Spatial Heterogeneity In Species Composition Research Articles

SPATIAL HETEROGENEITY EXPLAINS THE SCALE DEPENDENCE OF THE NATIVE–EXOTIC DIVERSITY RELATIONSHIP

While small-scale studies show that more diverse native communities are less invasible by exotics, studies at large spatial scales often find positive correlations between native and exotic diversity. This large-scale pattern is thought to arise because landscapes with favorable conditions for native species also have favorable conditions for exotic species. From theory, we proposed an alternative hypothesis: the positive relationship at large scales is driven by spatial heterogeneity in species composition, which is driven by spatial heterogeneity in the environment. Landscapes with more spatial heterogeneity in the environment can sustain more native and more exotic species, leading to a positive correlation of native and exotic diversity at large scales. In a nested data set for grassland plants, we detected negative relationships between native and exotic diversity at small spatial scales and positive relationships at large spatial scales. Supporting our hypothesis, the positive relationships between native and exotic diversity at large scales were driven by positive relationships between native and exotic beta diversity. Further, both native and exotic diversity were positively correlated with spatial heterogeneity in abiotic conditions (variance of soil depth, soil nitrogen, and aspect) but were uncorrelated with average abiotic conditions, supporting the spatial-heterogeneity hypothesis but not the favorable-conditions hypothesis.

Woody vegetation and canopy fragmentation along a forest-to-urban gradient

To identify patterns that can be used to predict vegetation and landscape characteristics in urban environments, we surveyed the species composition and size of woody plants, as well as the landscape structure of forest canopies, along a forest-to-urban gradient near Oxford, Ohio, USA. The gradient included six sites of increasingly urban land-use: a preserve, a recreational area, a golf course, a residential subdivision, apartment complexes, and a business district. We recorded species identity and stem diameter for all woody plants greater than 3 cm diameter at breast height (DBH) to examine the distribution of individual species as well as overall community composition. We used digitized aerial photographs to compare the spatial characteristics of the forest canopy at each site. We found predictable patterns in species diversity (Shannon index), spatial heterogeneity in species composition (mean percent dissimilarity), and all measures of patch fragmentation (canopy cover and patch number and size). There were clear differences in tree density and total basal area between forested sites and developed sites, but there was little resolution among developed sites. Species richness and average DBH showed no clear pattern, suggesting that landscaping preference largely determined these values. We present a modified version of an intermediate heterogeneity model that can be used to predict diversity patterns in urban areas. We discuss probable mechanisms that led to these patterns and the potential implications for animal communities in urban environments.

Spatial and temporal distribution patterns of drifting pupal exuviae of Chironomidae (Diptera) in streams of tropical northern Australia

SUMMARY1. Periodic collecting of floating cast pupal cuticles of chironomids (exuviae) in two tropical northern Australian streams demonstrates (i) spatial heterogeneity in species composition across a wide stream, (ii) temporal heterogeneity in the maximum abundance of each species, and that (iii) species accumulate as a function of sample size and duration of sampling,2. Spatial heterogeneity is ascribed to variation in larval microhabitat across the wide stream, combined with short exuvial drift duration and restricted upstream mixing.3. Temporal heterogeneity is ascribed to diel periodicity in adult emergence and, as with spatial heterogeneity, to the short floating life.4. The consequences of spatial and temporal variation for the sampling of exuvial drift are discussed in relation to the objectives of particular programmes. Thus, if the objective is assessment of chironomid species composition for inventory purposes such as faunistics or conservation, the large sample sizes attained by 24‐h sampling are necessary and appropriate. However, for rapid assessment that requires comparable samples at different sites, species numbers may be optimized by temporally and spatially restricted sampling of the maximal emergence period, which in this study is at dusk, or by subsampling from a 24‐h sample.

Factors Affecting the Spatial Distribution of Diatoms on the Surface Sediments of Three Precambrian Shield Lakes

Spatial heterogeneity in species composition of surface sediment diatom assemblages was examined in three Precambrian Shield lakes of different water chemistry and basin morphometry. The study revealed that one sample cannot adequately represent the "average" diatom flora of a lake. The results of cluster analysis, multivariate analysis of variance, and analysis of covariance indicated that the lakes exhibited different patterns of spatial heterogeneity in their sedimentary diatom assemblages. In the morphometrically simple Plastic Lake the amount of variability was substantially lower than in the other lakes. In the morphometrically more complex Blue Chalk Lake and Chub Lake there was considerably more variability; however, very little of the variability could be attributed to depth or basin shape. The variability in surface diatom assemblages was markedly higher in Chub Lake, probably as a result of the influx of diatoms from the wetlands surrounding the lake. Despite a significant amount of heterogeneity in sediment diatom composition within each lake, between-lake differences were sufficiently high to suggest that comparisons between lakes based on sedimentary diatom remains are probably valid if sampling sites are selected following prescribed criteria.