Field exclusion of large soil predators impacts lower trophic levels and decreases leaf‐litter decomposition in dry forests

Abstract

Shifts in densities of apex predators may indirectly affect fundamental ecosystem processes, such as decomposition, by altering patterns of cascading effects propagating through lower trophic levels. These top–down effects may interact with anthropogenic impacts, such as climate change, in largely unknown ways. We investigated how changes in densities of large predatory arthropods in forest leaf‐litter communities altered lower trophic levels and litter decomposition. We conducted our experiment in soil communities that had experienced different levels of long‐term average precipitation. We hypothesized that altering abundances of apex predators would have stronger effects on soil communities inhabiting dry forests, due to lower secondary productivity and greater resource overexploitation by lower trophic levels compared to wet forests. We experimentally manipulated abundances of the largest arthropod predators (apex predators) in field mesocosms replicated in the leaf‐litter community of Iberian beech forests that differed in long‐term mean annual precipitation by 25% (three dry forests with MAP < 1,250 mm and four wet forests with MAP > 1,400 mm). After one year, we assessed abundances of soil fauna in lower trophic levels and indirect impacts on leaf‐litter decomposition using litter of understorey hazel, Corylus avellana. Reducing densities of large predators had a consistently negative effect on final abundances of the different trophic groups and several taxa within each group. Moreover, large predatory arthropods strongly impacted litter decomposition, and their effect interacted with the long‐term annual rainfall experienced by the soil community. In the dry forests, a 50% reduction in the densities of apex predators was associated with a 50% reduction in decomposition. In wet forests, the same reduction in densities of apex soil predators did not alter the rate of litter decomposition. Our results suggest that predators may facilitate lower trophic levels by indirectly reducing competition and resource overexploitation, cascading effects that may be more pronounced in drier forests where conditions have selected for greater competitive ability and more rapid resource utilization. These findings thus provide insights into the functioning of soil invertebrate communities and their role in decomposition, as well as potential consequences of soil community responses to climate change.

Diversity and functional structure of soil animal communities suggest soil animal food webs to be buffered against changes in forest land use

“…Macrofauna predators may depend in part on aboveground prey (von Berg et al 2010 ) which more sensitively responds to forest management than belowground animals (Penone et al 2018 ). Loss of macrofauna predators potentially feeds back to lower trophic levels and can even impact leaf litter decomposition (Melguizo-Ruiz et al 2020 ).…”

“…Virtually all forests in Central Europe are managed and their natural growth dynamics and overall structure are, therefore, altered (MCPFE 2007;Fischer et al 2010). This has been shown to reduce aboveground biodiversity (Bengtsson et al 2000), which is primarily due to a reduction of species negatively affected by higher canopy closure, lower availability Communicated by Roland A. Brandl.…”

Biodiversity mediates the effects of stressors but not nutrients on litter decomposition

“…The effects of changes in decomposer diversity and abundance on decomposition found in the present study might also have channeled changes in community and food-web structure not captured by our biodiversity metrics. Changes in keystone species ( Hättenschwiler et al, 2005 ), functional diversity ( Cadotte et al, 2011 ; Dangles et al, 2012 ; Heemsbergen et al, 2004 ), vertical diversity ( Gessner et al, 2010 ; Melguizo-Ruiz et al, 2020 ; Wang and Brose, 2018 ; Zhao et al, 2019 ), or dominance patterns ( Dangles and Malmqvist, 2004 ) might have shifted concomitantly to taxonomic diversity and abundance. Moreover, these different components of diversity might act at different timings of decomposition ( Oliveira et al, 2019 ).…”

“…Using realistic extinction scenarios, experiments found contrasting effects of non-random shifts in biodiversity on ecosystem functioning (e.g. Cárdenas et al, 2017 ; Jonsson et al, 2002 ; Melguizo-Ruiz et al, 2020 ; Oliveira et al, 2019 ; Smith and Knapp, 2003 , Zavaleta and Hulvey, 2004 ). In addition, several variables that are not directly related to biodiversity control ecosystem functions (e.g.…”

“…Facilitation and complementarity through niche partitioning are primary mechanisms underlying the positive relationship between decomposer diversity and decomposition ( Gessner et al, 2010 ; Hättenschwiler et al, 2005 ; Tonin et al, 2018 ). Experiments conducted in natural conditions and reflecting realistic extinction scenarios are still relatively scarce, and demonstrate contrasting effects of non-random shifts in decomposer diversity on decomposition ( Cárdenas et al, 2017 ; Jonsson et al, 2002 ; Melguizo-Ruiz et al, 2020 ; Wenisch et al, 2017 ). The need to quantify environmental change effects on decomposer diversity, along with potential knock-on effects on litter decomposition, is therefore particularly pressing.…”