effects of chiton granosus (frembly, 1827) and other molluscan grazers on algal succession in wave...
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mid-intertidal algal succession and to isolate the effects of Chiton granosus, the most conspicuous member of the assemblage at thesetidal elevations. At sites heavily impacted by humans themolluscan grazer assemblage had strong negative effects on colonization and
Journal of Experimental Marine Biology andabundance of green algae such as ulvoids and Blidingia minima. In doing so, the grazer assemblage had a strong negative indirecteffect on the establishments of chironomid fly larvae, which were only observed on green algal mats and rarely on bare rock. Nosignificant effects were detected on epilithic microalgae, and effects on sessile invertebrates were highly variable over space and time.C. granosus also had significant negative effects on green algae but did not account for the total grazing pressure exerted by the guild.Limited foraging excursions (ca. 35 cm) from refuges and moderate site (crevice) fidelity in this species may contribute to thepatchiness in green algal distribution observed in the field. Nearly 13 months after rock surface were experimentally cleared,M. laminarioides appeared in all experimental plots, but increased over three times faster in enclosures containingC. granosus than inexclosures plots or controls, suggesting that moderate levels of herbivory could actually facilitate the establishment of this alga in thesuccession and that the green algal cover found in the absence of grazers may delay its establishment. 2007 Elsevier B.V. All rights reserved.
Keywords: Ecological redundancy; Epilithic algae; Foraging behaviour; Grazing; Refuges; Successiongrazers on algal succession in wave exposed mid-intertidalrocky shores of central Chile
Moiss A. Aguilera, Sergio A. Navarrete
Estacin Costera de Investigaciones Marinas, Las Cruces, and Center for Advanced Studies in Ecology and Biodiversity,Pontificia Universidad Catlica de Chile, Casilla 114-D, Santiago, Chile
Received 15 January 2007; received in revised form 28 April 2007; accepted 7 May 2007
Molluscan grazers can have important effects on the abundance, colonization rates, and successional pathways of algalassemblages and the entire intertidal community. In general, early successional algae are more readily consumed than corticated algaeand kelps, which usually get established later in the community succession. To generalize, however, the effect of different grazers onalgal assemblages must be examined on different coasts and under different scenarios. This information could help us understand themechanisms of ecosystem processes and situations in which general models do not apply. Along the coast of Chile, humans harvestlarge keyhole limpets, which seem to be the only invertebrate grazers capable of controlling the dominant corticated algaMazzaellalaminarioides, a canopy-forming species that can cover extensive areas of the mid intertidal zone. In this scenario, where large limpetsare harvested, the overall effects of the diverse molluscan assemblage of limpets, chitons and snails on algal succession and oncorticated algae in particular are not clear. We conducted a 26-month-long experiment to evaluate the effects of molluscan grazers onEffects of Chiton granosus (Frembly, 1827) and other molluscan Corresponding author. Tel.: +56 35 431670; fax:+56 35 431720.E-mail address: email@example.com (S.A. Navarrete).
0022-0981/$ - see front matter 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.jembe.2007.05.002Ecology 349 (2007) 8498www.elsevier.com/locate/jembeExperimental manipulations in different parts of theworld have demonstrated that invertebrate herbivores
mentacan determine the structure and diversity of intertidalalgal assemblages by controlling the abundance (bio-mass, cover) of established dominant macrophytes, bypreventing or severely decreasing colonization by algaeand sessile invertebrates, or by selectively removingedible algal species and, through this, altering succes-sional pathways (see Lubchenco and Gaines, 1981;Hawkins and Hartnoll, 1983; Santelices, 1990; Branchand Moreno, 1994; Fernndez et al., 2000; Duffy andHay, 2001 for reviews). Along most temperate coasts ofthe world, the intertidal grazer assemblage is numericallydominated by several species of mollusks and crusta-ceans (Lubchenco and Gaines, 1981; Brawley, 1992;Duffy and Hay, 2001). Several studies have shown thatmost of these grazers prefer early successional algae overthe more herbivore-resistant corticated algae and kelps(sensu Steneck and Dethier, 1994). These algal groupsusually become established later in succession. Excep-tions to these broad generalizations abound, however.Algal responses to grazers depend, among other things,on a) attributes of the grazer assemblage, such as speciescomposition, overall size and the range of feedingpossibilities, which is in part determined by the radularapparatus (Steneck and Watling, 1982), b) attributes ofthe algal species composition and their chemical ormechanical defenses (e.g. Branch, 1981; Gaines, 1985;Hay and Fenical, 1988), c) the rates of algal growth,which are usually determined by external environmentalfactors (Underwood and Jernakoff, 1981; Nielsen andNavarrete, 2004; Coleman et al., 2006; Wieters, 2005),and d) the relative effects of stress gradients on algalversus grazer species (Menge and Olson, 1990; Jenkinsand Hartnoll, 2001; Menge et al., 2002). Since humansare dramatically reducing the biomass of many grazersand top predator species, targeting mostly the largestspecies in these assemblages (Botsford et al., 1997;Jackson et al., 2001; Worm et al., 2002), there is anurgent need to understand how biodiversity and speciescomposition determines the dynamics of benthic com-munities (Coleman et al., 2006). Here, we evaluate theeffect of an entire molluscan grazer assemblage on algalcolonization and succession in an intertidal ecosystemstrongly influenced by humans on the central coast ofChile.
