Habitat productivity predicts the global distribution of social spiders

Marija Majer,Trine Bilde,Jens-Christian Svenning

doi:10.3389/fevo.2015.00101

Abstract

Permanently-social spiders share a common suite of traits, including cooperative foraging and brood care, elimination of pre-mating dispersal, and the transition to an inbreeding mating system. Social spiders are confined to tropic and subtropical habitats, suggesting environmental constraints on the evolution of group living in spiders. Because social spider groups are sedentary and dependent on arrival of insect prey in their capture webs, group living and the associated higher local density is expected to rely on a relatively resource rich environment. We used spatial statistical modelling to explore environmental factors underlying the macro-ecological patterns in the distribution and diversity patterns of social spiders. We found strong support for habitat productivity as a predictor of the distribution of social species, particularly in the Old World. We show that social species are restricted to more productive habitats relative to a set of closely related subsocial sister species with a solitary lifestyle. Within their distribution range, social species richness was higher where precipitation seasonality is lower. These macro-ecological patterns corroborate the underlying biological hypotheses that evolution of group living is facilitated in environments that provide more abundant insect prey and a more continuous supply of food resources.

Highlights

Cooperation includes a wide diversity of traits, for example predator avoidance by warning signals or nest defense, resource acquisition through harvesting, communication or cooperative hunting, or cooperative breeding through provisioning of reproductive females or offspring (Dugatkin, 1997; Davies et al, 2012)
globalized difference vegetation index (GVI) was best supported in both sets of models, and had a much stronger effect on the probability of occurrence of social spiders in the Old World
This effect of GVI could be attributed to the differences in the proportional occurrences of different species of social spiders across the Old and New World habitats (Figure 3)

Summary

Introduction

Cooperation includes a wide diversity of traits, for example predator avoidance by warning signals or nest defense, resource acquisition through harvesting, communication or cooperative hunting, or cooperative breeding through provisioning of reproductive females or offspring (Dugatkin, 1997; Davies et al, 2012). Cooperative breeding in birds is favored when ecological conditions are harsh and individual reproduction constrained, whereas individuals disperse and attempt own reproduction under benign conditions (Emlen, 1982, 1994). Our understanding of these relationships are, further challenged by a complex set of interactions between life history and environment on the probability of group formation and degree of social organization (Hatchwell and Komdeur, 2000). While these patterns show altitudinal or latitudinal and thereby ecological correlations with degree of sociality, nest size or other social traits, our understanding of the causal relationships between ecological factors and social traits is incomplete

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