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 consumers than inland sites on islands in the Gulf of California. Previous data suggested that subsidies of energy and nutrients from the ocean allowed large coastal populations. Stable carbon and nitrogen isotopes are 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 more strongly marine-based diets than inland individuals. Coastal spiders and scorpions were significantly more enriched in C-13 and N-15 than inland spiders and scorpions, suggesting that the coastal individuals consumed more marine-based foods than their inland counterparts. These patterns existed in both drought years and wet El Nino years. However, the marine influence was stronger in drought years when terrestrial productivity was nearly non-existent, than in wet years when terrestrial productivity increased by an order of magnitude.