EFFECTS OF BASAL RESOURCES, PREDATION, AND ALTERNATIVE PREY IN MICROCOSM FOOD-CHAINS

Ecological theorists propose that the species composition of trophic levels can influence the relative strengths of top-down (predation) or bottom-up (nutrient) effects in food chains. We tested this by constructing two- and three-level food chains of bacteria and protists. Bacteria made up the first trophic level. The second level contained Chilomonas paramecium alone, Colpidium cf. striatum alone, or both together. Predatory Euplotes patella occupied the third trophic level. These assemblages were cultured in microcosms containing either low- or high-nutrient medium. Manipulating nutrients and predation produced comparable changes in the abundance of bacteria. In the second trophic level, Chilomonas was usually driven extinct by predation, but Colpidium was affected more by nutrients than predation because it had a partial size refuge from predation. Both species survived more poorly with predatory Euplotes if the other was present, because the predator was more abundant and persistent when both prey were available. The predator was less persistent in high-nutrient microcosms, because additional nutrients boosted the proportion of Colpidium populations within the size refuge. This type of mechanism could limit trophic cascades in food chains where resources affect prey vulnerability.