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UBC Theses and Dissertations

Factors affecting the proportion of three-dimensional spider webs along a precipitation gradient Robertson, Mark


Our study considers how predation by ants and intense rainfall affect the proportion of three-dimensional (3D) versus two-dimensional (2D) spider webs along a precipitation gradient. We predicted that if predator protection benefits of 3D webs outweighed the costs of rain damage, the proportion of 3D webs would increase with annual rainfall, which is expected to correlate with predation intensity (the predation hypothesis). Alternatively, if the costs of rain damage were more significant, we expected a decrease in the proportion of 3D webs with annual precipitation (the rain intensity hypothesis). To assess how predation and rain intensity affect the proportion of 3D webs, we selected seven sites along a rain gradient in western Ecuador. First, we verified annual rainfall and January to April rain intensity data using geographic information systems (GIS). Second, we surveyed up to 120 webs along six separate transects at each site. In areas adjacent to the transects, we estimated ant predation intensity using tuna baits. Finally, using the same transects, we determined how vegetation lushness changed with rainfall. To do so, we measured leaf area, canopy cover, and the diameter at breast height of adjacent trees. We found that 3D webs increased in proportion with annual rainfall, which correlated positively with predatory ant abundance, consistent with the predation hypothesis, but counter to the rain intensity hypothesis. We found, however, that in areas of greater precipitation, lusher vegetation provided greater shelter under which spiders built their webs. As such, we suggest that greater availability of immediate cover in lusher and wetter habitats would mitigate the destructive power of intense rainfall, allowing the predator protection benefits of 3D webs to be realized despite the simultaneous occurrence of strong rains. Microhabitat factors may thus interact with broader-scale biotic and abiotic factors in structuring web-building spider communities.

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