干旱区生态环境知识资源中心

The tremendous increase in precipitation associated with the 1992-1993 El Nino profoundly affected terrestrial communities on arid islands in the Midriff region of the Gulf of California. In 1992, winter precipitation was 5.4 times the historical mean, and winter precipitation over the entire El Nino was the highest two-year amount ever recorded. Increased precipitation led to an explosion of annual plant growth on the previously barren (0-4% cover) islands: plant cover increased 10-160 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 onset of El Nino.

Insect abundance tracked this pulse in plant productivity and approximately doubled in 1992 and 1993 compared to 1991 levels. 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 affected the composition of the insect assemblage. In the dry years 1990-1991, the assemblage was dominated by insects feeding on products originating in the ocean: detritivores/scavengers on shore drift of marine algae and carcasses, avian parasites, and detritivores of bird products. Herbivores were extremely rare. The heavy plant growth in 1992 stimulated large (40-190 times pre-El Nino levels) increases in herbivores.

The great increases in land plant biomass and insect abundance are indicative of an important change in the dynamics of this system. Previously, most material flowing through the food webs of these islands originated directly or indirectly in the ocean. In contrast, during this El Nino, most material originated via productivity by terrestrial plants. Thus, wet El Ninos represent an agent that switches the system from one dependent primarily on allochthonous input to a system driven to a greater extent by in situ productivity. The influence of this pulse of terrestrial productivity extends beyond the El Nino years: the persistence and slow release of plant and detrital biomass reserves may also greatly affect dynamics for years after the El Nino event has passed. We suggest that large-scale climatic events such as El Nino may be long-lasting determinants of community dynamics rather than occasional disturbance events.