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I. Nowadays, tropical landscapes experience large-scale land use intensification and land conversion driven by increasing demand for resourses. Due to the continuously high demand for tropical timber and politically intended step increase in palm oil production, multiple rounds of logging and subsequent conversion to oil palm plantations became a regionally wide-spread land conversion pattern in Southeast Asia. Although many tree species and some animals are highly threatened by logging, a great number of species groups, such as birds or mammals, have been shown to persist in logged forests. Accordingly, many ecosystem services, such as dung removal, seed dispersal or the activity of scavengers, are functionally maintained in logged forests. In contrast, oil palm plantations have been shown to not only dramatically alter the species composition and reduce biodiversity, but also curtail many crucial biotic and abiotic ecosystem functions. The focus of this dissertation was to investigate the response of anuran species richness and community composition to logging and conversion to oil palm plantation in northern Borneo (chapter II). I analysed the diet of various frog species and their change with habitat degradation. Furthermore, I assessed the shift in the trophic position of the anuran community as well as the response of anuran phylogenetic, dietary, and functional diversity to logging and conversion to oil palm plantations (chapter III). Finally, the resilience of the predator-prey interaction between an ant-specialist toad and its ant prey was analysed using shifts in species-level interactions (chapter IV).
II. This part of the study compares the species richness, relative abundance and community composition of stream anuran assemblages among primary forests, repeatedly logged forests and oil palm plantations. I used a highly standardised sampling setup applying transect-based sampling. Surprisingly, most of the anuran species native to primary forests were able to survive in logged forest streams. In contrast, on average only one third of the forest species richness was found in oil palm plantation streams. However, a high percentage of canopy cover above the plantation streams was able to mitigate this loss substantially. This study demonstrates the high conservation value of logged forests for Southeast Asian anurans. In contrast, the conversion to oil palm plantations leads to a dramatic decline of forest species. However, they have a mainly unused potential to contribute to the protection of parts of the regional anuran biodiversity if conservation-oriented management options are implemented.
III. In this part, I analysed the shifts in trophic position and multiple diversity layers of Southeast Asian stream-dependent anuran species across a gradient of disturbance from primary forest through intensively logged forest to oil palm plantation. For this purpose, I identified the diet composition of 59 anuran species by means of stomach flushing. Furthermore, I use diet composition of frog species as well as species traits to calculate dietary and functional diversity, respectively. I found that the trophic position of the entire anuran community is elevated in heavily disturbed habitats. Furthermore, species diversity, phylogenetic species variation, dietary diversity, and functional diversity were reduced. However, beyond the effect of the decreased species richness, only phylogenetic species variability and functional diversity were significantly impacted by land conversion, indicating a non-random loss of phylogenetic groups and functionally unique species. Overall, the observed changes to species interactions and functional composition suggest a greatly modified role of anurans in altered habitats and major foodweb reorganisation. Such far-reaching changes to the way species groups interact are likely to threaten local biodiversity and ecosystem functioning in natural and particularly modified habitats. However, I could also show, that small-scale habitat quality, provided by riparian reserves, is able to mitigate the negative consequences of land conversion considerably.
IV. Here I assess how logging of rain forest and conversion to oil palm plantations affect the populations of the ant-specialist giant river toad (Phrynoidis juxtaspera), and availability and composition of its ant prey. I measured canopy cover as an estimate for the degree of disturbance. I found that toad abundance decreased with increasing disturbance. At the same time, ant community composition was altered, and local ground-foraging ant species richness increased with disturbance. However, for a given amount of canopy cover, primary forest supported more ant species than altered habitats. Despite these changes, composition of ants consumed by toads was only weakly affected by habitat change, with the exception of the invasive yellow crazy ant (Anoplolepis gracilipes), which was positively selected in oil palm plantations. This suggests that predator-prey interactions can be mostly maintained with habitat disturbance despite shifts in community composition, and even that some predators are capable of exploiting new prey sources in novel ecosystems.
V. I could show that anuran diversity and their trophic interaction is negatively impacted by logging and in particular by conversion to oil palm plantations. From species richness and community composition, my study expanded to phylogenetic, dietary and functional diversity. Furthermore, I investigated the interaction of a particular toad species with its preferred prey (ants), on species level. This increasing degree of detail in my study provided comprehensive results, beyond the detail of many related studies. Overall, conservation of the remaining forest in Southeast Asia is urgently required to protect anuran biodiversity and their trophic interactions.
