Habitat stress, species pool size and biotic resistance influence exotic plant richness in the Flooding Pampa grasslands

1. Theory and empirical evidence suggest that community invasibility is influenced by propagule pressure, physical stress and biotic resistance from resident species. We studied patterns of exotic and native species richness across the Flooding Pampas of Argentina, and tested for exotic richness correlates with major environmental gradients, species pool size, and native richness, among and within different grassland habitat types. 2. Native and exotic richness were positively correlated across grassland types, increasing from lowland meadows and halophyte steppes, through humid to mesophyte prairies in more elevated topographic positions. Species pool size was positively correlated with local richness of native and exotic plants, being larger for mesophyte and humid prairies. Localities in the more stressful meadow and halophyte steppe habitats contained smaller fractions of their landscape species pools. 3. Native and exotic species numbers decreased along a gradient of increasing soil salinity and decreasing soil depth, and displayed a unimodal relationship with soil organic carbon. When covarying habitat factors were held constant, exotic and native richness residuals were still positively correlated across sites. Within grassland habitat types, exotic and native species richness were positively associated in meadows and halophyte steppes but showed no consistent relationship in the least stressful, prairie habitat types. 4. Functional group composition differed widely between native and exotic species pools. Patterns suggesting biotic resistance to invasion emerged only within humid prairies, where exotic richness decreased with increasing richness of native warm-season grasses. This negative relationship was observed for other descriptors of invasion such as richness and cover of annual cool-season forbs, the commonest group of exotics. 5. Our results support the view that ecological factors correlated with differences in invasion success change with the range of environmental heterogeneity encompassed by the analysis. Within narrow habitat ranges, invasion resistance may be associated with either physical stress or resident native diversity. Biotic resistance through native richness, however, appeared to be effective only at intermediate locations along a stress/fertility gradient. 6. We show that certain functional groups, not just total native richness, may be critical to community resistance to invasion. Identifying such native species groups is important for directing management and conservation efforts.