Model selection for a subterranean trophic cascade: Root-feeding caterpillars and entomopathogenic nematodes

Conjecture abounds while evidence is limited concerning indirect protection afforded plants by carnivorous predators in terrestrial ecosystems, apropos of the Hairston-Smith-Slobodkin (HSS) hypothesis. We conducted a field experiment with a suspected trophic cascade. Could an entomopathogenic nematode protect bush lupine by killing rootfeeding ghost moth caterpillars? The experiment measured survival of lupine seedlings as a function of density of hatchling ghost moth caterpillars in rhizospheres with or without the entomopathogenic nematode. We modeled lupine survival with a hierarchical family of 'one-hit dose response' models to interpret the results of the experiment. We obtained maximum likelihood estimates of parameters and selected the best-fitting model using the Schwarz Information Criterion (SIC). The best model fit the data closely, and SIC model selection was consistent with classical likelihood ratio test results of models nested in the one-hit family. A parallel analysis performed upon a logistic family of models yielded results of poorer fit but largely consistent with results of the one-hit analysis. Finally, we compared our model-centered approach with the conventional methods-centered approach of logistic regression in statistical packages. While these packages give correct calculations, the implications of hypothesis tests are ecologically obscure in the absence of the explicit representation of models and their hierarchical relationships. For understanding ecological data, building an explicit statistical model of the process and testing parameters can be more informative than accepting the implicit model and testing variables in canned statistical packages. The ecological implications were that seedling survival decreased exponentially with increasing densities of root-feeding caterpillars, and the entomopathogenic nematode virtually canceled the negative effect of this herbivore upon seedling survival. However, the significance to the broader community of this trophic cascade remains to be demonstrated. This cascade is a module or vignette within the greater food web, and additional interactions affect its influence: intraguild predation by nematode predators, apparent competition from other herbivores of lupine (each with its own natural enemies), and even more complicated interactions through competing plant species all come into play. As well, genetic variation of both the lupine and ghost moth caterpillars affects these interactions. Evidence does not support the inference that protection from ghost moth caterpillars by the entomopathogenic nematode is key to the 'green' world of bush lupine.