Inconsistency and variation in the development of rocky intertidal algal assemblages

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<ul><li><p>LJournal of Experimental Marine Biology and Ecology,224 (1998) 265289</p><p>Inconsistency and variation in the development of rockyintertidal algal assemblages</p><p>*M.G. Chapman , A.J. UnderwoodCentre for Research on the Ecological Impacts of Coastal Cities and the Institute of Marine Ecology,</p><p>Marine Ecology Laboratories A11, University of Sydney, NSW 2006, AustraliaReceived 11 March 1997; received in revised form 22 July 1997; accepted 2 August 1997</p><p>Abstract</p><p>Experimentally-cleared patches were used to test hypotheses about the relative importance ofbroad-scale biogeographic processes and small-scale historical processes in the development oflow-shore algal assemblages on wave-exposed rocky coasts of New South Wales (Australia). Inaddition, the applicability of generalizing from patterns of recruitment and development at onetime was tested by providing similar cleared patches in the algal assemblage at three-monthlyintervals and quantifying early development of the algal assemblage in these clearings fifteentimes over a period of four years.</p><p>The early stages of development of these assemblages differed significantly from shore to shoreand time to time and there was no evidence for common broad-scale patterns of recruitment, norfor any biogeographic trend. Nevertheless, these assemblages developed from a limited commonpool of species, some of which recruited fairly regularly on most shores and some of which onlyarrived sporadically on some shores. Despite different starting points, assemblages convergedtowards the surrounding assemblages on most shores, so that within less than 12 years, clearedareas resembled the surrounding assemblages. Although the specific changes leading to conver-gence differed from shore to shore, there was a general pattern of early colonizers, such asephemeral algae and sessile animals being gradually replaced by larger perennial algae.</p><p>These results demonstrate no simple seasonal nor clear-cut biogeographical patterns in thedevelopment of algal assemblages on these shores and indicated the relative importance of localinfluences. Results are discussed with respect to the need to do experiments at numerous places inorder to examine responses of assemblages to, or recovery from, environmental disturbances. 1998 Elsevier Science B.V.</p><p>Keywords: Algal assemblages; Low-shore species; Cleared patches; Biogeograghical patterns</p><p>*Corresponding author.</p><p>0022-0981/98/$19.00 1998 Elsevier Science B.V. All rights reserved.PII S0022-0981( 97 )00202-5</p></li><li><p>266 M.G. Chapman, A.J. Underwood / J. Exp. Mar. Biol. Ecol. 224 (1998) 265 289</p><p>1. Introduction</p><p>Species of algae characteristically vary in abundance from time to time and place toplace, leading to assemblages that are very variable in structure and composition(Dayton, 1971; Lubchenco, 1980; Jernakoff, 1985; Foster, 1990). In an analysis oflowshore algal assemblages on wave-exposed rocky shores in New South Wales,Underwood and Chapman (1997) found variation at scales of replicate quadrats (metresapart), sites on a shore (tens of metres apart) and among shores (hundreds of kilometresapart). Differences were mainly due to the relative importance of a few dominantspecies, rather than changes in composition of the assemblage from place to place. Therewas no evidence for broad-scale, biogeographic trends in the assemblages or in theirpatterns of variability, although there were persistent differences among assemblages ondifferent shores. This is surprizing because the study spanned 68 of latitude (from TuraHead at 368 529 south to Scotts Head at 308 409 south; see locations in Fig. 1) and hasbeen described as spaning cooltemperate to subtropical biogeographical regions (e.g.Dakin et al., 1948; Dakin, 1987). Similarly, there was no evidence for consistentlong-term (years) or seasonal patterns of abundance of the dominant species or changesin the algal assemblage (Underwood and Chapman, 1997). In each site, assemblagesvaried from one sampling period to the next, again due to sporadic variations inabundance of the dominant species.</p><p>In many other studies, temporal or spatial variation in the structure of intertidalassemblages has been attributed to responses to disturbances (e.g. Sousa, 1979a, 1980;Connell and Sousa, 1983; Underwood et al., 1983; Underwood and Denley, 1984).Therefore, processes of disturbance and recruitment are out of phase from place to placeso that there are always differences in occupancy of different parts of a shore and fromshore to shore.