Microtopographic Heterogeneity and Floristic Diversity in Experimental Wetland Communities

Abstract

1 Coexistence is thought to be facilitated in heterogeneous environments due to interspecific differences in habitat preference. Microtopographic heterogeneity is a major factor structuring natural freshwater wetland communities and is thought to influence diversity. I tested whether interspecific differences existed in habitat preference for different microtopographic positions and whether diversity was associated with small-scale spatial microtopographic heterogeneity in experimental wetland communities. 2 I manipulated microtopographic heterogeneity to produce 'homogeneous' (flat surface) and 'heterogeneous' (hummock-hollow) treatments. Three propagule source treatments (seed bank, seeds, seed bank + seeds) were incorporated to determine if responses to heterogeneity depended on the propagule source. Habitat preferences for microhabitats within heterogeneous treatments were determined for each species by recording the number of individuals located on 'hummocks' or in 'hollows'. Responses to heterogeneity and propagule source were quantified by measuring richness, evenness, species abundances and above-ground biomass. 3 Small-scale variability in microtopography, on the order of 1-3 cm, produced highly significant differences in plant community structure. Both components of floristic diversity, species richness and evenness, were consistently greater in communities with heterogeneous microtopography. 4 Community composition, abundance of individuals and above-ground biomass responded differentially to heterogeneity and propagule source. Most species reached greater abundances in heterogeneous environments than homogeneous environments; however, species that were similarly abundant in both included Eleocharis ovata, Alisma triviale and Sparganium americanum. 5 Most species growing within the heterogeneous environments showed distinct habitat preferences for hummock or hollow microhabitats, with many rarer species, particularly woody perennials, favouring hummocks.