Low biotic resistance to cheatgrass invasion in Patagonia: evidence from competition experiments

Early competition between native and non-native species can greatly influence ecosystem integrity through its effect on species regeneration, which can be further modulated by environmental context. Bromus tectorum (cheatgrass , winter annual grass) has recently invaded the temperate grasslands of Patagonia Argentina ecosystems mainly used for extensive livestock raising. We evaluated the relative impact of competition from B. tectorum on two native species with different life-forms, from the seedling stage: Vulpia australis , the only winter annual grass in the community, and Bromus setifolius , a perennial grass highly palatable to livestock. The effect of asymmetric competition between B. tectorum (seedling or adult individuals) on B. setifolius was also assessed. A greenhouse experiment was conducted to simulate interspecific competition (two species per pot), and control (one species per pot) treatments. We regulated the recruitment timing of all studied species to represent realistic competition dynamics in the field. Bromus tectorum had a greater negative impact on the growth of both native species relative to the effect produced by a co-occurring native, even when that native was ecologically equivalent to the invader. Also, the impact of B. tectorum on B. setifolius depended on the invader’s developmental stage, with bigger individuals having the largest impact. In contrast, native species hardly affected B. tectorum development. Our findings indicate that native grasses may not hinder the spread of B. tectorum through competition. Moreover, B. tectorum could have a negative impact at the community level through its effect on the recruitment of native species, probably exacerbating degradation in overgrazed areas.