UNPALATABLE PLANTS PROTECT NEIGHBORS FROM GRAZING AND INCREASE PLANT COMMUNITY DIVERSITY

Abstract

Tasty plants can be protected from herbivores by unpalatable neighbors. We used experimental exclosures, removal of unpalatable species, and transplants of palatable and unpalatable species in subalpine meadows of the Caucasus Mountains of Georgia to study the effects of two unpalatable species on plant communities. We found that Cirsium obalatum and Veratrum lobelianum, two large native perennial herbs that invade after heavy grazing, had strong facilitative effects on communities through their indirect effects on livestock herbivores. These unpalatable invaders had different effects on community composition when livestock were present than when livestock were excluded. Furthermore, removing Cirsium and Veratrum where herbivory was permitted decreased the richness of associated communities, but inside a livestock exclosure removal of these species increased community richness. Transplanted palatable species (Anthoxanthum odoratum and Phleum alpinum) grew larger inside the exclosure, and in the exclosure Cirsium and Veratrum had no effect on their growth. However, outside of the exclosure, Cirsium and Veratrum had strong positive effects on the growth of A. odoratum and P. alpinum. Excluding livestock decreased the growth of Luzula pseudosudetica, another unpalatable species, and Cirsium and Veratrum had no effect on L. pseudosudetica outside the exclosure. In contrast, inside exclosures Cirsium and Veratrum had competitive effects on L. pseudosudetica. Our results indicate that Cirsium and Veratrum, which are in some ways undesirable rangeland weeds, may also play an important role in maintaining species and functional diversity of overgrazed plant communities in the Caucasus.