Effects of marine subsidies on terrestrial food webs in the Bahamas
How consumers cope with spatial and temporal variation in resource availability has implications for ecological dynamics at the population, community, and ecosystem levels. Resource pulses—infrequent, high magnitude, ephemeral increases in resource availability—provide a natural framework for exploring these dynamics. Our research focuses on how deposition of seaweed wrack directly and indirectly affects various components of the terrestrial food web on small islands in the Bahamas. Lizards are top predators in this system, exerting top-down control on herbivorous arthropods and thereby having an indirect positive effect on plants (i.e., a trophic cascade). The seaweed represents a pulsed resource subsidy that is consumed by amphipods and flies, providing an alternative prey source to lizards. Additionally, seaweed decomposes directly into the soil under plants. In a series of experiments, we have manipulated the presence/absence, timing, and magnitude of seaweed pulses, as well as the ability of consumers to respond via aggregation. We have found evidence for two causal pathways for the effects of marine subsidies on terrestrial plants: 1) the below-ground "fertilization effect" in which seaweed adds nutrients to plants, increasing their growth rate, and 2) the above-ground "predator-diet-shift effect" in which lizards shift from eating local prey to eating mostly marine detritivores. Here I will present results focusing on lizard consumer responses to seaweed pulses, and how these effects translate into variation in trophic cascade strength over time.