Changes in South African rocky intertidal invertebrate community ...
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MARINE ECOLOGY PROGRESS SERIESMar Ecol Prog SerVol. 340: 163171, 2007 Published June 18INTRODUCTIONThe spread of alien species is altering the composi-tion of marine communities on a global scale (Ruiz etal. 1999, Mack et al. 2000, Grosholz 2002) and hasbeen identified as a major threat to biodiversity (Occhi-pinti-Ambrogi & Savini 2003). Many studies have con-sidered direct interactions between alien and indige-nous species (Berman & Carlton 1991, Byers 2000,Byrnes & Witman 2003, Bachelet et al. 2004, Le Pape etal. 2004), but relatively little attention has been paid tothe impacts of alien species on the biological structureof the communities they invade.One alien species that has received substantial atten-tion at the level of species-specific effects is the mytilidmussel Mytilus galloprovincialis along the SouthAfrican coast. As the most abundant and widespreadinvasive marine species in this region (Robinson et al.2005), M. galloprovincialis has partially displaced thelocal mussels Choromytilus meridionalis and Aula-comya ater along the west coast (Hockey & Van ErkomSchurink 1992), while exhibiting spatial segregation Inter-Research 2007 www.int-res.com*Email: firstname.lastname@example.orgChanges in South African rocky intertidal invertebrate community structure associated withthe invasion of the mussel Mytilus galloprovincialisTamara B. Robinson1, 2,*, George M. Branch1, Charles L. Griffiths1, 2, Anesh Govender1, Philip A. R. Hockey31Marine Biology Research Institute, Zoology Department, University of Cape Town, Private Bag X 03, Rondebosch 7701, South Africa2Centre of Invasion Biology, Zoology Department, University of Cape Town, Private Bag X 03, Rondebosch 7701, South Africa3DST/NRF Centre of Excellence at the Percy FitzPatrick Institute, University of Cape Town, Private Bag X 03, Rondebosch 7701, South AfricaABSTRACT: Since the establishment of the alien mussel Mytilus galloprovincialis in South Africa,several authors have studied its interactions with individual indigenous species. However, thebroader implications of this invasion on the intertidal zone remain undocumented. This paper analy-ses the impacts of this mussel on the rocky-shore invertebrate community structure at Marcus Islandon the west coast of South Africa. The effects of the invasion were linked to 3 key elements and werenot consistently spread across the intertidal zone, but were focused within the mid-to-low shore.Firstly, physical stress in the mid-intertidal zones was ameliorated by the presence of M. galloprovin-cialis beds. Secondly, habitat complexity was increased where M. galloprovincialis replaced barerock or less complex secondary habitat. Thirdly, habitat became less patchy as mussel beds blan-keted the shore. Consequently, invertebrate density and species richness increased substantially, andcommunity composition changed significantly in the mid-shore. Lower on the shore, significantchanges in invertebrate community structure were driven by a switch from mono-layered beds of thesmall indigenous mussel Aulacomya ater to multilayered beds of M. galloprovincialis, despite nochange in total species richness.KEY WORDS: Alien mussel Community structure Marine invasions Mytilus galloprovincialis Rocky shoresResale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 340: 163171, 2007with the indigenous mussel Perna perna on the southcoast (Robinson et al. 2005). As a consequence of therapid growth rate, high fecundity and desiccation toler-ance of this invasive mussel (Van Erkom Schurink &Griffiths 1990, Hockey & Van Erkom Schurink 1992), itsarrival resulted in a net upshore shift in the zonation ofintertidal mussel beds. Due to extremely high recruit-ment rates (up to 20 000 recruits m2; Harris et al. 1998),M. galloprovincialis presently dominates primary rocksurfaces at the expense of various competitively inferiorlimpet species (Branch & Steffani 2004). By excludingthe limpet Scutellastra granularis from open rock,M. galloprovincialis has reduced the number of individ-uals that occur directly on rock, although at the sametime it offers the potential of increasing overall S. gran-ularis density by providing a favourable settlement andrecruitment substratum for juveniles (Griffiths et al.1992, Hockey & Van Erkom Schurink 1992). A secondlimpet species, Scutellastra argenvillei, has also beensignificantly affected by this invasion, although thestrength of the interaction between these 2 species ismediated by wave action (Steffani & Branch 2003a,b).On exposed shores, M. galloprovincialis outcompetesS. argenvillei and dominates the primary substratum,while, on semi-exposed shores, the mussel is relativelyscarce and S. argenvillei maintains dominance in openrock space (Steffani & Branch 2003a,b).Besides the biological role of mussels on rocky shores,they also form an important biotic substratum (Seed &Suchanek 1992). Mussel beds impact surroundingcommunity structure as the highly complex configura-tion of mussel matrices offers a multitude of micro-habitats, which ameliorate fluctuating environmentalconditions and provide protection from predation(Gosselin & Chia 1995). The physical presence of themussel shells also constitutes a suitable hard substratumfor settlement and development of co-occurring species.Despite substantial work on the ecological impacts ofMytilus galloprovincialis and the known role of mus-sels as biotic substratum, the impact of this invasion onthe intertidal community has not been considered. Inan effort to elucidate community impacts of such inva-sions, this study characterises changes in intertidal in-vertebrate community composition following invasionof South African rocky shores by M. galloprovincialis.MATERIALS AND METHODSThis study took place on the southern shores of Mar-cus Island in Saldanha Bay (33 02.59' S, 17 58.26' E)on the west coast of South Africa. The distribution andabundance of intertidal invertebrates was recorded in1980, before the invasion of Mytilus galloprovincialiswas recognised (although this species may have beenpresent in low numbers) and again in 2001, by whichtime M. galloprovincialis was well established and hadinvaded much of the South African coast (Robinson etal. 2005). In 1980, 7 intertidal zones were identifiedand sampled. They were (in descending order of tidalheight):(1) The Porphyra zone, consisting of patchy beds ofthe alga Porphyra capensis.