Results
A combined total of 3542 arthropod individuals were collected in June
and August, with an approximately equal number of spiders (1703) and
beetles (1839). The spider assemblage included 18 families and 114
species, with Linyphiidae, Lycosidae and Salticidae being the most
abundant families. The beetle assemblage included 22 families and 136
species, with Staphylinidae, Chrysomelidae and Carabidae being the most
abundant families.
When comparing spider and beetle responses based on the design
characteristics, we found that only total beetle density varied between
categories (Χ 5,59=18.8, P<0.005), which
occurred because sites classified as dry-dry had a higher beetle density
(Fig. 4). Spider density did not vary between the different flooding
categories (P>0.1), and neither did rarefied species richness
(P>0.2) of beetles and spiders. When comparing explanatory
power of the proportion of dry pixels between spatial scales, we found
that the variance explained (R2) for both spider and
beetle densities was highest when the proportion of dry pixels was
estimated at the scale of 10x10 m2 (Fig. 5a). At this
scale, both spider (LR=5.9, df = 56, p<0.02) and beetle (LR =
11.8, df = 56, p<0.0006) densities increased with the proportion
of surrounding dry pixels (Fig. 5b), and there was no effect of the
local (2x2 m2) wetness-dryness, as expressed by the
local inundation frequency. Rarefied species richness was not
significant at any scale for either spiders or beetles, and was
therefore not further analysed. When we included environmental variables
and the heterogeneity index, we found that none of these variables
explained variation in either the density or species richness of either
spiders or beetles and these variables are not further discussed.
The multivariate analyses through manyGLM similarly showed that the
proportion of dry pixels affected the community structure of both
spiders (Wald=4.6, p<0.007) and beetles (Wald=5.2,
p<0.001). These differences occurred because the densities of
Staphylinidae (Wald=3.6, p<0.005) and Chrysomelidae (Wald=2.9,
p<0.03) increased, with a similar non-significant trend
Linyphiidae (Wald=2.5, p<0.08), whereas Salticidae (Wald=2.7,
p<0.04) decreased with the proportion of dry pixels at the
respective optimal spatial scale (Fig. 6, adjusted p-values). Other
included groups (Lycosidae, Thomisidae, Tetragnathidae, Carabidae,
Curculionidae and Coccinellidae) did not vary with the proportion of dry
pixels. To further examine families with significant responses, we
repeated the manyGLM at a genus level for Linyphiidae, Salticidae and
Staphylinidae for genera with at least 20 specimens. For Linyphiidae, we
could include five genera (Bathyphantes, Oedothorax, Porrhomma,
Savignia, Silometopus ) and for these only Savignia (Wald=2.7,
p<0.03) increased with the proportion of dry pixels. For
Salticidae, we could only include two genera (Attulus andMarpissa ) and only Attulus (Wald=2.6, p<0.03)
decreased with the proportion of dry pixels. For Staphylinidae, we found
somewhat similar responses for all five included genera (Amischa ,Erichsonius , Paederus , Rybaxis , Stenus ), but
only Amischa (Wald=2.8, p<0.04) and Rybaxis (Wald=3.3, p<0.01) significantly increased with the proportion
of dry pixels. The other genera had 0.05<p<0.1.
When comparing habitat preferences, we found a relationship between the
proportion of dry pixels and the density of beetles (Wald=4.6,
p<0.002) but not of spiders (Wald=2.1, p>0.2). The
relationship for beetles was caused by a positive relationship between
density and the proportion of dry pixels for habitat generalists
(Wald=4.4, p<0.002). When comparing traits, we found positive
relationships between density and the proportion of dry pixels for both
beetles (Wald=4.6, p<0.001) and spiders (Wald=4.4,
p<0.02). For beetles, all three trophic groups (carnivores,
herbivores and other consumers) responded similarly
(Waldcarnivores=3.2, p<0.007;
Waldherbivores=2.7, p<0.009;
Waldother=3.9, p<0.002). For spiders, only sheet
web weaver densities (Wald=3.3, p<0.008) showed a relationship
to the proportion of dry pixels, matching the result for Linyphiidae
that includes the sheet web weavers.