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A Brown-World Cascade in the Dung Decomposer Food Web of an Alpine Meadow: Effects of Predator Interactions and Warming

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A Brown-World Cascade in the Dung Decomposer Food Web of an Alpine Meadow: Effects of Predator Interactions and Warming W&M ScholarWorks VIMS Articles 5-2011 A brown-world cascade in the dung decomposer food web of an alpine meadow: effects of predator interactions and warming XW Wu JE Duffy Virginia Institute of Marine Science PB Reich SC Sun Follow this and additional works at: https://scholarworks.wm.edu/vimsarticles Part of the Environmental Sciences Commons Recommended Citation Wu, XW; Duffy, JE; Reich, PB; and Sun, SC, "A brown-world cascade in the dung decomposer food web of an alpine meadow: effects of predator interactions and warming" (2011). VIMS Articles. 1711. https://scholarworks.wm.edu/vimsarticles/1711 This Article is brought to you for free and open access by W&M ScholarWorks. It has been accepted for inclusion in VIMS Articles by an authorized administrator of W&M ScholarWorks. For more information, please contact [email protected]. Ecological Monographs, 81(2), 2011, pp. 313–328 Ó 2011 by the Ecological Society of America A brown-world cascade in the dung decomposer food web of an alpine meadow: effects of predator interactions and warming 1 2 3 1,4,5 XINWEI WU, J. EMMETT DUFFY, PETER B. REICH, AND SHUCUN SUN 1ECORES Lab, Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041 China 2Virginia Institute of Marine Science, College of William and Mary, Gloucester Point, Virginia 23062-1346 USA 3Department of Forest Resources, University of Minnesota, 1530 Cleveland Ave. North, Saint Paul, Minnesota 55108 USA 4Department of Biology, School of Life Sciences, Nanjing University, Nanjing 210093 China Abstract. Top-down control has been extensively documented in food webs based on living plants, where predator limitation of herbivores can cascade to facilitate plant growth (the green-world hypothesis), particularly in grasslands and aquatic systems. Yet the ecosystem role of predators in detrital food webs is less explored, as is the potential effect of climate warming on detritus-based communities. We here show that predators have a ‘‘brown-world’’ role in decomposer communities via a cascading top-down control on plant growth, based on the results of an experiment that factorially manipulated presence and size of two predator species as well as temperature (warmed vs. unwarmed). The inclusion of predatory beetles significantly decreased abundance of coprophagous beetles and thus the rate of dung decomposition and productivity of plants growing surrounding the dung. Moreover, the magnitude of these decreases differed between predator species and, for dung loss, was temperature dependent. At ambient temperature, the larger predators tended to more strongly influence the dung loss rate than did the smaller predators; when both predators were present, the dung loss rate was higher relative to the treatments with the smaller predators but comparable to those with the larger ones, suggesting an antagonistic effect of predator interaction. However, warming substantially reduced dung decomposition rates and eliminated the effects of predation on dung decomposition. Although warming substantially decreased dung loss rates, warming only modestly reduced primary productivity. Consistent with these results, a second experiment exploring the influence of the two predator species and warming on dung loss over time revealed that predatory beetles significantly decreased the abundance of coprophagous beetles, which was positively correlated with dung loss rates. Moreover, experimental warming decreased the water content of dung and hence the survival of coprophagous beetles. These results confirm that the ‘‘brown-world’’ effect of predator beetles was due to cascading top-down control through coprophagous beetles to nutrient cycling and primary productivity. Our results also highlight potentially counterintuitive effects of climate warming. For example, global warming might significantly decrease animal- mediated decomposition of organic matter and recycling of nutrients in a future warmed world. Key words: alpine meadow; artificial warming; beetles; biodiversity and ecosystem function; Qinghai- Tibetan Plateau, China; coprophagy; dung decomposers; food webs; nutrient cycling; predator; top-down control; trophic cascade. INTRODUCTION al. 2005, Duffy et al. 2007, Bruno and Cardinale 2008). Trophic cascades have long been recognized in the Top-down control by predators has proven to be strong ecological literature (Hairston et al. 1960, Oksanen et al. in aquatic systems (Shurin et al. 2002), grasslands (Schmitz 2003), intertidal rocky shores (Paine 2002), 1981, Fretwell 1987, Stiling 1999), as predators indirect- and wetlands (Finke and Denno 2004, Duffy et al. ly affect plant community structure and ecosystem 2005), all of which are systems involving food webs functioning via a top-down control on the abundance based on living plants. In contrast, the controlling role and diversity of herbivores (Schmitz et al. 2000, Borer et of predators in decomposer food webs is surprisingly unexplored. This probably reflects the fact that food web Manuscript received 22 April 2010; revised 7 September theories have traditionally focused on living organisms 2010; accepted 16 September 2010. Corresponding Editor: but have neglected detritus since the development of A. M. Ellison. 