MARINE SUBSIDIES OF ISLAND COMMUNITIES IN THE GULF OF CALIFORNIA - EVIDENCE FROM STABLE CARBON AND NITROGEN ISOTOPES

Coastal sites support larger (2 to > 100 x) populations of many consum ers than inland sites on islands in the Gulf of California. Previous d ata suggested that subsidies of energy and nutrients from the ocean al lowed large coastal populations. Stable carbon and nitrogen isotopes a re frequently used to analyze diet composition of organisms; they are particularly useful to distinguish between diet sources with distinct isotopic signatures, such as marine and terrestrial diets. We analyzed the C-13 and N-15 concentrations of coastal versus inland spiders and scorpions to test the hypothesis that coastal individuals exhibited m ore strongly marine-based diets than inland individuals. Coastal spide rs and scorpions were significantly more enriched in C-13 and N-15 tha n inland spiders and scorpions, suggesting that the coastal individual s consumed more marine-based foods than their inland counterparts. The se patterns existed in both drought years and wet El Nino years. Howev er, the marine influence was stronger in drought years when terrestria l productivity was nearly non-existent, than in wet years when terrest rial productivity increased by an order of magnitude.