Metacommunities and biodiversity patterns in mediterranean temporary pondsthe role of pond size, network connectivity and dispersal mode

Resumen

Understanding how ecological communities are structured is a fundamental question on ecology which is still unanswered. In this line, the role of environmental and spatial processes operating in multiple scales to shape local community composition is explicit in the metacommunity framework. Further, metacommunity dynamics are strongly dependent on three factors ─scale, habitat heterogeneity and connectivity. Indeed, connectivity is considered an important attribute of natural ecosystems that sustains both biodiversity and ecosystem function. Moreover, there is evidence for variation in the spatial processes and patterns driving biodiversity at different spatial scales. Finally, habitat heterogeneity has been identified as a key factor for the distribution, abundance and diversity of species. Within this context, the aim of this thesis was to analyze how network connectivity, environmental characteristics and organisms’ dispersal mode act in shaping macrofaunal metacommunities, taking into account different spatial scales. Pond networks constitute an ideal scenario to study assembly processes in the metacommunity framework, since for aquatic organisms, ponds are suitable patches in an unsuitable habitat matrix. Thus, ponds act as “habitat islands” and this particular “isolation” can, further, modulate the effect of both environmental and spatial factors on their communities. Moreover, pond networks are also good candidate ecosystems in which to examine the mechanisms underlying biodiversity patterns at local and regional scale since, on one hand, individual ponds constitute discrete systems suitable for studying their local environmental factors and communities, and, on the other hand, the whole network allows observing the dynamics at metacommunity level. Therefore, we selected and sampled four networks of Mediterranean temporary ponds covering a wide range of pond sizes and different spatial extents. Firstly, we studied the smallest pond network and we observed that pond size was not related to any of the environmental factors taken into account, and neither to macrofaunal community structure despite a positive relationship between pond size and for instance, species richness, was expected. However, pond size had an effect on community composition since certain species (such as Gyraulus laevis, Piona sp. and Aeshna mixta) had higher abundances in larger ponds. Moreover, the effect of a high connectivity throughout the network and therefore, a likely homogenization of the communities, seemed to explain the absence of significant differences in community structure. Secondly, we analyzed the effect of the spatial extent of the network comparing the decay of community similarities with distance (both spatial and environmental) among the ponds from two pond networks (one four times larger than the other), and also possible differences due to organisms’ dispersal mode. In general, environmental distance had larger effects than spatial distance in both pond networks, probably due to the small extent of both networks. However, when carefully analyzing the responses observed, some differences arise linked to the spatial extent and the dispersal mode. Thus, in the smaller network, the different biotic groups showed different distance-decay patterns, that fit under different metacommunity archetypes: the mass effects in the case of the macrofaunal active dispersers, the species sorting for the macrofaunal passive dispersers, and undetermined for plants. In contrast, when increasing the spatial extent (i.e., in the larger network), a homogenization of the observed response occurred, and all the groups fit to the expected response under a species sorting archetype. Thirdly, we studied four pond networks applying the graph theory to obtain several connectivity network descriptors and centrality metrics. The network centrality metrics are descriptors of the degree of the isolation-centrality situation that a patch has with respect to the rest of the patches in the network. Then, we tried to identify if biodiversity patterns, at local and regional level, can be related to these centrality metrics. The environmental characteristics of the ponds, which included habitat condition, water characteristics and pond size, were also included in the analyses. We observed that the different centrality metrics, which measure connectivity within the network from a neighboring to a broader scale, affect both regional and local pond biodiversity. Moreover, most of the biodiversity metrics were not explained by environmental characteristics or network centrality metrics alone, but through a combination of them both. Overall, regional biodiversity metrics showed similar patterns in all the networks studied, whereas local biodiversity metrics showed greater network-dependent patterns. Therefore, considering the findings of this thesis, we argue that further studies on pond metacommunities at different spatial extents are needed and, especially, at small spatial extents, since there is a lack of works on metacommunities conducted at small scales, and we have demonstrated that the results can differ from those found in studies at larger scales, providing, then, new knowledge. Moreover, we demonstrated the effect on biodiversity of the connectivity within a pond network, highlighting the importance of preserving habitat connectivity. On the other hand, although we did not find support for the expected positive species-area relationship, we found differences at composition level. Thus, we think that it is important to maintain size diversity of ponds, since smaller waterbodies do not seem to harbour less species than larger ponds but, certain species are favoured in smaller ponds whereas other species are favoured in larger ones. Moreover, smaller ponds can further, act as stepping-stones helping to maintain metacommunity dynamics and stability.