Evidence for potential competition between carnivorous plants and spiders

Background/Question/Methods

Most ecological research on interspecific competition has been conducted on closely related taxa, even though a number of studies have demonstrated that competition can be equally strong, if not stronger, between disparate taxa. Surprisingly, no quantitative studies have been conducted on potential competition between the plant and animal kingdoms, even though it may ostensibly occur between carnivorous plants and animals for shared prey. To examine the potential for competition between these taxa, we studied dietary and microhabitat overlap between pink sundews (Drosera capillaris) and funnel-web building wolf spiders (Sosippus floridanus). Sampling was conducted from July to November 2008 using quadrat censuses along a total of four 20 m transects at two sites. In each quadrat, we measured sundew density, number of leaves per sundew, number of trichomes per sundew leaf, web area, distance of webs to the nearest sundew, prey captured per web and per sundew leaf, leaf area, and arthropod prey abundance/biomass.

Results/Conclusions

Diets of sundews and spiders had a statistically significant overlap, and they predominantly consisted of flies, springtails and ants, which were also the most abundant arthropod taxa found in their habitat. Associations between spiders and sundews depended on spatial scale. Spiders were found more frequently in quadrats with sundews present, but they constructed larger webs, and located them further away from sundews, as total sundew capture area increased within each quadrat. Web area was also negatively associated with arthropod biomass, suggesting that spiders constructed larger webs when fewer prey were available. For sundew phenotypic traits, we found a positive association between number of trichomes per individual leaf and arthropod biomass, but surprisingly no statistically significant association between density of trichomes per individual leaf area and arthropod biomass. There was also no evidence that spiders affected phenotypic traits of sundews at the scales studied, possibly because spiders are transient in comparison to sundews. Our results are consistent with the hypothesis that these species compete, but manipulative experiments will be necessary to test this hypothesis rigorously.