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

Seidler, Tristram^*,1, Garibaldi, Lucas², Chaneton, Enrique², ¹ NERC Centre for Population Biology, Ascot, Berkshire, United Kingdom² IFEVA, Buenos Aires, Argentina

ABSTRACT- 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.

SPANISH ABSTRACT- Muchas invasiones de plantas no implican la modificació® ¤rá³´ica de comunidades o ecosistemas nativos por una sola invasor (invasió® ¡guda). Má³ com?ero menos estudiada, es la invasió® °or muchas especies en densidad baja (invasió® £ró®©£a). Por contraste a invasiones agudas, invasiones cró®©£as pueden resultado en diversidad má³ alta, pero sus efectos de la comunidad y ecosistema se han considerado raramente. Predijimos eses efectos de ecosistema de invasiones cró®©£as dependerá® de la similitud funcional entre las plantas exó´©£as y las plantas nativas, esa resistencia de la comunidad al aumento de la fracció® ¥xó´©£a dependerá ´ambié® de la similitud funcional entre las exó´©£as y las nativas. Para probar estos, y hipó´¥³is relacionadas, nosotros realizamos un experimento de la eliminació® ¤e plantas de la Pampa Deprimida de Argentina, conteniendo 25% de especies exó´©£as. Los grupos funcionales se removido basados en la forma de vida (dicotiled󮥡s, pastizales C3, y pastizales C4), y la procedencia (nativo, exó´©£o). Para controlar para los disturbios creado durante la eliminaci󮬠creamos un serie de parcelas en que removimos las plantas al azar con respecto a especies. El reacció® ¤e la comunidad y la ecosistema a eliminaciones dirigidas fueron comparada a reacció® ¥n niveles equivalentes del disturbio. Efectos de la invasió® £ró®©£a fueron limitados en gran parte a la comunidad (biomasa, diversidad y uniformidad) con pocos efectos mensurables de ecosistema. ANPP era idé®´ico en parcelas con y sin exó´©£as, aunque productividad de nativas fuera má³ alta en ausencia de las exó´©£as, sugiriendo que con respecto a productividad, las exó´©£as y las nativas funcionan semejantes. La diversidad total era apreciablemente má³ baja en ausencia de las plantas exó´©£as, aunque la diversidad de las nativas era apreciablemente má³ alta. Las plantas exó´©£as causaron la reducció® ¥n biomasa de todos grupos funcionales de nativas durante el perí¯¤o temprano del verano de la mḩma de las exó´©£as. La resistencia a la invasió® ¥ra evidente principalmente en el grupo nativo y fí³©camente dominante (pastizales C4), aunque facilitació® °or pastizales C4 de otros grupos funcionales nativos pudiera aumentar apreciablemente su habilidad de resistir la invasió®®¼/textarea>