Print PageThe Midriff Islands of the Gulf of California, an archipelago that stretches from Baja California to mainland Sonora Mexico, occur in the midst of one of the most productive seas in the world. Nutrient rich cold-water upwelling leads to a diverse marine based-trophic web, while terrestrial biomes receive irregular precipitation that is dramatically arid at present, supporting a unique assemblage of desert species. The vegetation of each island is distinct, from cardón cactus forests to agave-studded slopes, the forcing topographic, climatic, and biological forces have sculpted a set of unparalleled desert worlds. Yet, the flora of the Midriff Islands shows evidence that climatic conditions were not stable during Pleistocene glaciation cycles, with the occurrence of surprisingly disjunct relict species. These two-dozen islands, with diverse geologic histories, offer a remarkable system in which to pursue (1) what was the predominant vegetation of the Midriff Islands during the Pleistocene and (2) how does the rich oceanographic setting of the Gulf of California shape the flora found there today?
Results/Conclusions
The flora of these desert islands are a manifestation of the climatic and geologic legacies of the Pacific coastal drylands. At the Last Glacial Maximum sea levels were 100m below present levels, substantially changing the physiographic realities of the Midriff region. Distributions of extant and fossil plant species on Midriff Islands provide evidence for two major biotic responses to climatic changes. First, a number of widespread Baja California plant species have distributions indicative of migration from the Peninsula to the mainland via the Midriff Islands. Second, the presence of a dozen species with disjunct distributions at high elevations on the largest landbridge island is indicative of historic vegetation different from the current suite of desert species. It is likely these vegetation patterns were shaped by a more equable climate with increased winter rainfall and cooler summers with diminished or non-existent monsoonal activity that permitted the occurrence of Baja California species and mesic mainland Mexico taxa. The arid desert flora of the islands today, especially that of the small oceanic islands, is directly linked to and controlled by marine conditions. Seabird dominated islands host an impoverished flora due to elevated levels of nitrogen and phosphorus derived from abundant bird guano, yet are dominated by succulent species, especially cacti. The pulse based cycle of El Niño/La Niña global climactic events are manifested via a flush of terrestrial biomass in wet El Niño years.
