Demography, competitive interactions and grazing effects of intertidal limpets in southern New Zealand

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<ul><li><p>Demography, competitive interactions and grazing</p><p>effects of intertidal limpets in southern New Zealand</p><p>R.A. Dunmore*, D.R. Schiel</p><p>Marine Ecology Research Group, Zoology Department, University of Canterbury,</p><p>Private Bag 4800, Christchurch, New Zealand</p><p>Received 14 September 2001; received in revised form 30 September 2002; accepted 27 November 2002</p><p>Abstract</p><p>Population dynamics and the effects of intraspecific competition on limpet growth and</p><p>maintenance of bare patches were investigated for the intertidal limpet Cellana ornata (Dillwyn) at</p><p>a boulder-dominated site and on a rocky platform near Kaikoura (South Island), New Zealand.</p><p>Distribution and abundance patterns of C. ornata were described in relation to other biota and tidal</p><p>height. C. ornata occurred almost exclusively in patches devoid of macroalgae, particularly in the</p><p>mid-tidal zone. Both adult and juvenile limpets were most abundant on the tops of boulders, where</p><p>their numbers were positively correlated with barnacle cover, which averaged 77%. The size</p><p>structure and growth patterns of C. ornata were different between populations. Mark-recapture</p><p>studies showed that the slopes of annual growth increments regressed on initial sizes were similar</p><p>at both sites but that the annual increments on the platform were about 6 mm greater than on</p><p>boulders. Growth virtually ceased at 27 mm for limpets at the barnacle-dominated boulder site and</p><p>at 40 mm at the platform site. Recruiting cohorts had 20% survival on boulders and 37% on the</p><p>platform during their first year. The largest size classes at both sites had around 57% annual</p><p>survival. To test the effects of varying limpet densities on the growth and mortality of limpets and</p><p>the maintenance of bare patches, densities of C. ornata were experimentally increased at both</p><p>sites. Beyond a density of 4 per 0.25 m2, sizes and survival of limpets were reduced at both sites,</p><p>but the effect was more pronounced at the boulder site. Limpets at the boulder site were more</p><p>effective at maintaining bare space than those on the reef platform. Enclosing limpets in plots with</p><p>and without barnacles showed that C. ornata and a co-occurring species (Cellana denticulata</p><p>(Martyn)) grazed more effectively and had greater growth in cleared plots. Overall, there was</p><p>0022-0981/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.</p><p>doi:10.1016/S0022-0981(02)00579-8</p><p>* Corresponding author. Tel.: +64-3-3197700; fax: +64-3-3197701.</p><p>E-mail addresses: (R.A. Dunmore),</p><p>(D.R. Schiel).</p><p></p><p>Journal of Experimental Marine Biology and Ecology</p><p>288 (2003) 1738</p></li><li><p>considerable variation in the demographics of C. ornata between populations driven by site-</p><p>specific characteristics.</p><p>D 2002 Elsevier Science B.V. All rights reserved.</p><p>Keywords: Cellana ornata; Limpets; Barnacles; New Zealand; Rocky intertidal; Grazing</p><p>1. Introduction</p><p>Understanding the life histories of grazers is important to understanding the dynamic</p><p>processes of intertidal communities. Numerous studies have shown that grazers can</p><p>greatly affect the distribution and abundance patterns of algae and barnacles, and also</p><p>provide and maintain much of the bare space in the intertidal zone (see reviews by</p><p>Branch, 1981; Lubchenco and Gaines, 1981; Hawkins and Hartnoll, 1983; Creese, 1988;</p><p>Underwood and Kennelly, 1990). Limpets are the dominant grazers on many temperate</p><p>intertidal shores (reviews by Underwood, 1979; Branch, 1981) and their varying grazing</p><p>intensities are a major component of models concerning community structure (Lub-</p><p>chenco and Gaines, 1981; Farrell, 1991; Raffaelli and Hawkins, 1996). The demographic</p><p>features of limpet populations can therefore have a direct bearing on community</p><p>dynamics.</p><p>Information on distribution, population structure, growth, mortality and reproduction</p><p>of grazers is required to clarify their role in the dynamics of intertidal shores. Because</p><p>these characteristics can vary spatially and temporally, comparisons at different shores</p><p>through time allow a greater understanding of the demography of species and variability</p><p>between populations. For example, the structure of limpet populations and associations</p><p>with other species can vary considerably among localized sites (Johnson et al., 1997).</p><p>Growth rates can be highly variable and are affected by the densities of conspecifics and</p><p>other co-occurring invertebrates, the seasonal supply of food, shore height and sub-</p><p>stratum complexity (Lewis and Bowman, 1975; Underwood, 1979; Branch, 1981;</p><p>Creese, 1988). Growth rates and maximum sizes can be reduced where barnacles and</p><p>other sessile organisms are abundant (Lewis and Bowman, 1975; Choat, 1977;</p><p>Thompson, 1980). Reproduction and recruitment are usually highly seasonal but this</p><p>varies considerably among species (Branch, 1981). Mortality rates also vary widely</p><p>within and between species. They are related not only to species life histories (Choat</p><p>and Black, 1979) but also to the characteristics of habitats, especially the presence of</p><p>sessile organisms in conjunction with competitive interactions (Lewis and Bowman,</p><p>1975; Branch, 1976; Choat, 1977; Underwood, 1978; Underwood and Jernakoff, 1981;</p><p>Creese and Underwood, 1982; Marshall and Keough, 1994). The integration of life</p><p>histories with habitat characteristics, therefore, determines demographic responses to the</p><p>local environment.