Evolution of Territoriality by Herbivores in Response to Host Plant Defenses

Plants may play an active role in the evolution of territorialityand associated animal behaviors such as spacing and cannibalism.I hypothesize that these behaviors have in part evolved in directresponse to plant defenses that would otherwise diminish individualsuccess. These defenses limit resource availability and concentrateherbivores at specific sites where they then suffer from inducedplant defenses, increased predation and competition. Plant traitsthat enhance the negative effects of competition and increasedpredation must be included in the suite of plant defenses againstherbivory. In a specific example with Pemphigus gall aphids, plant defensesresult in a strong selection pressure favoring territorial behavior.The negative effects of competition give territorial individualsa 2.3-fold advantage over non-territorial individuals. Induceddefenses (i.e., density dependent leaf abscission) can be justas important as competition as a selection pressure for territorialbehavior. With the addition of induced plant defenses territorialindividuals realize a 4.4-fold advantage. Furthermore, roughapproximations suggest that in the absence of territorial behaviorpredation would increase by 52%. The same traits which promote territoriality also encouragecannibalism, a surprisingly common herbivore behavior. To theextent cannibalism reduces the herbivore population, plant traitswhich promote this behavior may realize a selective advantage. Viewing plants as dynamic rather than passive in their interactionswith pests may lead to a better understanding of animal behaviors.