Density- and trait-mediated impacts of predators on prey and soil properties

Background/Question/Methods

Generalist arthropod predators have been shown to impact the grazing component of terrestrial food webs, but less attention has been given to detrital systems. Ground-dwelling spiders and beetles are common in terrestrial ecosystems, and detritivores have been found to compose a large portion of their diet. These predators have been shown to indirectly alter soil processes through consumption of microarthropods. However, studies of detrital systems have focused on density-mediated interactions between predators and detritivores, leaving trait-mediated interactions largely unexplored. We used the wolf spiders Pardosa milvina, Tigrosa helluo, and Rabidosa rabida along with the carabid beetle Scarites quadriceps to investigate their impacts on the survival of the collembolan Sinella curviseta, soil respiration, and soil carbon content. Additionally, we isolated trait-mediated effects between Pardosa and Sinella, as these species were believed to interact most strongly. We constructed air-tight mesocosms containing soil, Sinella, and a single predator or its chemotactile cues (silk, feces, and other excreta) (for Pardosa only). Total respiration was quantified using NaOH base traps, and we analyzed soil composition for total carbon, organic carbon, and total nitrogen. All Sinella were recovered after a five day period in the mesocosms to estimate density-mediated effects.

Results/Conclusions

Data for Sinella survival and total respiration support a strong, indirect link between Pardosa and the soil community. Predation on Sinella only occurred when Pardosa was present and was correlated with decreased respiration. Comparison with a treatment without Sinella or predators provides evidence for a trophic cascade from Pardosa through Sinella to the soil microbe community. Although Sinella survival was high in the treatment containing only chemotactile cues from Pardosa, changes in respiration were comparable to the treatment allowing Pardosa to consume Sinella. This suggests that trait-mediated interactions between Pardosa and Sinella may have similar effects on soil function as density-mediated interactions (i.e., trophic cascade). Tigrosa, Rabidosa, and Scarites had higher respiration rates, and only Scarites seemed to have an impact on the soil community. Scarites is frequently active below the soil surface, so this effect may be attributed to the physical disturbance of burrowing. These data help to clarify the importance of terrestrial predators as members of the soil community that indirectly alter soil function.