Facilitation by unpalatable weeds may conserve plant diversity in overgrazed meadows in the Caucasus Mountains

Positive interactions among plants have been demonstrated in many communiti es around the world, and appear to play important roles in maintaining spec ies coexistence, productivity, and species diversity. However, the potentia l 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 pr otection from herbivory. By associating with an unpalatable neighbor, a tas ty species may avoid being eaten and increase in size and reproductive fitn ess. We examined the role of two highly unpalatable plants, Cirsium obvalat um and Veratrum lobelianum, in subalpine meadow plant communities of the ce ntral Caucasus Mountains in the Republic of Georgia, where intense livestoc k grazing has occurred for over two thousand years. These two species are a voided 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 Guda uri region, bisected by the Russian-Georgian Military Road, was a focal poi nt of these migrations, and there we found that plant communities associate d with Cirsium and Veratrum were very different in composition than open me adows. 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 significantl y higher covers under Cirsium or Veratrum. Of the 38 species that were repr oducing sexually at our site, eight were found only under the unpalatable i nvaders. 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 out put differed substantially between Cirsium and Veratrum, indicating some de gree of species-specificity in their effects. These results indicate that u npalatable plants, which are generally indicators of unhealthy rangelands, have the potential to preserve plant diversity in overgrazed plant communit ies.