The relative contribution of non-selection and selection processes in marine benthic assemblages.

We tested the hypothesis that the ubiquity of marine meiofaunal nematodes and their indiscriminate passive dispersal create assemblages that are less limited by its environment; whereas the relatively smaller population sizes of macrofauna, associated with their ability to track environmental conditions before settlement, renders their distribution more environmentally-restricted. We compared the empirical distribution of macrofauna and nematode species with that of communities simulated under different assumptions of selection (e.g. environmental filtering) and non-selection (e.g. dispersal limitation) processes. Selection processes were the prime driver of both meio- and macrofauna assemblages, with rare species strongly contributing to this component. The total number of species explained by non-selection processes was 27% higher in nematodes than in macrofauna. Our results underline the importance of a species-level approach to determine the contribution of selection and non-selection assembly processes. Moreover, they highlight the important yet overlooked role of dispersal and stochastic processes in determining species dynamics.

A simulation-based framework to explore the importance of non-selection and selection processes in structuring ecological communities.

The purpose of this framework is to identify the relative importance of selection and dispersion processes in structuring ecological communities. Using a "pattern-oriented modelling" approach, it consists of five steps: (a) aggregate information from the empirical community and its environment, (b) simulate communities under different degrees of dispersal and selection, (c) select the best set of simulations into a composite model using the environmental boundary (EB) and niche breadth (NB) of each observed species, (d) validate the composite model by comparing expected and observed results from three additional community patterns and (e) classify each observed species along the selection/non-selection continuum. A free-living marine nematodes data set from a coastal bay was used as empirical example. A total of 20 parameterizations were applied varying selection and dispersion levels. In the absence of selection, species from high-dispersal parameter sets showed maximum EBs and NBs, whilst selection parameter sets generated species with narrower EB and NB values. EB and NB values declined with decreasing dispersal. The composite model encompassed 96% of the 194 nematode species and predicted all the three patterns evaluated without further calibration, i.e., they are independent: (1) abundance-rank distribution, the assemblage structures along both the (2) spatial and (3) environmental gradients. Non-selection and selection parameter sets accounted for 34% and 85% of the observed species, respectively. The main advantage of this approach is that empirical niche measurements are placed in the context of model-generated expectations, enabling a deeper understanding of community assembly processes and how they vary from species to species.

The importance of vertical and horizontal dimensions of the sediment matrix in structuring nematodes across spatial scales.

Intensive surveys have been conducted to unravel spatial patterns of benthic infauna communities. Although it has been recognized that benthic organisms are spatially structured along the horizontal and vertical dimensions of the sediment, little is known on how these two dimensions interact with each other. In this study we investigated the interdependence between the vertical and horizontal dimensions in structuring marine nematodes assemblages. We tested whether the similarity in nematode species composition along the horizontal dimension was dependent on the vertical layer of the sediment. To test this hypothesis, three-cm interval sediment samples (15 cm depth) were taken independently from two bedforms in three estuaries. Results indicated that assemblages living in the top layers are more abundant, species rich and less variable, in terms of species presence/absence and relative abundances, than assemblages living in the deeper layers. Results showed that redox potential explained the greatest amount (12%) of variability in species composition, more than depth or particle size. The fauna inhabiting the more oxygenated layers were more homogeneous across the horizontal scales than those from the reduced layers. In contrast to previous studies, which suggested that reduced layers are characterized by a specific set of tolerant species, the present study showed that species assemblages in the deeper layers are more causal (characterized mainly by vagrant species). The proposed mechanism is that at the superficial oxygenated layers, species have higher chances of being resuspended and displaced over longer distances by passive transport, while at the deeper anoxic layers they are restricted to active dispersal from the above and nearby sediments. Such restriction in the dispersal potential together with the unfavorable environmental conditions leads to randomness in the presence of species resulting in the high variability between assemblages along the horizontal dimension.