Using body mass, metabolism and stoichiometry to assess ecological impacts in a changing environment
von Malte Jochum
Datum der mündl. Prüfung:2016-02-15
Betreuer:Prof. Dr. Ulrich Brose
Gutachter:Prof. Dr. Ulrich Brose
Gutachter:Prof. Dr. Stefan Scheu
EnglischEarth’s ecosystems are composed of living organisms and their biotic and abiotic environment. In order to understand the structure and functioning of these ecosystems, ecologists study the interactions of organisms with one another and their environment. The body mass of an organism, its energy demand, and the elemental composition of the body tissue of itself and the resources it depends on are three fundamental aspects of its biology affecting its interactions with other organisms and its environment and, therefore, shaping ecological communities. While a large body of research has established the importance of these drivers, much less is known about how they jointly affect whole-ecosystem processes. This lack of knowledge is partly due to the lack of comprehensive approaches integrating body mass, metabolism and stoichiometry to assess ecosystem structure and functioning in diverse, multitrophic communities. Body size has fundamental effects on biological rates and ecological interactions and strongly affects living organisms across levels of organisation, from individuals to communities. One major reason for this importance is the effect of body size on an organism’s metabolic rate, the rate of energy uptake, transformation and allocation that, in turn, controls important aspects of its biology and defines the organism’s energy demand. Ecological stoichiometry is concerned with the balance of chemical substances in ecological interactions and thus puts constraints on consumer-resource interactions. As such, these three drivers play a key role in describing and explaining ecological processes. Over the past centuries, the growing human population has dramatically altered Earth’s ecosystems and climate with severe consequences on biodiversity and ecosystem functioning. In this thesis, I provide an important step towards jointly using body mass, metabolism and stoichiometry to assess ecological impacts of changing environmental conditions, as driven by anthropogenic alteration of Earth’s ecosystems. First, in Chapter 2, I review previous research on body size with a focus on insects. Initially, I discuss the historical underrepresentation of insects in body-size research and present recent developments toward a better representation of this important animal group enabled by technological improvements and the availability of high-resolution datasets. I discuss the importance of body size for animal movement and behaviour and highlight their importance for the strength and outcome of trophic interactions. Furthermore, I point to the importance of including both size and non-size effects, such as temperature,phylogeny, and stoichiometry, in future ecological experiments and theory. Finally, I emphasise the intersection of allometry effects on behaviour and functional-morphology effects on foraging success as promising directions of future research. In Chapter 3, I present whole-community energy flux as a measure of multitrophic ecosystem functioning and test it by assessing ecological consequences of anthropogenic land use on biodiversity and ecosystem functioning in tropical leaf-litter macro-invertebrate communities in forest, jungle rubber, rubber and oil-palm plantations. Combining metabolic theory and food web theory with previous advances in the energetic view of ecosystem processes, I develop a highly flexible measure that takes into account consumer metabolism, assimilation efficiency, network topology, feeding preferences and loss to higher trophic levels. It can now be used to easily assess and compare ecosystem funtioning across communities in different ecosystem types, carrying out a diverse range of functions that would otherwise be difficult to compare. After establishing consistent declines in species richness, animal density, and biomass from forest to oil-palm macro-invertebrate communities, I find that energy flux also decreases and is able to pick up more fine scale differences between trophic groups than, for example, standing stock biomass can detect. Additionally, I use the novel measure of ecosystem functioning to compare biodiversity ecosystem functioning relationships between land-use systems and find the relationship of species richness and energy flux to be steepest in oil-palm communities. However, different trophic guilds exhibit different patterns here. These results highlight the importance of including trophic complexity into future research on community-level processes and additionally emphasise the ability of the developed ecosystem functioning measure to describe community-level patterns based on only few easily obtainable parameters. In Chapter 4, I combine the energetic approach developed in the previous chapter with ecological stoichiometry theory to assess multitrophic consumer responses to changing resource quality. Specifically, I test for changes in consumer stoichiometry, biomass, and feeding rates in response to increasing resource carbon:nitrogen ratios. By slightly altering the energy flux calculations, I calculate consumer feeding rates based on metabolic demand and assimilation efficiency in response to varying resource stoichiometry without having to measure feeding rates in the field or laboratory. I find that, instead of altering their body stoichiometry or avoiding low-quality resources, detritivore and predator communities exhibit increased feeding rates when exposed to low-quality resources. Interestingly, detritivore species richness significantly decreases with decreasing resource quality, potentially indicating limited ability of consumer species to perform compensatory feeding due to physiological constraints. Thus, my findings suggest compensatory feeding to be much more common across trophic levels than was previously known. Additionally, the method of calculating consumer feeding rates in response to resource quality is a highly useful tool for future research on consumer-resource interactions. Finally, in Chapter 5, I use an information theoretic approach to investigate the effects of basal resource stoichiometry and habitat structure on multitrophic consumer biomass density and diversity. Using this standardised model averaging framework, I am able to directly compare the effects of three habitat structural and seven stoichiometric variables on ten major taxonomic groups and four functional feeding guilds. I find partial support for all specifically tested hypotheses relating certain consumer groups to different stoichiometric and habitat-structural drivers. The tropical macro-invertebrate consumer communities are co-limited by multiple, rather than single, variables with different taxonomic groups controlled by different sets of predictor variables. Interestingly, biomass density and diversity of a given consumer taxon do not always respond homogeneously to a given change in a certain stoichiometric variable, but exhibit a diverse range of response patterns, such as parallel and opposing effects, but also cases where only one of the community characteristics is affected. Consequently, I develop a conceptual framework explaining response patterns found across 80% of the taxonomic consumer groups by assuming a saturating response of biomass, but a hump-shaped response of diversity to increasing availability of a limiting resource. Thus, my findings suggest that tropical consumer communities are co-limited by multiple parameters and highlight the importance of looking at both consumer biomass and diversity when trying to understand community responses to changing environmental conditions. Additionally, I provide a conceptual framework explaining biomass and diversity responses that can now be tested in other ecosystem types. Taken together, in this thesis, I present novel methods and approaches that jointly use body mass, metabolism and stoichiometry to investigate ecological consequences of changing abiotic and biotic conditions. I develop whole-community energy flux and a method for calculating consumer feeding rates in response to resource stoichiometry and test the ability of these tools to describe ecological processes in complex, real-world communities. Furthermore, I integrate metabolic theory and ecological stoichiometry theory to study consumer-resource interactions across trophic levels. By combining ecological theory with state-of-the-art statistical approaches to develop and test novel methods of assessing ecological processes, this thesis provides a significant advance toward understanding and mitigating ecological impacts of anthropogenic alterations of Earth’s ecosystems.
Keywords: metabolic theory of ecology; ecological stoichiometry