Competitive Hierarchies Among Three Species of Juvenile Coral Reef Fishes
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Vol. 472: 239–248, 2013 MARINE ECOLOGY PROGRESS SERIES Published January 9 doi: 10.3354/meps10015 Mar Ecol Prog Ser Competitive hierarchies among three species of juvenile coral reef fishes Shane W. Geange1,*, Adrian C. Stier2, Jeffrey S. Shima1 1School of Biological Sciences, Victoria University of Wellington, PO Box 600, Wellington 6023, New Zealand 2Department of Biology, University of Florida, Gainesville, Florida 32611-8525, USA ABSTRACT: Interspecific competition is often asymmetric, and it can limit the spatial distributions of competitively inferior species within a community. When asymmetric competition involves 2 or more component species, the ranking of species’ competitive abilities may form competitive hier- archies (all species of higher rank out-compete all species of lower rank) or competitive networks (at least 1 species of lower rank out-competes ≥1 species of higher rank). Expectations of resource monopolization and patterns of distribution and abundance among species in competitive net- works are expected to differ from those in competitive hierarchies. We conducted a field experi- ment to evaluate the relative competitive abilities of juveniles of 3 closely related species of reef fish (bird wrasse Gomphosus varius, fivestripe wrasse Thalassoma quinquevittatum and the sixbar wrasse T. hardwicke) on Moorea, French Polynesia. We controlled for intrinsic variation in sur- vivorship among species and found that competition among these 3 species was highly asymmet- ric, resulting in a simple competitive hierarchy (sequence of competitive ability from superior to inferior competitors): fivestripe wrasse > bird wrasse > sixbar wrasse. We surveyed densities of the 3 reef fish species on 55 patch reefs and observed significant negative spatial covariation between superior and inferior competitors, consistent with competitive hierarchies that limit the spatial dis- tributions of the inferior species (sixbar wrasse). Our work demonstrates that intense asymmetric competition and the formation of competitive hierarchies may be an important determinant of resource monopolization and patterns of distribution and abundance in reef fishes. KEY WORDS: Asymmetric competition · Competitive hierarchies · Interspecific interactions · Wrasse · Gomphosus · Thalassoma · French Polynesia Resale or republication not permitted without written consent of the publisher INTRODUCTION Alternatively, interspecific competition may be asym metric (i.e. superior competitors dominate a lar - Interspecific competition can be intense in natural ger portion of the resource, excluding inferior com- communities and is often an important determinant petitors; sp. A > sp. B), with inferior competitors per- of resource monopolization as well as patterns of dis- sisting in the system via a range of mechanisms, tribution and abundance (e.g. Connell 1961, Gladfel- including competition-colonization tradeoffs (Levins ter et al. 1980, Anderson et al. 1981, Schmitt & Hol- & Culver 1971, Tilman 1994, Hurtt & Pacala 1995), brook 1990, Robertson 1996, Munday et al. 2001, disturbance (Connell 1978, Huston 1979, Tilman Munday 2004). Interspecific competition between 2 1982) and aggregation in patchy habitats (Atkinson & species can be symmetric (i.e. species are competi- Shorrocks 1981, Hanski 1981). tively interchangeable; sp. A = sp. B), with species When competition is asymmetric, a common hypo - coexistence determined by priority of arrival of re- thesis is that, all else being equal, superior competi- cruits (e.g. Munday 2004, Geange & Stier 2009). tors exclude inferior competitors (e.g. Hardin 1960), *Email: [email protected] © Inter-Research 2013 · www.int-res.com 240 Mar Ecol Prog Ser 472: 239–248, 2013 resulting in negative spatial (or temporal) covariation tled reef fishes at this important life-history bound- in the distribution of each competitor (Colwell & ary, and because those few studies focused primarily Fuentes 1975, Connell 1983, Schoener 1983, Munday on pairs of competitive species. As individuals make et al. 2001, Young 2004). However, when pairs of the transition from a pelagic environment (where inferior and superior competitors are embedded they develop as larvae) to benthic reef habitat, they within a suite of competing species, the outcome of encounter multiple fish species with highly similar competition and influences on species distributions resource requirements. Consequently, interference may be altered. For example, when relative competi- competition for habitat resources among suites of tive abilities take the form of competitive networks, juvenile reef fish species is commonplace (e.g. in which a loop in an otherwise hierarchical se - Munday et al. 