Relict grassland resistance to exotic species invasion disturbance and nutrient addition effects

TOGNETTI, P.M.; CHANETON, E.J.

Yucatán, Mexico

Workshop; Ecology in an Era of Globalization. Challenges and Opportunities for Environmental Scientists in the Americas; 2006

ESA

Grassland ecosystems around the world have been destroyed by human activity, and replaced by agriculture and cattle grazing systems. Natural systems conservation requires knowledge about factors affecting its maintenance and recovery, and those that determine the possibility of invasion by exotic species. Disturbances in grasslands systems were mainly related to cattle foraging, including foliage cut and nitrogen deposition. To study the controls of exotic species invasion in natural relict grassland of the Inland Pampa, Argentina, we carried out a factorial cutting and nitrogen addition experiment on homogeneous grassland plots immersed in an agricultural landscape matrix. Fertilization increased nitrogen availability, while cutting increased photosynthetically active radiation (PAR) at the ground level. Forbs biomass augmented with PAR and soil nitrogen availability. Nevertheless, invasive grasses did not respond directly to this resource increase, invading plots independently of the applied treatments. Exotic species invasion had and important spatial component, in which different plots were invaded by different exotic grasses species. Our results show that the invasion of grasslands relicts depends, aside from the space and temporary fluctuation of resource availability, on the invading species life histories.Exotics with traits analogous to those of the ecosystem dominant species (in this case perennial C4 grasses), can invade independent of the increase in resources availability. Conservation strategies must tend to limit the access of invading species to the protected sites as to control the regime of disturbances, depending on the type of species studied.