University of Dayton eCommons Biology Faculty Publications Department of Biology 2018 Tropical Herbivorous Phasmids, but Not Litter Snails, Alter Decomposition Rates By Modifying Litter Bacteria Chelse M. Prather University of Dayton, [email protected] Gary E. Belovsky University of Notre Dame Sharon A. Cantrell Universidad del Turabo Gurabo Grizelle Gonzalez International Institute of Tropical Forestry, Jardın Botanico Sur Follow this and additional works at: https://ecommons.udayton.edu/bio_fac_pub Part of the Biology Commons, and the Cell Biology Commons eCommons Citation Prather, Chelse M.; Belovsky, Gary E.; Cantrell, Sharon A.; and Gonzalez, Grizelle, "Tropical Herbivorous Phasmids, but Not Litter Snails, Alter Decomposition Rates By Modifying Litter Bacteria" (2018). Biology Faculty Publications. 238. https://ecommons.udayton.edu/bio_fac_pub/238 This Article is brought to you for free and open access by the Department of Biology at eCommons. It has been accepted for inclusion in Biology Faculty Publications by an authorized administrator of eCommons. For more information, please contact [email protected], [email protected]. Ecology, 99(4), 2018, pp. 782–791 © 2018 The Authors Ecology published by Wiley Periodicals, Inc. on behalf of Ecological Society of America. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Tropical herbivorous phasmids, but not litter snails, alter decomposition rates by modifying litter bacteria 1,2,3,6,7 1,2 2,4 2,5 CHELSE M. PRATHER, GARY E. BELOVSKY, SHARON A. CANTRELL, AND GRIZELLE GONZALEZ 1Department of Biological Sciences, Galvin Life Sciences, University of Notre Dame, Notre Dame, Indiana 46656 USA 2Luquillo Long-Term Ecological Research Site, Rıo Grande PR 00745 Puerto Rico 3Department of Biology and Biochemistry, University of Houston, Houston, Texas 77004 USA 4Department of Biology, Universidad del Turabo Gurabo, Gurabo PR 00778 Puerto Rico 5USDA Forest Service, International Institute of Tropical Forestry, Jardın Botanico Sur, 1201 Calle Ceiba, Rıo Piedras PR 00926 Puerto Rico Abstract. Consumers can alter decomposition rates through both feces and selective feeding in many ecosystems, but these combined effects have seldom been examined in tropical ecosystems. Members of the detrital food web (litter-feeders or microbivores) should presumably have greater effects on decomposition than herbivores, members of the green food web. Using litterbag experiments within a field enclosure experiment, we determined the relative effects of common litter snails (Megalomastoma croceum) and herbivorous walking sticks (Lamponius portoricensis) on litter compo- sition, decomposition rates, and microbes in a Puerto Rican rainforest, and whether consumer effects were altered by canopy cover presence. Although canopy presence did not alter consumers’ effects, focal organisms had unexpected influences on decomposition. Decomposition was not altered by litter snails, but herbivorous walking sticks reduced leaf decomposition by about 50% through reductions in high quality litter abundance and, consequently, lower bacterial richness and abundance. This relatively unexplored but potentially important link between tropical herbivores, detritus, and litter microbes in this forest demonstrates the need to consider autotrophic influences when examining rainforest ecosystem processes. Key words: ecosystem process; enclosure; herbivory; light gap; litter; litterbags. INTRODUCTION Macro-consumers may have differential effects on decom- position, depending on their trophic level, and these effects Decomposition is a crucial flux in recycling organic matter are largely context-dependent, thus making predictions about and nutrients, and ‘thus’ is inherently tied to understanding the effect of any one species in an ecosystem difficult. Litter- our changing global carbon cycle (van Groenigen et al. dwelling consumers may have positive or negative effects on 2014). Litter decomposition in tropical forests may be partic- decomposition depending on their dominant food. Generally, ularly important to the global carbon cycle because of the as members of the brown food web, litter-dwelling consumers rapid litter turnover and high rates of carbon sequestration can facilitate decomposition through their comminution of that occur there. Biotic control of decomposition may have litter that occurs from feeding directly on litter (detritivores) disproportionately large effects in tropical systems compared or on litter microbes (microbivores), and, thus, increasing sur- to their temperate counterparts because of optimal climatic face area available for microbes (Heneghan et al. 1998, conditions for decomposition to occur and some