</p><p>This model suggests that any biogeographical patterns in the structure or compositionof lowshore algal assemblages on the coast of New South Wales may be masked bystochastic disturbances and recoveries. This study investigates this model by testinghypotheses about responses to disturbances along the coast. Availability of propagulesand therefore recruitment may be similar everywhere, but vary through time. Ifdisturbances are by chance in different parts of the coast, temporal and spatialdifferences will occur because different places will get different recruits at the differenttimes they are disturbed. From this, it is predicted that similar assemblages will developover a wide geographic scale if cleared areas were made available at the same time ineach place. In contrast, if the sources and varieties of propagules were local, one wouldexpect assemblages to differ from place to place because of local differences in rates ofrecruitment (Underwood and Denley, 1984).</p><p>Even if the early stages of development of an assemblage following disturbance aresimilar from place to place, because of similarity of recruits, locally different late stagesof assemblages may develop from a common early stage. Local stochastic events (localhistory) may subsequently disturb or alter the developing assemblages (Sousa et al.,1981).</p><p>The model that local versus broad-scale geography determines observed patterns ofspatial differences in assemblages from shore to shore was tested by clearing patches in</p></li><li><p>M.G. Chapman, A.J. Underwood / J. Exp. Mar. Biol. Ecol. 224 (1998) 265 289 267</p><p>Fig. 1. Map of the coast of New South Wales, Australia showing locations of study sites.</p><p>the algal assemblage on each shore and quantifying development of the assemblage inthese clearings over four years. If the source of propagules were widespread, one wouldpredict similar early stages of the assemblages from shore to shore (in this caseseparated by hundreds of kilometres). If processes are local, the early stages of theseassemblages should vary from shore to shore because they will depend on a local supplyof larvae.</p><p>Similarly, the role of geography and local history in the long-term development ofthese assemblages was tested. It was proposed that these assemblages would either (a)develop to resemble those in the immediate surrounding area, i.e. the development of thelowshore algal assemblage is primarily determined by the position of the shore along the</p></li><li><p>268 M.G. Chapman, A.J. Underwood / J. Exp. Mar. Biol. Ecol. 224 (1998) 265 289</p><p>coast irrespective of early structure, or (b) be determined primarily by early patterns ofrecruitment, i.e. history primarily determines subsequent pattern.</p><p>Finally, processes that influence structure of assemblages are frequently interpretedfrom single surveys or experiments done in one place at one time (Foster, 1990;Underwood and Petraitis, 1993). The applicability of generalizing from patterns ofrecruitment and development of early stages of these assemblages found at one time wastested by clearing patches in the algal assemblage at three-monthly intervals andquantifying the early development of the algal assemblage in these clearings during threemonths. This was done fifteen times over four years.</p><p>2. Materials and methods</p><p>2.1. Study sites</p><p>Five locations were chosen to span the length of the coast of New South Wales,approximately 800 km. Locations were chosen in areas about equally spaced along thecoast. Criteria for choosing the particular shore in each area were simply that it bewave-exposed, accessible and have assemblages at different levels that were typical ofother shores in the area. The locations were, from south to north, Tura Head, Flat RockIsland, the Cape Banks Scientific Marine Research Area (Cape Banks), Blueys Headand Scotts Head (Fig. 1). The lowshore assemblage dominated by macroscopic, foliosealgae was sampled in each of two sites on each shore four times per year (atapproximately three-monthly intervals) from 1980 to 1983. These sites were chosen atrandom along a shore and separated by 30100 m depending on the size of the shore.The only criterion for choosing a site was that it have a dense cover of foliose algae (atleast, at the first time of sampling). Sites were, however, chosen to represent the algalassemblage on the shore. Each lowshore site discussed here was as low on the shore as</p><p>2could safely be reached; sites were between 10.5 and 14 m , except at Blueys Head</p><p>2where the topography of the shore only allowed sites of 4.5 and 7.5 m (details arebelow).</p><p>Scotts Head consists of steeply sloping basalt rocks (slopes of 708 or more) with alowshore algal area dominated by Corallina spp., although this varied greatly from timeto time. Several foliose algae were common on the shore. The two sites wereapproximately 50 m apart and each was about 4.5 m along-shore and 3 m downshore.The rocks are generally very smooth and there was little cover of encrusting algae inmidshore areas.</p><p>Blueys Head lies approximately 200 km south of Scotts Head. Like Scotts Head,these shores were very steep (nearly vertical). The surfaces were, however, pitted andcorrugated by irregular erosion. Lowshore regions were subject to occasional periods ofscour by sand. The dominant algae were Corallina spp. and a mixture of red foliosealgae, including species of Gracilaria, Ceramium and Laurencia. The two sites sampledwere approximately 80 m apart; the first was 2.5 m along-shore by 3 m downshore, thesecond was 1.8 m by 2.5 m downshore. Scotts and Blueys Heads were extremelydifficult to sample because of steepness and extreme wave-action at many times.