</p><p>In New Zealand, there is a relatively diverse fauna of limpets, which are prominent on</p><p>most rocky intertidal shores (Morton and Miller, 1973; Powell, 1979). However, the role</p><p>they play in intertidal community structure and the demographic features of even the most</p><p>common species have barely been investigated. As on rocky shores worldwide, limpets in</p><p>R.A. Dunmore, D.R. Schiel / J. Exp. Mar. Biol. Ecol. 288 (2003) 173818</p></li><li><p>southern New Zealand are usually associated with algal films, crusts and filamentous algae,</p><p>and there is considerable variation in species composition and abundances among sites</p><p>(Raffaelli, 1979; Creese, 1988). The limpet fauna is especially rich in southern NewZealand,</p><p>where several species of patellid limpets of the genus Cellana co-occur in the less-vegetated</p><p>portions of the intertidal zone (Morton and Miller, 1973; Powell, 1979). The ornate limpet</p><p>Cellana ornata is one of the most common and widely distributed limpets and is found along</p><p>the length of the country. It is greatly abundant along the shores of the central South Island</p><p>where it occurs on open reefs and boulder habitats and is the only abundant patellid limpet</p><p>species commonly associated with barnacles (cf., Choat and Black, 1979).</p><p>This work was prompted by the observations that C. ornata is the most abundant grazer</p><p>in many intertidal areas, particularly in patches with few macroalgae, and that it tended to</p><p>be smaller on boulders than on open reefs. We wished to understand its population</p><p>dynamics and variability between sites, and how it affects wider community processes</p><p>through maintenance of bare patches. To achieve this, the distribution, population</p><p>structure, growth and mortality of C. ornata were described at two sites. We experimen-</p><p>tally tested for variation in intraspecific competition and grazing ability of C. ornata by</p><p>manipulating densities of limpets at the two sites. The effect of barnacle cover on limpets</p><p>was also tested, using C. ornata and a co-occurring limpet, Cellana denticulata. It was</p><p>hypothesized that the limpets were not able to feed effectively in barnacle-covered areas,</p><p>resulting in decreased sizes.</p><p>2. Methods</p><p>2.1. Study sites</p><p>Studies on the distribution, abundance, growth, mortality and conspecific grazing of C.</p><p>ornata were done at two sites, First Bay and Blue Duck, situated near Kaikoura on the east</p><p>coast of the South Island, New Zealand (42j25VS, 173j42VE; Fig. 1). The barnacle/grazingexperiment involving both C. ornata and C. denticulata was done only at Blue Duck,</p><p>where barnacle cover was dense. First Bay and Blue Duck are 20 km apart and differ in</p><p>several characteristics. First Bay is a moderately exposed siltstone platform located on the</p><p>Kaikoura Peninsula. It is generally protected from severe swells by rocky outcrops</p><p>projecting from the peninsula. The platform is ca. 50 m from the low to high tide marks</p><p>and ca. 70 m wide. Blue Duck is moderately exposed at most times but is subjected to</p><p>strong wave action during southerly storms. It is composed mostly of large greywacke</p><p>boulders, extends ca. 35 m from the low to high tide marks and is several hundred meters</p><p>long. A major difference biologically between the sites was the presence of dense patches</p><p>of barnacles on the boulder tops at the Blue Duck site, while few barnacles occurred at the</p><p>First Bay reef site.</p><p>2.2. Distribution and abundance</p><p>To determine the abundance patterns seasonally of C. ornata, stratified random</p><p>sampling was done in three tidal zones (0.20.7, 0.71.2 and 1.22.0 m above chart</p><p>R.A. Dunmore, D.R. Schiel / J. Exp. Mar. Biol. Ecol. 288 (2003) 1738 19</p></li><li><p>datum), characterized by abrupt changes in habitat at similar tidal positions in both</p><p>sites. At both sites, limpets were entirely confined to patches where rock surfaces were</p><p>predominantly bare of foliose macroalgae, although barnacles, algal films and crusts</p><p>were common. Most of these patches looked superficially to be bare and all were &gt;1</p><p>m2 in area. Randomly placed 0.25 m2 quadrats were sampled within these patches in</p><p>winter (July 1995), spring (October 1995), summer (January 1996) and autumn (April</p><p>1996). At First Bay, 10 quadrats were sampled within each zone. At Blue Duck, the</p><p>tops and the sides of boulders (n = 10 quadrats each) were sampled separately within</p><p>each tidal zone. The sampling quadrat was divided into a 100-square grid, which was</p><p>used to estimate the percentage cover of algal species and barnacles. Juvenile (&lt; 10-</p><p>mm shell length; Dunmore and Schiel, 2000) and adult C. ornata were counted</p><p>separately to identify recruitment patterns. The other common limpet in the study sites,</p><p>C. denticulata, was also counted within quadrats. Analyses of variance were used to</p><p>test the various factors on limpet abundances. All data sets were first tested for</p><p>homogeneity of variances using Cochrans test, and data transformations were done if</p><p>necessary to stabilize variances. For ANOVAs of limpet abundances testing site</p><p>(n= 2), season (n = 4) and tidal height (n = 3), only data from the tops of boulders</p><p>at Blue Duck were used and compared to the flat reef at First Bay. Site was nested</p><p>within season for analysis, and both were treated as random factors. In this analysis,</p><p>therefore, season simply represents four different times of sampling spaced through</p><p>Fig. 1. Map of the Kaikoura Peninsula showing study sites and location within New Zealand.