LICHEN HERBIVORE PREFERENCE: A TEST OF TWO HYPOTHESES

Field data from a saxicolous lichen community in Shenandoah National Park, Virginia, suggested that the slug Pallifera varia grazed more frequently on certain lichen species than would be expected if it fed randomly. Two hypotheses might explain this grazing pattern. The preference hypothesis, that slugs select lichens of highest quality, was tested by measuring the concentration of essential elements in lichens eliciting high and low preference by slugs. Lichens with the highest element contents were assumed to be of the highest quality to herbivores. The avoidance hypothesis, that preference by slugs actually results from the rejection of unpalatable species, was tested by offering slugs choices of baited filter-paper disks impregnated with secondary products of the lichen species in question. Results suggested that preferred lichens had significantly lower concentrations of N, P, and Ca, and that avoided lichens produced secondary products that effectively inhibited Pallifera grazing activity. These results suggested that the avoidance hypothesis was the better explanation for nonrandom Pallifera grazing patterns. Furthermore, they suggested that lichens with the highest concentrations of essential elements are most likely to produce defense compounds, an observation that supports predictions to explain patterns of chemical defense in vascular plants.