Composition of Speciose Leaf Litter Alters Stream Detritivore Growth, Feeding Activity and Leaf Breakdown
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Oecologia (2006) 147: 469–478 DOI 10.1007/s00442-005-0297-8 PLANTANIMALINTERACTIONS Christopher M. Swan Æ Margaret A. Palmer Composition of speciose leaf litter alters stream detritivore growth, feeding activity and leaf breakdown Received: 22 April 2005 / Accepted: 2 November 2005 / Published online: 20 January 2006 Ó Springer-Verlag 2006 Abstract Leaf litter derived from riparian trees can ian plant species richness, in-stream secondary produc- control secondary production of detritivores in forested tion of detritivores and organic matter dynamics may be streams. Species-rich assemblages of leaf litter reflect related to species loss of trees in the riparian zone. Loss of riparian plant species richness and represent a heteroge- key species may be more critical to maintaining such neous resource for stream consumers. Such variation in processes than species richness per se. resource quality may alter consumer growth and thus the feedback on leaf breakdown rate via changes in feeding Keywords Mixed detritus Æ Non-additive effects Æ activity. To assess the consequences of this type of re- Plant species richness Æ Riparian Æ Stonefly source heterogeneity on both consumer growth and subsequent litter breakdown, we performed a laboratory experiment where we offered a leaf-shredding stream Introduction detritivore (the stonefly Tallaperla maria, Peltoperlidae) ten treatments of either single- or mixed-species leaf litter. Allochthonous inputs of detritus and nutrients can We measured consumer growth rate, breakdown rate and influence both the structure and function of food webs feeding activity both with and without consumers for (Hynes 1970; Polis and Hurd 1996; Anderson and Polis each treatment and showed that all three variables re- 1998). These inputs can increase the production of pri- sponded to speciose leaf litter. However, the number of mary consumers, in turn, creating more prey for higher leaf species was not responsible for these results, but leaf trophic levels (Strong 1992; Huxel and McCann 1998). species composition explained the apparent non-additive For example, terrestrial food webs on desert islands are effects. T. maria growth responded both positively and supported largely by arthropods that feed on ocean-de- negatively to litter composition, and growth on rived shore detritus (Polis and Hurd 1996). Similarly, mixed-litter could not always be predicted by averaging energy demands of many temperate stream ecosystems estimates of growth in single-species treatments. are met by terrestrially derived (e.g., riparian) leaf litter Furthermore, breakdown and feeding rates in mixed litter that enters the system during leaf fall (Fisher and Likens treatments could not always be predicted from estimates 1973; Cummins et al. 1989; Richardson 1991; Hall et al. of single-species rates. Given that species richness and 2001). Experimental exclusion of riparian leaf-fall to composition of senesced leaves in streams reflects ripar- streams decreases consumer production and predator density (Wallace et al. 1999). While the effects of the presence of allochthonous resource inputs on consumers Communicated by David Post have received considerable study (e.g., Richardson 1991; C. M. Swan (&) Polis et al. 1997; Gende and Willson 2001; Hall et al. Department of Geography & Environmental Systems, 2001; Power 2001; Murakami and Nakano 2002; University of Maryland, Baltimore County, Schindler and Scheuerell 2002; Takimoto et al. 2002), Baltimore, MD 21250, USA the extent to which variation in quality of these resources E-mail: [email protected] Tel.: +1-410-4553957 influences consumer growth and feeding in freshwater Fax: +1-410-4551056 ecosystems remains largely unexplored. Ecologists know that resource heterogeneity can af- M. A. Palmer fect trophic structure (Hilborn 1975) by altering inter- Chesapeake Biological Laboratory, specific interactions and feeding rates (Hilborn 1975; University of Maryland Center for Environmental Science, Solomons, MD 20688, USA Hanski 1981; Pacala and Roughgarden 1982; Naeem E-mail: [email protected] and Colwell 1991). For terrestrial herbivores, variability 470 in plant nutritional quality can reduce temporal vari- containing consumers. We asked: (1) How much does ability in feeding rates (Fox and Macauley 1977) and consumer growth rate, leaf breakdown rate, and/or possibly affect consumer growth. There is evidence from feeding rate change on mixtures of leaf litter compared both terrestrial and aquatic systems that the abundance with single-species leaf litter? (2) Is species composition of (Blair et al. 1990; Kaneko and Salamanca 1999) and leaf litter important in explaining patterns of consumer biomass (Leff and McArthur 1989; Swan and Palmer growth rate, leaf breakdown rate, and/or consumer 2005) of detritivores may vary between mixed-versus feeding rate? and (3) If species composition is important, single-leaf species litter. Taylor et al. (1989a) suggested can consumer growth rate, breakdown rate, and/or con- that, in terrestrial systems, this