The role of leaf traits and bird-mediated insect predation on patterns of herbivory in a semiarid environment in central Mexico

Abstract

Background. In arid environments plants face aridity and herbivory, therefore it has been proposed that both are convergent selective forces. However the drivers of insect herbivory in these ecosystems remain poorly understood.
 Question. Does insect herbivory vary in two plant associations subject to different levels of aridity? To what extent differences in herbivory are determined either by foliar traits, or predation by birds?
 Study species. Citharexylum tetramerum, Viguiera pinnatilobata, and Solanum tridynamum.
 Study site. We compared two-plant associations: the mezquital with a closed canopy and wettest conditions and the tetechera a dry place with an open canopy at the Zapotitlán Valley, México. 
 Methods. We evaluated leaf traits (N, C, water content, leaf strength and trichomes), herbivore insects abundance and the effects on herbivory when some predators are excluded.
 Results. Herbivory was higher in the drier site (tetechera) than in the more humid one (mezquital) in one year but not in the second one. In both plant associations herbivory increased when predators of herbivores were excluded. Plants in mezquital had more water and nitrogen content than plants in tetechera. The later had higher carbon concentration and carbon:nitrogen ratio, leaf strength and density of trichomes. Abundance of herbivore insects and insect predation were higher in the most arid site. 
 Conclusions. Our results show that insect herbivory increases in the most arid site and that predation by birds plays a role as a driver of herbivory, while resistance to aridity seems to be the main driver of leaf structural characteristics.