Several intertidal mollusks can control the abun-dance of corticated algae and kelps (Lubchenco, 1978;Moreno and Jaramillo, 1983; Hawkins et al., 1992;Bustamante et al., 1995), but many others can feedefficiently only on benthic microalgae. While they havethe capacity to diminish considerably or even totallyeliminate the thin algal films (diatoms, sporelings and
M.A. Aguilera, S.A. Navarrete / Journal of Expericyanobacteria), they have virtually no direct effects onestablished macroalgae (Castenholz, 1961; Nicotri,1977; Underwood, 1984; Hill and Hawkins, 1991;Hawkins et al., 1992). It has also been observed thatsome molluscan grazers can affect the settlement andrecolonization of sessile invertebrates (mostly barna-cles), by eating or exerting a bulldozing effect on larvaeor post-metamorphic stages (e.g. Lottia pelta and Tec-tura scutum Dayton, 1971; Berlow and Navarrete,1997; Scurria (=Collisella) digitalis Paine, 1981; Pa-tella vulgata Hill and Hawkins, 1991; Katharinatunicata Wootton et al., 1996; Collisella grata Chanand Williams, 2003). These differences are primarilydetermined by mechanical restrictions of the feedingapparatus and individual body size. Moreover, differentkinds of molluscan grazers, even those with similarfeeding apparatus and sizes can influence spatialpatterns of algal and sessile invertebrate abundance indifferent ways because of differences in foragingbehavior (see Creese and Underwood, 1982; Chelazziet al., 1987) and the spatial range of foraging excursions(Williams et al., 2000). For example, the mosaics ofsessile species in the intertidal seascape appear to begreatly modified by the distribution of grazers and theirforaging performances, which are directly related tospecific behavioral adaptations (eg. Mackay andUnderwood, 1977; Branch, 1981; Focardi and Chelazzi,1990; Chapman, 2000). In this context, homingbehavior, which is the repeated return of individualsto the same resting site after foraging excursions (seeStimson, 1970; Chelazzi et al., 1987; Focardi andChelazzi, 1990), can have important consequences foralgal community structure and spatial patterns ofvariance (Benedetti-Cecchi, 2000a,b; Coleman et al.,2004).
1.1. The system
A diverse array of herbivores inhabit the mid andupper intertidal zones of the wave-exposed coast ofcentral Chile, including several species of patellid,keyhole and pulmonate limpets, littorinid, turbinid andtrochid snails, as well as several chiton species(Santelices, 1990; Rivadeneira et al., 2002). A fewspecies of grapsid crabs also form part of thisheterogeneous grazing guild, but their abundance isgenerally low in central Chile. At high tide fish also formpart of this guild during some life stages and their effecton macroalgae can be very important, especially in semi-protected areas (Muoz and Ojeda, 1997). The coast isintensively harvested by humans, who target largeindividuals of the fissurellids, Fissurella crassa and
85l Marine Biology and Ecology 349 (2007) 8498F. limbata (Oliva and Castilla, 1986). Inside the marine
mentareserve of Las Cruces, in central Chile, where noharvesting takes place, Oliva and Castilla (1986)suggest that large keyhole limpets are responsible forthe low abundance of the corticated alga Mazzaellalaminarioides as well as green algae. This result agreeswell with results inside a marine reserve in southernChile involving another large keyhole limpet species(Jara and Moreno, 1984). On the other hand, at sites incentral Chile impacted by human harvesting, andtherefore with low abundances of large limpets, Nielsenand Navarrete (2004) observed that molluscan grazerscan exert strong control on the abundance of ephemeral(mostly green) algae that settled soon after the rocksurface is cleared. However, they also noted that thesegrazers had no significant effects on late successionalspecies, such as Mazzaella, regardless of variation innutrient loadings produced by coastal u