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  • Demography, competitive interactions and grazing

    effects of intertidal limpets in southern New Zealand

    R.A. Dunmore*, D.R. Schiel

    Marine Ecology Research Group, Zoology Department, University of Canterbury,

    Private Bag 4800, Christchurch, New Zealand

    Received 14 September 2001; received in revised form 30 September 2002; accepted 27 November 2002

    Abstract

    Population dynamics and the effects of intraspecific competition on limpet growth and

    maintenance of bare patches were investigated for the intertidal limpet Cellana ornata (Dillwyn) at

    a boulder-dominated site and on a rocky platform near Kaikoura (South Island), New Zealand.

    Distribution and abundance patterns of C. ornata were described in relation to other biota and tidal

    height. C. ornata occurred almost exclusively in patches devoid of macroalgae, particularly in the

    mid-tidal zone. Both adult and juvenile limpets were most abundant on the tops of boulders, where

    their numbers were positively correlated with barnacle cover, which averaged 77%. The size

    structure and growth patterns of C. ornata were different between populations. Mark-recapture

    studies showed that the slopes of annual growth increments regressed on initial sizes were similar

    at both sites but that the annual increments on the platform were about 6 mm greater than on

    boulders. Growth virtually ceased at 27 mm for limpets at the barnacle-dominated boulder site and

    at 40 mm at the platform site. Recruiting cohorts had 20% survival on boulders and 37% on the

    platform during their first year. The largest size classes at both sites had around 57% annual

    survival. To test the effects of varying limpet densities on the growth and mortality of limpets and

    the maintenance of bare patches, densities of C. ornata were experimentally increased at both

    sites. Beyond a density of 4 per 0.25 m2, sizes and survival of limpets were reduced at both sites,

    but the effect was more pronounced at the boulder site. Limpets at the boulder site were more

    effective at maintaining bare space than those on the reef platform. Enclosing limpets in plots with

    and without barnacles showed that C. ornata and a co-occurring species (Cellana denticulata

    (Martyn)) grazed more effectively and had greater growth in cleared plots. Overall, there was

    0022-0981/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

    doi:10.1016/S0022-0981(02)00579-8

    * Corresponding author. Tel.: +64-3-3197700; fax: +64-3-3197701.

    E-mail addresses: zoolkk2@it.canterbury.ac.nz (R.A. Dunmore), d.schiel@zool.canterbury.ac.nz

    (D.R. Schiel).

    www.elsevier.com/locate/jembe

    Journal of Experimental Marine Biology and Ecology

    288 (2003) 1738

  • considerable variation in the demographics of C. ornata between populations driven by site-

    specific characteristics.

    D 2002 Elsevier Science B.V. All rights reserved.

    Keywords: Cellana ornata; Limpets; Barnacles; New Zealand; Rocky intertidal; Grazing

    1. Introduction

    Understanding the life histories of grazers is important to understanding the dynamic

    processes of intertidal communities. Numerous studies have shown that grazers can

    greatly affect the distribution and abundance patterns of algae and barnacles, and also

    provide and maintain much of the bare space in the intertidal zone (see reviews by

    Branch, 1981; Lubchenco and Gaines, 1981; Hawkins and Hartnoll, 1983; Creese, 1988;

    Underwood and Kennelly, 1990). Limpets are the dominant grazers on many temperate

    intertidal shores (reviews by Underwood, 1979; Branch, 1981) and their varying grazing

    intensities are a major component of models concerning community structure (Lub-

    chenco and Gaines, 1981; Farrell, 1991; Raffaelli and Hawkins, 1996). The demographic

    features of limpet populations can therefore have a direct bearing on community

    dynamics.

    Information on distribution, population structure, growth, mortality and reproduction

    of grazers is required to clarify their role in the dynamics of intertidal shores. Because

    these characteristics can vary spatially and temporally, comparisons at different shores

    through time allow a greater understanding of the demography of species and variability

    between populations. For example, the structure of limpet populations and associations

    with other species can vary considerably among localized sites (Johnson et al., 1997).

    Growth rates can be highly variable and are affected by the densities of conspecifics and

    other co-occurring invertebrates, the seasonal supply of food, shore height and sub-

    stratum complexity (Lewis and Bowman, 1975; