Evolution of territoriality by herbivores in response to host plant defenses

Abstract

Plants may play an active role in the evolution of territoriality and associated animal behaviors such as spacing and cannibalism. I hypothesize that these behaviors have in part evolved in direct response to plant defenses that would otherwise diminish individual success. These defenses limit resource availability and concentrate herbivores at specific sites where they then suffer from induced plant defenses, increased predation and competition. Plant traits that enhance the negative effects of competition and increased predation must be included in the suite of plant defenses against herbivory.In a specific example with Pemphigus gall aphids, plant defenses result in a strong selection pressure favoring territorial behavior. The negative effects of competition give territorial individuals a 2.3-fold advantage over non-territorial individuals. Induced defenses (i.e., density dependent leaf abscission) can be just as important as competition as a selection pressure for territorial behavior. With the addition of induced plant defenses territorial individuals realize a 4.4-fold advantage. Furthermore, rough approximations suggest that in the absence of territorial behavior predation would increase by 52%.The same traits which promote territoriality also encourage cannibalism, a surprisingly common herbivore behavior. To the extent cannibalism reduces the herbivore population, plant traits which promote this behavior may realize a selective advantage.Viewing plants as dynamic rather than passive in their interactions with pests may lead to a better understanding of animal behaviors. © 1987 by the American Society of Zoologists.