Cannibalism is common ecological interaction and ubiquitous in aquatic and terrestrial food webs. It is a unique ecological interaction in that it creates a trophic structure and feed-back loops between cannibals and conspecific prey within the population.  Because of this trophic structure, both density-mediated and trait-mediated indirect interactions (DMII and TMII) are possible even in simple one predator-one prey systems. As a consequence, cannibalism results in complex interactions between and within species with dynamics that cannot occur in systems without cannibalism.  However, the consequences of cannibalism for food web structure and dynamics are still poorly understood. 

Size determines most key life history processes, such as feeding and growth rates, food and habitat choice, and it determines the type and strength of ecological interactions with other species in the system.  For example when interacting with a single species, individuals might start out as prey, become competitor and finally predator when they reach their final size.  Thus, different sized individuals of the same species often differ more in their ecology than similar sized individuals of different species and represent distinct functional groups.  In addition, the behavior of individuals towards conspecifics will depend on their relative size, i.e. if they are predator (larger) or prey (smaller than conspecifics). If we are to understand the dynamic of interacting populations and communities in general we need to recognize that populations are not homogeneous but show distinct size structures and that we need to account for the functional differences between size classes within a population.

In my research I examine how the size structure within a population and the lethal and non-lethal interactions between different size-cohorts of a species determine predator-prey dynamics, and how these interactions alter the structure of communities and how they respond to environmental influences.  To answer these questions, I use a combination of theoretical and empirical work focusing on different species and communities. The theoretical framework provides a predictive framework to estimate the short-and long-term consequences of cannibalism in different trophic position on the dynamics and structured of food webs.  In particular I am interested in how cannibalism in simple food webs alters the conditions for coexistence, relative abundance, species invasion, food web stability and the effect of enrichment and top-down cascades. 

These models are closely connected to my empirical research that focuses on two different aquatic systems: a stream-salamander system and a pond-dragonfly system.  In these systems I examine the importance of cannibalism at different trophic levels.  In the salamander system I mainly focus on the impact of cannibalism and size structure in the predator on predator-prey interactions.  Cannibalism in the predator has the potential to change the way a predator regulates prey densities.  For example my results show that cannibalism and behavioral interaction between cohorts alter the predator's functional response and thereby the way it affects its own abundance and the abundance of the prey. Overall, the results demonstrate that predictions solely based on numerical changes can be strongly misleading and that we need to account for the size structure within predator populations.

Previous research has focused on cannibalism in the predator, ignoring that cannibalism is very common in intermediate predators, herbivores and detritivores which are subject to predation themselves.  Because of the trophic structure within the cannibalistic population, indirect interactions between cannibals and predator can lead to non-linear effects that are likely to reduce or enhance the effect of predators on the prey populations when compared to non-cannibalistic populations.  In the dragonfly system, I use different field and laboratory experiments to examine how cannibalism in the prey alters the impact of a predator in predator-prey systems, and to determine the role and relative importance of trait- and density-mediated indirect interactions. 

Relevant publications:

V.H.W. Rudolf (2006): The influence of size-specific indirect interactions in predator-prey systems. – Ecology 87: 362-271