Wetlands DOI 10.1007/s13157-012-0326-4 ARTICLE Predator/Prey-Interactions Promote Decomposition of Low-Quality Detritus Christine Ewers & Anika Beiersdorf & Kazimierz Więski & Steven C. Pennings & Martin Zimmer Received: 29 February 2012 /Accepted: 10 July 2012 # Society of Wetland Scientists 2012 Abstract Predation on detritivores is expected to decelerate were present, probably owing to predation on detritivorous detritivore-mediated decomposition processes. In field mes- snails. Thus, the effects of predator/prey-interactions on ocosms, we studied whether the decomposition of leaf and decomposition processes are context-dependent and are needle litter of live oak (Quercus virginiana) and loblolly controlled by resource quality. pine (Pinus taeda), respectively, was affected by saltmarsh detritivores (Gastropoda: Littoraria irrorata and Melampus Keywords Decomposition processes . Predator/prey- bidentatus) and predacious omnivores (Decapoda: Armases interaction . Omnivory . Saltmarsh . Spatial subsidy cinereum) and their interactions. Both crabs and snails alone increased decomposition (mass loss) rates of oak litter, while a combination of both resulted in the same mass loss Introduction as in animal-free controls, probably due to crabs feeding on snails rather than litter. Neither crabs nor snails alone affect- Primary production of angiosperms is primarily con- ed mass loss of pine litter, but a combination of both signif- sumed by detritivores rather than herbivores (Teal icantly increased decomposition rates. Irrespective of the 1962;Cebrian1999). It has long been studied how litter type, crabs significantly increased mortality of the detritivores are affected by interactions with microbes snails but gained biomass only on pine litter and only when (e.g., Newell and Bärlocher 1993; Newell 1996;Zimmer detritivorous snails were present. Our findings suggest that and Topp 1999; Kautz et al. 2000;Laaksoetal.2000; unidirectional facilitation of omnivorous semi-terrestrial Zimmer et al. 2003;Airaetal.2008), and with other crabs by their detritivorous prey (saltmarsh snails) promotes detritivores (e.g., Gonzalez and Seastedt 2001; Jonsson the decomposition of low-quality (pine) litter. On high-quality et al. 2001;Bradfordetal.2002;DeDeynetal.2003; (oak) litter, by contrast, negative effects of the predator pre- Jonsson and Malmqvist 2003; Wright and Covich 2005; vail, resulting in a drop of decomposition rates when crabs Zimmer et al. 2005; Bobeldyk and Ramirez 2007). Top-down effects of predators on detritivores, and thus on decomposition processes, have been studied as well (e.g., Lawrence and Wise Christine Ewers and Anika Beiersdorf contributed equally. : : 2000, 2004; Wu et al. 2011), but rarely have omnivorous C. Ewers A. Beiersdorf M. Zimmer detritivores, feeding on both detritus and other detritivores, Zoologisches Institut, Christian-Albrechts-Universität zu Kiel, been considered in this respect. Am Botanischen Garten 9, 24118 Kiel, Germany Some food web models predict that omnivory will desta- : bilize food chain dynamics (Pimm and Lawton 1977, 1978; K. Więski S. C. Pennings Holt and Polis 1997), while others suggest that omnivory Department of Biology and Biochemistry, University of Houston, can be stabilizing in some circumstances (Matsuda et al. Houston, TX 77204, USA 1986; Fagan 1997; McCann and Hastings 1997). The result- M. Zimmer (*) ing uncertainty underscores the need for detailed studies of FB Organismische Biologie, AG Ökologie, trophic interactions of omnivores and their prey and how Biodiversität & Evolution der Tiere, Paris-Lodron-Universität, they affect ecosystem processes (Diehl 2003;Arimand Hellbrunner Str. 34, 5020 Salzburg, Austria Marquet 2004; van de Wolfshaar et al. 2006). To this end, e-mail: [email protected] effects of omnivores in detrital food webs may differ from Wetlands those in herbivore food webs because herbivores usually compounds. To this end, these detritivores cannot be induce defences of host plants, reducing plant quality regarded as functionally redundant with respect to decom- (Grime et al. 1996; Cornelissen et al. 2002), whereas detri- position processes. Owing to its wide range of activity along tivores may increase food quality of detritus by fragmenting the marine-terrestrial gradient and its omnivorous feeding litter or enhancing microbial activity (c.f. Zimmer et al. strategy, Armases cinereum may be of particular signifi- 2004). As has been shown for the interaction of endogean cance in this habitat as an omnivore that transports matter earthworms and epigean woodlice (Zimmer et al. 2005), the and energy between intertidal and terrestrial ecosystems mutual effects of different detritivores depend upon the (Zimmer et al. 2004). decomposability of the