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Understanding how habitat structures species assemblages in a community is one of the main goals of community ecology. To relate community patterns to particular factors defining habitat conditions, ecologists often use canonical ordinations such as canonical redundancy analysis (RDA). It is a common practice to use dissimilarity coefficients to perform canonical ordinations through distance-based RDA (db-RDA) or transformationbased RDA (tb-RDA). Dissimilarity coefficients are measures of resemblance where the information about species communities is condensed into a symmetric square matrix of dissimilarities among sites. In this study, we compared 16 of the most commonly used dissimilarity coefficients to evaluate if the species-abundance distribution (SAD) of a community can be used to select an appropriate coefficient. Of these, 11 are designed to be used primarily with abundance data, although they can also be used with presence-absence data, whereas five can only be applied to presence-absence data. Using simulations, we compared the explained variance of RDAs differing only by their coefficients to evaluate how the abundance patterns of communities influence coefficient choice. We found that coefficients are largely equivalent, independently of the community SAD. In light of these findings, we propose the consensus RDA method, a new canonical ordination procedure that performs a consensus of RDAs across several coefficients. This new method focuses on the common relations found by independent RDAs differing only by their dissimilarity coefficients; this ensures the absence of a coefficient-related bias when interpreting the canonical ordination result. Also, because in our simulations the presence-absence data were directly derived from the abundance data, we were able to evaluate if the information in presence-absence data was equivalent to that in abundance data. We found that although some information was lost by converting abundance data into presence-absence, both data formats may be complementary. When applying consensus RDA to abundance and presence-absence data independently, a more complete understanding and interpretation of the ecological patterns is obtained. An ecological example illustrating consensus RDA and the conclusions of our simulations is presented, using Carabidae data collected at the Ecosystem Management Emulating Natural Disturbances (EMEND) project in northwestern Alberta, Canada. [ABSTRACT FROM AUTHOR]

Landscape patterns influence a range of ecological processes at multiple spatial scales. Landscape pattern metrics are often used to study the patterns that result from the linear and nonlinear interactions between spatial aggregation and abundance of habitat., However, many class-level pattern metrics are highly correlated with habitat abundance, making their use as a measure of habitat fragmentation problematic., We argue that a class-level pattern metric should be (1) able to differentiate landscapes across a range of spatial aggregations, and (2) independent of habitat abundance, if it is to be used to distinguish between effects of habitat amount and fragmentation., Based on these criteria and using both simulated and actual landscapes, we evaluated 64 class-level pattern metrics. These metrics were reclassified into four groups based on their correlation with aggregation and abundance., Among all these metrics, nine were considered robust for fragmentation measurements, which cover most of the characteristics that define pattern, including core area, shape, proximity / isolation, contrast, and contagion / interspersion., Optimal metrics for individual studies will depend on both biological rationales and statistically robust metrics that are appropriate for achieving each study objectives. [ABSTRACT FROM AUTHOR]

Ground beetle (Carabidae) assemblages are speciose and frequently employed as indicators of ecosystem function. It is thus important to understand the factors that affect their spatial distributions so as to better interpret how ecosystem variation influences the structure of their assemblages. We evaluated how anthropogenic disturbances, habitat heterogeneity, and spatial autocorrelation (unmeasured spatially structured habitat variables and/or dispersal limitation) influence the distribution of carabids on a mature boreal forest landscape. We worked with a large sample of carabids from 194 sites from a near-regular grid covering 70 km2. Data about forest floor cover, vegetation structure, soil drainage, and topography were associated with each site. We modelled the structure of carabid assemblages using these variables together with Moran's eigenvector maps (MEM) as spatial constraints. Overall, our model explained about half of the variation in ground beetle assemblages. Forest floor cover, soil drainage, and vegetation structure were the principal factors useful for explaining carabid assemblages. The spatial patterns described by the MEMs for the ground beetle assemblages seem to result from spatial autocorrelation in soil drainage, floor cover, topography, and vegetation structure; however, this spatial autocorrelation independently and uniquely explained only 1.4% of the variation in assemblages. This was much less important than the explanatory power of the environmental variables considered; e.g. ground cover descriptors and vegetation structure by themselves accounted, respectively, for 12.1 and 4.5% of the variation in the carabid data. The spatial distribution of ground beetles in undisturbed forest was only mildly affected by surrounding anthropogenic disturbances, but clearly small patches were invaded by species characteristic of open habitat. We conclude that to help conserve ground beetle diversity at the landscape scale in boreal forests, mature forest patches should be large and connected to other patches and the surrounding forest. [ABSTRACT FROM AUTHOR]

Habitat heterogeneity and dispersal limitation are widely considered to be the two major mechanisms in determining tree species distributions. However, few studies have quantified the relative importance of these two mechanisms at different life stages of trees. Moreover, rigorous quantification of the effects of dominant tree species in determining species distributions has seldom been explored. In the present study, we tested the hypothesis that the distribution of tree species is regulated by different mechanisms at different life history stages. In particular, we hypothesised that dispersal limitation regulates the distribution of trees at early life stages and that environmental factors control the distribution of trees as they grow, because of niche differentiation resulting from environmental filtering. To test this, trees in 400-m2 quadrats in a 20-ha plot in Xishuangbanna, southwest China were grouped into four classes on the basis of the diameter at breast height (DBH) that roughly represent different stages in the life history of trees. A neighbourhood index was computed to represent a neutral spatial autocorrelation effect. We used both biotic (dominant species) and abiotic (topography and soil) predictor variables to model the distribution of each target species while controlling for spatial autocorrelation within each of the DBH classes. To determine which factors played the largest role in regulating target species distribution, the simulated annealing method was used in model selection based on Akaike information criterion (AIC) values. The results showed that the relative importance of neutral and niche processes in regulating species distribution varied across life stages. The neutral neighbourhood index played the most important role in determining the distributions of small trees (1 cm ≤ DBH ≤ 10 cm), and dominant species, as biotic environmental predictor variables, were the next most important regulators for trees of this size. Environmental predictor variables played the most important role in determining the distributions of large trees (10 cm ≤ DBH). This finding builds on previous research into the relative importance of neutral and niche processes in determining species distributions regardless of life stages or DBH classes. [ABSTRACT FROM AUTHOR]