Time and space in the community structure of a species‐rich limestone grassland
Authors: Wilson J. Bastow , Sykes Martin T. , Peet Robert K. 
Published: May 2006

Abstract. The community structure of a species‐rich grassland was investigated at a small spatial scale (0.001 m2), to determine whether evidence suggesting assembly rules could be found in temporal or spatial variation in either species richness or guild proportions. The community was alvar limestone grassland on the island of Öland, Sweden. Three sites were sampled: two lightly grazed, the other recently ungrazed. Plots with and without fertilizer were compared.

Evidence was sought for restriction on the ability of species to co‐occur within a limited spatial area. Restriction due to a limited number of niches available,‘niche limitation’, could be manifest as lower variance in quadrat richness than expected under a null model (i.e. RVr, the ratio of observed: expected variance in richness, would be < 1.0). In several cases, RVr values were significantly < 1.0, even using a patch model to allow for possible spatial variation in the environment. Low RVr values were found only at the smallest square quadrat size, 10 cm2. On Fertilized plots in the years immediately after application of fertilizer, low RVr could not be demonstrated. Explanations of low RVr other than niche limitation are considered, such as environmental heterogeneity (present and/or historical) and limitations to the co‐occurrence of individual plant modules.

Assembly rules based on guild membership were sought by looking at the variance across quadrats in the proportions of species from morphological guilds. An assembly rule would be seen as relatively constant proportion, estimated via RVgp, the ratio of observed: expected variances in guild proportions. Significant guild proportionality was found in some cases. There was no evidence of guild proportionality in the years after the application of fertilizer.

The significant effects in RVr were more numerous than expected on a random basis, though not observed in every site in every year. Similar trends were seen in RVgp. At the space/time scales examined, the species in a plant community may be constrained by assembly rules only intermittently, e.g. when resources are more limiting (Wiens 1977). Under this concept, when competition is relaxed, such as following fertilizer application, there is a temporary microhabitat ‘waterhole’ in which more species can coexist, and the assembly rules break down, at least temporarily whilst the species composition adjusts. There was some indication of a return to more deterministic community structure four years after fertilization commenced.

Variants of van der Maarel's Carousel model were tested. A Niche‐limited Carousel Model (i.e. a model in which there is some limitation in the number of species that can occupy a microsite) would imply restricted variation in richness through time for a single quadrat (temporal RVr). Overall differences between years in species richness were demonstrated, and their effect removed; after this adjustment there was support for the Niche‐limited Carousel Model. The extent of this limitation varied between sites. There were also consistent differences between quadrats in species richness. There was little evidence for constancy of guild proportions through time.

The site that showed the strongest community structure in time and space, least year‐to‐year variation in mean species richness, and least response to fertilizer perturbation, is that on the shallowest soil. Possibly the thin soil results in greater resource limitation, supporting suggestions that assembly rules are stronger when resources are more limited.