Fulvio Cerfolli

Spatial network structure and robustness of detritus-based communities in a patchy environment

The mechanisms that regulate the spatial distribution of species are an essential aid to understanding the effects of the environment on the persistence of populations and communities. The effects of spatial structure on the persistence and robustness of ecological communities can, in turn, prove useful in uncovering their functioning, e.g., in the decomposition of leaf detritus. We applied the framework of complex networks to evaluate the effects of spatial structure on the colonization process of leaf detritus in a patchy aquatic environment, with a spatial network of six pools at different salinity. We found three well-defined modules formed by groups of taxa sharing the same pools, observing an association between modularity and spatial proximity of pools. Modules maximize the number of links within modules, and minimize the number of links among modules, showing the presence of a strong site-specific association between taxa and pools. The topological characteristics of the network show robustness against random perturbations and a lower tolerance of targeted perturbations. These findings suggest that random events, such as flooding or heavy rains, slightly affect the robustness of the system, while localized perturbations on the most connected nodes could have a negative effect on the connectivity of the whole network. The consequences could lead to a structural and functional homogenization of the system, with potential effects for the entire trophic chain. Here we discuss the topological properties of the network in relation to the spatial distribution of pools, showing how network analysis can yield valuable insight for conservation and management.

The interplay between network structure and functioning of detritus-based communities in patchy aquatic environment

Understanding models of networks formation is fundamental to explore the role of the structure in the functioning of the systems they describe, and their ability to respond to change. In this work, we aimed to understand whether and how the modular (or compartmented) structure of a network composed by macroinvertebrates and leaf detritus in six pools of aquatic system was related with the decomposition process. First, we evaluated the relationship between the temporal patterns of leaf detritus colonization and the modular subdivision of the network. Modularity was then related with the clustering of the pools based on convergent traits of leaf detritus consumption and environmental conditions. A significant relationship between the colonization patterns and the distribution of taxa and pools in well-defined modules was found. The modular and clustering subdivision of the pools showed a significant overlap, which revealed the intimate linkage between the structure and functioning of the system. Modularity shapes the functional architecture of the network, by increasing the spatial differences of leaf litter decomposition over time and the diversity of functional traits among detritus feeder. As a consequence, modularity influences the variability of communities’ responses to disturbance, increasing the diversity and robustness of functional processes. Our results have also implications from a conservation point of view, showing the importance of habitat heterogeneity for the robustness of ecosystem functioning, potentially enhancing biodiversity with positive, long-term effect on the whole food web.

Infaunal macrobenthic community dynamics in a manipulated hyperhaline ecosystem: a long-term study

BackgroundUnderstanding the responses of ecological communities to human-induced perturbations is crucial for establishing conservation goals. Ecological communities are dynamic entities undergoing fluctuations due to their intrinsic characteristics as well as anthropogenic pressures varying over time. In this respect, long-term studies, based on large spatial and temporal datasets, may provide useful information in understanding patterns and processes influencing the communities’ structure. Theoretical evidence suggests that a role of biodiversity is acting as a compensatory buffer against environmental variability by decreasing the temporal variance in ecosystem functioning and by raising the level of community response to perturbations through the selection of better performing species. Therefore, the spatial and temporal changes in the specialization of the community components may be used as an effective tool to monitor the effects of natural and anthropogenic alterations of the environment in dynamic systems. We examined the temporal dynamics of macroinvertebrate community structure in the hyperhaline habitat of Tarquinia Saltworks (central Italy). We aimed at: (i) investigating the relationships between the level of community specialization and the alterations of the environment across fourteen years; (ii) comparing the ability of aggregate community parameters such as the average abundance vs. species specialization in describing patterns of community composition.ResultsWe arranged the data in three sub-sets according to three periods, each characterized by different environmental conditions. The mean abundance of sampled macroinvertebrates showed a significant change (p < 0.01) only in the community inhabiting the saltwork basin closely connected to the sea, characterized by the highest environmental variation (i.e. the coefficient of variation, CV, of the aggregate environmental variability over the study period, CVrange = 0.010 - 0.2). Here we found marine species like Modiolus adriaticus (Lamarck, 1819), Neanthes irrorata (Malmgren, 1867), and Amphiglena mediterranea (Leydig, 1851), which inhabited the saltworks during the halt period but disappeared during the subsequent eutrophication phase. Conversely, species specialization showed a significant decrease for each sampled community in the presence of habitat degradation and a recovery after ecological restoration. The widest fluctuations of specialization were recorded for the community inhabiting the saltwork basin with the highest long-term environmental variability.ConclusionsRecent advances have shown how the increased temporal and spatial variability of species’ abundance within the communities may be a signal of habitat disturbance, even in the absence of an apparent decline. Such approach could also be used as a sensitive monitoring tool, able to detect whether or not communities are subjected to increasing biotic homogenization. Also, the increased functional similarity triggered by habitat degradation may impact on species at higher trophic levels, such as the waterbirds wintering in the area or using it as a stopover during migration.

