Ecological variability, either across space or through time, is central to coexistence and is well-known to maintain biodiversity. Most experimental studies of coexistence, however, focus on the role of temporal variability. Here, we examine the role of spatial heterogeneity in coexistence within metacommunities by quantifying the strength of coexistence and the spatial niche in experimental metacommunities. We use protist microcosms consisting of Paramecium bursaria and caudatum in in two-patch metacommunities with between-patch spatial heterogeneity. We create spatial heterogeneity by altering light conditions and patch size, and we track local and regional Paramecium abundances through time across three dispersal treatments. We quantify the strength of fluctuation-independent coexistence mechanisms, or non-spatial coexistence, by calculating low-density growth rates when each species invades into an unoccupied environment (i.e. with no intra- or inter-specific competition) and averaging across space. We quantify the strength of variation-dependent coexistence mechanisms (e.g. the spatial storage effect, fitness-density covariance, and nonlinear competitive variance) by determining low-density growth rates with each species as an invader when the other is at equilibrium abundances across the metacommunity and then comparing this to growth rates in unoccupied patches.
Heterogeneity between metacommunity patches led to local competitive exclusion in the absence of dispersal, where P. bursaria outcompeted P. caudatum in the light patches, while P. caudatum outcompeted P. bursaria in the dark patches within 25 days. In contrast, low dispersal between metacommunity patches led to coexistence of both species in the light patches but maintained competitive exclusion in the dark patches. A high dispersal rate led to coexistence at both the local and regional scales. Variation-independent coexistence mechanisms yielded positive growth rates for all species in all environments, although P. caudatum’s intrinsic growth rate was 25x higher in the dark environment and 2x higher in the light environment than P. bursaria’s intrinsic growth rate, implying that variation-dependent mechanisms overcame variation-independent mechanisms for P. bursaria to coexist regionally. In comparison, both variation-dependent and independent mechanisms led to a positive low-density growth rate of P. caudatum. We conclude that spatial coexistence in the metacommunities is the outcome of both abiotic variation in environmental conditions and biotic variation in dispersal rates, and provide a framework for calculating the relative roles of variation-independent and variation-dependent spatial coexistence mechanisms.