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Contact to nature and greenspace is important for emotional well-being and can promote human health. Forest landscapes provide such access to greenspace, especially in protected areas. However, forested protected areas are impacted by natural disturbances such as bark beetle infestations. On the one hand, such disturbances have positive impacts on ecological processes and biodiversity. On the other hand, they have allegedly negative impacts on the recreational value of a landscape. Limited knowledge about the public’s perception of forests subject to natural disturbances still hampers forest management to balance ecological functions and visitors’ recreational experience. Thus, our aim was to determine how attitudes towards nature influence the personal well-being in a naturally disturbed landscape. We investigated self-reported well-being and attitudes towards nature in a standardized questionnaire-based survey of 1008 German inhabitants in an experimentally adapted landscape visualization. Self-reported well-being was generally highest in landscapes with relatively few bark-beetle-killed trees. This was especially the case for people who felt included with nature and preferred an appreciative use or preservation of nature. Conversely, people who had previously visited a national park with visible bark beetle infestations rated their personal well-being highest in landscapes with larger proportions of beetle-killed trees. Our results indicate that it is necessary to analyze people’s knowledge about and relations to forest landscapes as well as concepts of nature conservation, natural landscapes, and biodiversity to gain a better understanding of people’s perceptions of natural disturbances.
The recently observed consistent loss of β-diversity across ecosystems indicates increasingly homogeneous communities in patches of landscapes, mainly caused by increasing land-use intensity. Biodiversity is related to numerous ecosystem functions and stability. Therefore, decreasing β-diversity is also expected to reduce multifunctionality. To assess the impact of homogenization and to develop guidelines to reverse its potentially negative effects, we combine expertise from forest science, ecology, remote sensing, chemical ecology and statistics in a collaborative and experimental β-diversity approach. Specifically, we will address the question whether the Enhancement of Structural Beta Complexity (ESBC) in forests by silviculture or natural disturbances will increase biodiversity and multifunctionality in formerly homogeneously structured production forests. Our approach will identify potential mechanisms behind observed homogenization-diversity-relationships and show how these translate into effects on multifunctionality. At eleven forest sites throughout Germany, we selected two districts as two types of small ‘forest landscapes’. In one of these two districts, we established ESBC treatments (nine differently treated 50x50 m patches with a focus on canopy cover and deadwood features). In the second, the control district, we will establish nine patches without ESBC. By a comprehensive sampling, we will monitor 18 taxonomic groups and measure 21 ecosystem functions, including key functions in temperate forests, on all patches. The statistical framework will allow a comprehensive biodiversity assessment by quantifying the different aspects of multitrophic biodiversity (taxonomical, functional and phylogenetic diversity) on different levels of biodiversity (α-, β-, γ-diversity). To combine overall diversity, we will apply the concept of multidiversity across the 18 taxa. We will use and develop new approaches for quantification and partitioning of multifunctionality at α- and β- scales. Overall, our study will herald a new research avenue, namely by experimentally describing the link between β-diversity and multifunctionality. Furthermore, we will help to develop guidelines for improved silvicultural concepts and concepts for management of natural disturbances in temperate forests reversing past homogenization effects.
Over the past centuries, anthropogenic utilization has fundamentally changed the appearance of European forest ecosystems. Constantly growing and changing demands have led to an enormous decline in ecological key elements and a structural homogenization of most forests. These changes have been accompanied by widespread declines of many forest-dwelling and especially saproxylic, i.e. species depending on deadwood. In order to counteract this development, various conservation strategies have been developed, but they primarily focus on a quantitative deadwood enrichment. However, the diversity of saproxylic species is furthermore driven by a variety of abiotic and biotic determinants as well as interactions between organisms. A detailed understanding of these processes has so far been largely lacking. The aim of the present thesis was therefore to improve the existing ecological knowledge of determinants influencing saproxylic species and species communities in order to provide the basis for evidence-based and adapted conservation measures.
