Biotic and abiotic factors interact to create environmental heterogeneity. A simple null model, particularly germane to small and vulnerable organisms such as parasites, is that local abiotic conditions set the stage upon which biotic conditions can act. Local soil moisture strongly influences survival of entomopathogenic nematodes (EPN), which in turn drive trophic cascades by protecting vegetation from root-feeding herbivores. Wet years and moist rhizospheres foster these nematodes, while dry rhizospheres in drought years lead to high mortality and local population extirpation. Our null model was a causal chain where severe abiotic conditions increases mortality and reduces survival, thereby lowering long term incidence. Using a 14-year (1993-2007) data set on nematode occurrence, we calculated colonization, extinction, and persistence rates at six sites on a spatial landscape. In 2005-06, we conducted an experiment to measure nematode mortality across the same sites.
Results/Conclusions
Mean mortality over the course of one year, the longest period that nematodes have to survive & infect a new cohort of hosts, was identical and low among five of six sites. Further, mortality did not correlate strongly with spatial patterns of moisture or persistence. Sites with high long-term persistence spanned the entire range of moistures and mortality rates. Paradoxically, sites with low persistence experienced the lowest overall mortality. Irrespective of local mortality or moisture, colonization rate strongly predicted long-term persistence. Sites with high persistence also had the highest colonization (mean=0.016, SE = 0.0009, n=14) and extinction rates (mean=0.05, SE = 0.003, n=14). Despite favorable conditions and low mortality other sites were rarely colonized (mean=0.0001, SE= 0.0001, n=14) and experienced very few extinctions (mean=0.015, SE = 0.0001, n=14). The inherent patchiness and metapopulation structure of naturally-occurring EPN populations may be a key to the persistence and stability of EPN-prey interactions
