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Allochthonous marine input is a key component of the dynamics of islands and terrestrial coastal ecosystems on islands in the Gulf of California, Mexico, where an unproductive desert juxtaposes a highly productive ocean. In this area, seabirds are a major conduit bringing marine productivity to land. Seabirds leave guano on roosting and nesting islands, and carrion on nesting islands.

We analyze how seabirds affect the abundance of a dominant consumer group, tenebrionid beetles, on 25 islands in the Gulf of California over a three-year period. Tenebrionid densities vary by three orders of magnitude among islands. Beetles are most abundant in areas influenced by seabirds: they are approximately five times more dense on nesting and roosting islands than on other islands and on mainland sites, and approximately six times more dense inside vs. outside colonies. Path analyses show that seabirds significantly affect beetles by two distinct pathways. On roosting islands, effects are mainly indirect: guano, a fertilizing nutrient, significantly enhances plant productivity; beetles eat plant detritus. On nesting islands, significant effects are primarily direct: beetles eat seabird carrion; the indirect pathway (guano and plants) contributes little on these islands.

By providing energy and nutrients to fuel a diverse array of consumer populations, seabirds are central to the dynamics of these island ecosystems. Scavengers and avian parasites directly increase by eating seabird products. Likewise, populations of consumers that eat detritus, plant tissues, and seeds are enhanced indirectly via the fertilizing effects of guano on plants. Increases in these primary consumers indirectly facilitate high densities of many predators. Thus, consumer populations on these islands are deeply subsidized by substantial input from seabirds. Because consumers cannot influence the renewal rate of their allochthonous resources, the dynamics of these consumers and their food webs are largely donor controlled.