Total herbivore exclusion alters plant species dominance and trait expression in endangered grassy woodlands.

Abstract

Herbivores are important drivers of plant community composition and dominance in grassy ecosystems, yet few studies have attempted to understand how different types of herbivores impact plant dominance and function. This study examined the biomass of dominant grass species and other plant functional groups after 3 years of nested herbivore exclusion of non-native livestock, native herbivores, and invertebrates in endangered grassy woodlands of southeastern Australia. We also measured the leaf traits of the two dominant perennial grass species, the native Themeda triandra and the non-native Eragrostis curvula, and how their investment in growth and defense changed with herbivore exclusion. The exclusion of all aboveground herbivores, including invertebrates, affected plant biomass and traits more than vertebrate herbivore exclusion alone. Specifically, with release from invertebrate herbivory we found a shift in the hierarchy of graminoids, with subdominant grass species surpassing the dominant T. triandra as the highest contributor of biomass. Aboveground herbivores appear to sustain T. triandra dominance by promoting a cycle of defoliation and compensatory regrowth, which reduces self-shading and maintains tussock vigor. When released from all herbivores, E. curvula traits shifted in favor of growth rather than defense, which may explain E. curvula's maintenance of biomass levels with herbivore exclusion and its invasive potential more generally. The findings of this study underscore the collective role of herbivore guilds in sustaining native dominance and ecological function in grassy ecosystems and highlight the need for conservation strategies that consider both vertebrate and invertebrate herbivory.