Gradients of precipitation and ant abundance may contribute to the altitudinal range limit of subsocial spiders: insights from a transplant experiment

Abstract

Species range boundaries often form along environmental gradients that dictate the success of the phenotypes present in each habitat. Sociality may allow colonization of environments where related species with a solitary lifestyle cannot persist. Social spiders in the genus Anelosimus appear restricted to low-and mid-elevation moist environments in the tropics, while subsocial spiders, common at higher elevations and latitudes, appear to be absent from the lowland tropical rainforest. Here, we seek factors that may simultaneously prevent subsocial Anelosimus species from colonizing the lowland rainforest while favouring species with large social groups in this habitat. To this end, we transplanted small groups of a subsocial species, which contain the offspring of a single female, from cloud forest habitat in the centre of its natural range to lower montane rainforest on the range margin and to lowland rainforest outside of the species range. Groups transplanted at the range margin and below their range limit were less likely to disperse and experienced increased mortality. This was correlated with greater rainfall intensity and ant abundance. We show that protection from rainfall enhances the performance of small groups of spiders in the lowland rainforest, and suggest that predation or disturbance by ants may influence the geographical range limits of this species.

Ecological predictors of spider sociality in theAmericas

“…Rain intensity was indeed a significant predictor of Anelosimus sociality at both spatial scales (Table ), with subsocial species being less likely to occur under greater rain intensities (Figs & ), thus lending support to the rain intensity hypothesis. In interaction with the uniquely dense tri‐dimensional webs characteristic of the genus Anelosimus , rain intensity may play a direct role in preventing subsocial species in this genus from occupying the lowland tropical rain forest (Avilés et al ., ; Purcell & Avilés, ). Given the apparently costly nature of Anelosimus webs, a premise we are in the process of testing, in areas where webs are frequently destroyed by strong rains web maintenance may be untenable for subsocial Anelosimus females living alone with their offspring (Purcell & Avilés, ).…”

“…In interaction with the uniquely dense tri‐dimensional webs characteristic of the genus Anelosimus , rain intensity may play a direct role in preventing subsocial species in this genus from occupying the lowland tropical rain forest (Avilés et al ., ; Purcell & Avilés, ). Given the apparently costly nature of Anelosimus webs, a premise we are in the process of testing, in areas where webs are frequently destroyed by strong rains web maintenance may be untenable for subsocial Anelosimus females living alone with their offspring (Purcell & Avilés, ). In contrast, groups of females may fare better, as per capita expenditure in silk may decrease with colony size (Riechert et al ., ; Purcell & Avilés, ).…”

“…In such environments, web maintenance may be energetically untenable for subsocial Anelosimus females living alone with their offspring, but not for groups of females, as per capita expenditure in silk may decrease with colony size (Riechert et al ., ; Purcell & Avilés, ). Finally, the predation hypothesis (Purcell & Avilés, ; Purcell, ) predicts that high predation rates in the lowland tropical rain forest, in particular by ants (Kaspari et al ., ), may prevent the smaller and more vulnerable subsocial spider colonies from establishing in these areas. The latter two hypotheses fall directly under a more general ‘maternal death’ hypothesis (Avilés et al ., ; Jones et al ., ), which postulates that cooperative breeding (i.e.…”

Sociality level correlates with dispersal ability in spiders

“…Both these behaviours are important in the lowland wet tropical environments where social Anelosimus species occur. Here, strong rains require constant maintenance of webs (Purcell & Aviles 2008), which, relative to the body size of the spiders, may be orders of magnitude larger than in subsocial species. Likewise, efficient silk‐wrapping of prey is important as social spiders have in their habitats access to much larger prey compared with non‐social species at higher elevations or latitudes (Guevara & Aviles 2007; Powers & Aviles 2007).…”

Subsocial spiders in space and time: A fine scale approach to the dynamics of dispersal

“…Changes in the behaviour of spiders can be influenced by the decrease in temperature (Foelix, ). Several predators such as spiders (Perkins, Riechert, & Jones, ; Rao & Aceves‐Aparicio, ) and ants (Hoffman & Avilés, ; Purcell & Avilés, ) have been previously identified in Anelosimus colonies (Perkins et al., ; Powers & Avilés, ; Purcell & Avilés, ); for A. baeza, a subset of these have also been previously reported (Rao & Aceves‐Aparicio, ). Although night movements may reduce possible encounters with predators, we observed several pirate spiders (Araneae: Mimetidae) feeding on Anelosimus spiders during the night, and only a couple during the day.…”