How spatial structure alters population and community dynamics in a natural plant community

Summary We quantify the effects of spatial structure on individual, population and community biomass within a natural community of annuals for two years (1994 and 1996) in which competition operated differently. Using field‐parameterized neighbourhood models in which we had previously estimated the spatial scale and magnitude of competitive effects, we perform virtual experiments in which we manipulate spatial structure relative to that observed. We first remove segregation by randomly re‐labelling all annual plants in a focal plant's neighbourhood and then give individuals of all species the same average number of neighbours by re‐locating individuals within the overall pattern of annuals. The natural spatial structure generally acted to promote species coexistence. In 1994, when the strength of individual‐level competition within species was already greater than that between species, the natural spatial structure enhanced this effect for six out of seven species by increasing the relative frequency of intraspecific contacts. Average plant biomass increased by 0.24–38% when spatial locations were randomized. In 1996, when a single competitive hierarchy operated, the natural spatial structure systematically benefited small‐seeded, weakly competitive species while having weak negative effects on large‐seeded, strong competitors. Randomizing spatial locations decreased the biomass of the species with the smallest seeds by 12% but increased the biomass of the species with the largest seeds by < 1%. This asymmetry increased dramatically when we used the model to increase the competitive advantage of large seeds artificially. Randomizing spatial locations decreased the biomass of the species with the smallest seeds by 37% but increased the biomass of the species with the largest seeds by only 2.7%. Effects of spatial structure on total community biomass were always small (maximum 7%). This was because common species always have weak spatial structure, and they draw down the effect on the community as a whole. Our analysis, for the first time, quantifies the magnitude and direction of the effects of spatial structure in a natural community, and supports the conclusions of theoretical models that spatial structure can have substantial impacts on component species and community dynamics.