Predators and prey-derived necromass impact local ecosystem processes

Yaya Tang and Dror Hawlena, Ecology, Evolution and Behavior, The Hebrew University of Jerusalem, Jerusalem, Israel, Shawn M. Wilder, Department of Integrative Biology, Oklahoma State University, Stillwater, OK

Background/Question/Methods
Detrital inputs provide soil communities with organic matter essential for recycling nutrients. Plants provide the majority of this necromass entering the soil and are typically high in quantity yet low in nutrient quality. In contrast, animals provide necromass with higher concentrations of limiting nutrients. Predators can directly alter animal necromass quality by consuming prey, transforming, and redistributing prey-derived nutrients. This higher quality animal necromass can in turn indirectly affect soil microbial community function to decompose plant necromass. However, similar predators may differentially produce necromass (such as egesta and prey remains) due to differences in their feeding strategy. In this experiment we examined how prey nutrients and predator consumption can affect broader ecosystem function via direct effects on animal necromass and indirect effects on plant necromass.
We evaluated the effects of predators and their resulting prey necromass on local rates of decomposition, primary productivity, and nutrient cycling using field mesocosms. Specifically, we installed 40 cages (65cm height x 40cm diameter) into a grassland habitat and added homogenized soil 10cm deep with a 2cm top layer of the local seed bank. We also installed four litterbags containing 15N labeled wheatgrass in each cage. We systematically allocated the cages into four treatments: Control (no animals or animal-derived necromass), Necromass Control (grasshopper carcass), and Spider (spider predator and its prey-derived necromass), and Mantid (mantid predator and its prey-derived necromass). We used spiders (Argiope lobata) and mantids (Mantis religiosa), as these sit-and-wait predators consume their prey in distinct ways: spiders digest prey extra-orally, while mantids chew prey piece by piece. Our field study ran from July 2019-April 2020. We collected soil samples from each cage at three timepoints throughout the study. At the conclusion of the study, we collected all of the plant above- and below-ground biomass in each cage.
Results/Conclusions
Preliminary results suggest the addition of prey-derived necromass fundamentally altered local plant community composition. We found that prey-derived necromass increased the abundance of cereals and decreased the abundance of legumes and other forbs compared to control cages. This overall effect of prey-derived necromass on plant community structure was unaffected by predators. Furthermore, spider inputs increased soil C, decomposition, and soil N but decreased N from plant litter. In contrast, mantid inputs were not different from controls. Thus, it is important to consider higher trophic levels and their indirect effects on large-scale ecosystem processes, as their effects may generate spatial variation in ecosystem function.