Spatial and historical effects shape a wide-scale rice field ostracod metacommunity pervaded by invasive species

Metacommunity structure is shaped by a complex interplay of environmental conditions, spatial factors, and historical contingencies. To determine the processes that govern community assembly and biodiversity patterns, it is necessary to know the relative importance of these elements. In this study, we examine the ostracod metacommunity of rice fields across the Iberian Peninsula, aiming to assess the influence of spatial constraints, historical contingency (as derived from taphocoenoses remains), and environmental variables on the present metacommunity (biocoenoses) structure. We hypothesized that while all three components would affect the metacommunity, spatial effects would be particularly relevant due to the large geographical scope and associated dispersal limitations. Our results indicate that space was the most relevant factor in structuring the metacommunity, operating at broad and fine scales. Historical contingency also contributed substantially, as indicated by the high concordance between taphocoenoses and biocoenoses, suggesting that past community composition influences present assemblages, possibly through colonization advantages associated with egg banks. Although the pure environmental fraction was comparatively weak and nonsignificant, likely due to the low environmental variability of these homogeneous and artificially controlled aquatic habitats, the substantial shared environmental-spatial fraction could indicate that environmental conditions covaried with broad-scale spatial structure among regions. A notable number of invasive ostracods were identified, suggesting ongoing, albeit infrequent, colonization processes, probably facilitated by waterbirds, land ploughing, movement of agricultural machinery, and farmer activities, which support the establishment of invasive species within each community egg bank. These findings indicate that biodiversity management in seasonal agricultural landscapes, such as rice fields, should extend beyond local environmental conditions, taking into account regional connectivity, the persistence of egg-bank legacies, and human-mediated dispersal pathways that may facilitate the establishment of non-native species.