Trade-Offs in Resistance to Competitors and Predators, and Their Effects On

Journal of Animal Blackwell Publishing, Ltd. Ecology 2005 Trade-offs in resistance to competitors and predators, and 74, 313–321 their effects on the diversity of tropical marine sponges JANIE L. WULFF Department of Biological Science, Florida State University, Tallahassee, FL 32306–1100, USA Summary 1. Consistently very high regional diversity of tropical marine sponges reflects a com- bination of high within-habitat diversity and distinctness of species composition among habitats. Distinctive sponge faunas of Caribbean coral reefs and mangroves seem to support the assumption that abiotic factors determine sponge diversity within habitats and faunal differences between habitats because these habitats differ greatly in abiotic characteristics and because lower species diversity on mangroves appears to reflect their inferiority as sponge habitat. 2. A way to test this assumption is provided by unusual mangrove cays in Belize that are inhabited by the typical Caribbean reef sponge fauna. Reciprocal transplant experiments, combined with caging (predator-free space) and artificial substrata (competitor-free space), demonstrated control of community membership by biological interactions for 12 common species: spongivorous predators excluded typical mangrove sponges from reef sponge assemblages, and reef sponges were excluded from mangrove sponge assemblages by competition. 3. Variation in growth rate was related inversely to variation in defences against predators in the species studied, suggesting a trade-off between resistance to competitors and to predators. 4. This trade-off influences community structure, as the key importance of competition for space among mangrove species results in lower within-habitat diversity, while multiple challenges, including predation, may maintain high diversity of reef sponges. Differences in species composition between habitats are maintained, as this trade-off precludes success of individual species as members of both faunas. 5. Most surprising is that typical faunas of mangroves and reefs are not tied to these habitats by abiotic factors. Greater sponge species diversity on the reef does not neces- sarily indicate superior conditions for sponges. Instead reefs may be a refuge for species that grow too slowly to coexist with typical mangrove species. Key-words: Caribbean, coral reefs, mangroves, Porifera, spongivory. Journal of Animal Ecology (2005) 74, 313–321 doi: 10.1111/j.1365-2656.2004.00925.x bolstering regional species counts in tropical seas (e.g. Introduction Knowlton & Jackson 1994; Ogden 1997; Paulay 1997). Intriguingly high regional diversity of tropical marine If species are sorted into habitats by how gracefully organisms has been attributed to competition, predation, they cope with habitat-characteristic abiotic factors, mutualism, disturbance, recruitment patterns, geolog- the regional species pools for different habitats are ical age, physical habitat structure, areal extent of the functionally independent of each other. However, if region and various combinations of these. One consist- biotic interactions influence species distributions ent pattern among taxa and regions is that distinct across habitats, species could live in different habitats species composition in each habitat (i.e. high between- as distributions of their predators, competitors and habitat diversity) plays an especially important role in mutualistic partners shift. A metacommunity view- point, acknowledging potential linkage of all species Correspondence: Janie L. Wulff, Department of Biological in the region, is then required. Thus as coral reefs © 2005 British Science, Florida State University, Tallahassee, FL 32306– and associated ecosystems slide more desperately Ecological Society 1100, USA. E-mail: [email protected] into disrepair, knowing the relative importance of 314 abiotic and biotic factors in sorting species into differ- by spongivores and are therefore restricted to typical J. L. Wulff ent habitats and maintaining within-habitat diversity mangrove stands from which spongivores are absent of key groups is crucial (e.g. Bellwood & Hughes 2001). and (2) sponge species typical of coral reefs are pre- Sponges, like the much better-studied corals, fishes vented from inhabiting typical mangroves by competi- and shelled molluscs, are especially diverse and abun- tion from mangrove sponge species, which grow more dant, and are players of key functional roles in tropical rapidly. I used the direct approach of reciprocal trans- marine systems (e.g. Paulay 1997; Diaz & Rützler 2001; plant experiments between the Pelican Cays and Twin Wulff 2001). However, sponges are set apart by inti- Cays, augmented by cages (predator-free space) and midating identification and quantification