results from enhanced sumer feeding rate on mixed resources be predicted by consumer growth due to elevated nutrient cycling in averaging the estimates from single-species diets? mixed litter. However, it is also possible that changes in consumer growth are influenced by the length of the time when detritus is available to invertebrates (Golladay Materials and methods et al. 1983; Webster and Benfield 1986). Rapidly decomposing leaf species are palatable shortly after As larvae, the stonefly T. maria (Needham and Smith; entering the detrital pool and may only be available for a Plecoptera, Peltoperlidae) feeds mainly on leaf detritus brief period of time. Slower decomposing leaf species (i.e., shredder sensu Cummins and Klug 1979) gaining may not be immediately palatable but over time may the majority of its energy demands from leaf tissue ra- become more digestible, providing an important re- ther than resident microbial flora (Findlay et al. 1986). source for consumers. Given that there is temporal This species is found in cold, Piedmont streams in both variation when leaf species are most palatable, litter that the mid-Atlantic and northeastern United States (Stew- is made up of leaf mixtures—some rapidly decomposing art and Harper 1996). In November 2000, we collected and some more slowly decomposing—may be a more larvae from a second-order Piedmont stream (Fishing temporally stable resource base (e.g., over months) than Creek) located within the Frederick County Wildlife single species-litter. Management area of Maryland, USA (lat 39°31¢N, long The relationship between resource species richness and 77°28¢E; elevation 375 m asl). The tree species employed consumer dynamics likely involves complex interactions were chosen because they represent the dominant species between resource quality and the effects of consumer in local riparian habitats and vary in C:N content feeding on the rate of leaf breakdown. For example, litter (analyzed with an automated CHN analyzer, University consumption by invertebrates may fluctuate in response of Maryland Soils Testing Laboratory, College Park, to litter availability (Cummins et al. 1989), leaf chemistry MD, USA): Boxelder (Acer negundo L., C:N=21.1), (Campbell and Fuchshuber 1995; Findlay et al. 1996) and American Sycamore (Platanus occidentalis L., microbial conditioning (Ba¨ rlocher and Kendrick 1975). C:N=47.5), Black Willow (Salix nigra Marsh., Despite this complexity, the ‘‘quality’’ of the leaf as a C:N=47.2), Black Walnut (Juglans nigra L., C:N=37.8) resource for decomposers can be related to litter break- and Slippery Elm (Ulmus rubra Muhl., C:N=41.1). The down rate (Melillo et al. 1982; Taylor et al. 1989b; leaf litter was dried at 60°C, and allocated to ten leaf Ostrofsky 1997). Since the chemical attributes of a leaf species treatments: five single-species treatments, and (e.g., C:N or lignin:N) influence the rate of breakdown five four-species treatments (hereafter, ‘‘mixed’’ treat- and leaf chemistry varies among leaf species (Webster and ments). We held total initial dry mass of the leaf treat- Benfield 1986; Ostrofsky, 1993, 1997; Haapala et al. ments constant at 200 mg (e.g., 50 mg per species in the 2001), the energy available to the invertebrate consumers ‘‘mixed’’ treatments) and examined single-species versus can vary between leaf species (Iversen 1974; Herbst 1982; mixed-species treatments. Additional treatment combi- Irons et al. 1988; Sweeney 1993; Motomori et al. 2001). nations (e.g., two- and three-species) were difficult to Therefore, variation in leaf litter resource quality, defined manage in the laboratory with adequate replication and here as leaf species richness, may change consumer thus were not used in the present study. growth and resource use, with a subsequent feedback on Chambers to raise individual shredders were con- the rate of leaf litter breakdown. We predicted changes in structed out of plastic containers (64 mm H · 65 mm consumer growth and resource use on mixed litter, which dia.). Two lateral windows (27 mm dia.) and a single top would be associated with altered breakdown rates of window were cut into each chamber, covered with mixed litter versus single-species litter. We tested these fiberglass screening (1.6 mm mesh), and secured with hypotheses using a common stream invertebrate silicone cement. Each chamber received 200 mg of leaf detritivore and riparian leaf litter. material as coarsely broken fragments and ten standard Leaf litter species richness was manipulated as a food aquarium rocks (5 mm dia.). The chambers were resource for a stream detritivore, larvae of the stonefly randomly assigned to plastics bins (55 cm L · 37 cm W Tallaperla maria. We created ten leaf litter treatments: · 15 cm D), 30 chambers per bin, with each bin filled to five single-species leaf litter resources, and five mixture 4.5 cm with deionized water. Each bin was inoculated treatments comprised of all four-species combinations. with 500 ml of stream water, filtered twice at 45 lmto We measured breakdown rate in the presence/absence of remove invertebrates, while still adding microbial bio- consumers, and growth and feeding activity in treatments mass.