Introduction
Arthropod species living in wetlands often face more or less regular
periods of flooding, during which the habitat is temporally submerged,
compromising the life of terrestrial organisms (Batzer and Wu 2020, Plum
2005). Terrestrial species inhabiting these shifting habitats have
developed a range of adaptations that allow them to persist despite the
regular disturbances (Kolesnikov, et al. 2012, Lafage, et al. 2015,
Marx, et al. 2012, Ramey and Richardson 2017, Rothenbücher and Schaefer
2006). Some species simply withstand the floods until the water recedes,
which may require special adaptations to avoid drowning. Other species
adapt their life cycle to account for regular flooding regimes or
retreat to drier grounds, returning when the water is again gone. Among
arthropods, previous studies from large flood plains suggest that plant-
and leafhoppers (Auchenorrhyncha) often tolerate prolonged submersion
and therefore remain in the wetland during floods, whereas most spiders
and ground beetles instead tend to move in and out of wetlands in
response to fluctuating water levels (Lafage, Sibelle, Secondi, Canard
and Petillon 2015, Rothenbücher and Schaefer 2006). However, even among
beetles and spiders, some species are well able to survive at least
short-term flood events (Kolesnikov, Karamyan and Hoback 2012).
The occurrence of topographical heterogeneities including vegetation
that create local dry ground refuges during floods likely influences
species survival and recolonization ability. For instance, wetlands
often contain a mixture of hummocks and hollows, with the former
providing arthropods with safe ground during high-water events. These
small-scale heterogeneities have seldom been considered in studies of
local arthropod communities and, are in fact rarely quantified even in
hydrological studies (but see Diamond, et al. 2021). A study by Datry et
al. (2014) demonstrated that heterogeneities in flood disturbance at a
larger scale within a flood plain can significantly affect arthropod
communities by creating spatial differences in plant successional
patterns. However, the two only studies known to us that combined a
quantification of small scale heterogeneities in a large wetland and
arthropod communities are Åhlén et al. (2023), which quantified
inundation frequencies that reflect local wetness-dryness at a 2x2
m2 resolution and used SLAM (sea, land, air –
Malaise) traps for arthropod quantification, and Åhlén et al. (2024).
While other studies have incorporated flood dynamics, they generally do
so at a coarse scale (but see O’Callaghan, et al. 2013 for an example
from the riparian zone of a large river). These studies have documented
both positive and negative relations between the inundation frequency
and arthropod densities. For instance, Åhlén et al. (2023) found that
groups such as leafhoppers were more abundant in frequently flooded
areas, whereas dance flies were more prevalent in drier parts of the
wetland. This study was however biased by the choice of sampling method.
SLAM-traps, like all Malaise traps, primarily capture flying insects and
are therefore less suited for quantifying the densities of cursorial
species such as spiders and beetles, that are less able to recolonize
after disturbances.
In wetlands, spiders and predatory beetles commonly occur at high
densities, utilizing the high prey production in these habitats (Ramey
and Richardson 2017). Many previous studies have also explored
mechanisms underlying community responses of both spiders and predatory
beetles, showing the prime importance of the hydrological regime at
larger spatial scales (Bonn, et al. 2002, Cartron, et al. 2003, Gerisch,
et al. 2006, Jachertz, et al. 2019, Sienkiewicz and Zmihorski 2012,
Uetz, et al. 1979). In fact, it has been suggested that the flooding
regime is more important than habitat management for both spiders and
carabid beetles (Lafage and Petillon 2016). There are also potential
differences in responses between spiders and beetles, where these
communities are filtered by different environmental factors in wetlands
(Åhlén, Jarsjö, et al. 2024). For instance, Lambeets et al. (2008)
showed that spider densities decreased strongly with all types of
flooding whereas carabid beetles seemingly benefit from intermediate
flooding regimes. Their study however used pitfall traps which bias
spider catches towards wolf spiders, and carabid beetles, and provide
poor density estimates of most other spider groups (Amiar, et al. 2023),
and of small staphylinid beetles. Nevertheless, other studies similarly
show that carabid beetles are often good indicators of local flooding
regimes (Gerisch, Schanowski, Figura, Gerken, Dziock and Henle 2006,
Moran, et al. 2012), whereas spider communities seem to be more
determined by habitat structure (Bonn, Hagen and Wohlgemuth-Von Reiche
2002, Åhlén, Jarsjö, Jonsell, Klatt, Schneider, Strand and Hambäck
2024), which may directly or indirectly be affected by flooding regimes.
In this study, we first identified sites with varying inundation
patterns in a large wetland based on the quantification of small-scale
inundation patterns from Åhlén et al. (2023). We then estimated the
density and species diversity of beetles and spiders in these sites
using suction sampling, which reflect total communities better than
pitfall traps, to connect arthropod communities and inundation patterns.
We selected sites along two gradients, with wet or dry local (2x2
m2, centered on the arthropod collection point)
conditions and wet or dry conditions at a larger spatial scale (6x6
m2). To further identify relevant spatial scale of
responses, we calculated inundation patterns also at larger distances
from the arthropod collection sites. In addition, we measured local
habitat characteristic such as vegetation height and soil carbon content
that are known to affect arthropod communities in wetlands (Ye, et al.
2024). For an overview of the methods see Figure 1. Our prediction was
that arthropod densities in locally very wet sites depend on the
presence of dry sites nearby, whereas the opposite pattern would be less
important.