Abstract
Spatial patterning is a key natural history attribute of sessile organisms that frequently emerges from and dictates potential for interactions among organisms. We tested whether bunchgrasses, the dominant plant functional group in longleaf pine savanna groundcover communities, are nonrandomly patterned by characterizing the spatial dispersion of three bunchgrass species across six sites in Louisiana and Florida. We mapped bunchgrass tussocks of >5.0 cm basal diameter in three [Formula: see text] plots at each site. We modeled tussocks as two-dimensional objects to analyze their spatial relationships while preserving sizes and shapes of individual tussocks. Tussocks were overdispersed (more regularly spaced than random) for all species and sites at the local interaction scale (<0.3 m). This general pattern likely arises from a tussock-centered, distance-dependent mechanism, for example, intertussock competition. Nonrandom spatial patterns of dominant species have implications for community assembly and ecosystem function in tussock-dominated grasslands and savannas, including those characterized by extreme biodiversity.
Highlights
Nonrandom spatial patterns are frequently observed in natural populations (Levin 1992; Rietkerk and van de Koppel 2008; fig. 1)
When we analyzed all tussocks collectively, as a functional guild, the results were qualitatively similar to the overdispersed patterns we found for dominant species only, and we found no cases of aggregation
Bunchgrass tussocks were overdispersed at the local neighborhood scale at all sites and for all species
Summary
Nonrandom spatial patterns are frequently observed in natural populations (Levin 1992; Rietkerk and van de Koppel 2008; fig. 1). Individuals in a community are often assumed to encounter other individuals, whether conspecifics or heterospecifics, in proportion to their average population densities. This is referred to as the mean-field assumption (Murrell et al 2001). In communities with species-specific nonrandom spatial patterning and ecological processes (e.g., competition, dispersal, natural enemies) occurring at relatively small spatial scales, the mean-field assumption may not be an accurate reflection of what most individuals experience
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