Hydrobiologia 434: 11–16, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.
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Species interactions between estuarine detritivores: inhibition or facilitation? Charles T. Chong1 , Scott T. Larned2 , Alan P. Covich3 & Robert A. Kinzie III4,∗ 1 Zoology
Department, University of Hawai‘i, Honolulu, HI 96822, U.S.A. Ecology Branch, United States Environmental Protection Agency, Newport, OR 97365, U.S.A. 3 Department of Fishery and Wildlife Biology, Colorado State University, Fort Collins, CO 80523, U.S.A. 4 Zoology Department and Hawai‘i Institute of Marine Biology, University of Hawai‘i, Honolulu, HI 96822, U.S.A. E-mail:
[email protected] (∗ Author for correspondence) 2 Coastal
Received 9 March 2000; in revised form 9 March 2000; accepted 21 April 2000
Key words: ecological redundancy, competition, detritus processing, species interactions, tropical estuaries
Abstract Native Hawaiian estuarine detritivores; the prawn Macrobrachium grandimanus, and the neritid gastropod Neritina vespertina, were maintained in flow-through microcosms with conditioned leaves from two riparian tree species, Hau (Hibiscus tiliaceus) and guava (Psidium guajava). Their ability to beak down leaf detritus was determined when alone and when they were together. In single-species treatments, N. vespertina processed leaves from both trees at higher rates than M. grandimanus, but in combined treatments, facilitation occurred when the substrate consisted of Hau leaves, and interference occurred when the substrate consisted of guava leaves. From this, we conclude that whether detritivore species are functionally redundant, facilitating or inhibiting in their processing of detritus depends not only on the detritivore species, but also on the species composition of the detritus food source.
Introduction Questions of species-specific functions, species interactions and species redundancy are of general and increasing interest to ecologists studying ecosystem processes (Jones & Lawton, 1995; Finlay et al., 1997; Ehrlich & Walker, 1998; Naeem, 1998). This interest stems from concerns about the stability and diversity of natural ecosystems and how such systems respond to loss (or addition) of species (Ehrlich & Ehrlich, 1981; Robinson & Dickerson, 1987; Lawton & Brown, 1993; Bond, 1993; Lawton, 1994). While most attention has been given to terrestrial systems, the underlying ecological principles should be similar in aquatic habitats. Low species diversity, limited spatial complexity and dependence on a limited range of food sources make tropical estuaries ideal for the study of questions of species interactions and ecosystem function. In estuarine ecosystems, benthic
food webs are often linked to terrestrial as well as in situ production, with biotic processing of coarse particulate organic matter of terrestrial origin being a fundamental pathway of energy flow (Odum, 1980, 1984; Day et al., 1989). Rates at which coarse detritus is converted to fine particles and dissolved matter in estuarine food webs are influenced by the composition of the detritus and the species composition of the macrofauna. In estuaries, there are typically a few generalist animal species, most of which carry out similar ecosystem functions. There is, however, a possibility that interactions between and among these species could increase or decrease rates of detritus processing (Day et al., 1989). The term facilitation describes cases where two or more species have combined effects on the rates of detritus processing that are greater than the sum of their individual effects. The term inhibition describes cases in which rates of detritus
12 processing are less than the sum of the rates attributed to individual species. Inhibition could result from interference competition.
Materials and methods In this study, we measured rates of mass loss of leaf detritus due to the actions of two native Hawaiian detritivores, alone and together, to compare the rates at which they processed leaves from riparian trees, and to determine their effects on the combined processing rate when they co-occurred. We used the indigenous palaemonid prawn Macrobrachium grandimanus (Randall) and the endemic neritid gastropod Neritina vespertina (Sowerby). Both species are common in estuaries on all Hawaiian Islands with perennial streams. Our study addresses the question of whether these species interact in either a facilitative or inhibitory manner when functioning together as detritivores. M. grandimanus moves actively about the estuarine benthos, using its periopods to pick at the substratum and transfer food particles to the mouth parts which grind the particles before they are ingested. N. vespertina is a scraper, using its rhipidoglossan radula to remove material from the substratum. Because the prawn functions by using its mandibles to rapidly shred entire leaves, while the scraping action of N. vespertina is limited to leaf surfaces, we anticipated that M. grandimanus would be more effective in processing leaf litter. We further expected that the combined shredding and scraping actions of the two species would possibly show some facilitation. Leaves from two trees, Hau (Hibiscus tiliaceus L.) and guava (Psidium guajava L.), were used in the experiments. Hau and guava are among the most common riparian plants at lower elevations in Hawai‘i, and their leaves were abundant in the benthic detritus of estuaries along Hamakua coast on the island of Hawai‘i (pers. obs.). Guava is alien to Hawai‘i, introduced in the early 1800s from tropical America (Wagner et al., 1990). Hau may be indigenous but was probably also transported to the Hawaiian Islands by early Polynesians (Wagner et al., 1990). Microcosm experiments were carried out at the Hawai‘i Department of Natural Resources aquaculture facility near the mouth of the Wailoa River, Hilo, Hawai‘i. Water was taken from the estuarine reach of the Wailoa River 200 m from the river’s entry into Hilo Bay. The water intake was at a depth of 50–100 cm depending on tide stage. Water was filtered through
a Hayward S360SX sand filter filled with 450–550 µm silica sand, then pumped to the upstream end of a concrete channel (1 m wide × 0.4 m high × 16 m long). The microcosm experiments were carried out in the concrete channel with flowing water at a depth of 25 cm. Microcosms were made from square plastic buckets (23 cm on a side and 35 cm high with a capacity of 13 l. Large (16.5 cm diameter) holes were cut in two opposite sides of the microcosms and plastic mesh (Vexar, 0.6 cm2 openings) was glued across both openings with silicon cement. The microcosms were set in the channel with the openings on the upstream and downstream sides to facilitate water flow through them. Each microcosm contained a slotted concrete brick (10 × 10 × 20 cm) to provide shelter for the invertebrates. Microcosms were arranged across the channel in rows of four. Water flow rate in the channel, measured with a Swoffer 2100 flow meter, was approximately 1.6 m s−1 . To prevent algal growth in the microcosms, the entire channel was covered by shade cloth, and the microcosms were fitted with opaque plastic lids. Fresh Hau and guava leaves were picked from riparian trees 2–3 days before each experimental run. Leaves, with petioles removed, were air-dried for 2 days. After drying, packs of 2–3 Hau leaves or 6–7 guava leaves were weighed and assigned randomly to experimental treatments. The variability of starting leaf pack masses was minimized within each run (range
