Discussion

Our results supported the importance of connectivity for multiple organism groups in a delineated habitat network covering a small spatial scale of a few hundred metres. We found that pond taxonomic richness scales with network position and that spatial configuration has an imprint on metacommunity structure across multiple taxonomic groups. The local environment explained a larger share of the variance in both richness and metacommunity structure than space, indicating a predominant role of species sorting, which is not surprising given the existence of strong environmental gradients (e.g., salinity, vegetation cover). Despite this, we found a clear indication that space also played an important role in structuring the metacommunity of the bomb crater pond network with such a small spatial extent. This held for most of the studied organism groups and for both taxonomic richness and metacommunity structure. Overall, our results highlight the importance of dense habitat networks in sustaining biodiversity. Moreover, the various organism groups differed in the amount of variance explained in their richness and composition by space, and it was likely related to their dispersal abilities according to our expectations.
While metacommunity processes are highly context-dependent, species sorting is expected to be predominant over short spatial scales, especially for microbes (Bie et al. , 2012; Hanly & Mittelbach, 2017; Langenheder & Lindström, 2019; Mony et al. , 2022). In line with this, we found clear evidence for the predominance of species sorting for all organism groups, but with slightly different environmental variables being relevant for each group. Overall, conductivity, pH, and TSS were among the most important environmental predictors, which is in line with data from similar saline temporary waters from the region (Horváth et al. , 2014; Horváth, Vad, & Ptacnik, 2016; Márton et al. , 2023). But while there was a strong environmental signal underlying community patterns, a significant spatial effect also emerged in multiple groups. These included the larger-bodied passively dispersing rotifer and crustacean zooplankton and the weak flyer dipterans, suggesting some level of dispersal limitation in their case.
The evidence for spatial structuring has been confirmed with complementary analyses, as we both tracked similarities based on spatial eigenvectors and explored the predictive role of relative spatial position (closeness centrality). The different sets of analyses gave consistent results, revealing the importance of spatial effects in the richness and metacommunity structure of the three organisms groups expected to be the weakest dispersers (i.e., large passive dispersers and weak active dispersers). For metacommunity structure, the first five MEM eigenvectors were the most frequently selected spatial explanatory variables across the organism groups. These correspond to the largest eigenvalues indicating coarse-scaled spatial structuring. Furthermore, multiple of these significant MEM eigenvectors (MEM 3, 4, and 5) illustrated the main spatial structuring between central and peripheral sites, similar to our results on centrality and richness. These overall indicated an important network effect in several organism groups, where richness is enhanced by a more central position of a local habitat via a higher number of surrounding patches, presumably related to the higher frequency of dispersal.
Overall, the richness and community structure of prokaryotes and microeukaryotes showed only weak spatial structuring, which is in line with other studies on spatial patterns in prokaryotes across various spatial scales (Beisner et al. , 2006; Van Der Gucht et al. , 2007; Bie et al. , 2012; Padial et al. , 2014). These taxa are easily dispersed by the wind even across vast distances (Smithet al. , 2013; Mony et al. , 2020) or via zoochory at small spatial scales (Lindström & Langenheder, 2012; Szabó et al. , 2022). Similarly, we did not find any evidence for spatial structuring in communities of actively flying macroinvertebrates (excluding dipterans), amphibians and reptilians. This was in line with our expectation, as these groups are unlikely to show patterns related to dispersal limitation due to their abilities to move over distances larger than in our study system (Ficetola et al. , 2004; Smith & Green, 2005; Heino, 2013; Florencio et al. , 2014; Godet & Clauzel, 2021). At the same time, spatial signals in zooplankton richness and community structure were evident. These indicated some level of dispersal limitation at the spatial scale of our study in a passively dispersing group with a large body compared to microbes. This finding aligns with previous studies that showed the importance of pond centrality for Daphnia metapopulation structure over a comparable spatial scale (Holmeset al., 2020) and the role of connectivity in structuring zooplankton metacommunities over larger scales of hundreds of square kilometers (Cottenieet al., 2003; Soininen et al., 2007).
Therefore, connectivity is important in pondscapes even across small spatial scales for at least some members of the metacommunity. The different groups of organisms that make up a metacommunity differ in their traits and the scales relevant to them in terms of spatial processes. Therefore, in multi-group studies, it is worth considering small spatial scales even if some groups are not expected to show spatial patterns. Additionally, the varying responses of the different groups indicate different structuring processes, i.e., the small but significant spatial effect on community structure may indicate a certain level of mass effect for microeukaryotes, while the largest-bodied actively dispersing groups with the best dispersal abilities showed efficient species sorting at the same spatial scale. In several groups, the effect of connectivity was found to be more important when metacommunity structure was considered compared to taxonomic richness indicating that more connected patches do not necessarily hold more species but connectivity within the pondscape is an important determinant of metacommunity structure.
There remains a large portion of unexplained variation in taxonomic richness and metacommunity structure for all groups, similar to other metacommunity studies (Soininen et al. , 2007; Vanormelingenet al. , 2008; Bie et al. , 2012). Our analyses were based on an exhaustive dataset of the environmental variables, and three complementary metrics of spatial configuration and both environmental and spatial variables were well-represented making their effects less likely to be underestimated. At the same time, biotic interactions (competition, grazing, predation, mutualism, parasitism etc.) are likely to play a role in metacommunity structuring and may mask the effects of space or environment (Van De Meutter, Stoks and De Meester, 2008; Verreydt et al., 2012, Mony et al., 2022). Apart from such trophic relationships, interspecific competition at the same trophic level can also affect spatial patterns (Thompson et al., 2020; Guzman et al., 2022). Finally, our study is only a snapshot in a presumably highly dynamic system and temporal aspects may need to be explored along with historical processes (Vyverman et al. , 2007; Vanormelingen et al. , 2008; Thompson et al. , 2020; Guzman et al. , 2022). Dormant eggs and other resting stages integrate past dispersal events and this could have masked recent dispersal events in passive dispersers in our study (Wisnoski, Leibold, & Lennon, 2019; Holyoak, Caspi, & Redosh, 2020; Wisnoski & Shoemaker, 2022). Incorporating these effects in future studies would provide novel in-depth knowledge of the additional processes acting in such pondscapes, further increasing our understanding of how these habitat networks function.
In conclusion, we found that both space and environment shape the metacommunity of a pondscape even when the spatial extent is relatively small. Differences in dispersal traits between organism groups are likely attributable to group-specific differences in spatial patterns, with communities of actively dispersing larger animals and small passive dispersers showing weak or no spatial signals. In contrast, communities of weak-disperser macroinvertebrates and especially large-bodied passive dispersers are structured by space to a greater extent. The fact that spatial patterns occur in metacommunity structure and central ponds in the network host higher richness highlights the importance of studying and protecting ponds as parts of a network. This needs to be taken into consideration during conservation planning to maximise the protection of overall biodiversity at both the local and landscape levels.