PARENT SESSION
Oral Session #9: Trophic structure and community interactions. Presiding: B. Menge.
Monday, August 6, 2001. 8:00 AM to 11:45 AM. Hall of Ideas I.

Bottom-up influences in a Hawaiian arthropod community.

Gruner, Daniel¹, ¹

ABSTRACT- On young basaltic lava flows on the island of Hawaii, nutrients severely limit primary productivity and plant diversity, but higher-level trophic effects have yet to be evaluated. This study focuses on arthropods associated with the dominant species in these young successional systems, Metrosideros polymorpha (Myrtaceae), also the dominant tree in the Hawaiian Islands. In August and September of 1998 on a 120-year-old flow, nutrient limitation was removed by fertilization and combined with bird predator removal cages in a large-scale, well-replicated, crossed factorial design. After one year, foliar nitrogen content and several measures of M. polymorpha growth rate were increased in fertilized relative to unfertilized plots. Arthropod densities were measured from foliage clipping samples, and herbivory and gall densities were observed in situ over the course of the experiment. Arthropod densities, primarily detritivores, were increased in fertilized relative to unfertilized plots. In contrast, herbivory and gall (Homoptera: Psyllidae) densities were unaffected by the treatments. In both in situ and clipping samples, arthropods and their damage to plants were higher on glabrous rather than pubescent tree morphotypes. The effects of top avian predators on arthropods are not yet apparent-these processes probably operate on longer temporal scales. However, top-down effects may not be particularly strong in this system because much of the plant biomass passes directly through arthropod and microbial detritivores that are not prominent components of avian diets in the region.

KEY WORDS: Metrosideros polymorpha, Hawaii, top-down and bottom-up processes, herbivory