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.