Since more than two centuries naturalists are fascinated by the profound changes in biodiversity observed along climatic gradients. Although the theories explaining changes in the diversity and the shape of organisms along climatic gradients belong to the foundations of modern ecology, our picture on the spatial patterns and drivers of biodiversity is far from being complete. Ambiguities in theory and data are common and past work has been strongly concentrated on plants and vertebrates. In the last two decades, interest in the fundamental processes structuring diversity along climatic gradients gained new impetus as they are expected to improve our understanding about how ecosystems will respond to global environmental changes. Global temperatures are rising faster than ever before; natural habitats are transformed into agricultural land and existing land use systems get more and more intensified to meet the demands of growing human populations. The fundamental shifts in the abiotic and biotic environment are proclaimed to affect ecosystems all over the world; however, precise predictions about how ecosystems respond to global changes are still lacking. We investigated diversity, traits and ecosystem services of wild bees along climate and land use gradients on Mount Kilimanjaro (Tanzania, East Africa). Wild bees play a major role in ecosystems, as they contribute to the reproduction and performance of wild and crop plants. Their responsiveness to environmental changes is therefore of high ecological and economic importance.
Temperature and energy resources have often been suggested to be the main determinants of global and local species richness, but the mechanisms behind remain poorly understood. In the study described in chapter II we analyzed species richness patterns of wild bees along climate and land use gradients on Mount Kilimanjaro and disentangled the factors explaining most of the changes in bee richness. We found that floral resources had a weak but significant effect on pollinator abundance, which in turn was positively related to species richness. However, temperature was the strongest predictor of species richness, affecting species richness both directly and indirectly by positively influencing bee abundances. We observed higher levels of bee-flower-interactions at higher temperatures, independently of flower and bee abundances. This suggests that temperature restricts species richness by constraining the exploitation of resources by ectotherms. Current land use did not negatively affect species richness. We conclude that the richness of bees is explained by both temperature and resource availability, whereas temperature plays the dominant role as it limits the access of ectotherms to floral resources and may accelerate ecological and evolutionary processes that drive the maintenance and origination of diversity.
Not only species numbers, but also morphological traits like body size are expected to be shaped by both physiological and energetic constraints along elevational gradients. Paradoxically, Bergmann´s rule predicts increases of body sizes in cooler climates resulting from physiological constraints, while species-energy theory suggests declines in the mean body size of species caused by increased extinction probabilities for large-bodied species in low-energy habitats. In chapter III we confronted this ambiguity with field data by studying community-wide body size variation of wild bees on Mt. Kilimanjaro. We found that along a 3680 m elevational gradient bee individuals became on average larger within species, while large species were increasingly absent from high-elevational communities. This demonstrates, on the one hand, how well-established, but apparently contrasting ecological theories can be merged through the parallel consideration of different levels of biological organization. On the other hand it signals that the extinction risk in the course of environmental change is not equally distributed among species within a community.
Land use intensification is known to threaten biodiversity, but the consequences for ecosystem services are still a matter of debate. In chapter IV, we experimentally tested the single and combined contributions of pest predators and pollinators to coffee production along a land use intensification gradient on Mount Kilimanjaro. We found that pest predation increased fruit set by on average 9%, while pollination increased fruit weight of coffee by on average 7.4%. Land use had no significant effect on both ecosystem services. However, we found that in coffee plantations with most intensified land use, pollination services were virtually exclusively provided by the honey bee (Apis mellifera). The reliance on a single pollinator species is risky, as possible declines of that species may directly lower pollination services, resulting in yield losses. In contrast, pollination services in structurally complex homegardens were found to be provided by a diverse pollinator community, increasing the stability of pollination services in a long term.
We showed that on Mount Kilimanjaro pollinator communities changed along elevational gradients in terms of species richness (chapter II) and trait composition (chapter III). Temperature and the temperature-mediated accessibility of resources were identified as important predictors of these patterns, which contributes to our fundamental understanding about the factors that shape ectothermic insect communities along climatic gradients. The strong temperature-dependence of pollinators suggests that temperature shifts in the course of global change are likely to affect pollinator communities. Pollinators might either profit from rising temperatures, or shift to higher elevations, which could result in related biotic attrition in the lowland with consequences for the provision of ecosystem services in cropping systems. Up to now, land use intensification had no significant impact on the diversity of pollinator communities and their ecosystem services. Pollinators might profit from the strong landscape heterogeneity in the region and from the amount of flower resources in the understory of cropping systems. However,progressing homogenization of the landscape and the pronounced application of pesticides could result in reduced diversity and dominance of single species, as we already found in sun coffee plantations. Such shifts in community compositions could threaten the stability of ecosystem services within cropping and natural systems in a long term.
