Land-use responses of dung beetle communities and their ecosystem services

Abstract

Global urbanization and rising human population densities result in a constant need for resources and food. Land use and land cover are inevitably bound to human population dynamics and thus remain the major drivers of continuous environmental change. Agricultural land use and forestry affect species communities and consequently their ecological functions – such as nutrient cycling or protection against soil erosion – resulting in a loss of ecosystem services. Indicator species, therefore, provide valuable contributions for the observation of anthropogenic disturbance, as they respond sensitively towards changes of their habitat and living conditions. The presence or absence of such species affects the evaluation of disturbances and predictions of ecosystem changes and thus might reveal functional consequences. Detritivores (Saprobionts) utilize and process organic material, which otherwise would be inaccessible for higher trophic levels. Their unique way of recycling organic material renders them important contributors on the base of every food web. Hence, in context of agricultural land use and forestry in differing management intensities, I focused within this thesis on the occurrence, ecosystem services, community structure and behavioral patterns of a basal superfamily of insects: dung beetles (Coleoptera: Scarabaeoidea). Dung beetles are widespread in most habitats around the globe and represent important ancient and current detritivores. These functionally important beetles are faced with a wide range of anthropogenic disturbances and changes in environmental conditions due to land use. I thus conducted quantitative surveys of the abundance of dung beetles and their dung removal rates in forest and grassland sites with varying land-use intensity, to focus on following research questions: (Q1) Does land use affect dung beetles and their ecosystem services? (Q2) In which ways do dung beetle-resource connectivity and the complexity of this trophic network respond to (rising) land-use intensity? (Q3) Do dung beetle – resource interactions change in specificity along a global, latitudinal gradient? Besides these applied and ecosystem-service related questions I found patterns and results for the beetles’ resource preference, which were discussed controversially in the literature and still remain to be fully understood. For a better and more basic understanding of this detritivorous group of insects I addressed the following questions: (Q4) Is the nutritional value of dung a driving force for dung type attractiveness and dung beetle preference? (Q5) Which roles have volatile organic compounds in dung beetle attraction? In chapter 2 I used dung from livestock and game animals to provide a characteristic spectrum of dung resources and sampled 300 experimental sites, including forests and grasslands. Since every sampling site differed in management intensity, I was able to calculate the effects of rising land-use and forest management and highlight contrary, but foremost negative effects on dung removal for both habitats. Chapter 3 is a more indepth analysis of the beetles’ community structure and the complexity of dung beetle networks. The species’ abundance and distribution across sites revealed a generalization of beetle-resource interactions, which led to more even and higher decomposition rate. Additionally, I found that a rising dung beetle network complexity translates into an enhanced robustness against land use. To test the beetles’ resource specificity on a global scale, I conducted in chapter 4 a meta-analysis of 110 dung beetle-resource interaction networks along a latitudinal gradient. Despite a significant increase of dung beetle diversity towards the equator, overall the dung beetle networks remained highly generalistic. In chapter 5 I conducted nutritional analyses (amino acids, fatty acids, sterols, C/N contents) of different dung types to unravel patterns in dung type preferences I observed in the field. Albeit differences in nutritional composition on a feeding guild level, these results did not predict patterns of dung preference. Subsequently, I analyzed volatile organic compounds of different dung types in chapter 6. Dung scent components (as described in the literature or elucidated by my own gas chromatographic measurements) were used in single and mixed baits to test for attractivity, compared to natural dung samples. Dung scent analyses revealed both, unique bouquets and ubiquitous volatiles for different dung types. This leads to specific volatile blends – including key volatiles – for the beetles’ resource localization. In summary, this thesis contributes to applied issues regarding land use and forestry, conveys an enhanced understanding of the (global) community structure and faces issues in basic dung-beetle research. Anthropogenic disturbance, like habitat dependent management, often negatively affects ecosystem services of various taxa; also true for some responses of dung beetles i.e. the beetles’ dung removal in terms of deforestation. Yet, dung beetle communities show an unexpected robustness against land-use intensity. Due to the high generalization level of dung beetle networks, so far, a balanced dung removal is assured. Even on a global basis, this generalistic character of dung beetle-resource interactions remains similar across a latitudinal gradient. Observed resource specificity cannot be explained by (differing) nutritional values of the dung types, but certain mixtures and single (key) volatile organic compounds seem crucial for specific patterns in dung beetle attraction.