In chapter II of this thesis, I first investigated the impact of sun exposure, tree species, and their combination on saproxylic beetles, wood-inhabiting fungi, and spiders. Therefore, logs and branches of six tree species were set up under different sun exposures in an experimental approach. The impact of sun exposure and tree species strongly differed among single saproxylic taxa as well as diameters of deadwood. All investigated taxa were affected by sun exposure, whereby sun exposure resulted in a higher alpha-diversity of taxa recorded in logs and a lower alpha-diversity of saproxylic beetles reared from branches compared to shading by canopy. Saproxylic beetles and wood-inhabiting fungi as obligate saproxylic species were additionally affected by tree species. In logs, the respective impact of both determinants also resulted in divergent community compositions. Finally, a rarefaction/extrapolation method was used to evaluate the effectiveness of different combinations of tree species and sun exposure for the conservation of saproxylic species diversity. Based on this procedure, a combination of broadleaved and coniferous as well as hard- and softwood tree species was identified to support preferably high levels of saproxylic species diversity.
The aim of chapter III was to evaluate the individual conservational importance of tree species for the protection of saproxylic beetles. For this, the list of tree species sampled for saproxylic beetles was increased to 42 different tree species. The considered tree species represented large parts of taxonomic and phylogenetic diversity native to Central Europe as well as the most important non-native tree species of silvicultural interest. Freshly cut branches were set up for one year and saproxylic beetles were reared afterwards for two subsequent years.
The study revealed that some tree species, in particular Quercus sp., host a particular high diversity of saproxylic beetles, but tree species with a comparatively medium or low overall diversity were likewise important for red-listed saproxylic beetle species. Compared to native tree species, non-native tree species hosted a similar overall species diversity of saproxylic beetles but differed in community composition.
In chapter IV, I finally analysed the interactions of host beetle diversity and the diversity of associated parasitoids by using experimentally manipulated communities of saproxylic beetles and parasitoid Hymenoptera as a model system. Classical approaches of species identification for saproxylic beetles were combined with DNA-barcoding for parasitoid Hymenoptera. The diversity of the host communities was inferred from their phylogenetic composition as well as differences in seven functional traits. Abundance, species richness, and Shannon-diversity of parasitoid Hymenoptera increased with increasing host abundance. However, the phylogenetic and functional dissimilarity of host communities showed no influence on the species communities of parasitoid Hymenoptera. The results clearly indicate an abundance-driven system in which the general availability, not necessarily the diversity of potential hosts, is decisive.
In summary, the present thesis corroborates the general importance of deadwood heterogeneity for the diversity of saproxylic species by combining different experimental approaches. In order to increase their efficiency, conservation strategies for saproxylic species should generally promote deadwood from different tree species under different conditions of sun exposure on landscape-level in addition to the present enrichment of a certain deadwood amount. The most effective combinations of tree species should consider broadleaved and coniferous as well as hard- and softwood tree species. Furthermore, in addition to dominant tree species, special attention should be given to native, subdominant, silviculturally unimportant, and rare tree species.
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.
Spatiotemporal dynamics of freshwater macrophytes in Bavarian lakes under environmental change
(2022)
Macrophytes are key components of freshwater ecosystems because they provide habitat, food, and improve the water quality. Macrophyte are vulnerable to environmental change as their physiological processes depend on changing environmental factors, which themselves vary within a geographical region and along lake depth. Their spatial distribution is not well understood and their importance is publicly little-known. In this thesis, I have investigated the spatiotemporal dynamics of freshwater macrophytes in Bavarian lakes to understand their diversity pattern along different scales and to predict and communicate potential consequences of global change on their richness.
In the introduction (Chapter 1), I provide an overview of the current scientific knowledge of the species richness patterns of macrophytes in freshwater lakes, the influences of climate and land-use change on macrophyte growth, and different modelling approaches of macrophytes.
The main part of the thesis starts with a study about submerged and emergent macrophyte species richness in natural and artificial lakes of Bavaria (Chapter 2). By analysing publicly available monitoring data, I have found a higher species richness of submerged macrophytes in natural lakes than in artificial lakes. Furthermore, I showed that the richness of submerged species is better explained by physio-chemical lake parameters than the richness of emergent species. In Chapter 3, I considered that submerged macrophytes grow along a depth gradient that provides a sharp environmental gradient on a short spatial scale. This study is the first comparative assessment of the depth diversity gradient (DDG) of macrophytes. I have found a hump-shaped pattern of different diversity components. Generalised additive mixed-effect models indicate that the shape of the DDG is influenced mainly by light quality, light quantity, layering depth, and lake area. I could not identify a general trend of the DDG within recent years, but single lakes show trends leading into different directions. In Chapter 4, I used a mechanistic eco-physiological model to explore changes in the distribution of macrophyte species richness under different scenarios of environmental conditions across lakes and with depths. I could replicate the hump-shaped pattern of potential species richness along depth. Rising temperature leads to increased species richness in all lake types, and depths. The effect of turbidity and nutrient change depends on depth and lake type. Traits that characterise “loser species” under increased turbidity and nutrients are a high light consumption and a high sensibility to disturbances. “Winner species” can be identified by a high biomass production. In Chapter 5, I discuss the image problem of macrophytes. Unawareness, ignorance, and the poor accessibility of macrophytes can lead to conflicts of use. I assumed that an increased engagement and education could counteract this. Because computer games can transfer knowledge interactively while creating an immersive experience, I present in the chapter an interactive single-player game for children.
