Leaf traits mediate herbivory across a nitrogen gradient differently in extirpated vs. extant prairie species.

Abstract

Increasing nitrogen deposition threatens many grassland species with local extinction. In addition to the direct effects of nitrogen deposition, nitrogen can indirectly affect plant populations via phenotypic shifts in plant traits that influence plant susceptibility to herbivory. Here, I test how herbivory varies across an experimental nitrogen gradient and whether differences in susceptibility to herbivory might explain patterns of local species loss. Specifically, I examine how increasing nitrogen availability in a restored prairie influences leaf traits and subsequent herbivory (by leaf-chewers like insects/small mammals versus deer) and the severity of herbivore damage on confamiliar pairs of extirpated versus extant species from Michigan prairies. Nitrogen increased herbivory by both leaf-chewers and deer as well as herbivore damage (proportion of leaves damaged). Leaf hairiness and specific leaf area affected patterns of herbivory following nitrogen addition, although patterns varied between extirpated vs. extant taxa and herbivory type. Nitrogen increased leaf hairiness. At high levels of nitrogen addition, hairy extant plants experienced less herbivory and damage than smooth-leaved plants. In contrast, hairy extirpated plants were more likely to experience leaf-chewer herbivory. Extirpated plants with thin leaves (high specific leaf area) were less likely to experience leaf-chewer herbivory; the opposite was true for extant species. Generally, extant species experienced more herbivory than locally extirpated species, particularly at high levels of nitrogen addition, suggesting that increasing herbivory under nutrient addition likely does not influence extirpation in this system. This study suggests that trait-mediated responses to nitrogen addition and herbivory differ between extant and extirpated species.