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.