Ontogenetic omnivory in a top-level predator

Background/Question/Methods

Ecological theory predicts that multi-channel omnivory, the consumption of resources from multiple food webs by a consumer, has the potential to stabilize consumer and resource populations. However, this prediction is based on an assumption that all consumer individuals are ecologically equivalent, and it may not hold if resource use changes with consumer age or life stage. Such ontogenetic omnivory results in serial specialization that can negate the stabilizing effects of multi-channel omnivory. A top arthropod predator in salt marsh food webs, the wolf spider Pardosa littoralis, utilizes both grass-based and algal-based food webs, and is capable of suppressing densities of the most abundant herbivores, thereby indirectly benefitting marsh grass. However, Pardosa’s effect on herbivores is inconsistent and as a result the occurrence of trophic cascades varies spatially and temporally for reasons that are not well understood. As a first step in determining whether ontogenetic omnivory plays a role in reducing the dependability of prey suppression by Pardosa, we assessed the use of algal and grass-based food webs by different Pardosa size classes. We measured stable isotopic values of various-sized Pardosa and potential prey individuals in both food webs in the two dominant marsh grasses monthly during the summer of 2009.

Results/Conclusions

Preliminary results show that in both grasses, δ¹³C of small Pardosa was significantly closer to that of the algal food web than the δ¹³C of larger Pardosa, indicating a heavier reliance on the algal food web by small Pardosa. In addition, small Pardosa had lower δ¹⁵N than adults, indicating a lower trophic position. This partial division of resources among size classes may reduce the ability of Pardosa to move between food webs in response to changes in prey density. In particular, it may diminish the ability of Pardosa to suppress herbivore outbreaks in the grass-based food web, especially early in the season when the majority of these spiders are small. In addition, low densities of algivores could create a bottleneck among small Pardosa, depressing densities of larger Pardosa later in the season regardless of how abundant prey for adults may be.