence patterns of distribution. The match between the environmental conditions available and each species requirements can also influence patterns of distribution. We currently have a poor understanding of the variance in environmental conditions provided in springs, how environmental conditions change in space and time, and how patterns of abundance and distribution of species endemic to the system relate to environmental variance. Whilst dispersal limitation is undoubtedly an important process in this system, Autecological information is also needed to assess the importance of potential environmental limits on these specialised species. This thesis addresses environmental heterogeneity across GAB fed springs and its relationship with patterns of diversity and abundance in species endemic to the system, but particularly endemic macro-invertebrates. It focuses on three relevant scales: across all spring complexes at a basin- wide scale, across springs within a single complex and across microhabitats within springs. It begins with a review of current knowledge concerning springs in Australia and the primary processes thought to be influencing the distribution and persistence of springs species (Chapter 1). It then presents the results of a basin-wide review of the taxa endemic to GAB springs and their distributions at a basin-wide scale, and assesses whether there are environmental correlates of high endemic diversity. This review demonstrates that macro-invertebrates are the most diverse, that the vast majority of species are geographically restricted and that the invertebrates are under- represented in conservation legislation (Chapter 2). Spring complexes vary considerably, and spring complexes with more springs within close Euclidean distance of other complexes are more likely to contain endemics, however environmental characteristics at this broad geographic scale are not strongly correlated with the number or diversity of endemic species (Chapter 2). The findings of Chapters 1 and 2 guide a subsequent research focus in this system on macro- invertebrates, and particularly narrow-range endemic species of gastropod. Chapter 3 addresses the need to rapidly collect data in springs whilst ensuring the methods used are appropriate for sampling the full diversity of species and are comparable across different studies. Alternative methods used to sample the diversity and abundance of spring macro-invertebrates are compared, and for each taxon an optimal sampling strategy is proposed. These methods are used to survey gastropod species richness across one of the GAB spring systems biodiversity ‘hot spots’ (Pelican Creek), in order to assess how environmental heterogeneity across springs relates to the distribution of six snail species endemic to this site from three different families (Chapter 4). The majority of springs are unoccupied by any endemic snail species. Some species occupy many springs (>20% at the complex) and some occupy few (<10%), and all have suffered extirpations and colonisations since the last survey in 2006-2008. Most species endemic to Pelican Creek only occupy the largest and deepest springs with higher numbers of connections to other springs via wetland. This suggests that deep spring pools are as important as connectivity for maintaining diversity in these springs. In Chapter 5 I ask whether this pattern occurs because deep spring pools provide different conditions to shallow tail areas, and whether species with limited distributions are restricted to these pool areas. Most species were absent from, or had lower abundance in, areas of high environmental variability and had a distribution restricted to deep pool environments. These pool areas are unique because they had stable water chemistry, temperature and water depth. Some species persisted in areas of high environmental variability and endured the variable conditions that prevailed in tail areas, and were observed tracking these areas as they shifted through the seasons. This led me to ask if those species restricted to deep pools were less able to endure the environmental extremes that prevail in tail areas (Chapter 6). No differences in the ability to endure short-term exposure (up to 24h) to any one factor (elevated conductivity, pH and desiccation) concordant with tail conditions were found across species. However, one pool- restricted species (Glyptophysa sp.) suffered sub-lethal effects within 24h exposed to elevated conductivity, and experienced mortality within 7 days if high conductivity and pH were experienced in combination. This thesis has revealed previously overlooked insights into the species-specific environmentally mediated processes that compound with dispersal limitations to maintain the narrow distributions of GAB endemic species. The majority of species studied here perished without the environmental stability afforded by strong flow from the basin into deep spring pools. However, two species on opposite sides of the GAB system have escaped this dependence, and occupy ephemeral tail areas in high abundance. Springs are not only diverse because species from a wide range of taxa are endemic to the ecosystem, but also because species have adapted different ways of persisting in the face of the inherent variability of environmental conditions in the system. This has applications for our understanding of ecological processes in the GAB springs system and are discussed in light of the evolutionary history of species in springs and their conservation (Chapter 7).