Impacts of and resistance to chronic invasion by exotic plants in Argentine flooding pampa.

SEIDLER, T.; GARIBALDI, L. A.; CHANETON, E. J.

Mérida, México.

Congreso; International Conference, Ecological Society of America.; 2006

Invasions by exotic plants are a major component of global change, but not all plant invasions involve drastic alteration of native communities and ecosystems by monocultures of an invader ('acute' invasion). More common, but far less studied, is invasion by many species at relatively low density ('chronic' invasion). In contrast to acute invasions, chronic invasions may result in higher overall species diversity, but their community and ecosystem effects have rarely been considered. We predicted that ecosystem effects of chronic invasions will depend on the functional dissimilarity of exotics from natives, particularly of the most abundant exotics, and that community resistance to the increase of the exotic fraction will also depend on functional overlap between exotics and natives. To test these and related hypotheses, we performed a plant removal experiment in a species-rich grassland in Argentina, containing 25% exotic species. Functional groups were removed based on life form (forbs, C3 grasses, C4 grasses), and provenance (native, exotic). To control for the disturbance created during removal, we also created a disturbance gradient in which plant cover was removed at random with respect to species. Community and ecosystem responses to targeted removals were compared to responses at equivalent levels of random disturbance. Effects of chronic invasion in this system were largely confined to the community (biomass, diversity, evenness) with few measurable ecosystem effects. ANPP was identical in plots with and without exotics, although productivity of natives was higher in the absence of exotics, suggesting that with respect to productivity, exotics and natives are functionally similar. Diversity was significantly lower in the absence of exotics, although diversity of natives was significantly higher. Exotics reduced biomass of all native functional groups during the early summer exotic maximum. Resistance to invasion was evident mainly in the physically dominant native functional group (C4 grasses), although facilitation by C4 grasses of other native functional groups may significantly increase their ability to resist invasion. We term an invasion chronic when many exotic species are present at low density, with various effects on the native community but low to moderate ecosystem impacts.