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.