The flow of resources from one habitat to another can significantly impact recipient food webs. Mutualisms play an underappreciated role in structuring ecological communities. We conducted a field experiment examining how seaweed subsidies from the marine environment influence the strength of a terrestrial mutualism between ants and plants. Whole islands were the replicates for the seaweed treatment – seaweed was added to six islands and removed from six islands. Lizards were present on half of the experimental islands and absent on the other half, forming a 2x2 factorial design in which each seaweed/lizard combination was represented by three islands. Ant exclusions were conducted on four experimental plants on each island. Two experimental branches were selected on each plant, one of which was randomly chosen for ant-exclusion while the other served as a control. Leaf damage, plant growth, and plant-defense traits were measured on each experimental branch. Arthropods were sampled on each experimental island using baits, pan traps and sticky traps. The experiment was initiated in October 2008 and ran until May 2009.

Results/Conclusions

Overall, ants had a beneficial effect on plants, indicating that they do function as mutualists. However, the effect of ants differed on islands with different combinations of the seaweed and lizard treatments. On islands with lizards, ants had a beneficial effect on plants when seaweed was removed but not when it was added, while on islands without lizards, ants did not have detectable effects on plants in either seaweed treatment. Ant abundance in pan traps placed on the ground was 4.5 times higher on seaweed-addition than on seaweed-removal islands, suggesting that ants increase foraging activity on the ground when allochthonous resources are available. This may decrease foraging activity on the plants and reduce the beneficial effects of ants on islands with seaweed. The densities of some arthropod predators (such as spiders) are reduced by lizards. Spiders can be more abundant on plants from which ants have been excluded, potentially compensating for the lack of ants. Thus, lizards may facilitate the ant-plant mutualism by reducing compensatory predation by other arthropods. Taken together, these two mechanisms (prey-switching by ants and compensatory predation by spiders) may explain why the beneficial effect of ants was most pronounced on lizard islands from which seaweed had been removed.