Lakes as food sources for bats : evidence from stable isotopes and acoustic monitoring

Abstract

This thesis deals with aquatic–terrestrial interactions, specifically in the use of freshwater insect fluxes by bats. Emerging aquatic insects are an important source of energy for bats and other terrestrial consumers. Many bat species depend on aquatic bodies not only for drinking water, but also partly or entirely for food. The main question of this thesis is how important are lakes as food sources for bats.With a literature review (Chapter I), I first present an overview of the current knowledge on bats’ use of aquatic habitats, emphasizing how anthropogenic impacts on water bodies affect bats. This review shows that the majority of the studies have been conducted in Europe and North America, and most indirectly describe the use of aquatic resources from bats. The most common method used is acoustic monitoring, sometimes combined with other methods such as radio-tracking. It does not appear that research is focused on threatened or endangered species. The effects of water pollution and eutrophication on bats are unclear, as different effects are reported for different species and areas. Thus, this topic needs further investigation. More studies are also needed for understudied areas such as Africa, South America and Asia, and also for areas with limited water resources.For the research of the topic, we used only non-invasive methods, for the bats. The first approach was biochemical tracers, i.e. stable isotope and fatty acid analysis of bat faeces. Since stable isotope analysis has so far been used in bat ecology mostly on hair, blood, muscles and claws, we wanted to explore faeces as samples, as they do not require catching the animals. First, we tested the effectiveness of the stable isotope analysis method on faeces, with a diet-shift experiment on captive bats of two species (Chapter II). We shifted the bats’ mealworm diet from light to heavy isotope labels and after seven days we shifted it back to the light isotopic label. The stable carbon (δ13C), nitrogen (δ15N) and sulphur (δ34S) values on the faeces reflected the signature of the last diet within three hours after the last meal. We also calculated the isotopic difference (Δ) between diet and faeces which was significant for nitrogen, but not for carbon and sulphur, and did not differ for diet or species. These isotopic difference values are necessary for reconstructing diet from wild individuals and when the diet is unknown. Our results, that faecal stable isotopes reflect the isotopic signature of the last consumed food, showed that stable isotope analysis in faeces is a suitable method for investigating questions concerning short-term shifts in diet or habitat of bats, and possibly other insectivorous small mammals.The next step was to apply these biochemical tracers on bat faeces from wild bats (Chapter III). Our question —always linked to the main question of this thesis— was to investigate whether it is possible to use stable isotopes on faeces to identify aquatic or terrestrial origins of the prey. In addition to stable isotopes, we used another chemical tracer, fatty acids. Both stable isotopes and fatty acids have different signatures between different habitats, such as between aquatic and terrestrial systems. We collected fresh faeces from the roosts of three bat species with known preferences, on the borders of Germany and Switzerland. The species Myotis daubentonii is known to feed almost exclusively on aquatic insects (mainly Chironomidae), M. myotis feeds on terrestrial arthropods (beetles) and M. mystacinus has been reported to feed on both aquatic and terrestrial insects. Thus, we expected that the stable isotope and fatty acid values of their faeces would reflect their feeding preferences. In line with our expectations, we found higher δ15N and omega-3 and lower δ13C and omega-6 in M. daubentonii’s faeces, as is characteristic for freshwater systems. The opposite was true for M. myotis, while M. mystacinus—as expected— had intermediate values, indicating that it indeed feeds on both aquatic and terrestrial food.The second approach, for investigating the relationship between bats and aquatic insects and shedding more light on the effect of season on aquatic insect emergence and bat activity, was a field study at three lakes in South Germany during the three seasons when bats are active (Chapter IV & Chapter V). We used passive acoustic monitoring, during the whole night, which is an effective, non-invasive method for assessing bat activity. For the nights of recording, as well as the preceding days, we collected emerging insects using floating emergence traps, and caught aerial flying insects using a Malaise trap. In all lakes, Chironomidae constituted the highest number of emergent insects; seasonal patterns of emergence were unimodal or bimodal with peaks at different times (beginning of summer, end of summer, beginning of autumn). Insect emergence had a positive relation with the water temperature in all lakes, but not with any other water parameters. In general, we found weak correlations between bat activity and insect emergence in the two lakes (Constance and Siechenweiher) and no correlations in Mindelsee. Bat activity also showed seasonal fluctuations that did not always follow insect emergence, probably because other factors (e.g., season, habitat characteristics, or energy requirements) played an important role. Bats were active throughout night, and the pattern of their activity also differed among lakes and seasons.In conclusion, the results of this thesis show that lakes and their shores are important habitats for bats, as they support a high number of bat species. Aquatic fluxes to terrestrial systems have a considerable seasonal variation. Bat activity is influenced by season, insect availability and probably other factors (e.g., habitat structure, bats energy requirements) that we did not examine here. We suggest acoustic monitoring of bat activity and biochemical methods, i.e. stable isotope and fatty acid analysis of faeces, may be used to answer questions related to short-term diet or habitat shifts. These methods are non-invasive and efficient in studying aquatic-terrestrial trophic interactions and the use of aquatic resources by bats.The findings of this thesis have a value for studying ecological questions related to food web dynamics, interactions between different habitats and animals or animal behaviour related to diet and habitat (with stable isotopes and fatty acids on faeces revealing short-term changes). The present results and conclusions may also prove useful for conservation, not only in the local region and for the studied species, but also for other insectivorous mammals or other species that rely on aquatic resources.