Taxonomic Distinctness as a Measure of Diversity of Stream Insects Exposed to Varying Salinity Levels in South-Eastern Australia

Abstract

Average taxonomic distinctness (delta+) is a measure of the mean taxonomic breadth of a sample and is thus a measure of biodiversity. It has the great advantage of not being sensitive to variations in sampling effort, unlike measures of species richness, and is thus potentially of great value for examining the effects of disturbance on biodiversity. In addition, it is possible to test delta+ for departures from expectation. The increasing level of salinity in rivers in Victoria and South Australia is a well known environmental hazard. Delta+ was calculated using all insect species or the subset of Ephemeroptera, Plecoptera and Trichoptera (EPT) species from the bank and main channel habitats at 941 and 607 sites respectively. Samples from these sites were classified into 16 salinity categories (ranging from 0.05 to 30 mS cm-1). Delta+ values for the insects showed minor change and were close to the expected value until salinity levels reached 7-14.9 mS cm-1; at higher salinities delta+ declined and became significantly different from expectation. When EPT species were used trends were less consistent and the decline at higher salinities was weaker. Feeding group diversity within each salinity category declined more or less linearly as delta+ decreased. Overall, the diversity of aquatic insects, as measured by delta+, began to decline at higher salinities than suggested by previous work. Delta+ responded somewhat differently to salinity increase compared with species richness. In addition, trophic diversity diminished continuously as delta+ declined, indicating that this index of biodiversity was explicitly linked to an aspect of functional diversity. This last characteristic may make delta+ a more interpretable measure of disturbance than other measures of biodiversity. Delta+ is well suited for giving a preliminary view of the effects of a specific disturbance over a large region, but needs to be tested with other stressors. © 2012 John Wiley & Sons, Ltd.