Do species incidence matrices reflect different degrees of plant community integration?

Abstract

Background and aims - Assuming the asymmetrical hump model of the stress-gradient hypothesis, higher degree of species aggregation in foristically similar communities is expected under conditions of stronger environmental severity.Methods - Several species incidence-based metrics were calculated for two large, homogeneous matrices pertaining to predefned community types: weed assemblage of trampled sites (S-P) and nutrient-rich pastures (F-A). For each metric and matrix, a standard effect size (SES) was estimated through a null model with fxed species frequencies and equiprobable sites. Key results - All metrics related to co-occurrence, nestedness and clustering of species range boundaries have significant SES in both matrices, but they are all larger in S-P. No significant checkerboardness was detected in S-P, whereas F-A displays non-random segregation patterns that are exclusively due to species turnover. The core species effect is positive but not significant in S-P, and the between-site pairwise similarities present a monotonic distribution. The higher SES of coherence and the significant SES of homotoneity in F-A make it more clearly delineated as a syntaxon. In fact, between-site pairwise similarities are significantly higher in F-A and their distribution approaches a reversed J-shape. Conclusions - The stronger species association and nestedness patterns revealed in weed assemblages are consistent with the hypothesis of higher community integration in harsh environments. However, the species-rich and productive grassland communities appear overall more structured. Regular and moderate disturbances produced by herbivores may be an important requisite for plant community organisation according to the core-satellite species hypothesis in fertile habitats. Although not a test for inferring underlying ecological processes, the joined use and comparison of complementary, species incidence-based metrics represents a useful exploratory tool for distinguishing between various patterns of species assembly, at least for relatively homogeneous, presence-absence matrices of different plant community types. © 2014 Botanic Garden Meise and Royal Botanical Society of Belgium.