</p><p>R.A. Dunmore, D.R. Schiel / J. Exp. Mar. Biol. Ecol. 288 (2003) 173820</p></li><li><p>the year. Correlation analysis between C. ornata and the other main organisms was</p><p>done.</p><p>2.3. Growth and mortality</p><p>To examine the population size structure through time, C. ornata were measured bi-</p><p>monthly from February 1995 to February 1996 at both sites. A 0.25-m2 quadrat was used</p><p>to sample randomly and all limpets within each quadrat were measured to the nearest 0.5</p><p>mm using Vernier calipers. Quadrats were repeated across all shore heights until 500</p><p>limpets had been measured. Data were plotted as histograms to show the size structure of</p><p>each population through time and to identify the arrival of recruits.</p><p>To determine growth rates, 200 C. ornata were tagged in February 1995. Individuals</p><p>were chosen from the full size range across all shore heights at each site. A small plastic</p><p>numbered tag was glued with superglue to each shell and then covered with clear epoxy</p><p>resin for further protection. C. ornata is a homing species, and the location of each limpet</p><p>was mapped to facilitate later recovery. Tagged limpets were re-measured bi-monthly from</p><p>February 1995 to February 1996. Missing tags were replaced. Individuals that had tags</p><p>missing were easily identified from their mapped position and from a residual glue mark</p><p>left on the shell from the previous tag.</p><p>Annual growth rates were calculated using only those limpets that survived the whole</p><p>year. Size-specific survival was calculated for the year, based on the recovery of tagged</p><p>and mapped limpets.</p><p>2.4. Intraspecific effects</p><p>The null hypothesis that limpet density had no effect on growth and mortality of C.</p><p>ornata and on algal abundance was tested at First Bay and Blue Duck. The experiment</p><p>was initiated in October 1995 and continued for 7 months until May 1996. There were five</p><p>experimental treatments and three controls, each with four replicates. All plots were 0.25</p><p>m2 (50 50 cm). The perimeter of experimental plots was painted with a copper-basedpaint (5 cm wide) to include and exclude limpets. These plots were scraped to the bare</p><p>substratum using a chisel and wire brush.</p><p>The average densities of C. ornata in 20 random quadrats in midshore areas were</p><p>15.9F 0.36 per 0.25 m2 at Blue Duck and 7.6F 0.11 per 0.25 m2 at First Bay. Using thesedensities as a guide, treatments were set at 0, 4, 8, 16 and 24 limpets per 0.25 m2. Limpets</p><p>were collected from the same tidal height from areas adjacent to each experimental area</p><p>and were 1924 mm in size. Only limpets that were introduced to plots in October 1995</p><p>were used in analyses. Limpet densities were checked and maintained at least once every 3</p><p>weeks.</p><p>Three control treatments were used. Unmanipulated controls were untouched and were</p><p>indicative of the natural state of the substratum during the experiment. Scraped control</p><p>plots were cleared in the same way as the experimental plots, allowing the effects of the</p><p>scraping on subsequent recruitment to be identified. These two control treatments were not</p><p>bordered by paint, but had their corners marked with plastic tags nailed to the substrate.</p><p>The third controls (paint controls) were scraped and were bordered with dashed paint,</p><p>R.A. Dunmore, D.R. Schiel / J. Exp. Mar. Biol. Ecol. 288 (2003) 1738 21</p></li><li><p>allowing the movement of limpets in and out of the plots while the plot was still subjected</p><p>to any effects of the paint. Algal coverage and chlorophyll concentration were measured</p><p>each month as described below.</p><p>The percentage cover of algae and bare space was estimated monthly using a 0.25-m2</p><p>quadrat sub-divided into 100 squares. Analysis of variance (ANOVA) was used to test for</p><p>treatment effects 2, 4 and 7 months after the initiation of the experiment. Chlorophyll a</p><p>was analysed as a measure of microalgal concentration on what appeared to be bare rock</p><p>(Hill and Hawkins, 1990). Three replicate 1-cm2 rock scrapings were taken at random</p><p>from each plot monthly using a flat-edged chisel. The scrapings were ground with 10 ml of</p><p>90% acetone, and left in a darkened fridge for 24 h for the chlorophyll to be extracted. The</p><p>mixture was then put through a glass filter and the absorption level read using a Kontron</p><p>Uvikon Spectrophotometer set at 665- and 750-nm wavelen...</p></li></ul>


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