10 A General Rule for Predicting When Insects Will Have Strong Top-Down Effects on Plant Communities: On the Relationship Between Insect Outbreaks and Host Concentration

We provide a new and general rule that predicts when native insect herbivores will have a major influence on dominant native plant species in communities and ecosystems worldwide. We argue that native insect herbivores will func-tion as classic keystone species whenever their hosts become abundant and form large, persistent, dense stands. Specifically, our Host Concentration Model predicts that the impact of specialist insect herbivores will be more severe on a per individual basis as host species build up to form large and dense stands. The impact of these native insect herbivores, while important at non-outbreak levels, will be most important during major bouts of defoliation that occur during periodic insect outbreaks. Our review of the literature sug-gests that such outbreaks are common from a phytocentric perspective. Con-sequently, these insect outbreaks will have a major influence on ecosystem function via their ability to regulate and reduce the abundance of host species that may typically be the superior competitor across the landscape. Finally, we believe that this Host Concentration Model will predict when specialis