1. Since the early nineteenth century describing (and understanding) patterns of distribution of biodiversity across the Earth has represented one of the most significant intellectual challenges to ecologists and biogeographers. Among the most striking patterns of species richness are: the latitudinal and elevational gradients, with peaks in number of species at low latitudes and somewhere at mid altitudes, although other patterns, e.g. declines with increasing elevation, are often observed. Even in highly diverse tropical regions, species richness is not evenly distributed but there are “hotspots” of biodiversity where an exceptional number of species, especially endemics, are concentrated. Unfortunately, such areas are also experiencing dramatic loss of habitat. Among vertebrate taxa, amphibians are facing the most alarming number of extinctions. Habitat destruction, pollution and emergence of infectious diseases such as chytridiomycosis, are causing worldwide population declines. Responses to these drivers can be multidirectional and subtle, i.e. they may not be captured at the species but at the genetic level. Moreover, present patterns of diversity can result from the influence of past geological, climatic and environmental changes.
In this study, I used a multidisciplinary and multilevel approach to understand how and to which extent the landscape influences amphibian diversity. Mount Kilimanjaro is an exceptional tropical region where the landscape is rapidly evolving due to land use changes; additionally, there is a broad lack of knowledge of its amphibian fauna. During two rainy seasons in 2011, I recorded anurans from the foothills to 3500 m altitude; in addition, I focused on two river frog species and collected tissue samples for genetic analysis and swabs for detection of chytridiomycosis, the deadly disease caused by Batrachochytrium dendrobatidis (Bd).
2. I analyzed how species richness and composition change with increasing elevation and anthropogenic disturbance. In order to disentangle the observed patterns of species diversity and distribution, I incorporated inferences from historical biogeography and compared the assemblage of Mt. Kilimanjaro and Mt. Meru (both recent volcanoes) with those of the older Eastern Arc Mountains. Species richness decreased with elevation and locally increased in presence of water bodies, but I did not detect effects of either anthropogenic disturbance or vegetation structure on species richness and composition. Moreover, I found a surprisingly low number of forest species. Historical events seem to underlie the current pattern of species distribution; the young age of Mt. Kilimanjaro and the complex biogeographic processes which occurred in East Africa during the last 20 million years prevented montane forest frogs from colonizing the volcano.
3. I focused on the genetic level of biodiversity and investigated how the landscape, i.e. elevation, topographic relief and land cover, influence genetic variation, population structure and gene flow of two ecologically similar and closely related river frog species, namely Amietia angolensis and Amietia wittei. I detected greater genetic differentiation among populations in the highland species (A. wittei) and higher genetic variation in the lowland species (A. angolensis), although genetic diversity was not significantly correlated with elevation. Importantly, human settlements seemed to restrict gene flow in A. angolensis, whereas steep slopes were positively correlated with gene flow in A. wittei. This results show that even ecologically similar species can respond differently to landscape processes and that the spatial configuration of topographic features combined with species-specific biological attributes can affect dispersal and gene flow in disparate ways.
4. River frogs of the genus Amietia seem to be particularly susceptible to chytridiomycosis, showing the highest pathogen load in Kenya and other African countries. In the last study, I collected swab samples from larvae of A. angolensis and A. wittei for Bd detection. Both species resulted Bd-positive. The presence of Bd on Mt. Kilimanjaro has serious implication. For instance, Bd can be transported by footwear of hikers from contaminated water and soil. Tourists visiting Mt. Kilimanjaro may translocate Bd zoospores to other areas such as the nearby Eastern Arc Mts. where endemic and vulnerable species may still be naïve to the fungus and thus suffer of population declines.
5. My study significantly contributed to the knowledge of the amphibian fauna of Mt. Kilimanjaro and of East Africa in general, and it represents a valuable tool for future conservation actions and measures. Finally, it highlights the importance of using a multidisciplinary (i.e. community ecology, historical biogeography, landscape genetics, disease ecology) and multilevel (i.e. community, species, population, gene) approach to disentangle patterns of biodiversity.