2001, Almany 2003), and such interac- quence of competitive abilities occurs (e.g. sp. A > sp. tions may determine the success of newly arrived B, sp. B > sp. C, but sp. C > sp. A), the presence of indi viduals, e.g. if superior competitors exclude infe- intransitives may stabilize coexistence (i.e. addition rior competitors from mutually preferred habitats of species may disrupt expectations of negative (Robertson & Gaines 1986, Munday et al. 2001, spatial co-variation between the original 2 species Geange 2010) and increase their exposure to preda- because a lower ranked competitor can out-compete tors (Holbrook & Schmitt 2002). Ultimately, a pattern ≥1 higher ranked competitors) (Buss & Jackson 1979, of negative spatial co-variation between superior and Levine 1999, Allesina & Levine 2011). Competitive inferior competitors may ensue, with important de - networks can lead to indefinite coexistence of the 3 mographic consequences for subsequent life-history species (Laird & Schamp 2006, Reichenbach et al. stages. 2007) and facilitate the coexistence of many more Here, we conduct a field experiment to determine species with appropriate resource–consumer inter- whether competitive interactions are symmetric or actions (Huisman et al. 2001). Conversely, relative asymmetric for 3 species of juvenile coral reef competitive abilities can take the form of hierarchical fishes. Our experiments facilitate estimates of rela- sequences in which all competitors of a higher rank tive competitive abilities for all pairwise combina- out-compete all competitors of a lower rank (e.g. sp. tions and the placement of pairs of competitors A > sp. B, sp. B > sp. C and sp. A > sp. C). In this within a 3-species suite of competitors. This enables scenario, patterns of negative spatial co-variation us to evaluate whether competitive interactions take between pairs of inferior and superior competitors the form of a competitive network or a competitive should be maintained (or amplified) when embedded hierarchy. We compare the results of our experi- within suites of competing species. We therefore ments with surveys of reef fish density to evaluate expect a single winner, with the best competitor driv- whether our expectations of competitive interactions ing all others extinct unless some other mechanism (based upon the experiments) match our observa- facilitates coexistence. tions of spatial covariation among these 3 species of In reef fishes, competitive interactions have histor- reef fish. ically been considered more or less symmetric, with species coexistence determined by a number of mechanisms, including priority of arrival (Sale 1974, MATERIALS AND METHODS 1976, 1977, 1978, 1980, Talbot et al. 1978) or storage effects (Chesson & Warner 1981, Chesson 1994, Study system 2000). Over the last few decades, it has become clear that competition is often asymmetric and can influ- We conducted the study in the northern lagoon of ence the distribution and abundance of reef fishes Moorea, French Polynesia (17° 30’ S, 149° 50’ W). The (e.g. Hixon 1980, Larson 1980, Ebersole 1985, Schmitt northern lagoon is ~800 to ~900 m wide, as measured & Holbrook 1990, 1999, Holbrook & Schmitt 1995, from the shore to the reef crest, and is interspersed Robertson 1996, Munday 2004). Although competi- with patch reefs separated from one another by a tion in reef fishes may occur among suites of species, mixture of sand, fine coral rubble and coral pave- rankings of relative competitive abilities and docu- ment (Galzin & Pointer 1985). Patch reefs are pre- mentation of competitive networks or competitive dominantly Porites spp., often surmounted by smaller hierarchies are rare. Juvenile reef fishes provide an colonies of branching corals, dead coral skeletons, exciting opportunity to examine relative competitive patches of macroalgae or lush stands of filamentous abilities because few studies have examined the role algae. The focal reef fishes in our study are closely and importance of competition among recently set- related wrasses (Family Labridae: bird wrasse Gom- Geange et al.: Competitive hierarchies among juvenile coral reef fishes 241 phosus varius, fivestripe wrasse Thalassoma quin- supp.pdf for background community structure). We quevittatum and sixbar wrasse T. hardwicke) that co- used the fish anesthetic eugenol (clove oil) and hand occur on reefs throughout the Indo-Pacific (Myers nets to collect fish. 1999). Collectively, these 3 species constitute nearly We conducted pairwise comparisons of competitive 70% of all juvenile labrids within the northern asymmetry between bird wrasse, fivestripe wrasse lagoon of Moorea (S. W. Geange unpubl. data). After and sixbar wrasse. To each reef, we randomly an approximately 39 to 68 d planktonic larval dura- assigned 1 of 6 treatments: (1) 6 bird wrasses; (2) tion (Victor