Environmental Thresholds of Nepomorpha in Cerrado Streams, Brazilian Savannah.

Determining thresholds of species loss in response to gradients of human impact is becoming an increasingly important component of stream conservation and management. We analyzed the thresholds of change in populations and communities of the infraorder Nepomorpha along a gradient of environmental integrity in streams of the Brazilian savannah, based on samples from 20 sites. We used the Threshold Indicator Taxa Analysis (TITAN) to identify the location and magnitude of changes associated with the environmental quality and the frequency and abundance of the different taxa. We also used TITAN to calculate the positive and negative thresholds of the community and species along the habitat integrity gradient. Nepomorphan species were generally associated with sites of higher environmental quality, which presented larger populations and greater species richness. Only a few nepomorphan species groups were associated with impacted streams. Thresholds were evident in some aquatic nepomorphan populations, but not for the community as a whole. This indicates that species have idiosyncratic responses to the environmental gradient, but in general nepomorphans appear to be a good indicator of healthy environments or less impacted streams in the Brazilian savannah.

Effect of Environmental and Temporal Factors on Patterns of Rarity of Ephemeroptera in Stream of the Brazilian Cerrado.

Patterns of species' abundance and occurrence over time and space allow division of species into (i) common species, which are abundant, but have a low diversity, and (ii) rare species, which are far more diverse and less abundant. Understanding the relationships among these two species groups and how they are affected by environmental conditions is a major challenge for ecologists, especially considering the distinction between local environmental factors and regional factors and variations in abundance over the course of the year. In this study, we focused on the long-term relationship between the abundance of rare and common ephemeropterans and abiotic factors on local and regional scales. Our hypotheses are that common species will be affected primarily by regional environmental variables (i), whereas rare species will be influenced more by temporal variation (ii). Together, both local and regional abiotic variables, plus temporal variation, best explained the abundance of the common species, whereas temporal variation was the best predictor of rare species. Considering the theoretical aspects and the empirical evidence, we discuss the results based on the plasticity of the common species and the life cycle of the rare ones. We believe that our findings reinforce the need for the deconstruction of communities for a deeper understanding of their relationships with abiotic variables and, in particular, the specific aspects of these relationships in the context of the different guilds of the community.