Rapid decline of a four-trophic-level system over a 15-year period.

Abstract

Recent climate change has produced a wide range of shifts in the phenology of species and consequent changes in the relationships among them. However, a dearth of studies exists that evaluates an entire trophic pyramid over an extended period. Here I characterize changes in several important variables on such a multitrophic suite of species, which contains two primary producers (sensitive and marsh ferns Onoclea sensibilis and Thelypteris palustris), one herbivore (caterpillars of the fern moth Herpetogramma theseusalis), one primary parasitoid (the braconid wasp Alabagrus texanus), and three facultative hyperparasitoids (including the eulophid wasp Aprostocetus sp.). I ask how the abundance of these species changes over time, how parasitism and mortality change along with the plants an herbivore selects, how the relationships between the different trophic levels change, and what effect these results play in the makeup of a trophic pyramid. Herbivores prospered most on sensitive fern, while levels of parasitism and mortality on herbivores and primary parasitoids remained relatively similar over the study period. However, facultative hyperparasitoids declined strikingly, and the few remaining individuals increased their exploitation of caterpillars (vs. Alabagrus), further decreasing the impact of the fourth trophic level. Since a previous study demonstrated that phenological change by these primary parasitoids exceeds that of their herbivore, further breakdown of the system appears likely.