干旱区生态环境知识资源中心

Positive interactions among plants have been demonstrated in many communities around the world, and appear to play important roles in maintaining species coexistence, productivity, and species diversity. However, the potential for positive interactions to conserve biological diversity in ecosystems that are disturbed by humans is poorly understood and often overlooked. One of the most important positive effects one plant can have on another is protection from herbivory. By associating with an unpalatable neighbor, a tasty species may avoid being eaten and increase in size and reproductive fitness. We examined the role of two highly unpalatable plants, Cirsium obvalatum and Veratrum lobelianum, in subalpine meadow plant communities of the central Caucasus Mountains in the Republic of Georgia, where intense livestock grazing has occurred for over two thousand years. These two species are avoided by livestock because of spines and toxicity, respectively, and have increased dramatically in abundance recently due to seasonal trans-Caucasus migrations of vast herds of domestic sheep during the Soviet era. The Gudauri region, bisected by the Russian-Georgian Military Road, was a focal point of these migrations, and there we found that plant communities associated with Cirsium and Veratrum were very different in composition than open meadows. Forty-four percent (15/34) of all species at our site were found at only "trace" ( < 1.0%) cover values in the open meadow, but at significantly higher covers under Cirsium or Veratrum. Of the 38 species that were reproducing sexually at our site, eight were found only under the unpalatable invaders. Communities associated with Cirsium and Veratrum had 78-128% more species in flower or fruit than open meadow communities, respectively, than open meadow sites. Furthermore, community composition and reproductive output differed substantially between Cirsium and Veratrum, indicating some degree of species-specificity in their effects. These results indicate that unpalatable plants, which are generally indicators of unhealthy rangelands, have the potential to preserve plant diversity in overgrazed plant communities.