Resistencia biótica a la invasión de exóticas en las comunidades de pastizales en Uruguay

Resumen de tesis doctoral

Abstract

Biological invasions by non-native species affect natural ecosystems, causing changes in its composition, structure and functioning. In Uruguay, natural grasslands are mostly under grazing regime, but the increasing pressure from agricultural activity constitutes an important source of propagules of alien species. The general objective of this study was to analyze the patterns of invasion in natural grasslands of Uruguay at a regional scale and to explore mechanisms of biotic resistance at a small scale. At regional scale, 313 vegetation census from the four geomorphological regions with the greatest cover of natural grasslands were analyzed. At small-scale it was evaluated whether community species richness and growth form of the C4 grasses affect invasion success in a mesocosm experiment where patches of natural grasslands were seeded with four exotic species. At regional scale, exotic species constitute the 7% of the total richness and cover 4.7% of natural grasslands of Uruguay. The relationship between native and exotic richness was negative for Centro Sur region while the effect of native species richness on exotic plant cover was negative for Centro Sur and Cuenca Sedimentaria regions. In most analyses, the functional type C4 grasses was negatively associated with the exotic richness and cover It was also observed that the availability of propagules and productivity were positively related to invasion. The results at the small- scale experiment showed that C4 grasses with erect growth form had an inhibitory effect on seedling establishment of exotics, probably due to competition for light. However, contrary to expected, species richness was not associated to invasion success. Overall results suggest that propagule pressure and productivity may contribute to the arrival and establishment of exotics at larger scales, but their success would be largely controlled by C4 perennial grasses, which constitutes the plant functional type with the highest cover in the Uruguayan grasslands. This may explain the low incidence of invasive species in these communities.