Species diversity has two components - number of species and spatial turnover in species composition (beta-diversity). Using a field experiment focusing on a system of Mediterranean grasslands, we show that interspecific competition may influence the two components in the same direction or in opposite directions, depending on whether competitive exclusions are deterministic or stochastic. Deterministic exclusions reduce both patch-scale richness and beta-diversity, thereby homogenising the community. Stochastic extinctions reduce richness at the patch scale, but increase the differences in species composition among patches. These results indicate that studies of competitive effects on beta diversity may help to distinguish between deterministic and stochastic components of competitive exclusion. Such distinction is crucial for understanding the causal relationship between competition and species diversity, one of the oldest and most fundamental questions in ecology.

SignificanceA basic challenge of ecology is to understand the mechanisms that generate changes in the composition of ecological communities. At the most fundamental level, any change in species composition results from the interplay between two contrasting forces: selection (representing deterministic forces) and drift (representing stochastic forces). Here we provide experimental evidence that the relative importance of these two forces depends on the magnitude of dispersal. Specifically, increasing dispersal increases the effective size of the communities, thereby diminishing the relative importance of demographic stochasticity and increasing the relative importance of selective forces as drivers of community assembly. Our findings, supported by computer simulations, have important implications for understanding the ecological consequences of dispersal.

The enormous variation in seed mass along gradients of soil resources has fascinated plant ecologists for decades. However, so far, this research has focused on the description of such variation, rather than its underlying mechanisms. Here we experimentally test a recent model relating such variation to two fundamental properties of plant growth: allometry of biomass growth and size‐asymmetry of light competition. According to the model, mean seed mass should increase, and the variance of seed mass should show a unimodal response, to increasing soil resource availability (productivity). We test these predictions and their underlying assumptions using a combination of field observations, mesocosm experiments and greenhouse experiments focusing on Mediterranean annual plants. Our results support the predictions and assumptions of the model, and allow us to reject alternative models of seed mass variation. We conclude that growth‐allometry and size‐asymmetric light competition are key drivers of seed‐mass variation along soil resource gradients.

The “habitat‐specific species pool hypothesis” proposes that differences between habitats in the sizes of their species pools are the main drivers of diversity responses to habitat heterogeneity. Empirical tests of this hypothesis are not trivial as species might be missing from ecologically suitable habitats due to limited dispersal, while others may occur in unsuitable habitats by means of source–sink dynamics and mass effect. We tested the habitat‐specific species pool hypothesis in a local, environmentally heterogeneous community of annual plants using a novel “ecological selection” experiment. Mixtures of seeds representing the whole community were sown in each habitat, and the emerging species were exposed to six generations of selection by environmental filtering and competition while being blocked from dispersal. A comparison of the total number of species that were able to survive in each habitat (i.e. to pass the selection test) with data on species richness in the natural community allowed us to test the degree to which observed differences in species richness between habitats could be explained by differences in the sizes of the respective species pools. Results supported the species pool hypothesis, showing that differences in the sizes of the habitat‐specific species pools were important in determining diversity responses to habitat heterogeneity. Moreover, species richness showed a unimodal response to local‐scale gradients in habitat productivity, and this response could be attributed to underlying differences in species pool sizes. Both results were robust to properties of the data and the method of analysis. Synthesis. Our results provide a strong experimental evidence that differences in the sizes of habitat‐specific species pools might be important in shaping the diversity of local communities. Future theoretical and empirical studies in community ecology should explore the potential sources and implications of such differences.