Introduction

Connectivity in habitat networks is a major driver of local and regional biodiversity patterns via species dispersal (Taylor et al. , 1999; Leibold et al. , 2004; Fletcher et al. , 2016). Among discrete habitat patches, such as ponds, connectivity is frequently approximated as a function of distance, where the highest levels of dispersal are assumed among neighbouring habitats (Calabrese & Fagan, 2004). Across small spatial scales, i.e., up to a few hundred metres, dispersal levels are generally assumed to be high enough to efficiently counteract local extinction events (Langenheder & Lindström, 2019), thereby contributing to efficient species sorting. Within a few meters, e.g., in rock pool metacommunities, occasional biotic homogenisation may occur via mass effects, resulting from extremely high dispersal rates (overspill; Vanschoenwinkel et al. , 2007). The relevance of these processes, however, decays rapidly with increasing distance and most natural metacommunities are rather assembled along a gradient of intermediate to low dispersal, depending on the actual spatial scale, landscape connectance, and species traits (Leibold & Chase, 2018). While there is a general agreement on the importance of sufficient connectivity in habitat networks for the sustenance of biodiversity, its above-mentioned scale and context dependence and their effect on biodiversity patterns remain lesser understood.
Pondscapes are regional networks of ponds clustered in a landscape (Biggs et al. , 1994; Boothby, 1997; Baguette et al. , 2013). Since the introduction of the term, it has been frequently argued that ponds function as habitat networks, e.g., by serving as stepping stones for aquatic taxa (Pereira, Segurado, & Neves, 2011), and that their value for biodiversity lies in their numbers (Oertli et al. , 2002; Martínez-Sanz et al. , 2012). Pondscapes are shrinking worldwide, which increases the isolation between the remaining habitats (Thornhill et al. , 2018). These changes are predicted to result in biodiversity loss both at the local (Horváth et al. , 2019; Holmes et al. , 2020) and the landscape scale (Horváth et al. , 2019). This calls for a better understanding of connectivity within their remaining networks, both for conservation and efficient restoration measures. Graph-based measures can be important connectivity metrics to understand the functioning of habitat networks (Urban & Keitt, 2001), and for such analyses, well-delineated networks of pools and ponds could provide ideal test cases. Surprisingly though, such explicit network analyses involving ponds are still scarce (exceptions include e.g., Thornhill et al. , 2018; Godet & Clauzel, 2021) including their utilisation for metacommunity theory in understanding drivers of biodiversity patterns (Borthagaray, Berazategui, & Arim, 2015; Cunillera-Montcusí et al. , 2021).
While there is an upsurge in studies focusing on the role of spatial configuration within pond networks in shaping the structure and dynamics of aquatic metacommunities, there is a significant knowledge gap in understanding the role of pond configuration in networks over smaller spatial scales (within a few km, where most of the dispersal events supposedly happen). Most of the previous works come from networks over large distances, which at the same time did not capture all ponds in the landscape (Soininen et al. , 2007; Florencio et al. , 2014; Gálvez et al. , 2023). While these studies are very informative on distance-based community similarities and general community patterns, they cannot offer a full view of the functioning of the habitat network per se. A central-peripheral connectivity gradient in a pondscape might underlie the main biodiversity gradients, with central ponds hosting more species or playing a relatively more important role (Borthagaray, Berazategui, & Arim, 2015; Cunillera-Montcusí et al. , 2020; Holmes et al. , 2020). However, these patterns are largely masked in such study designs and can only reliably be revealed in pondscapes where the entire network is considered for the calculation of network properties (i.e., measures of centrality).
In addition to spatial distances, the dispersal capacity of organisms among habitat patches also varies according to key traits such as dispersal mode and body size. Passive dispersers, such as microscopic organisms, rely on dispersal vectors, such as wind or animals (Bilton, Freeland, & Okamura, 2001; Mony et al. 2022). Their dispersal potential is expected to decrease with body or propagule size (Bieet al. , 2012). Active dispersers include organisms with life stages that allow movement outside of water independent of vectors, such as amphibians or flying insects. As these groups can actively select suitable habitat patches, their distribution patterns are expected to be predominantly determined by the local environmental conditions rather than spatial processes (Heino, 2013). In contrast to passive dispersers, the dispersal ability of active dispersers is expected to increase with body size (Alzate & Onstein, 2022; Cote et al. , 2022). While small-bodied chironomids mostly stay within 200 m of their emergence site (Khan 2012), larger aquatic heteropterans disperse readily over to 1.6 km (Briers 1998; Choi & Kim 2009), and most amphibian species cover distances up to a few kilometres (Smith & Green, 2005).
Small passive dispersers and active, good dispersers are assumed to disperse well over large distances and thus their communities are not expected to show spatial patterns across small spatial scales related to dispersal limitation. Consequently, small scales stretching over only a few kilometres are often neglected in metacommunity studies. Nonetheless, studying community patterns at small spatial scales could still be informative: large passive dispersers may display spatial patterns across small scales due to low dispersal ability (Cottenie & De Meester, 2003; Cottenie et al. , 2003) and even active dispersers might show patterns from limited dispersal due to territorial behaviour (McCauley, 2010) or differences in the surrounding matrix (Gall, Chaput-Bardy, & Husté, 2017). Even so, recent studies tend to concentrate on a single or a limited number of organism groups, while multi-group studies, covering a wide range of taxonomic groups with diverse dispersal traits are still scarce, even though they have been increasing in recent years (e.g. Bie et al., 2012, De Marco et al., 2014, Gálvez et al., 2023). Such studies are crucial as they can corroborate the overall functioning of connectivity networks within pondscapes by highlighting congruent patterns across diverse taxa.
The aim of the current study was to investigate how the relative position of ponds in a well-delineated pond network affects local species richness and metacommunity structure in metacommunities of multiple organism groups, encompassing a variety of dispersal traits. As a model system, we used a cluster of closely-spaced bomb crater ponds, with over 50 habitats situated on a spatial scale of a few hundred meters. We specifically tested whether local species richness scales positively with the centrality of the ponds in a graph-based approach. In addition, we quantified the extent to which spatial configuration predicts metacommunity structure based on eigenvector analysis. We expected to detect spatial signals despite the small spatial extent, linked to the presumed higher dispersal rates among more central ponds, which should result in a gradient of biodiversity along the gradient of connectivity. Further, we also predicted that the strength of spatial signals will vary between organism groups linked to body size and dispersal mode as key traits. Specifically, we expected weaker spatial signals in the richness and community structure of taxa assumed to be good dispersers (i.e., especially for large active and small passive taxa) and stronger for groups more constrained due to larger size or lower motility.