Fuzziness and Heterogeneity of Benthic Metacommunities in a Complex Transitional System

<div><p>We propose an extension to the metacommunity (MC) concept and a novel operational methodology that has the potential to refine the analysis of MC structure at different hierarchical levels. We show that assemblages of species can also be seen as assemblages of abstract subregional habitat-related metacommunities (habMCs). This intrinsically fuzzy concept recognizes the existence of habMCs that are typically associated with given habitats, while allowing for the mixing and superposition of different habMCs in all sites and for boundaries among subregions that are neither spatially sharp nor temporally constant. The combination of fuzzy clustering and direct gradient analysis permits us to 1) objectively identify the number of habMCs that are present in a region as well as their spatial distributions and relative weights at different sites; 2) associate different subregions with different biological communities; and 3) quantitatively assess the affinities between habMCs and physical, morphological, biogeochemical, and environmental properties, thereby enabling an analysis of the roles and relative importance of various environmental parameters in shaping the spatial structure of a metacommunity. This concept and methodology offer the possibility of integrating the continuum and community unit concepts and of developing the concept of a habMC ecological niche. This approach also facilitates the practical application of the MC concept, which are not currently in common use. Applying these methods to macrophytobenthic and macrozoobenthic hard-substrate assemblages in the Venetian Lagoon, we identified a hierarchical organization of macrobenthic communities that associated different habMCs with different habitats. Our results demonstrate that different reference terms should be applied to different subregions to assess the ecological status of a waterbody and show that a combination of several environmental parameters describes the spatial heterogeneity of benthic communities much better than any single property can. Our results also emphasize the importance of considering heterogeneity and fuzziness when working in natural systems.</p> </div

Indicator taxa of the three metacommunities’ CSs.

<p>Only taxa with significative (p-level<0.05, 499 permutations) IndVal >25% are shown.</p

Marginal effects and partial effects in the RDA parsimonious model of environmental variables on FKM membership grades.

<p>Marginal effects were conditioned to substrate type, which explained 14% of total variance. Variables are ordered according to decreasing marginal and decreasing partial effects. Asterisks mark variables included in the parsimonious model. P = significance level of variables’ contribution in the parsimonious model.</p

Percentage abundance for taxa in the centroids of the three habMCs.

<p>Only the most abundant taxa are shown, i.e. those which cumulatively add up for >90% of the total abundance in the centroids representative of each of the three habMCs. Taxa are ordered by decreasing alphabetic order.</p

Spatial distribution of membership grades for the three habitat metacommunities habMC1 (A); habMC2 (B); habMC3 (C).

<p>Dominant and characteristic species are also indicated. All habMCs are present (with different relative weights) in most of the sites, indicating the importance of migration and dispersal processes through water mixing. Areas with high membership for a given habMC represent its CS. CSs do not overlap, indicating the importance of sorting by environmental gradients. Areas in which two or three habMCs have similar membership grades present mixed traits and are areas in which dispersal processes and environmental sorting are balanced.</p

The Venetian Lagoon and the macrobenthos sampling sites.

<p>The Venetian Lagoon and the macrobenthos sampling sites.</p

FMV for 15 taxa with highest affinities for each of the three habMCs.

<p>Taxa are ordered by decreasing FMV in habMC1, habMC2, habMC3, FMV values are shown for each habMC.</p

Conceptual diagram of the hierarchical spatial organization of macrobenthic communities.

<p>A) Different levels of biological aggregation (regional metacommunity, habitat metacommunity, species). B) Spatial scales (region, habitat, site). C) Environmental gradients and mixing/dispersal processes.</p

Principal statistics of the environmental parameters used in the study.

<p>Min = minimum, 25 perc = 25^th percentile, 75 perc = 75^th percentile, Max = maximum, IQR = Inter-quartile range, StdDev = standard deviation. Substrate type was coded as three dummy variables: Concrete, Iron, Wood.</p

Biplot of the RDA parsimonious model explaining fuzzy memberships for the three habMCs.

<p>Biplot of the RDA parsimonious model explaining fuzzy memberships for the three habMCs.</p