Taxonomic and functional structure of odonata larvae assemblages in neotropical savanna headwater streams

Abstract

Freshwater ecosystems have been subjected to an increased and continuous process of degradation, mainly due to anthropogenic pressures. Advances in research for assessing and monitoring these ecosystems have not been performed in sufficient time to prevent or reduce the loss of aquatic biodiversity. In this context, it is necessary and urgent to understand and measure how biological assemblages respond to these human pressures. Like other aquatic insects, larval Odonata assemblage structure and functions are closely associated with the environmental conditions of freshwater ecosystems. Therefore, changes in these conditions affect the structure and function of Odonata assemblages, which makes them excellent indicators of environmental condition. However, studies addressing the responses of Odonata larvae to anthropogenic stressors are still scarce, while knowledge about Odonata adults is relatively more advanced and well-established. Given the above, the general aim of this thesis was to evaluate the taxonomic, structural and functional responses of Odonata larvae to a gradient of environmental conditions in Brazilian Savanna headwater streams. This thesis is divided into three chapters. Chapter 1 evaluates the distribution patterns of the two larval Odonata suborders along a gradient of riparian vegetation disturbance. Chapter 2 describes how anthropogenic stressors affect larval Odonata taxonomic and functional structure. Chapter 3 identifies the ecological thresholds of larvae Odonata genera along a gradient of environmental disturbance. We assessed the responses of Odonata larvae to gradients of disturbance considering traditional ecological structure approaches, such as richness and abundance, as well as complementary functional approaches, such as traits and ecological thresholds. We found that, like adults, Zygoptera larvae are more sensitive to changes in environmental conditions, being more dependent on the presence of riparian vegetation, good water quality, and heterogeneous in-stream substrate. Zygoptera genera have specific traits related to these conditions and narrower ecological thresholds and, therefore are more often found in reference condition streams. On the other hand, Anisoptera larvae are more tolerant to anthropogenic pressures, including removal of riparian vegetation, increased erosion and siltation, and higher levels of total dissolved solids and electrical conductivity. Anisoptera genera have specific traits and wider ecological thresholds, and were found in intermediate or degraded streams. I conclude that anthropogenic disturbances, mostly related to the removal or reduction of riparian vegetation, affect larval Odonata assemblages in the Neotropical Savanna. I expect that the products generated by this thesis will support the use of Odonata larvae as assessment and monitoring tools for headwater stream ecosystems.