Metapopulation Dynamics Override Local Limits on Long‐term Parasite Persistence

Abstract

Abstract. A simple null model, particularly germane to small and vulnerable organisms such as parasites, is that local conditions set a stage upon which larger-scale dynamics play out. Soil moisture strongly influences survival of entomopathogenic nematodes (EPN), which in turn drive trophic cascades by protecting vegetation from root-feeding herbivores. In this study, we examine the mechanisms responsible for patchy occurrence of an entomopathogenic nematode, Heterorhabditis marelatus, in a California coastal prairie. One hypothesis proposes that biotic factors such as competition and natural enemies could regulate occurrence of EPN populations. We found that fungi and other enemies of EPN, although locally potent, did not explain the patterns of incidence across sites. Abiotic factors also have strong effects on EPN persistence, especially for vulnerable free-living stages. Thus, we tested the hypothesis that patchy occurrence of EPN on a large landscape was driven by differences in soil moisture.Our research uses long-term data on nematode incidence in combination with a landscapelevel experiment to demonstrate the lack of a correlation between soil moisture and long-term persistence. A year-long experiment showed EPN mortality was weakly correlated with soil moisture among our study sites. Thirteen years of data, however, showed that colonization rates were highly correlated with long-term persistence. Sites with highest long-term persistence experienced the highest rates of rhizosphere colonization, extinction, and turnover. As a result, we concluded that metapopulation dynamics override limitations set by local and short-term abiotic conditions to determine long-term persistence in this parasite-driven trophic cascade.

Individual heterogeneity in mortality mediates long-term persistence of a seasonal microparasite

“…We designed our experiment to measure survivorship under the yellow bush lupine ( Lupinus arboreus ), the favorable habitat for H. marelatus , and under grasslands, the surrounding matrix habitat (Preisser et al 2005; Ram et al 2008b). In previous iterations of this experiment, we inoculated experimental tubes with a large number (ca.…”

“…Thus, the individuals that endure abiotic stress and survive are more likely to find a host and replenish the local population. Additional natural history about this system can be found in Strong (1999), Preisser et al (2006), and Ram et al (2008a, b)…”

“…IJ do not transition into other physiological states, and reproduction and development can occur only inside a host. A single IJ is capable of producing upwards of 100,000 offspring from a single host (Preisser 2003), although hosts are typically infected by more than one IJ (Ram et al 2008b). Each IJ must therefore rely on a fixed lipid reserve for survival and movement until it can locate and infect a host (Qiu and Bedding 2000).…”

“…In previous efforts, our research group estimated mortality of IJ by placing an approximate number (via a dilution series) in enclosures and counting the number of survivors over time (Preisser et al 2006; Ram et al 2008b). With such a design, we were unable to separate the measurement error of initial population size from process stochasticity.…”

Host resistance reverses the outcome of competition between microparasites

“…We measured the relative incidence of the two species in targeted surveys from six sites on the reserve established for long-term monitoring for H. marelatus (Ram et al 2008b). At each sample point in April 2005 we deployed four centrifuge tubes drilled with minute bottom holes, filled with 6-cc saturated soil, and stocked with two larvae of the greater wax moth Galleria mellonella (Lepidoptera: Pyralidae; hereafter, ''Galleria'') (see Gruner et al [2007] for description of sampling apparati ).…”