5 classical community ecology and ecosystem ecology in Corresponding author. Present address: Department of the 1950s (Moore et al. 2004). It may also be because Biology, School of Life Sciences, Nanjing University, 22 Hankou Road, Nanjing 210093 China. predators are often assumed to have weak effects on E-mail: [email protected] decomposers, and perhaps assumed to be not important 313 314 XINWEI WU ET AL. Ecological Monographs Vol. 81, No. 2 enough to alter ecosystem properties (e.g., Dyer and focused solely on direct consumers of detritus (e.g., Letourneau 2003). Cebrian 2004, Srivastava et al. 2009). The lack of attention to detritus-based food webs is Another issue is whether the cascading trophic effects curious because detritus actually constitutes a large of predators are ubiquitous and consistent between proportion of organic matter (particularly structural living and detritus food webs. As shown in several materials) and supports a great deal of biodiversity, reviews (Schmitz et al. 2000, Shurin et al. 2002, 2006, which is important in the energy flow of ecosystems Borer et al. 2005, Duffy et al. 2007), the strength of (Lindeman 1942, Odum 1969). Most terrestrial primary trophic cascades depends on herbivore species diversity, production is not consumed by herbivores (Cebrian carnivore type (vertebrate vs. invertebrate), ecosystem 2004), and even of that part consumed by large type (terrestrial vs. aquatic systems), primary produc- herbivores, only very little is assimilated, e.g., in alpine tivity, and predator metabolic factors. The top-down pastures (Wu and Sun 2010). Thus, the decomposition effect of predators is generally believed to be less rate of plant detritus (e.g., plant litter and excrement of frequent and weaker in detritus-based than in living large herbivores such as cattle) is critical to ecosystem food webs (Dyer and Letourneau 2003, Moore et al. nutrient cycling and primary production (Cebrian 2004, 2004, Wardle et al. 2004, Wardle 2006), in part because Nichols et al. 2008). of some particular properties of predator–detritivore Interestingly, top-down control by predators in interactions. For example, in a soil detritus food chain, decomposer food webs, as we will explain, could have microbes grew faster when they were grazed upon important and perhaps opposite effects on ecosystem frequently by nematodes, indicating a compensatory process rates (e.g., detritus decomposition rate and plant growth in the microbes (Mikola and Seta¨la¨1998). growth rate) to those of the predators of living food Therefore, microbial biomass might remain unchanged webs. In plant-based food webs, where energy originates even though the number of nematodes was reduced by from living primary producers, predator control often their predators, suggesting a limited trophic cascading cascades through herbivore density and diversity to effect. In contrast, another distinction between living facilitate plant growth (Schmitz et al. 2000, Shurin et al. and detritus food webs is that herbivores often induce 2002, Duffy et al. 2007), as suggested by the Green antiherbivore defenses and compensatory growth of World Hypothesis (Hairston et al. 1960), which posited living plants that, in turn, reduce the impact of that in three-level food chains herbivores are more likely herbivores on plant populations (Schaller 2008), whereas to be predator limited but seldom food limited. Later detritus food webs are donor-controlled (Stiling 1999), models like the Exploitative Ecosystem Hypothesis with detritus quantity and quality being little affected by (Oksanen et al. 1981, Fretwell 1987, Oksanen and consumers. This difference may lead to a stronger Oksanen 2000) generalized the idea of predator top- cascading effect of predators in detritus-based webs down control to food chains with more than or fewer than in food webs based on living plants (Srivastava et than three trophic levels and argued that predators with al. 2009). Accordingly, it is not clear whether the an odd number of trophic levels increase the abundance predicted ‘‘brown-world’’ role of predators is significant of producers in food chains (e.g., the three trophic levels in detritus food webs. of Hairston et al. 1960). The importance of predator control in brown world Although little effort has focused on extending these webs
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