Top-Down Cascade Effects of the Long-Clawed Shrew (Sorex unguiculatus) on the Soil Invertebrate Community in a Cool-Temperate Forest

Abstract

In a soil ecosystem, bottom-up control is generally considered more influential than topdown control, although some empirical studies have suggested that predators have a trophic cascade effect on soil animals at lower trophic levels. In the present study, the effects of the long-clawed shrew, a mammalian predator at a high trophic level, on the soil invertebrate community and litter decomposition were investigated in a field experiment using enclosures. In the presence of the shrew, the population densities of earthworms, isopods and spiders tended to decrease, whereas that of large springtails and centipedes appeared to increase. This result might have been caused by the shrew’s direct predation on the former invertebrates and the release from predation by spiders on springtails. The reason of the increase of centipedes was unknown. Shrew had no effects on litter decomposition rates both by litter trap analysis and litter-bag test. The top-down effects of shrews on litter decomposition might have been diluted through a complex food-web and the observing period of the present study might be too short to detect litter decomposition process. Experiment for longer time might demonstrate more explicit effect of the shrew on the soil ecosystem

Indirect control of decomposition by an invertebrate predator

“…There may be several reasons we detected a cascading effect of predation on decomposition, where many other studies (including those on large-scales) did not (Cates et al, 2021;Denmead et al, 2017;Hocking & Babbitt, 2014;Namba & Ohdachi, 2016;Sitvarin et al, 2016). First, the extremely high abundance and ubiquity of the predators (ants) in this study far outweighs that of previous large-scale studies, which have examined effects of ants in temperate regions (where they are naturally less abundant; Cates et al, 2021;Kendrick et al, 2015), or focussed on vertebrate predators (such as salamanders and shrews; Hocking & Babbitt, 2014;Namba & Ohdachi, 2016), which are present at far lower abundances than ants. Ants are by far the most abundant predatory group in our system, comprising 80% of invertebrates collected in pitfall traps (A. Walker, unpublished data), and are much more ubiquitous than termite-feeding vertebrates such as aarvarks and lizards.…”

“…The marginally non-significant increase in the abundance of termites when ants were suppressed (Figure 1a,b) is likely influenced by the small sample size and sampling challenges (as termite abundance is difficult to accurately measure and compare locally; Davies et al, 2021), but is suggestive that ants exert top-down control on termites in this environment. There may be several reasons we detected a cascading effect of predation on decomposition, where many other studies (including those on large-scales) did not (Cates et al, 2021;Denmead et al, 2017;Hocking & Babbitt, 2014;Namba & Ohdachi, 2016;Sitvarin et al, 2016). First, the extremely high abundance and ubiquity of the predators (ants) in this study far outweighs that of previous large-scale studies, which have examined effects of ants in temperate regions (where they are naturally less abundant; Cates et al, 2021;Kendrick et al, 2015), or focussed on vertebrate predators (such as salamanders and shrews; Hocking & Babbitt, 2014;Namba & Ohdachi, 2016), which are present at far lower abundances than ants.…”

“…Why predation has been found to have variable effects on decomposition is not fully understood but may be caused by differences in predator trophic guild -that is, predators that directly consume decomposer species may have negative cascading effects on decomposition (Lawrence & Wise, 2000;Wu et al, 2011;Wyman, 1998), whereas higher-trophic level predators that prey on the predators of decomposers, or on microbivores, may release decomposers from predation and, therefore, have positive cascading effects (Lawrence & Wise, 2004;McGlynn & Poirson, 2012;Melguizo-Ruiz et al, 2020). Moreover, a number of studies reported no predation effect on decomposition, which was attributed to high levels of functional redundancy within brown food webs (Cates et al, 2021), the complexity of food web pathways (Miyashita & Niwa, 2006;Namba & Ohdachi, 2016) and other biotic or abiotic factors overshadowing any predation effect (Denmead et al, 2017;Hocking & Babbitt, 2014;Homyack et al, 2010;López-Rodríguez et al, 2018). Notably, there is a lack of large-scale (>8 × 8 m), open plot experiments in previous literature (though see Parr et al, 2016 andCates et al, 2021), as the vast majority of studies employed mesocosms, which run the risk of artefacts arising due to small scales and enclosure of the study organisms (Petersen & Hastings, 2001).…”

“…Although there is consistent evidence to suggest that predation can regulate abundances of herbivores and have cascading effects on herbivory (the green food web; reviewed in Schmitz et al, 2000), evidence for the impact of predation on decomposers and decomposition (the brown food web) is somewhat mixed. Studies have found positive (Lawrence & Wise, 2000;Melguizo-Ruiz et al, 2020), negative (Liu et al, 2014;Wu et al, 2011;Wu et al, 2014) and neutral (Cates et al, 2021;Denmead et al, 2017;Hocking & Babbitt, 2014;Namba & Ohdachi, 2016) relationships between predation and decomposition across studies (reviewed in Sitvarin et al, 2016). Why predation has been found to have variable effects on decomposition is not fully understood but may be caused by differences in predator trophic guild -that is, predators that directly consume decomposer species may have negative cascading effects on decomposition (Lawrence & Wise, 2000;Wu et al, 2011;Wyman, 1998), whereas higher-trophic level predators that prey on the predators of decomposers, or on microbivores, may release decomposers from predation and, therefore, have positive cascading effects (Lawrence & Wise, 2004;McGlynn & Poirson, 2012;Melguizo-Ruiz et al, 2020).…”