Relationship between phylogenetic and nutritional diversity in Arctic (Kandalaksha Bay) seawater planktonic bacteria

Due to huge yearly variations of environmental stressing conditions, Kandalaksha Bay (Arctic Circle, White Sea, Russia) could represent a model to study microbial adaptation in extreme environments. This peculiar estuarine system has been scarcely investigated for its microbial diversity. In this work, to gather information on their nutritional competencies, seawater planktonic bacteria were studied for their ability to use different carbon sources by the Biolog phenotype microarray assays. Nestedness, a useful statistical tool used in ecology, was employed to underline nutritional differences among microbial groups. In particular, nestedness was used to understand the complex relationship that is established when many nutrients are available for various microorganisms, and to highlight presence of specialists and generalists. Among the studied bacteria, which showed very diverse nutritional abilities, 47% belonged to Pseudomonas, 21% to Serratia and 32% to other Genera. Within Pseudomonas, both highly generalist and highly specialist strains were discovered. However, most of them used organic and/or amino acids as principal carbon sources. In contrast, Serratia strains typically preferred sugars and appeared to be more generalist. On the whole, important differences in specialization levels and nutritional competencies were recorded in strains belonging to the same species. Correlations between phylogenetic and nutritional data were validated by Procrustes analysis.

Detritus-based assemblage responses under salinity stress conditions in a disused aquatic artificial ecosystem

BackgroundDespite the plethora of approaches, the sensitivity of the methods to measure the relationship between the abundance and biomass curves in stressed detritus-based ecosystems still remain to be refined. In this work, we report the comparison between biomass and abundance in a set of detritus-based macrozoobenthic assemblages located in six sampling pools with different salinity in an artificial aquatic ecosystem (disused Tarquinia Saltworks), using two diversity/dominance approaches (Abundance/Biomass Comparisons, or ABC, and Whittaker plots). We also evaluated the contribution of abundances and biomasses diversity (Simpson index) and nestedness, which measures the order by which macroinvertebrates colonized the detrital resource.ResultsThe outputs obtained by both ABC curves and Whittaker plots highlight two different thresholds in assemblage structure: between about 44 and 50 practical salinity unit (psu) and between 50 and 87 psu, respectively. The first threshold was due to a turnover in taxon composition between assemblages, the second threshold (evidenced by Whittaker plots) was due to a change in taxon richness (lower in pools with higher salinity: i.e. > 50 psu). Moreover, a normal-shaped pattern in diversity (Simpson index) emerged, suggestive of an intermediate disturbance effect. The nested pattern did not show significant differences when considering the density and biomass of the sampled taxa, providing similar threshold of salinity in the relative contribution of macrozoobenthos on nestedness.ConclusionsThe use of detailed (ABC and Whittaker plots) and macroscopic (Simpson index and nestedness) approaches is proposed to identify thresholds in the structuring and functioning of detritus-based community of disused aquatic ecosystems: in particular, the inclusion of the parameter of biomass (scarcely utilized in community-based research) appears crucial. The responses of macrozoobenthic assemblages to the salinity stress conditions, in term of abundance and biomass, using a detritus food source (Phragmites australis leaves), may also highlight, by comparing macroscopic and detailed approaches, structuring and functioning patterns to consider for the management of disused artificial ecosystems.