Discussion
The spider community showed large regional changes along the Baltic Sea
seashore despite comparatively small salinity differences (5 vs. 7‰).
Several spider species (Pardosa agrestis , P. agricola,
Arctosa leopardus and Alopecosa cuneata ) were almost exclusively
located on the higher salinity sites compared to lower salinity sites by
the Baltic Sea shore and by inland lakeshores. At the same time, other
taxa (Pardosa prativaga, P. amentata and Pirata spp.) had
the opposite distribution pattern, and this pattern was seemingly not
explained by either prey availability or actual spider diets. In fact,
there were no detectable diet differences between spider species or
between spiders captured on shores with different salinity levels.
Instead, spider diets varied between shores with or without thick beds
of stranded wrack, a gradient that did not affect spider community
structure. Consequently, and because the species shift only occurred on
coastal sites and not on corresponding inland sites, it seems that
coastal spider communities are directly affected by the saline
conditions.
High salinity has several negative impacts on spiders and other
arthropods, by reducing both survival and reproduction (Pétillonet al. 2011; Puzin et al. 2011; Foucreau et al.2012). Even though none of the species found on the Baltic shorelines
can be considered true halophilic and are usually not found on more
marine seashores (Pétillon et al. 2008), it seems reasonable to
assume that species vary in their sensitivity to saline conditions.
However, please note that previous studies on wolf spiders tested the
responses of individuals at a much higher salinity (>30‰)
than in our gradient, and it is unclear to what extent that their
conclusions could be extrapolated to our study. Irrespective of the
mechanisms, our data in combination with previous studies suggest a
gradient in salinity thresholds of the dominant wolf spider species on
marine shorelines in northwestern Europe where P. prativagatypically dominates low salinity sites, P. agricola dominates
intermediate salinity sites and P. purbeckensis dominates high
salinity sites. The species abundance distributions of wolf spider
communities are often highly skewed with one dominant species having
more than 60% of all individuals and a tail of rare species. Even
though low salinity sites are not always dominated by P.
prativaga , two-thirds are dominated by this species and then more
rarely by P. amentata , P. palustris and some other species
(see also Meriste, Helm & Ivask 2016).
Whereas the restriction to low salinity sites can likely be explained by
salt sensitivity, the corresponding absence of other species at the same
low salinity sites seems more puzzling. First, it is evident that the
absence from low salinity sites is not absolute as both Pardosa
agrestis and P. agricola are frequently reported also from
inland habitats in central Europe and more rarely inland also from
northern Europe (GBIF.org). Moreover, studies on P. purbeckensis ,
perhaps the most halophilic species, suggest that fitness is not reduced
on low salinity sites (Pétillon et al. 2011). It is possible that
some other habitat characteristics restrict the occurrence in low
salinity sites or that distributions are restricted by species
interactions. Several wolf spider species are known for intraguild
predation of other wolf spider species, at least in the laboratory, and
dominance is mainly governed by size differences (Buddle, Walker &
Rypstra 2003; Rypstra & Samu 2005; Rickers, Langel & Scheu 2006;
Rypstra et al. 2007; Turney & Buddle 2019), but no study this
far has evaluated the role of intraguild predation on the spatial
distribution of wolf spiders.
Whatever the reason is for the difference in wolf spider community
composition, the patterns are not likely explained by different dietary
niches among spider species or by differences in prey availability. Both
this, and previous studies using either molecular gut content analysis
or other methods, indicate large overlaps in the diet of wolf spider
species (Mellbrand & Hambäck 2010; Verschut et al. 2019). Diet
differences observed in this study instead seem to depend on whether
spiders were collected on sites with or without accumulated wrack, but
these diet shifts did not coincide with shifts in the wolf spider
community. By far the most abundant prey group in the wolf spider guts
on sites with either wrack or no wrack were dipterans (typically taxa
with smaller individuals) and to some extent homopterans. This general
prey composition of wolf spiders is of course well-known from
non-molecular studies (e.g., Nyffeler 1999), but the relative importance
of small dipterans is perhaps larger in our study habitats. Some
differences between molecular and non-molecular studies may occur
because the former provide an improved representation of small prey
items, which are easily overlooked in non-molecular studies due to more
rapid consumption. In either case, wolf spiders are likely quite
opportunistic predators where prey choice perhaps depend more on
encounter probabilities and catchability of prey in their selected
habitat than on prey qualities. This opportunistic behavior is perhaps
also reflected in the different number of prey species, where the number
is higher in southern sites, as expected, and in sites with no wrack.
Similarly, diet consistency was also higher on wrack sites, and both
patterns observed for wrack sites may reflect that wrack beds are
dominated by a small set of detritivorous species. More surprising was
the higher diet consistency of spiders on southern non-wrack sites
compared with northern non-wrack sites, despite the lower total prey
diversity observed for the spiders in the southern region.
Even though opportunism seems to be a dominant pattern, particularly
dark-winged fungus gnats (Sciaridae) are underrepresented in wolf spider
guts despite their comparatively high occurrence at these sites, similar
to what was found previously (Verschut et al. 2019). The reason
for spiders to avoid fungus gnats may be that they represent low quality
food (as suggested by Toft & Wise 1999b; Toft & Wise 1999a). Diet
differences between sites with or without accumulated wrack otherwise
reflect availability, even though we refrained from testing the
availability-use relationship due to the bias in SLAM traps. Many small
flies often occurring on wrack beds, such as Drosophilidae, Ephydridae,
Sepsidae and Sphaeroceridae are underrepresented in Malaise type traps
on shore lines because these flies tend not to stick to the ground. In
either case, these small detritivorous flies that likely developed in or
close to the decomposing wrack made up more than 75% of all prey in
spider guts when collected from sites with heavy wrack beds and the diet
composition was surprisingly similar for spiders collected on northern
and southern wrack beds. More unexpected was perhaps the low frequency
of chironomids in the spider gut contents, particularly in the non-wrack
sites. In a previous study (Verschut et al. 2019), not far from
the sites included in this paper, chironomids dominated the spider gut
contents and particularly late in the season. In this study, there were
no seasonal differences and spiders on non-wrack sites instead consumed
a range of terrestrial prey groups, such as Homoptera and various
terrestrial Diptera (Chloropidae, Empididae, Dolichopodidae etc.), and
it seems that spiders were less strongly connected to the nearby marine
environment than previously assumed. In either case, this variability
among studies indicate how dynamic food choice of spiders may be.
To summarize, our study indicates that quite a small difference in
salinity caused the species composition of wolf spider communities to
change almost completely. The mechanism underlying this community shift
is less obvious, both why species disappear in the high salinity and in
the low salinity ends, but we can conclude that prey availability or
differences in the trophic niche between species is likely not involved.