EL-NINO EFFECTS ON THE DYNAMICS AND CONTROL OF AN ISLAND ECOSYSTEM INTHE GULF OF CALIFORNIA

The tremendous increase in precipitation associated with the 1992-1993 El Nino profoundly affected terrestrial communities on arid islands i n the Midriff region of the Gulf of California. In 1992, winter precip itation was 5.4 times the historical mean, and winter precipitation ov er the entire El Nino was the highest two-year amount ever recorded. I ncreased precipitation led to an explosion of annual plant growth on t he previously barren (0-4% cover) islands: plant cover increased 10-16 0 times over what it had been. With the resumption of arid conditions in 1994, live plant cover returned to the low levels seen before the o nset of El Nino. Insect abundance tracked this pulse in plant producti vity and approximately doubled in 1992 and 1993 compared to 1991 level s. In 1994, the crash of annual plants caused insect densities to drop to the lowest levels recorded during the 5-yr study. El Nino also aff ected the composition of the insect assemblage. In the dry years 1990- 1991, the assemblage was dominated by insects feeding on products orig inating in the ocean: detritivores/scavengers on shore drift of marine algae and carcasses, avian parasites, and detritivores of bird produc ts. Herbivores were extremely rare. The heavy plant growth in 1992 sti mulated large (40-190 times pre-El Nino levels) increases in herbivore s. The great increases in land plant biomass and insect abundance are indicative of an important change in the dynamics of this system. Prev iously, most material flowing through the food webs of these islands o riginated directly or indirectly in the ocean. In contrast, during thi s El Nino, most material originated via productivity by terrestrial pl ants. Thus, wet El Ninos represent an agent that switches the system f rom one dependent primarily on allochthonous input to a system driven to a greater extent by in situ productivity. The influence of this pul se of terrestrial productivity extends beyond the El Nino years: the p ersistence and slow release of plant and detrital biomass reserves may also greatly affect dynamics for years after the El Nino event has pa ssed. We suggest that large-scale climatic events such as El Nino may be long-lasting determinants of community dynamics rather than occasio nal disturbance events.