TROPHIC CONSEQUENCES OF A POSITIVE PLANT INTERACTION

Positive interactions (mutualisms and commensalisms) are becoming more widely known as important processes in natural and agricultural commu nities. They can have strong and direct effects on associated species by creating hospitable environments for organisms lacking the morpholo gical and/or physiological ability to cope without neighbors. Despite the recent recognition of the importance of positive interactions, we know little about how they influence the population dynamics of higher trophic levels. In this study, we have coupled our previous research on positive plant interactions among salt marsh plants with experiment s on higher trophic levels to investigate the relative contribution of positive interactions to herbivore and predator population dynamics i n a southern New England salt marsh, We experimentally manipulate salt marsh host plants with and without the physical buffering effects of plant neighbors to investigate the colonization dynamics and populatio n growth rates of aphid herbivores, We show that, without plant neighb ors, aphid colonization increases on host plants but that negative pop ulation growth rates ensure local extinction at all levels of the food chain, Our study demonstrates that positive interactions between neig hboring plants can have strong community-wide repercussions and can ul timately determine the integrity of interactions at higher trophic lev els.