Testing the performance of bats as indicators of habitat quality in riparian ecosystems

Abstract

In the last 30 years, bioindication has become one of the pillars of modern environmental sciences and an essential part of conservation biology. In riparian ecosystems, a variety of biological assemblages have been used worldwide as bioindicators to assess the ecological status of lakes and rivers. Benthic macroinvertebrates, periphyton and fishes are the most commonly employed in national monitoring programs. For better understanding the dynamics in riparian ecosystems, it is important to study interactions between terrestrial and aquatic systems. Insectivorous bats are a promising link between both of them. Several studies showed that bats depend strongly on water habitat, not only for drinking. As linear landscape elements, rivers are used as preferential pathways for movement and migration. Their general activity is higher on rivers and lakes than over other habitats and some species forage exclusively over water or close to riparian vegetation. Moreover, bats have all the characteristics of a “good bioindicator”. Their taxonomy is stable, they are geographically widespread and are among the mammal orders with higher diversity, with more than 1300 species covering several ecological niches. They provide crucial ecosystem services worldwide, i.e. seed dispersal and pollination in tropic regions or pest control in agricultural systems. Bats are also highly sensitive to human-driven environmental alteration. However, few studies have tested their bioindication performances for main habitat categories, including rivers. The main objective of this thesis was to test bats as bioindicators in river ecosystems. We sampled bat community over five rivers in central-south Italy, using stationary and automatically triggered bat detectors, to measure species composition, commuting, foraging and drinking activity. For a comparison of biological responses, we evaluated the ecological status using the multimetric STAR_ICM index, based on the macrobenthic community. Contemporary we calculated the Fluvial Functionality Index (IFF), which considers biotic and abiotic factors for a comprehensive survey of river and riparian ecosystem functionality. In the studied area, the ecological status quality varies from “High” to “Moderate” in the different parts of the five rivers and the most part of them has a level of functionality “Good” or “Good-Moderate”. We found that bat species richness is not related to river quality or functionality, while total activity is negatively associated with STAR_ICMi values. Pipistrellus pygmaeus/Miniopterus schreibersii activity is also negatively related to river quality, probably because they mostly feed on nematoceran dipteran that are usually abundant in eutrophic waters. Eptesicus/Nyctalus activity is the only one that increases with increasing IFF values. The width of riparian forests is one of the parameters included in evaluation of IFF and riparian trees such as Salix alba or Populus may bear cavities used as roosts by Eptesicus/Nyctalus species, so the link with IFF might reflect roost availability. We have not found species exclusive of particular quality levels and the general increase of bat activity with lower ecological status shows that the effects of river ecosystems degradation on bats are more complex and probably at the opposite of what expected. Further studies on different areas will help explain the relations we found till now. For bioindication purpose, according to our results it is possible to lay the foundations for a bioacoustic index based on the analyses of total bats activity or bioacoustic group activity, which could be successfully applied by other than bat experts.