Spiders in tropical forest floors have two common distinct foraging strategies: wandering (species that do not build webs to capture prey) which is adopted by actively hunting predator and web-building which is adopted by sit-and-wait predator with continuous presence at fixed locations. Spiders can cause trophic cascades affecting litter decomposition rates. However, it remains unclear how spiders’ foraging strategies influence soil fauna communities and, consequently, have cascading effects on litter decomposition rates in tropical systems.

To answer these questions, Dr. LIU Shengjie, together with his teachers from Xishuangbanna Tropical Botanical Garden (XTBG) of Chinese Academy of Sciences, manipulated densities of spiders with two different foraging strategies (actively hunting (AH) and sit-and-wait (SW)), in microcosms mimicking the tropical rainforest floor ecosystem in Xishuangbanna. The experiment included 2 rainfall treatments (ambient and drought) × 4 spider treatments (Control, SW spider, AH spiders and SW + AH spider treatment) × 10 replicates, producing a total of 80 microcosms.

The researchers found a positive trophic cascade on litter decomposition rates triggered by actively hunting (AH) spiders under ambient moisture, which could be explained by their indirect positive effects on Oribatid abundance. However, sit-and-wait (SW) spiders had no effect on the soil fauna except Psocoptera, thus SW spiders had no cascading effects on litter decomposition under ambient moisture.

In contrast, drought reversed the cascading effects of spiders on litter decomposition rates. Under drought conditions, they observed negative trophic cascade effects on litter decomposition in all three spider treatments, and decreased Entomobrya densities. Increased dry periods should have important consequences for trophic interactions in detritus-based food webs and forest floor ecosystems.

The study revealed that different spider-foraging strategies had different trophic cascade effects under ambient rainfall, but spiders with both foraging strategies slowed litter decomposition under drought conditions. Furthermore, changes in soil-moisture content can affect interactions between soil fauna and spiders in the detrital food web by altering population densities and activity of soil fauna in tropical forest floors.

The study entitled “Spider foraging strategy affects trophic cascades under natural and drought conditions” has been published in Scientific Reports.