Finally, I discuss the findings of this thesis in the light of their implications for ecological theory, their implications for conservation, and future research ideas (Chapter 6). The findings help to understand the regional distribution and the drivers of macrophyte species richness. By applying eco-physiological models, multiple environmental shaping factors for species richness were tested and scenarios of climate and land-use change were explored.
Anthropogenic activities are causing air pollution. Amongst air pollutants, tropospheric ozone is a major threat to human health and ecosystem functioning. In this dissertation, I present three studies that aimed at increasing our knowledge on how plant exposure to ozone affects its reproduction and its interactions with insect herbivores and pollinators.
For this purpose, a new fumigation system was built and placed in a greenhouse. The annual plant Sinapis arvensis (wild mustard) was used as the model plant.
Plants were exposed to either 0 ppb (control) or 120 ppb of ozone, for variable amounts of time and at different points of their life cycle. After fumigation, plants were exposed to herbivores or pollinators in the greenhouse, or to both groups of insects in the field.
My research shows that ozone affected reproductive performance differently, depending on the timing of exposure: plants exposed at earlier ages had their reproductive fitness increased, while plants exposed later in their life cycle showed a tendency for reduced reproductive fitness. Plant phenology was a key factor influencing reproductive fitness: ozone accelerated flowering and increased the number of flowers produced by plants exposed at early ages, while plants exposed to ozone at later ages tended to have fewer flowers. On the other hand, the ozone-mediated changes in plant-insect interactions had little impact on plant reproductive success.
The strongest effect of ozone on plant-pollinator interactions was the change in the number of flower visits received per plant, which was strongly linked to the number of open flowers. This means that, as a rule, exposure of plants to ozone early in the life cycle resulted in a higher number of pollinator visits, while exposure later in the life cycle resulted in fewer flower visits by potential pollinators. An exception was observed: the higher number of visits performed by large syrphid flies to young ozone-exposed plants than to the respective control plants went beyond the increase in the number of open flowers in those plants. Also, honeybees spent more time per flower in plants exposed to ozone than on control plants, while other pollinators spent similar amounts of time in control and ozone-exposed plants. This guild-dependent preference for ozone-exposed plants may be due to species-specific preferences related to changes in the quality and quantity of floral rewards.
In the field, ozone-exposed plants showed only a tendency for increased colonization by sucking herbivores and slightly more damage by chewing herbivores than control plants. On the other hand, in the greenhouse experiment, Pieris brassicae butterflies preferred control plants over ozone-exposed plants as oviposition sites. Eggs laid on ozone-exposed plants took longer to hatch, but the chances of survival were higher. Caterpillars performed better in control plants than in ozone-exposed plants, particularly when the temperature was high.
Most of the described effects were dependent on the duration and timing of the ozone exposure and the observed temperature, with the strongest effects being observed for longer exposures and higher temperatures. Furthermore, the timing of exposure altered the direction of the effects.
The expected climate change provides ideal conditions for further increases in tropospheric ozone concentrations, therefore for stronger effects on plants and plant-insect interactions. Acceleration of flowering caused by plant exposure to ozone may put plant-pollinator interactions at risk by promoting desynchronization between plant and pollinator activities. Reduced performance of caterpillars feeding on ozone-exposed plants may weaken herbivore populations. On the other hand, the increased plant reproduction that results from exposing young plants to ozone may be a source of good news in the field of horticulture, when similar results would be achieved in high-value crops. However, plant response to ozone is highly species-specific. In fact, Sinapis arvensis is considered a weed and the advantage conferred by ozone exposure may increase its competitiveness, with negative consequences for crops or plant communities in general. Overall, plant exposure to ozone might constitute a threat for the balance of natural and agro-ecosystems.