Time and space in the community structure of a species-rich limestone grassland

• Published in: Journal of vegetation science 1995-10, Vol.6 (5), p.729-740
• Main Authors: Wilson, J. Bastow, Sykes, Martin T., Peet, Robert K.
• Format: Article
• Language: English
• Subjects: Assembly rule; community ecology; Community structure; Ecological competition; Ecological modeling; Grasses; Guild; limestone grasslands; microhabitats; Negative feedback; Niche limitation; pastures; Perturbation; Plant communities; plant competition; plant density; plant ecology; Plants; Spatial models; Species; species diversity; Species richness; Statistical variance; Synecology; Tutin et al. (1964-1980)
• ISSN: 1100-9233; 1654-1103
• DOI: 10.2307/3236444

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 lime-stone 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 yea