Despite belonging to the best described patterns in ecology, the mechanisms driving biodiversity along broad-scale climatic gradients, like the latitudinal gradient in diversity, remain poorly understood. Because of their high biodiversity, restricted spatial ranges, the continuous change in abiotic factors with altitude and their worldwide occurrence, mountains constitute ideal study systems to elucidate the predictors of global biodiversity patterns. However, mountain ecosystems are increasingly threatened by human land use and climate change. Since the consequences of such alterations on mountainous biodiversity and related ecosystem services are hardly known, research along elevational gradients is also of utmost importance from a conservation point of view. In addition to classical biodiversity research focusing on taxonomy, the significance of studying functional traits and their prominence in biodiversity ecosystem functioning (BEF) relationships is increasingly acknowledged. In this dissertation, I explore the patterns and drivers of mammal and dung beetle diversity along elevational and land use gradients on Mt. Kilimanjaro, Tanzania. Furthermore, I investigate the predictors of dung decomposition by dung beetles under different extinction scenarios.
Mammals are not only charismatic, they also fulfil important roles in ecosystems. They provide important ecosystem services such as seed dispersal and nutrient cycling by turning over high amounts of biomass. In chapter II, I show that mammal diversity and community biomass both exhibited a unimodal distribution with elevation on Mt.Kilimanjaro and were mainly impacted by primary productivity, a measure of the total food abundance, and the protection status of study plots. Due to their large size and endothermy, mammals, in contrast to most arthopods, are theoretically predicted to be limited by food availability. My results are in concordance with this prediction. The significantly higher diversity and biomass in the Kilimanjaro National Park and in other conservation areas underscore the important role of habitat protection is vital for the conservation of large mammal biodiversity on tropical mountains.
Dung beetles are dependent on mammals since they rely upon mammalian dung as a food and nesting resource. Dung beetles are also important ecosystem service providers: they play an important role in nutrient cycling, bioturbation, secondary seed dispersal and parasite suppression. In chapter III, I show that dung beetle diversity declined with elevation while dung beetle abundance followed a hump-shaped pattern along the elevational gradient. In contrast to mammals, dung beetle diversity was primarily predicted by temperature. Despite my attempt to accurately quantifiy mammalian dung resources by calculating mammalian defecation rates, I did not find an influence of dung resource availability on dung beetle richness. Instead, higher temperature translated into higher dung beetle diversity.
Apart from being important ecosystem service providers, dung beetles are also model organisms for BEF studies since they rely on a resource which can be quantified easily. In chapter IV, I explore dung decomposition by dung beetles along the elevational gradient by means of an exclosure experiment in the presence of the whole dung beetle community, in the absence of large dung beetles and without any dung beetles. I show that dung decomposition was the highest when the dung could be decomposed by the whole dung beetle community, while dung decomposition was significantly reduced in the sole presence of small dung beetles and the lowest in the absence of dung beetles. Furthermore, I demonstrate that the drivers of dung decomposition were depend on the intactness of the dung beetle community. While body size was the most important driver in the presence of the whole dung beetle community, species richness gained in importance when large dung beetles were excluded. In the most perturbed state of the system with no dung beetles present, temperature was the sole driver of dung decomposition. In conclusion, abiotic drivers become more important predictors of ecosystem services the more the study system is disturbed.
In this dissertation, I exemplify that the drivers of diversity along broad-scale climatic gradients on Mt. Kilimanjaro depend on the thermoregulatory strategy of organisms. While mammal diversity was mainly impacted by food/energy resources, dung beetle diversity was mainly limited by temperature. I also demonstrate the importance of protected areas for the preservation of large mammal biodiversity. Furthermore, I show that large dung beetles were disproportionately important for dung decomposition as dung decomposition significantly decreased when large dung beetles were excluded. As regards land use, I did not detect an overall effect on dung beetle and mammal diversity nor on dung beetle-mediated dung decomposition. However, for the most specialised mammal trophic guilds and dung beetle functional groups, negative land use effects were already visible. Even though the current moderate levels of land use on Mt. Kilimanjaro can sustain high levels of biodiversity, the pressure of the human population on Mt. Kilimanjaro is increasing and further land use intensification poses a great threat to biodiversity. In synergy wih land use, climate change is jeopardizing current patterns and levels of biodiversity with the potential to displace communities, which may have unpredictable consequences for ecosystem service provisioning in the future.
