Density‐Dependent Host‐Pathogen Dynamics in Soil Microcosms

Abstract

Temporal density—dependent parasitism and a host threshold density are important features of disease induced by infectious parasites in populations of aboveground, macroscopic organisms. We determined whether these features also occur in soil microcosms containing a microscopic host (the nematode Heterodera schachtii) and its parasite (the nematophagous fungus Hirsutella rhossiliensis). Soil microcosms are especially interesting because (1) the environment and scale are quite different from conventional host—parasite systems and may result in considerably different disease dynamics, (2) the small size of the soil microcosms, although biologically appropriate, facilitates experimentation and parameter estimation, and (3) some soil—borne, microscopic organisms (such as H. schachtii) are important agricultural pests. Temporal density—dependent parasitism was directly assessed with laboratory experiments in which host density and environment were controlled. A theory, which complements and extends the experiments, was developed to enable direct comparison of observed and predicted dynamics and to provide a stringent test of our understanding of processes underlying the dynamics. The theory was simple, yet explicitly described the essential biology. Parameters for the theory were measured with short—term experiments. We found that the disease dynamics in soil microcosms exhibited both temporal density—dependent parasitism and a host threshold density. However, epidemics were slow to develop. Observed and predicted dynamics were quite similar, indicating that our understanding of the underlying biology was correct.

Nematophagous Fungi

“…This raises the question whether the parasitism of nematophagous fungi can regulate the population size of soil nematodes. Experiments in soil microcosms using the endoparasitic fungus H. rhossoliensis and plant parasitic nematodes have shown that the level of fungal parasitism is dependent on the nematode density, although there is a relatively long time lag in the response of the fungal population to changes in the number of nematodes (Jaffee et al, 1992). See also: Parasitism: the variety of parasites Mostly, plant-parasitic nematodes attack plant roots and, therefore, the ability of the nematophagous fungi to grow in the rhizosphere is of great importance for their capacity to control these nematodes.…”

Density‐dependent parasitism in biological control of soil‐borne insects, nematodes, fungi and bacteria

“…For free-living propagules, density dependence arises because the probability of transmission is a function of parasite density and parasite density is a function of past host density (e.g. Jaffee et al, 1992). Host density seems clear enough for nematodes, bacteria and insects, but variable size and morphology complicate the definition of what comprises an individual fungus; for fungi, the term biomass-dependent parasitism might be more appropriate than density-dependent parasitism (Cook & Baker, 1983).…”

The anthropogenic environment lessens the intensity and prevalence of gastrointestinal parasites in Balinese long-tailed macaques (Macaca fascicularis)

“…Although the effect of the environment on the transmission dynamics of gastrointestinal parasites is evident (Haufstater and Meade 1982;Jaffee et al 1992;Roepstorff and Murrell 1997;Slifko et al 2000;Patz et al 2000), many studies compare host populations that differ drastically in only one important environmental component (i.e. water availability or habitat) (Anderson and Beaudoin 1966;Grutter 1998;Baird 2003;Ferrari et al 2004;TrejoMacias et al 2007).…”