The Princeton Guide to Ecology

Over the past few decades there has been an explosion of interest in the ecology of infectious diseases and their roles in ecosystem function. Many studies have focused on the dynamics of pathogens within human, other animal, and plant populations and their role in causing mass mortality events and population declines. Other researchers have focused on diseases that are increasing in incidence or geographic or host range—“emerging” infectious diseases—of humans, wildlife, and plants.

However, relatively few researchers have approached disease ecology in the context of ecosystem services, where diseases, parasites, or pathogens perform functions potentially useful to humans. In this chapter, we review the literature on three aspects of parasites and pathogens in the field of ecosystem services. The first is probably the most well known: their role in morbidity and mortality to their hosts, through which they disrupt the host’s ability to provide an ecosystem service, i.e., disrupting the survival or life-history success of ecosystem service providers. The second is the role of pathogens as regulating service providers by suppressing populations of pest species, resisting invasion, and acting as biocontrol agents. The third aspect is poorly understood but a subject of growing interest: rather than parasites performing the service per se, it is the role of species, communities, and biodiversity in regulating the risk of infectious diseases to people, i.e., performing a regulating service, that reduces disease risk. The thrust of our chapter is to review the state of the field, and we have paid particular attention to highlighting those areas where future research is likely to be most fruitful and to identifying strategies to take the field forward. We have therefore added a fourth section that discusses efforts and strategies to estimate the value of pathogens and the cost of their impact on natural capital and ecosystem services.

density dependent. A density-dependent process varies with the population density of the species concerned. For instance, below a certain host population size, parasitic infections may not occur (there are not enough hosts for the parasite to be transmitted between them), whereas above a certain host population size, parasitic infections may become prevalent. The probability of any individual host getting infected depends on the density of surrounding hosts.

emerging infectious disease. A disease that has recently and significantly increased in impact, in the number of cases it causes, or in its geographic range; a disease that is caused by a newly evolved pathogen or has recently been transmitted from one species to another to result in an outbreak in the new host species.

parasite. An organism that resides within or on, and is nutritionally dependent on, another organism. In this article, we include all forms of infectious microbes, including viruses, prokaryotes (e.g., bacteria), and eukaryotic parasites (e.g., roundworms).

pathogen. An infectious agent or parasite that causes illness in its host, usually defined as clinical illness, i.e., causing significant pathology or damaging physiological change.

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