| dc.description.abstracteng | Global change affects ecosystems worldwide and has already caused a massive decline
in the world's biodiversity. As the processes behind environmental change continue at
ever-accelerating rates, this leads to a severe threat of ecosystem functioning, ecosystem
services, and, in the end, human well-being. The most prominent drivers of global
biodiversity loss are climate change, increasing nitrogen deposition, land-use change
and biotic exchange. Their correlation with species extinctions has been documented in
numerous studies some of which have identified the underlying mechanisms they
operate on. However, it still remains difficult to predict the exact effects of specific
drivers of environmental change on populations. This makes it hard to identify
particularly endangered species and to develop adequate conservation strategies.
In my thesis, I focus on small-scale effects of global-change drivers on single
individuals or populations. I use bioenergetic modelling to show how these low-level
effects scale up to higher levels of ecological organisation and influence the stability of
food-web motifs. Finally, I provide experimentally testable hypotheses on
environmental-change effects and their compensation. Throughout the research chapters
of my thesis, I study the effect of different environmental-change drivers on the stability
of different trophic motifs.
In Chapter 2, I focus on single consumer-resource interactions and how environmental
warming influences their stability. The relationship between temperature and species'
biological rates (metabolism, growth and feeding) is well-known from empirical
warming experiments. However, their interactive effects on the stability of consumer-
resource systems are still under debate. I show that warming leads to dynamic
stabilization of biomass oscillations. These results are based on an extensive literature
research about temperature scaling of metabolism, feeding rates and maximum
population size. Implementing these relationships into a generalized bioenergetic model
yields information on the dynamical consequences of the different scaling relationships.
The vast majority of possible parameter combinations predicts a dynamic stabilization
of consumer-resource interactions at the risk of predator starvation. Consequently, this is
tested in a microcosm experiment using bacterial prey (Pseudomonas fluorescens) and a
cilliate predator (Tetrahymena pyriformis). Time-series analyses of these experiments
confirmed the hypothesis of warming leading to an increased population stability while,
at the same time, undermining species diversity.
In Chapter 3, I investigate the effect of nutrient enrichment which has been reported to
induce unstable dynamics in consumer-resource systems. The resulting oscillations have
been shown to endanger species persistence in trophic systems of low complexity.
However, in more complex natural systems this effect seems to be dampened which
indicates that some intrinsic properties of complex systems prevent unstable dynamics.
Identifying these “ecosystem buffers” is crucial for our understanding of the stability of
ecosystems and an important tool for environmental and conservation biologists. Earlier
theoretical studies suggested that this stabilization might be caused by so-called “weak
interactions”. However, their relevance has rarely been tested experimentally. I use
network and allometric theory for an a-priori identification of species that buffer against
externally induced instability of increased population oscillations via weak interactions.
Afterwards, the hypotheses are tested in a microcosm experiment using a soil food-web
motif. I show that large-bodied species feeding at the food web's base, so called “trophic
whales”, can buffer ecosystems against unstable dynamics induced by nutrient
enrichment.
In Chapter 4, I investigate the combined effects of habitat fragmentation and nutrient
enrichment as they occur under increasing land-use intensity. Moreover, this chapter
tackles the challenges of an integrative ecological theory on how different drivers of
global change interact. I thus study the combined effects of habitat isolation and nutrient
enrichment on the stability of a tri-trophic food-chain. Therefore, I expand bioenergetic
models towards spatially explicit systems of two habitat patches using empirically-
derived allometric scaling relationships of animal migration. I find that extinctions that
occur at high levels of habitat fragmentation are caused by reduced bottom-up energy
supply. Thus, conservation activities that focus only on single species might not prevent
biodiversity loss if they ignore the respective lower trophic levels. The starvation effects
of isolation are counteracted by nutrient enrichment which increases energy fluxes
along the food chains. Thus, habitat isolation stabilizes eutrophic systems but
undermines species diversity in oligotrophic systems.
The three research chapters provide good examples of how a generalized bioenergetic
modelling approach provides an in-depth understanding and can generate testable
hypotheses on the behaviour of simple trophic systems under global change. The
general findings are combined and discussed in the Synopsis which also provides a
categorization of environmental stressors according to their respective influence on
ecosystem stability. The Synopsis elucidates the interplay of multiple environmental
stressors and how their combined effects endanger biodiversity. In an ever changing
world, our understanding of ecosystem processes and their underlying mechanisms is of
striking importance. This conceptual work will foster future research by (1) applying
general modelling tools to investigate the effects of different environmental stressors,
(2) testing the generated hypotheses in experimental systems, and (3) synthesizing the
findings according to their respective influence on systems stability. Furthermore, it will
contribute to new and well-founded conservation approaches. | de |