COS 86-4 - CANCELLED - Antipredator strategies of spiders as a function of web architecture across a predation gradient

Background/Question/Methods

Organisms have evolved a variety of context-dependent antipredator strategies in response to the intensity of the predation pressure to which they are subject. As predation rate decreases along elevational gradients, it follows that there should be a decrease in the extent of antipredator adaptations of potential prey. Here, we test these hypotheses, and the interaction between the two, by examining the antipredator adaptations of web-building spiders along a gradient of predation on the eastern slopes of the tropical Andes. Spiders build webs of three architectures: the two-dimensional (2D) orbs, which leave spiders relatively exposed, and the three-dimensional (3D) tangles or sheet-and-tangle, which enclose spiders within a volume of silk. As 2D orbs also tend to occupy relatively open microhabitats, and 3D types occur in more secluded ones, spiders of the three types may be exposed to different levels of predation as a function of the architecture of their webs and microhabitat use. We collected data on web (type, presence/absence of a refuge) and spider characteristics (size, colouration on a scale from cryptic to aposematic, and level of spikiness) on over two thousand spiders from eight locations ranging from 200—4000 m elevation on the eastern Andes.

Results/Conclusions

We show that spiders displayed the greatest degree of antipredator morphologies at the lowest elevations, where predation risk is greatest, but also depended on web type and presence/absence of a refuge. Orb weavers without a refuge tended to be spikey with bright, background-contrasting colouration (aposematism), whereas orb weavers with a refuge tended to exhibit colouration that matched their refugia (crypsis). The 3D web-builders, presumably better protected by their webs and microhabitat use, were neither cryptic nor brightly coloured, nor armored. The extent of antipredator adaptations decreased with elevation, as predicted given declining predation rates. Thus, as elevation increased, 2D web builders had fewer spikes and duller colouration and refuges for both web geometries disappeared above 2500 m. Our results support the hypothesis that antipredator adaptations depend on the level of predation pressure web-building spiders are exposed to either as a result of external predation rates or the architecture of their webs and associated microhabitat use. By demonstrating how antipredator strategies change along a predation gradient on web-building spiders, this study illustrates the interaction between external environmental factors and intrinsic features of organisms in shaping their morphology and behaviour.