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

PERELMAN, S. B.; ENRIQUE JOSE CHANETON; BATISTA, W. B.; BURKART, S. E.; LEON, R. J. C.

JOURNAL OF ECOLOGY (PRINT)

Blackwell Publishing

Año: 2007 vol. 95 p. 662 - 673

0022-0477

Summary1Theory and empirical evidence suggest that community invasibility is influenced bypropagule pressure, physical stress and biotic resistance from resident species. We studiedpatterns of exotic and native species richness across the Flooding Pampas of Argentina,and tested for exotic richness correlates with major environmental gradients, speciespool size, and native richness, among and within different grassland habitat types.2Native and exotic richness were positively correlated across grassland types, increasingfrom lowland meadows and halophyte steppes, through humid to mesophyte prairies inmore elevated topographic positions. Species pool size was positively correlated withlocal richness of native and exotic plants, being larger for mesophyte and humid prairies.Localities in the more stressful meadow and halophyte steppe habitats containedsmaller fractions of their landscape species pools.3Native and exotic species numbers decreased along a gradient of increasing soil salinityand decreasing soil depth, and displayed a unimodal relationship with soil organic carbon.When covarying habitat factors were held constant, exotic and native richness residualswere still positively correlated across sites. Within grassland habitat types, exotic andnative species richness were positively associated in meadows and halophyte steppes butshowed no consistent relationship in the least stressful, prairie habitat types.4Functional group composition differed widely between native and exotic speciespools. Patterns suggesting biotic resistance to invasion emerged only within humidprairies, where exotic richness decreased with increasing richness of native warm-seasongrasses. This negative relationship was observed for other descriptors of invasion suchas richness and cover of annual cool-season forbs, the commonest group of exotics.5Our results support the view that ecological factors correlated with differences ininvasion success among grassland sites change with the range of environmental heterogeneityencompassed by the analysis. Within narrow habitat ranges, invasion resistance may beassociated with either physical stress or resident native diversity. Biotic resistancethrough native richness, however, appeared to be effective only at intermediate locationsalong a stress/fertility gradient.6We show that certain functional groups, not just total native richness, may be criticalto community resistance to invasion. Identifying such native species groups is importantfor directing management and conservation efforts.Key-words: abiotic stress, diversity, environmental heterogeneity, functional groups,grasslands, invasibility, niche overlap, recruitment limitation, spatial scales, speciespool size