PARENT SESSION
Oral Session # 84: Trophic Structure II: Terrestrial and Aquatic Systems.
Presiding: MD Moran
Thursday, August 7. 1:30 PM to 5:00 PM, SITCC Meeting Room 202.
Influences of resources and bird predation on species richness and diversity of Hawaiian arboreal arthropods.
Gruner, Dan*,1, 1 University of Hawaii at Manoa, Honolulu, HI, USA
ABSTRACT- Predator (top-down) and resource (bottom-up) influences in food webs are strong and pervasive. Both resource enrichment and predation, in isolation, are predicted to increase community diversity overall, but few studies have investigated their interactive effects on diversity in real terrestrial ecosystems. This study focuses on arthropods associated with the dominant species in 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, severe nutrient limitation was removed by fertilization and combined with bird predator removal cages in a large-scale, well-replicated, randomized block design. Arthropods were taken from foliage clipping samples at the outset and conclusion of the 2.75 year experiment. I sorted and enumerated all arthropods, well over 100 species from 14 orders, to the species level. Previous reports documented increased plant growth with fertilization and positive and interactive effects of treatments on arthropod densities. Fertilization also increased species richness of arthropods, but diversity declined due to disproportionate increases in dominant species. Bird exclusion increased species richness to an intermediate level, and strongly influenced species composition and densities of several restricted groups, such as spiders. Ordination revealed distinct compositional differences among treatments. Among trophic groups, parasitoids and predators represented the most species, but were generally transient, while detritivores were the most abundant but species poor. Top-down effects in this system are interactive, not purely additive, with bottom-up influences. These results support the prediction that bottom-up constraints set the range of possibilities, while top-down forces determine the realized community structure.
Key words: Hawaiian Islands, top-down and bottom-up forces, Metrosideros polymorpha, arthropod diversity