(2) The Ulva zone, characterised by mixed beds of thealgae Ulva capensis and Ulva (=Enteromorpha) linza.(3) The Granularis zone, dominated by the limpetScutellastra granularis.(4) The algal turf zone, covered by a moss-like red al-gal community dominated by Caulacanthus ustulatus.(5) The Gigartina zone, characterised by the algaeGigartina radula and Pterosiphonia cloiophylla.(6) The Aulacomya zone, dominated by the ribbedmussel Aulacomya ater.(7) The Choromytilus zone, comprising beds of theblack mussel Choromytilus meridionalis.In 1980, 10 to 16 quadrats, each of 0.01 m2, wereselected randomly in each zone from within areas of100% algal or mussel cover. These quadrats werecleared, and all mobile and sessile invertebrates>1 mm in size were counted and identified to specieslevel. In the Granularis zone, where invertebrates tendto be large and sparsely distributed, animals werecounted in situ in 27 quadrats of 0.5 m2.In 2001, the same survey protocol was used, with 2exceptions. Firstly, Mytilus galloprovincialis had over-run most of the Granularis zone, making it inappropri-ate to employ the 0.5 m2 quadrats previously used tosample this zone, and 0.01 m2 quadrats were cleared.Secondly, 7 samples were taken per zone. These wererandomly, horizontally interspersed between the 1980samples. To ensure equivalent areas were analysedin 1980 and 2001, in each zone a randomly selectedsubset of 7 samples from 1980 was compared with the7 samples taken in 2001. All calculations, except thoseof rarefaction curves, were conducted using the ran-dom sub-sample.Prior to univariate analyses, data were tested for nor-mality using the Kolmogorov-Smirnov 1-sample testand for homogeneity of variances using Levenes test.All univariate analyses were conducted using STATIS-TICA for Windows (Version 6), StatSoft Inc. (2004),with set at 0.05.Densities per square metre of mussels and otherinvertebrates were compared before and after theMytilus galloprovincialis invasion (1980 versus 2001)using the Mann-Whitney U-test. Each intertidal zonewas considered separately.To estimate the sufficiency of our sample size andcompare species richness between times in the respec-tive zones, sample-based rarefaction curves (Gotelli &164Robinson et al.: Changes in community structure associated with an exotic musselColwell 2001) and the incidence-based richness esti-mate Chao 2 (Chao 1987) were calculated using theprogramme EstimateS (Colwell 2005).Community composition (based on numerical abun-dance) was analysed separately for each intertidalzone using multivariate techniques in the PRIMERsoftware package (Plymouth Marine Laboratory)and non-standardised, fourth-root transformed data.ANOSIM was employed to detect significant changesin community structure between 1980 and 2001. SIM-PER resolved which species were responsible for thesechanges. Non-metric multidimensional scaling wasused to generate graphic illustrations of the differencesbetween the 1980 and 2001 communities in each zone.RESULTSIn 2001, only 6 of the original 7 intertidal zones couldbe detected. The algal turf zone could no longer be dis-tinguished and thus could not be resampled. Despitethe exclusion of this zone from the following analyses,it should be noted that the disappearance of a zone initself represents a major change in community struc-ture. As the vertical heights of the respective zoneswere not recorded in 1980, it was not possible to deter-mine if this zone had become dominated by Mytilusgalloprovincialis, or if it had been incorporated into thezones previously occurring above or below it.The densities of the various mussel species in eachzone in 1980 and 2001 are shown in Fig. 1. In 1980,Choromytilus meridionalis occurred at relatively lowdensities of 2000 to 5000 m2 across most of the shore,except in the Granularis zone and in the algal-domi-nated zones higher on the shore. The smaller Aulacomyaater attained much higher densities, but was confined tothe lower intertidal zone. In 2001, Mytilus gallopro-vincialis was recorded in all sampling zones, with the ex-ception of the Porphyra zone, and dominated 4 out of 5 ofthese zones, reaching densities of 2000 to 10 000 m2. Inthe mid-shore Ulva and Granularis zones, the M. gallo-provincialis invasion increased the total number of mus-sels present, but did not replace those present prior to itsinvasion. This was, however, not the case in the Aula-comya and Choromytilus zones, where the invasionmarkedly decreased the densities of indigenous musselspecies, particularly A. ater. By 2001, there had been ashift in the distribution and abundance of mussels fromthe Aulacomya zone to higher up the shore, with allzones except the Aulacomya zone showing an increasein overall mussel density (Fig. 1).In the Porphyra and Choromytilus zones there were nosignificant differences in the overall densities of inverte-brates between 1980 and 2001 (Mann-Whitney U-tests,p < 0.05; Fig. 2). In the Ulva and Granularis zones therewere substantial and significant increases in inverte-brate density (p < 0.01), whereas decreases occurred inthe Gigartina and Aulacomya zones (respectively, p
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