Document: LEE-3-48-22

Trophic cascades in tropical versus temperate systems.

DYER, L.A.*

Mesa State College, Grand Junction, CO 81502, USA ¹

Abstract:
Current hypotheses about tritrophic interactions predict that indirect positive effects of predators and parasitoids on plant fitness (top-down trophic cascades) are less likely to occur in complex versus simple trophic webs. However, studies in complex tropical ecosystems have demonstrated that top-down cascades can account for at least some of the variation in plant biomass and can affect plant fitness. To directly compare effect sizes of natural enemies on lower trophic levels in complex versus simple terrestrial food webs, I conducted a meta-analysis on 15 years of data collected from tropical and temperate systems. I also used the meta-analysis to compare top-down forces with the bottom-up effects of resource variability on plants and upper trophic levels. Latitudinal trends across the three trophic levels of plant, herbivore, and natural enemies indicate that compared to temperate ecosystems, the tropics exhibit more intense pressure from natural enemies, higher levels of herbivory, and more effective plant defenses. There were no differences in the effects of resources on plants, and despite predictions to the contrary from current hypotheses, there were no latitudinal differences in indirect effects of enemies on plants. In tropical systems, the effects of natural enemies on herbivores were greater than the effects of tropical plant defenses on herbivores. In contrast, while the effects of enemies on herbivores were large for temperate studies, the effects of temperate plant defenses on herbivores were significantly larger. Results from empirical studies that I have conducted with others corroborate these general trends. This major difference between tropical and temperate communities has very important implications for ecological and evolutionary relationships between plants, herbivores and natural enemies. For example, some current hypotheses suggest that natural enemies are more important in limiting the diet breadth of herbivores than plant chemistry. However, this might not be as tenable in temperate systems where, according to the meta-analysis, plant chemistry has a greater effect on herbivore biomass and survivorship than natural enemies.

Keywords: trophic cascades, latitudinal gradients, herbivory, predation, parsitism, plant defenses

This abstract is being presented at: 12:00 PM in session:
Oral Session #4: Herbivore Responses to Plants.