Allometry, temperature, and the stability of food webs

Understanding the mechanisms driving stability in natural ecosystems is of crucial importance, especially in the current context of global change. A classic paradigm in ecology was that complex food webs (the “who eats whom” of natural ecosystems) should be unstable. This paradigm, however, was based on simple mathematical models. Throughout the last decades, scientists proposed solutions to the contradictions between the predictions of simple models and the observation of the complexity of nature. However, the fundamental mechanisms driving these stabilizing effects are still rather unexplored. Especially, exploring and predicting the reaction of natural ecosystems to changes of the environment is a pressing issue of our time. Forecasting models predicted global warming up to 8°C until 2100, also nutrient enrichment is caused by anthropogenic land use. This causes changes in species composition and may lead to species extinctions. A fundamental unit of natural ecosystems is the interaction between species. The most obvious interaction is the feeding interaction between a predator and its prey. This interaction is mainly influenced by the metabolism and the feeding rate of the predator, as well as by the population density of the prey. Combining a mechanistic understanding of these interactions and traditional population models led to ground-breaking insights into the mechanisms stabilizing food-webs. For example, a non-random distribution of feeding interactions in a food web increases its resistance against destabilizing effects. This might be caused by strong constraints introduced by the distributions of body masses across the species in a food web. Additionally, relatively weak interactions are known to have a positive effect on stability, if they occur in a specific way within small food-web motifs (e.g., a weak interaction from a top predator to the basal species and a strong interaction to its main prey, the intermediate predator). Also, models suggested that the stability of natural populations may change, if the feeding capacity and the metabolism (or the death rate) of a predator are not equally influenced by the environmental temperature. However, empirical support for this is still scarce. In this thesis, I explored the impact of body masses and environmental temperature on feeding interactions (Chapters 3.1., 3.2.& 4.1.). Additionally, I explored the influence of these constraints on population and food-web stability by using mathematical models (Chapters 3.3., 3.4., 4.1. & 4.2.). The body-mass dependence of metabolism generally followed the 3/4 power laws as predicted by the Metabolic Theory of Ecology (Chapter 3.1.). However, the strength of the feeding rates follows a hump-shaped curve with the body mass ratio of the predator to its prey (Chapters 3.1.& 3.2.). This leads to the phenomenon that a predator would not be able to fulfil its metabolic demands if only insufficient small prey would be available (Chapter 3.2.). Moreover, with increasing temperature, the metabolism increases more than the ability of the predator to consume food (Chapter 4.1.). These findings have fundamental implications for food web stability. Predators only are able to exist within a given range of body mass ratios to their prey. Approximately 97% of all tri-trophic food chains existing in natural food webs fall within this range (Chapter 3.3.). Additionally, at high body-mass ratios an additional interaction from the top predator to the basal species (omnivory) leads to a higher stability when incorporating the results from chapters 3.1. & 3.2. into the population models. Together with the distribution of the interactions as given in natural food webs (Chapter 3.3.), omnivory motifs are stabilised within the whole range of natural body-mass ratios (Chapter 3.4.). The different temperature dependencies found for metabolism and feeding in chapter 5.1 led to more stable population cycles but may also lead to extinction events caused by starvation of the predators. In addition, warming affects the food web structure, increasing or decreasing these starvation effects, as found in chapter 4.1. Also, enrichment effects on population stability and food-web persistence can be overcome by incorporating naturally plausible feeding interactions (Chapter 5.1.). Overall, incorporating naturally relevant feeding interactions from laboratory studies into population and food-web models provides important insights into the functioning of populations and their stability in the context of food webs and their response to global change