Chapter 1 – General Introduction
One of the greatest challenges of ecological research is to predict the response of ecosystems to global change; that is to changes in climate and land use. A complex question in this context is how changing environmental conditions affect ecosystem processes at different levels of communities. To shed light on this issue, I investigate drivers of biodiversity on the level of species richness, functional traits and species interactions in cavity-nesting Hymenoptera. For this purpose, I take advantage of the steep elevational gradient of Mt. Kilimanjaro that shows strong environmental changes on a relatively small spatial scale and thus, provides a good environmental scenario for investigating drivers of diversity. In this thesis, I focus on 1) drivers of species richness at different trophic levels (Chapter 2); 2) seasonal patterns in nest-building activity, life-history traits and ecological rates in three different functional groups and at different elevations (Chapter 3) and 3) changes in cuticular hydrocarbons, pollen composition and microbiomes in Lasioglossum bees caused by climatic variables (Chapter 4).
Chapter 2 – Climate and food resources shape species richness and trophic interactions of cavity-nesting Hymenoptera
Drivers of species richness have been subject to research for centuries. Temperature, resource availability and top-down regulation as well as the impact of land use are considered to be important factors in determining insect diversity. Yet, the relative importance of each of these factors is unknown. Using trap nests along the elevational gradient of Mt. Kilimanjaro, we tried to disentangle drivers of species richness at different trophic levels. Temperature was the major driver of species richness across trophic levels, with increasing importance of food resources at higher trophic levels in natural antagonists. Parasitism rate was both related to temperature and trophic level, indicating that the relative importance of bottom-up and top-down forces might shift with climate change.
Chapter 3 – Seasonal variation in the ecology of tropical cavity-nesting Hymenoptera
Natural populations fluctuate with the availability of resources, presence of natural enemies and climatic variations. But tropical mountain seasonality is not yet well investigated. We investigated seasonal patterns in nest-building activity, functional traits and ecological rates in three different insect groups at lower and higher elevations separately. Insects were caught with trap nests which were checked monthly during a 17 months period that included three dry and three rainy seasons. Insects were grouped according to their functional guilds. All groups showed strong seasonality in nest-building activity which was higher and more synchronised among groups at lower elevations. Seasonality in nest building activity of caterpillar-hunting and spider-hunting wasps was linked to climate seasonality while in bees it was strongly linked to the availability of flowers, as well as for the survival rate and sex ratio of bees. Finding adaptations to environmental seasonality might imply that further changes in climatic seasonality by climate change could have an influence on life-history traits of tropical mountain species.
Chapter 4 – Cryptic species and hidden ecological interactions of halictine bees along an elevational Gradient
Strong environmental gradients such as those occurring along mountain slopes are challenging for species. In this context, hidden adaptations or interactions have rarely been considered. We used bees of the genus Lasioglossum as model organisms because Lasioglossum is the only bee genus occurring with a distribution across the entire elevational gradient at Mt. Kilimanjaro. We asked if and how (a) cuticular hydrocarbons (CHC), which act as a desiccation barrier, change in composition and chain length along with changes in temperature and humidity (b), Lasioglossum bees change their pollen diet with changing resource availability, (c) gut microbiota change with pollen diet and climatic conditions, and surface microbiota change with CHC and climatic conditions, respectively, and if changes are rather influenced by turnover in Lasioglossum species along the elevational gradient. We found physiological adaptations with climate in CHC as well as changes in communities with regard to pollen diet and microbiota, which also correlated with each other. These results suggest that complex interactions and feedbacks among abiotic and biotic conditions determine the species composition in a community.
Chapter 5 – General Discussion
Abiotic and biotic factors drove species diversity, traits and interactions and they worked differently depending on the functional group that has been studied, and whether spatial or temporal units were considered. It is therefore likely, that in the light of global change, different species, traits and interactions will be affected differently. Furthermore, increasing land use intensity could have additional or interacting effects with climate change on biodiversity, even though the potential land-use effects at Mt. Kilimanjaro are still low and not impairing cavity-nesting Hymenoptera so far. Further studies should address species networks which might reveal more sensitive changes. For that purpose, trap nests provide a good model system to investigate effects of global change on multiple trophic levels and may also reveal direct effects of climate change on entire life-history traits when established under different microclimatic conditions. The non-uniform effects of abiotic and biotic conditions on multiple aspects of biodiversity revealed with this study also highlight that evaluating different aspects of biodiversity can give a more comprehensive picture than single observations.