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, apropo s of the Hairston-Smith-Slobodkin (HSS) hypothesis. We conducted a field ex periment with a suspected trophic cascade. Could an entomopathogenic nemato de protect bush lupine by killing root-feeding 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 en tomopathogenic nematode. We modeled lupine survival with a hierarchical family of "one-hit dose resp onse" models to interpret the results of the experiment. We obtained maximu m likelihood estimates of parameters and selected the best-fitting model us ing the Schwarz Information Criterion (SIC). The best model fit the data cl osely, and SIC model selection was consistent with classical likelihood rat io 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 b ut largely consistent with results of the one-hit analysis. Finally, we com pared our model-centered approach with the conventional methods-centered ap proach of logistic regression in statistical packages. While these packages give correct calculations, the implications of hypothesis tests are ecolog ically obscure in the absence of the explicit representation of models and their hierarchical relationships. For understanding ecological data, buildi ng an explicit statistical model of the process and testing parameters can be more: informative than accepting the implicit model and testing variable s in canned statistical packages. The ecological implications were that seedling survival decreased exponenti ally with increasing densities of root-feeding caterpillars, and the entomo pathogenic nematode virtually canceled the negative effect: of this herbivo re upon seedling survival. However, the significance to the broader communi ty of this trophic cascade remains to be demonstrated. This cascade is a mo dule or vignette within the greater food web, and additional interactions a ffect its influence: intraguild predation by nematode predators, apparent c ompetition from other herbivores of lupine (each with its own natural enemi es), and even more complicated interactions through competing plant species all come into play. As well, genetic variation of both the lupine and ghos t moth caterpillars affects these interactions. Evidence does not support t he inference that protection from ghost moth caterpillars by the entomopath ogenic nematode is key to the "green" world of bush lupine.