Infection in ecosystems: Data, Models and Effects

Abstract

Infectious agents are ubiquitous in nature. They can be broadly distinguished into microparasites (viruses, bacteria, fungi, protozoa), macroparasites (nematodes, trematodes and cestodes), ectoparasites (fleas and ticks), parasitic castrators and parasitoids. Although these types of infectious agents are very different in the way they affect their host, they all tend to live on or in their hosts for most of their life while benefiting from its nutrients. Ecologically speaking, infectious agents are part of food webs (networks of species that have trophic consumer-resource relations). They are in essence consumers of resources (their hosts), but they differ from typical consumers in having only one victim per life stage and not necessarily killing or fully consuming their victims. The interaction between infectious agents and their hosts can result in subclinical or clinical disease in infected host individuals. Subclinical, infectious agents possibly have an impact on life-history traits, behaviour, feeding or other individual-level aspects of their hosts because of increased energy use. Infected predators may have a reduced ability to hunt and catch prey, while infected prey may be easier to catch or less nutritious. Clinical, infectious agents may increase mortality in their hosts. By changing behaviour or survival of their hosts, infectious agents indirectly influence other species, including non-host species, of their ecological community. These effects could be measured through an infectious agent’s influence on energy flow, biodiversity, community interactions and the abiotic part of the ecosystem. In that way infectious agents may affect structure, functioning and stability of ecological communities. In this thesis we use data collection, data analysis, and mathematical and computational modelling to study the potential impact of different types of infectious agents in food webs and ecosystems. First, we show results of field research on macroparasites of top predators. The research provides an illustration of the diversity of parasites and their hosts in real life systems. Second, we systematically classify the effects of infectious agents on energy flow, community interactions, diversity, and at the ecosystem level, covering a broad range of infectious agents in a broad range of host species. We also classify their diversity, types and functional roles. We discuss a concept of new indirect approach of modelling infectious agents in food webs and give a simple model of a microparasite in a very basic Lotka-Volterra consumer-resource system. Third, we quantify the clinical and subclinical impacts of infectious agents on food web structure and stability, using an indirect approach by adding infectious agents not as stand-alone species, but through the effects they have on their hosts. Finally, we show an example of a new approach based on multiplex networks that could be used in future for modelling the effects of infectious agents in food webs, explicitly recognizing different types of interaction between infectious agents and its hosts that occur in complex systems. This thesis makes an attempt to shed the light on the importance of infectious agents as potentially major players in food webs. They can play a much more positive, and even essential, role, in addition to their already established negative image. A healthy ecosystem may be one that is rich with infectious agents.