</p></li><li><p>M.G. Chapman, A.J. Underwood / J. Exp. Mar. Biol. Ecol. 224 (1998) 265 289 269</p><p>Cape Banks is approximately 230 km south of Blueys Head. The sandstone shores atCape Banks have been described in detail elsewhere (Underwood, 1975; Fairweatherand Underwood, 1991). The two sites in this study were on the exposed Point Shore,approximately 50 m apart, each extending 3 m downshore. The first was 4.7 m, thesecond 4 m along-shore. The lowshore algal assemblage was on a very gently slopingsurface, with extensive cover of foliose macro-algae, including about 60% of Ulvalactuca.</p><p>Flat Rock Island is approximately 230 km south of Cape Banks. The two sites at FlatRock Island were approximately 40 m apart, on a moderately-sloping (30408) part ofthe sandstone shore. Midshore regions persistently had large numbers of the barnacleTesseropora rosea and a variety of grazing molluscs. None of the grazers was abundantin the lowshore areas dominated by foliose algae, where Corallina spp. and Sargassumspp. were the dominant algae. The two sites extended 3 m downshore; one was 4 m, theother 3.5 m along-shore.</p><p>Tura Head is approximately 150 km south of Flat Rock Island. The sandstone shoresat Tura Head varied in shape. The lowshore regions were about 408 from horizontal.Although midshore areas were noticeably patchy in domination by the barnacles T. roseaand Catomerus polymerus, the lowshore areas were quite homogeneous in cover offoliose algae, primarily Corallina spp. and a variety of other algae, including Ptero-cladia lucida, Laurencia sp. and Ceramium spp. The two sites sampled were about 60 mapart; one was 4.5 m along-shore by 3 m downshore, the second was 4 m along-shore by3 m downshore.</p><p>2.2. Development of algal assemblages on each shore</p><p>At the first time of sampling, in one site on each shore, five randomly-chosen14-cm 3 16-cm quadrats in the algal assemblage were cleared by scrubbing and scrapingthe substratum to remove all foliose macro-algae, all sessile animals and most encrustingalgae. At each subsequent time, half of each of these quadrats was re-cleared after it wassampled, while the other half was left undisturbed. This method of clearing reproducesthe sort of patchy clearing found in lowshore areas of exposed shores in New SouthWales (unpub. data). Development of algae in cleared areas thus mimicked the naturalresponses to disturbances. Care was taken to ensure that clearing was equally effectiveon all shores. The sizes of clearings were a compromise between simulating sizes ofpatches found in nature and the need to get photographs of sufficient resolution withminimal time spent taking them because of the dangers of wave-action in these exposed,lowshore habitats. Clearings were of a size typical of many occurring naturally in theselowshore assemblages on exposed headlands.</p><p>At each sampling time, prior to reclearing half of each quadrat, the cleared quadratsand five randomly-placed quadrats in the surrounding assemblage were photographed(justification for the use of photographic quadrats is given in Underwood and Chapman,1997). The percentage covers of algae and sessile animals in each quadrat wereestimated from 100 points from each photograph. Species with a cover smaller than 1%were not recorded. It was not possible to distinguish between primary and secondary</p></li><li><p>270 M.G. Chapman, A.J. Underwood / J. Exp. Mar. Biol. Ecol. 224 (1998) 265 289</p><p>cover, unless primary cover was visible below the secondary cover; encrusting algaewere grouped into a single taxon.</p><p>The halves of the cleared quadrats that were left undisturbed after the first samplingperiod were used to test the hypotheses that early stages of development of theassemblage would be similar (regional processes determine them) or different (localprocesses determine them) and that the algal assemblages would (a) develop to besimilar to the surrounding assemblage or (b) develop differently from surroundingassemblages. These quadrats also provide estimates of the time taken for these algalassemblages to develop.</p><p>The halves of the quadrats that were repeatedly cleared were used to test thealternative predictions that early stages of the developing assemblages on each shore areconsistent from time to time, or vary stochastically from time to time and place to place.</p><p>2.3. Analyses of data</p><p>The early stages of development of the algal assemblages were compared amongshores at Time 2 (i.e. three months after the quadrats were first cleared) using ANOSIMon the BrayCurtis measures of dissimilarity calculated from 4th root transformed datafor percentage cover of all taxa. These comparisons tested the alternative hypotheses thatsimilar (or different) assemblages would develop on each shore.</p><p>To examine for convergence of the developing assemblages with the surroundingareas, Bray-Curtis dissimilarity measures were calculated for two set of quadrats foreach site and each time of...</p></li></ul>