litter. To gain more insight into interactions between detritivores We examined the effects of an omnivorous detritivore on and omnivores during the decomposition of spatial subsidies, decomposition processes in saltmarshes on the Atlantic we studied the individual and joint effects of detritivores and Coast of the United States. A limited number of macro- omnivores on mass loss of two types of terrestrially-derived invertebrate species are involved in decomposition process- litter in high-marsh habitats. We hypothesized that (1) preda- es in saltmarshes (Rietsma et al. 1982; Valiela et al. 1984; tion by the omnivorous Armases impairs both (a) leaf litter Zimmer et al. 2004). Among these, the periwinkle Littoraria mass loss and (b) survival of detritivorous snails, while, in irrorata (Gastropoda: Prosobranchia), the coffeebean snail turn, (2) the omnivorous Armases benefits from the presence Melampus bidentatus (Gastropoda: Pulmonata), and the of snails because they provide an alternate food source; to this wharf crab Armases cinereum (Crustacea: Decapoda) are end, we expected Armases to reduce snail-mediated decom- particularly abundant along the North American Atlantic position rates and to gain more mass from feeding on snails coast (Lee and Silliman 2006; Buck et al. 2003; Zimmer et than on detritus. However, (3) the significance of predator/ al. 2004). Past studies have investigated competitive inter- prey-interactions will depend on the quality of the litter as actions between Littoraria and Melampus (Lee and Silliman food source for the omnivorous predator; thus, we expected 2006). Here, we consider how interactions between snails Armases to gain more from the presence of detritivorous snails (as a group) and Armases might mediate decomposition. when low-quality detritus (pine) was available. Armases is an omnivore that not only feeds on detritus and plants (Pennings et al. 1998; Zimmer et al. 2004) but also preys upon other animals (Buck et al. 2003; Ho and Pennings Material and Methods 2008). Thus, Armases may exert contrasting effects on salt- marsh decomposition processes: through feeding on leaf litter, Study Site and Species it may accelerate litter mass loss, but through intra-guild predation on detritivores, it may simultaneously decelerate All field work was done at Sapelo Island, Georgia, U.S.A. litter mass loss. (31°27′ N; 81°15′W). Saltmarshes around this island are In most studies of decomposition in saltmarshes, the typical of the southeastern U.S. Atlantic Coast (Pomeroy focus has been on autochthonous detritus derived from salt- and Wiegert 1981). There, terrestrial habitats immediately marsh angiosperms (Newell and Bärlocher 1993; Newell adjacent to the marsh are usually dominated by trees, espe- 2001a, b; Silliman and Zieman 2001; Silliman and Newell cially live oak (Quercus virginiana), loblolly pine (Pinus 2003). In the upper marsh, where semi-terrestrial and even taeda) and juniper (Juniperus virginiana). The branches of terrestrial detritivores are abundant and contribute to decom- these trees often hang several meters out over the high position processes (Rietsma et al. 1982; Valiela et al. 1984; marsh, and consequently drop large amounts of leaf litter Zimmer et al. 2002), significant amounts of leaf and needle as spatial subsidies into the high marsh system. The result- litter from trees that border the marsh spatially subsidizes ing ecotonal transition (marine-terrestrial) zone is common the high-intertidal region (Zimmer and Pennings, personal along the landward margin of many North Atlantic coastal observations). Only a few studies, however, have examined saltmarsh systems. the interactions between semi-terrestrial detritivores and The snails Littoraria irrorata and Melampus bidentatus terrestrial detritus in saltmarshes. In laboratory microcosms are the most abundant macro-gastropods in saltmarshes on with litter from different saltmarsh and woody plant species the Atlantic and Gulf coasts of the United States. Littoraria that differ in their quality as food for detritivores (Zimmer et is most abundant at lower latitudes (Silliman and Zieman al. 2002, 2004), isopods, snails and crabs exhibited species- 2001). Melampus occurs along the entire U.S. East coast, specific effects on litter chemistry and on the activity of but its densities are suppressed at many low-latitude sites litter-colonizing microbiota: Melampus improved the quali- due to negative interactions with Littoraria (Lee and Silliman ty of litter as substrate for decomposition through promoting 2006). Both feed on microalgae and particulate organic matter microbial activity; by contrast, feeding by Armases resulted on the marsh surface (Daiber 1977;Thompson1984)andon in the accumulation of nitrogen and several phenolic