subsocial species approaching dispersal vs. social species
Insects sampled from the social species lowland tropical rainforest habitat were considerably larger on average than those from the cloudforest or Arizona riparian habitats (Fig. 1a) (F2,54 = 7·4, P = 0·0014, R = 0·33, for the comparison across the three habitats). Insect density (insects h−1 m−2) and overall biomass (mg h−1 m−2), however, were greater in samples from the cloudforest than in samples from the other two habitats (Fig. 1b,c) (insect density F2,56 = 13·1, P < 0·0001, R = 0·36; total biomass: F2,56 = 6·1, P = 0·004, R = 0·23).
Figure 1. Insect size, density and total biomass in the habitats of A. eximius and A. domingo (rainforest), A. baeza (cloudforest) and A. arizona (temperate riparian). Shown are least square means of malaise trap sample averages (insects collected by a trap during a day or night period) estimated using mixed model anovas (see Methods) on (a) natural log-transformed average insect sizes (dry mg), (b) square root-transformed insect density (number of insects flying per hour through a 1 m2 area) and (c) square root-transformed total insect biomass (insect dry mg flying per hour through a 1 m2 area). Means that are significantly different in a posteriori multiple comparisons (Tukey–Kramer honest significance difference test) shown with different letters.
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Considering prey caught by colonies (social species all year, subsocial species approaching dispersal), the social species captured much larger insects relative to the average size of spiders in their nests than did the subsocial species (contrast A. eximius vs. A. baeza: F1,75= 33·2, P < 0·0001; vs. A. arizona: F1,75 = 42·9, P < 0·0001; contrast A. domingo vs. A. baeza: F1,75 = 5·0, P = 0·03; vs. A. arizona: F1,75 = 13·9, P = 0·0004) (Fig. 2a). The two social species captured prey on average six times their body size (prey to spider ratio, A. eximius: 6·1, 4·8–7·7, 95% CI; A. domingo: 6·3, 1·6–24·0, 95% CI), while the subsocial species captured prey the same size or slightly smaller than the average spider in their nests (A. baeza: 1·3, 0·8–1·9, 95% CI; A. arizona prey to spider ratio: 0·3, 0·1–0·8, 95% CI). Prey to spider ratios (prey mg−1 spider mg−1) reflected both differences in the size of the spiders and of the insects they caught (Fig. 2b,c).
Figure 2. Sizes of prey captured by colonies in relation to spider body mass during colony census periods (all year for socials, period approaching dispersal for subsocials). Shown are back-transformed (from natural logarithms) mean colony averages of (a) relative prey mass (back-transformed from mean ln(mean prey mg/mean spider mg), (b) mean prey mg and (c) mean spider mg. Spider mass reflects number and size of the spiders present in a nest at the time of prey capture observations; insect biomass are colony averages obtained over several day/night observation periods. Bars represent standard error.
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Failure of a colony to capture any prey during the observation periods was also much less frequent in the social than in the subsocial species, despite the lower insect density in the environment of the social species (% colonies failing to capture any prey: 13 vs. 48 vs. 70 vs. 77, for A. eximius, A. domingo, A. baeza and A. arizona, respectively, likelihood ratio χ2 = 499·0, P < 0·0001; all pairwise comparisons P < 0·008, which adjusts for six comparisons). On a per capita basis, however, because prey had to be divided by a greater number of individuals, social species overall did not necessarily perform better than subsocial ones. Variance in colony prey capture success (number of insects and biomass captured h−1 per spider or per spider mg) was greatest in A. arizona, followed by A. baeza and A. eximius, and was lowest in A. domingo (Barlett test F3,211 = 42·5, P < 0·0001, for prey numbers; F3,211 = 7·9, P < 0·0001, for prey biomass).
early vs. late colony stages in subsocial species
In both subsocial species, colonies in the early communal stages containing young juveniles and their mother exhibited greater probability of capturing prey (A. baeza likelihood ratio χ2 = 88·7, P < 0·0001; A. arizonaχ2 = 130·0, P < 0·0001) and captured more prey items per capita (A. baeza: χ2 = 10·7, 1 d.f., P = 0·001; A. arizona: χ2 = 6·2, 1 d.f., P = 0·01) and greater prey biomass per colony biomass (A. baeza: χ2 = 13·5, 1 d.f., P = 0·0002; A. arizona: χ2 = 11·2, 1 d.f., P = 0·0008) than later-stage colonies with older juveniles and subadults approaching dispersal. These differences were present, even though the environments of both species did not differ in the density or overall biomass of insects caught by traps at these two times (insect numbers h−1 m−2, cloudforest F1,26 = 1·4, P = 0·26; Arizona riparian F1,27 = 1·2, P = 0·28; mass h−1 m−2, cloudforest F1,26 = 0·1, P = 0·74; Arizona riparian F1,27 = 0·01, P = 0·91).
In both species colonies in their early communal phase caught prey that was considerably larger relative to the average size of the spiders in the nests than colonies approaching dispersal (prey to spider ratio A. baeza, early: 4·4, 2·7–7·1, 95% CI; late: 1·3, 0·8–1·9; F1,61 = 15·4, P = 0·0002; A. arizona, early: 4·8, 2·5–9·0; late: 0·3, 0·1–0·7; F1,27 = 28·0, P < 0·0001) (Fig. 3). In A. baeza, this effect reflected the smaller size of spiders in young colonies, as prey caught by colonies did not differ in size between the early and late colony stages (F1,83 = 2·3, P = 0·13). In A. arizona the effect was due both to the smaller size of young spiders as well as to the larger absolute size of prey caught by colonies in the early communal stages (F1,28 = 30·4, P < 0·0001). The latter effect is particularly interesting, given that insects in the Arizona riparian trap samples did not differ in size between these two time-periods (F1,20 = 0·3, P = 0·61).
Figure 3. Size of captured prey relative to mean spider size in colonies of the subsocial species A. baeza and A. arizona during their early communal phase vs. the stage when spiders were approaching the dispersal phase. Units as in Fig. 2a.
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