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Predator Effects on Fouling Community Development MARINE ECOLOGY PROGRESS SERIES Vol. 337: 93–101, 2007 Published May 14 Mar Ecol Prog Ser Predator effects on fouling community development Marie Nydam1, 2,*, John J. Stachowicz1 1Section of Evolution and Ecology, University of California Davis, One Shields Avenue, Davis, California 95616, USA 2Department of Ecology and Evolutionary Biology, Corson Hall, Cornell University, Ithaca, New York 14853, USA ABSTRACT: Predation by small consumers has a demonstrable effect on the recruitment and devel- opment of sessile invertebrate (fouling) communities. However, few data are available to assess the degree to which different micropredators are functionally equivalent, and whether initial effects on recruitment translate into lasting effects on adult communities. In the present study, we used field surveys and manipulative experiments to examine the effect of 2 molluscan predators on both the recruitment of sessile invertebrates and the ultimate community development under continuous pre- dation pressure for 12 mo. The mossy chiton Mopalia muscosa dramatically reduced recruits of some species and increased those of others, but ultimately total adult cover of all species was reduced by over half relative to predator-free controls. The file limpet Lottia limatula had minimal effects on recruitment, slightly reducing recruitment of some colonial ascidians. However, limpets had no effect on the total cover of invertebrates, instead altering species composition by slightly delaying overall successional trajectories. Field surveys suggested that both predators have measurable effects on community cover, as the abundance of each predator was positively correlated with an increase in free space. Plots with both chitons and limpets had nearly 4 times the open space of no-predator plots, and 2 times the open space of plots with limpets only. Therefore, both field surveys and experimen- tal manipulations highlight the substantial impact of micropredators in determining the composition of this fouling community. KEY WORDS: Predation · Fouling community · Succession · Recruitment Resale or republication not permitted without written consent of the publisher INTRODUCTION salinity) factors have been suggested to contribute to the development of particular fouling communities. Fouling communities are composed primarily of ses- For example, numerous studies have debated the sile filter-feeding invertebrates and mobile epifauna importance of predation on early post-settlement lar- such as ascidians, bryozoans, hydroids, sponges, bar- val survival (Stoner 1990, Osman et al. 1992, Osman & nacles, and other crustaceans that live on docks, pil- Whitlatch 1995), on early community development ings, and natural rock substrates worldwide. These (Connell 2001), and on the adult community (Suther- communities often support numerous non-native spe- land 1974, Karlson 1978, Keough & Butler 1979, Vance cies, and, given their proximity to commercial shipping 1988). ports, they are likely establishment sites for new Predators, especially large mobile fish, crabs, and invaders before they spread to the open coast. Thus, sea urchins can shift community composition from the factors that regulate community composition and early fast-growing, competitively superior species to resource availability in these systems are critical for slower growing, but predator-resistant groups (Suther- understanding the invasion process. A broad range land 1974, Karlson 1978, Stoner 1990). Other studies of biotic (competition, predation, larval supply) and have found that even when predation by mobile abiotic (ocean proximity, hydrodynamics, temperature, predators appears high, this has a minimal effect on *Email: [email protected] © Inter-Research 2007 · www.int-res.com 94 Mar Ecol Prog Ser 337: 93–101, 2007 species composition (e.g. Keough & Butler 1979, Vance the dominant species, which include several colonial 1988, Connell 2001), either because predators are non- ascidians (Botrylloides diegensis, Botrylloides viola- selective or because other processes dominate. ceus, Botryllus schlosseri, Didemnum sp., Diplosoma While large predators can impact fouling assem- listerianum, Distaplia occidentalis), solitary ascidians blages, these communities also often host a diverse ar- (Ascidia ceratodes, Ciona intestinalis), encrusting bryo- ray of smaller predators, including small crabs and zoans (Schizoporella unicornis, Watersipora subtor- snails that may consume early post-settlement life- quata), and arborescent bryozoans (Bugula californica, history stages of fouling invertebrates. In New England, Bugula neritina). In addition, the community contains small gastropods (micropredators) can dramatically al- several bivalves (Hinnites giganteus, Mytilus