Spatial and temporal subsidies to insular communities in the Gulf of California.
Stapp, Paul*,1, Polis, Gary2, 1 Department of Biological Science, Fullerton, CA, USA2 Department of Environmental Science and Policy, Davis, CA, USA
ABSTRACT- Desert islands are model systems for studying the combined effects of spatial and temporal resource subsidies on species interactions. During dry years, inputs from the ocean (nesting seabirds, littoral resources) are a key source of nutrients and energy to food webs on arid islands in the Gulf of California. Rainfall associated with episodic El Niño (ENSO) events markedly increase land-based resources (plants, insects) for higher-level consumers; ENSO events also significantly alter marine productivity, with as-yet unknown effects on recipient terrestrial systems. Although their temporal dynamics differ, changes in marine and terrestrial productivity represent pulses of resources that enter insular food webs as water and nutrients to plants, or as detritus or prey to consumers, which may then exert top-down effects on local prey. These effects may ultimately be manifest in patterns of density, life-history traits or body size, i.e. the insular syndrome, or biogeography. Here, I review what we have learned about the effects of resource subsidies on consumers on Gulf islands, and in particular, about the consequences for trophic and competitive interactions in insular food webs. To simulate the effects of ENSO-related changes in terrestrial foods on species interactions, I added seeds to replicated plots on a Gulf island, and recorded changes in diet, population density, habitat use and body size of two, co-occurring rodent species over a 22-month period. In contrast to patterns observed during an actual ENSO event, granivorous rodents (Chaetodipus) monopolized supplemental food and increased in both number and body size, apparently restricting omnivorous rodents (Peromyscus) to areas near shore. Differences in the abundance of rattlesnakes (Crotalus), a major rodent predator, suggest that competitive interactions between these species may also be influenced by top-down effects. Comparisons of rodent densities in dry and wet years revealed strong ENSO-related effects, but also, a potentially greater role for predators than was previously envisioned. Future comparative studies, combined with experimental manipulations of terrestrial and marine resources, offer the best opportunity for elucidating the effects of pulsed resources on interactions in insular communities.
Key words: pulses, subsidies, islands, foodwebs
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