challenges artificial substrata (competitor-free space). that have slowed studies of processes underlying their Patterns of growth and survival of reciprocally trans- diversity patterns. Comprehensive faunal surveys of planted sponges address the importance of biotic fac- tropical sponges (e.g. Alcolado 1994; Zea 1994, 2001; tors for sponge distribution and diversity on three levels, Reed & Pomponi 1997; Hooper, Kennedy & Quinn as follows. (1) Region: do competition and predation 2002) demonstrate distinctive species compositions in influence the striking distinction between the mangrove different habitats. Abiotic factors have generally been and coral-reef sponge faunas in the Caribbean? (2) Com- held responsible for determining both habitat-distinctive munity: does within-habitat sponge diversity reflect the species composition and within-habitat diversity. role of biological interactions in determining faunal Universally acknowledged unusual chemistry of sponges membership? (3) Species: do trade-offs between resistance has been invoked to explain apparent relative immu- to competitors and resistance to predators preclude nity to control by interactions (e.g. Kubanek et al. 2002 success of individual species as members of both faunas? and references therein). Reports of sponges being outcompeted are rare (Sutherland 1980; Thacker et al. Methods 1998), although sponges often outcompete neighbours of other taxa (e.g. Jackson & Buss 1975; Vicente 1990; Rützler & Muzik 1993; Aerts & van Soest 1997). Similarly, predators restrict some sponge species to refuges (e.g. Twin Cays, a mangrove stand with a typical tropical Wulff 1988, 1995b, 1997b; Dunlap & Pawlik 1996, 1998), western Atlantic mangrove sponge fauna, is a set of off- but the influence of predation on sponge diversity has shore cays built on peat banks, pervaded by channels barely been considered (Wulff 1994, 1997c; van Dam & and bays and surrounded by sediment and sea-grass Diez 1997). meadows (Rützler et al. 2000; Wulff 2000, and references Caution in interpretation is advised by the possibil- therein; maps in both papers). In the Pelican Cays, ity that biotic influences can be invisible if they curtail where a sponge fauna typical of shallow Caribbean distributions so effectively that potentially competing coral reefs inhabits mangrove roots (Wulff 2000), the species, or predator and prey species, rarely coexist. mangroves are embedded in peat banks on the tops of Experiments are required. Coral reefs and mangroves coral reefs. All but four of the 30 most common sponge have distinct sponge faunas that contribute substan- species on the Pelican Cays mangroves are common tially to the region-wide Caribbean tally of over 640 inhabitants of shallow coral reefs (Wulff 2000). Prop described sponge species (van Soest 1994). An ideal situ- roots at Twin Cays and the Pelican Cays together ation for determining the relative importance of bio- harbour a total of 167 sponge species and distinct forms, logical and abiotic factors for within-habitat diversity but 78% of those are found in only one of these two sets and habitat-distinctive sponge faunas of mangroves of cays, and many of the species that live in both are and coral reefs is provided by two sets of mangrove cays rare in one or the other (Rützler et al. 2000). on the Belize Barrier Reef. The sponge fauna on man- The site chosen for experiments at Twin Cays, 3 km grove roots at Twin Cays is typical of mangrove stands north-west of the Carrie Bow Cay research station of throughout the western Atlantic (Rützler et al. 2000; the Smithsonian Institution, is a channel known as Wulff 2000, and references therein), but the sponge fauna Hidden Creek. In the Pelican Cays, 16 km south-west on the Pelican Cays mangroves is unusual in that it is of Carrie Bow Cay, two sites were chosen: one at Man- typical of shallow Caribbean coral reefs (Wulff 2000). atee Cay, on the far side of the pond (200 m away) from The many abiotic differences that otherwise confound the entrance, and the other at Cat Cay, to the left of and comparisons between mangroves and coral reefs (reefs just 80 m inside the pond entrance. At Carrie Bow Cay, generally have more light, less turbidity, fewer nutrients, experiments were established among large corals on more substratum continuity and more exposure to the shallow reef south-west of the island. physical disturbance) are minimized, facilitating experi- mental evaluation of biological factors. The proximate focus of this study was to determine if © 2005 British biological interactions maintain the distinction between Six of the most common sponge species at the Pelican Ecological Society, Journal of Animal the typical sponge faunas of mangroves and coral reefs. Cays and six of the most common sponge species at

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