PARENT SESSION
Oral Session #6: Insect Population and Community Ecology: Interactions, foodwebs.
Presiding: M. Hunter
Monday, August 5. 8:00 AM to 11:30 AM. Apache Meeting Room, TCC.

Top-down and bottom-up influences in a Hawaiian arboreal arthropod food web.

GRUNER, DANIEL^*,1, ¹ Univerity of Hawaii at Manoa:, Honolulu, HI

ABSTRACT- Predator (top-down) and resource (bottom-up) influences in food webs are strong and pervasive, but few studies have investigated their interactive effects 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. Arthropod densities were measured from foliage clipping samples at the outset and conclusion of the experiment. After 2.75 years, foliar nitrogen content and M. polymorpha growth were directly increased in fertilized relative to unfertilized plots, but were unaffected by indirect effects of top predator exclusion. Fertilization increased densities of detritivores and bird exclusion increased spider densities. Arthropod combined densities, besides spiders, were reduced in the interaction treatment, suggesting cascading effects of these intermediate level predators to the detritivores. Herbivore numbers were unchanged by either treatment, but herbivores were sensitive to another bottom-up influence, foliar pubescence, as confirmed by separate arthropod collections. Top-down effects in this system are complementary, not purely additive, to bottom-up influences, and the details are dependent on the structure of the food web.

KEY WORDS: top-down vs. bottom-up, trophic dynamics, Hawaiian Islands, Metrosideros polymorpha