gallo- ter the magnitude of recruitment and alter species com- provincialis), cnidarians (Metridium senile, Obelia position from ascidians to bryozoans by selectively con- longissima), polychaete worms (Eudistylia poly- suming juvenile ascidians (Osman & Whitlatch 1995). morpha, Myxicola infundibulum), and a sponge (Hali- When multiple species of snails are present, they can clona permolis). Numerous mobile predators and reduce total cover and increase bare space availability, grazers also live on these docks, including crustaceans whereas individual species may simply alter species (Pachygrapsus crassipes, Pugettia producta), mollusks composition or even have no effect (Stachowicz & Whit- (Lottia limatula, Mopalia muscosa), and a scale worm latch 2005). However, these studies generally lasted (Halosydna brevisetosa). only a few weeks to several months, and it is not clear Field survey. We assessed the potential for predators whether these effects on early life-history stages cause to affect fouling community biomass through a survey persistent changes in adult community composition. in which we recorded the percent of open (unoccu- Given the variation in the outcome of exclusion experi- pied) space, as well as the number and species identity ments with large mobile predators, it is clear that in- of predators in 350 quadrats of 25 × 25 cm. Quadrats tense consumption need not lead to long-term effects were randomly located among the 5 main docks of the on community composition (e.g. Connell 2001). While marina, with ‘dock’ retained as a blocking factor in the separate studies have addressed the effects of micro- analysis. Chitons Mopalia muscosa and limpets Lottia predation on recruits and the adult community, few limatula were the most abundant potential predators of studies have explicitly examined the link between sessile invertebrates in these surveys; thus, we focus initial predation on recruits and the subsequent compo- on these species in the rest of the paper. Both have sition of the adult community. been observed to either consume sessile invertebrates In the present study we addressed the impacts of 2 or, at least, bulldoze juveniles off the surface while abundant predatory mollusks on both the immediate grazing on algae (Morris et al. 1980). post-settlement survival of fouling recruits and the Recruitment and community development. We as- development of the fouling community of Bodega sessed community development on 100 cm2 (10 × Bay, California, under sustained predation pressure for 10 cm) PVC plates, with surfaces sanded to simulate 12 mo. Specifically we asked: (1) How is the density of the rough surface of a fouling dock and to promote set- potential predators related to the availability of unoc- tlement. Preliminary experiments found no differences cupied space in the field? (2) What are the short-term in settlement rates among several different substrate effects of these predators on settlement and early post- types (R. C. Coates unpubl. data). Plates were sus- settlement survival? (3) What are the consequences of pended from the dock in blocks and randomly chronic, continuous predation in the context of sea- assigned to one of the following treatments: 1 Mopalia sonal variation in the abundance and composition of muscosa (the mossy chiton), 1 large (1.27 to 3.3 cm) larval settlement for fouling community development? Lottia limatula (the file limpet), 2 small (<1.269 cm) L. (4) How do intra- and inter-annual variation in the limatula, or no predator on each plate. We separately recruitment patterns of fouling organisms affect the assessed the effects of small and large limpets because strength of predation pressure on fouling community they had a bimodal size distribution (M. Nydam & development? J. Stachowicz pers. obs) suggesting the presence of 2 distinct groups. We enclosed either 1 large or 2 small limpets into each treatment to equalize the area of MATERIALS AND METHODS the plate covered by the predator’s shell among all 3-predator inclusion treatments. The area covered by Study location. We chose to conduct this experiment 1 large or 2 small limpets was roughly equivalent to at Spud Point Marina, Bodega Harbor, California that covered by a single chiton. (38° N, 123° W). This site supports a diverse community We then completely enclosed each plate in a 10 × of sessile filter-feeding species, characteristic of pro- 10 × 10 cm cage made from 1 × 1 cm Vexar mesh and tected harbors in northern California. We focused on attached both the cage and the plate to a PVC pipe Nydam & Stachowicz: Predator effects on fouling community development 95 suspended off the dock. These mesh cages enclosed 60 a F = 16.55 C chitons and limpets and excluded crab and fish, while 50 p < 0.0001 allowing larvae to settle on the plates.
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