Results
The analysis on spider communities included 3931 spider individuals
separated into 16 taxa (Fig. 2). The variation in community composition
was explained by a region-by-inland/coast interaction (Wald
statistics=8.3, p<0.001) and not by the presence or absence of
a thick wrack bed (Wald statistics=4.7, p>0.1). The
region-by-inland/coast interaction arose because of a larger difference
between southern and northern coastal sites compared with southern and
northern inland sites (Fig. 2). When comparing abundances at the species
level (Table 1), four species (Pardosa agrestis , P.
agricola, Arctosa leopardus and Alopecosa cuneata ) were found
almost exclusively at southern coastal sites and three taxa
(Pardosa prativaga, P. amentata and Pirata spp. [mainlyP. piraticus ]) almost never occurred in these sites but were
abundant elsewhere (Fig. 2). In addition, one species (P.
monticola ) was mainly coastal whereas another species (Pardosa
palustris ) occurred mainly inland, irrespective of region.
The variation in the prey community was explained by a region-by-wrack
interaction (Wald statistics=11.4, p<0.001) and by season
(Wald statistics=11.6, p<0.002) (Fig. 3). The region-by-wrack
interaction occurred because Coleoptera (Deviance=14.1,
p<0.02) and Sciaridae (Deviance=20.3, p<0.003) were
positively affected by wrack availability only in northern sites,
whereas Dolichopodidae (Deviance=11.5, p<0.05) was negatively
affected by wrack availability only in southern sites (Fig. 3). The
seasonal effect occurred because Empididae (Deviance=12.4,
p<0.04) and Homoptera (Deviance=12.8, p<0.03) were
more abundant during early season in June, whereas Chironomidae
(Deviance=14.7, p<0.03) and Trichoptera (Deviance=12.5,
p<0.04) were more abundant during August.
The number of prey species encountered in the gut of spider individuals
varied between one and 15, with an average of 3.9 prey species. The
dominant order in the guts was Diptera, both Brachycera (60%) and
Nematocera (18%), with minor amounts of other groups; Homoptera (10%,
mainly Cicadellidae and Delphacidae), Collembola (4%), other flying
prey (3%, Hymenoptera and Lepidoptera), Formicidae (2%), Acari (2%)
and Heteroptera (1%) (Fig. 4, SI Table 2). The diet contents varied
considerably among sites and were mainly explained by wrack
(Lawley-Hotelling trace statistics=6.1, p<0.001) and season
(Lawley-Hotelling trace statistics=2.6, p<0.003), with an
almost significant effect from a region-by-wrack interaction
(Lawley-Hotelling trace statistics=1.7, p<0.07) (Fig. 4).
However, there was no effect of spider species either when including
this variable alone or in combination with other variables, or when run
separately for region. Because of the almost significant region-by-wrack
interaction on gut contents, we repeated the analysis for sites with or
without wrack separately. In this analysis, region was significant for
sites without wrack (p<0.03) but not for sites with wrack
(p>0.2). The prey groups explaining the region difference
for sites without wrack were Sphaeroceridae (F1,6=141,
p<0.003) and Enchytraeidae (F1,6=111,
p<0.05), that both had a higher frequency in spider guts from
southern sites (Fig. 4). Finally, the comparison between sites with or
without wrack suggested that mainly Sphaeroceridae
(F1,11=17.2, p<0.06) and Heteroptera
(F1,11=19.9, p<0.05) were more abundant in
spider guts from site with wrack.
The species accumulation curves indicated that prey diversity was higher
in southern sites and in sites with no wrack compared with northern
sites and wrack sites (Fig. 5). When comparing diet consistency, we
found that individual spiders had, on average, a Jaccard similarity of
diets = 0.056 (sharing approximately 5.6% of the prey species consumed
by two individuals). Diet consistency between any pair of individual
spiders varied with region (β =0.025, p<0.001), wrack
(β=0.080, p<0.001), and their interaction (β=-0.042,
p<0.001). The interaction arose because spider from northern
wrack sites had higher diet consistency than spiders from southern wrack
sites, whereas spiders from northern non-wrack sites had lower
consistency than those from southern non-wrack sites (Fig. 6). These
diet similarities were larger when performed for pairs of the same
spider species (Fig. 6) but patterns were otherwise similar (region:
β=0.062, p<0.001; wrack: β=0.108, p<0.001;
interaction: β=-0.099, p<0.001) and when diets were estimated
at the prey family level both for all spider individuals (region:
β=0.038, p<0.001; wrack: β=0.094, p<0.001;
interaction: β=-0.059, p<0.001) and for pairs of the same
spider species (region: β=0.070, p<0.001; wrack: β=0.136,
p<0.001; interaction: β=-0.124, p<0.001).