geologic fac- Gonzalez and Seastedt 2000, Gonzalez et al. 2001). Detriti- tors that usually have an important role in decomposition vores are largely assumed to have positive effects on the rates being negligible (e.g., high amounts of iron that render clays of decomposition by directly processing litter. Microbivores inactive for adsorption of nutrients) (Lavelle et al. 1993, may also affect decomposition through their selective feeding Fonte and Schowalter 2005). However, studies examining on functional groups within the litter layer (Moore et al. how the biota affects decomposition in these systems have 1988, Wardle et al. 2002). Additionally, highly labile inverte- largely focused on members of the detrital “brown” food web brate frass is often stimulating to decomposition, but can also (Milton and Kaspari 2007, Richardson et al. 2010). With so inhibit microbial activity thereby reducing decomposition much litterfall year-round and high litter turnover rates, this rates, particularly if the frass is tightly compacted and has a dynamic brown food web is thought to regulate many ecosys- low internal porosity (e.g., Hanlon and Anderson 1979). tem processes, including decomposition. This assumption has Litter-dwelling organisms may also alter microbial communi- left the potential effects of interactions within the autotrophic ties by acting as dispersal agents for fungal and bacterial “green” food web largely ignored. Moreover, the potential for propagules that colonize and, subsequently, decompose litter cascading, interactive effects between macro-consumers in (Behan and Hill 1978). green and brown food webs have not been examined, espe- Herbivore effects can also stimulate or inhibit decomposi- cially in tropical rainforests. tion due to context dependent mechanisms. Herbivores’ frass has been shown to increase nitrogen cycling (Sirotnak and Manuscript received 14 September 2017; revised 1 January 2018; Huntly 2000, Rinker et al. 2001), stimulate microbial activity accepted 4 January 2018. Corresponding Editor: Steven D. Allison. (Frost and Hunter 2004), and ultimately increase decomposi- 6 Present address: Department of Biology, University of Dayton, Dayton, Ohio 45469 USA tion rates (Fonte and Schowalter 2005), but these effects may 7 E-mail: [email protected] also be influenced by frass quality and internal porosity. 782 April 2018 TROPICAL HERBIVORES ALTER DECOMPOSITION 783 Their selective feeding can stimulate or reduce decomposi- 2000, Richardson et al. 2010, Gonzalez et al. 2014), the inter- tion, partially depending on the herbivores’ preferred foliage. active effects of herbivores and litter macro-consumers have For example, if herbivores prefer to consume faster decom- seldom been examined. In nature, litter-dwelling consumers posing plants, slower decomposing, poorer quality plants would constantly experience any potential effects of herbi- increase in abundance, resulting in poorer resources for vores on litter quality, and the possibility for interactions decomposers, and thereby decreasing decomposition rates between these trophic levels would be high. (Pastor et al. 1988, Brown and Gange 1992, de Mazancourt We hypothesized that: (1) macro-consumers from both and Loreau 2000, Schmitz et al. 2000, Feeley and Terborgh green and brown food webs will alter decomposition rates; 2005). The opposite pathway can also occur where herbivore (2) these consumers will have interactive effects; and (3) preference for slower decomposing plants can increase rates of canopy cover presence will alter consumer influences. We decomposition (McNaughton 1985, Holland 1995, Belovsky used a field enclosure experiment to test how common and Slade 2000, 2002). The effects of herbivores on decompo- brown macro-consumers (litter snails, Megalomastoma cro- sition may also be specific to the type of defensive strategy ceum Gmelin) and green macro-consumers (walking sticks, that the plant species involved use (inducible vs. constitutive Lamponius portoricensis Rehn) alter litter production, defenses), and this trait can be plastic depending upon decomposition rates, and microbial communities in a distur- microclimatic conditions under which the plant is growing bance-driven rainforest in Puerto Rico. We carried out this (Cardenas et al. 2014, Bixenmann et al. 2016). Because of the experiment in both light gaps and closed canopy sites to differential effects of these different trophic levels, the combi- determine how increased light inputs that accompany distur- nation of herbivores and detritivores could result in either bances may alter these influences. Specifically, we predicted synergistic or negative feedbacks on the process of decomposi- that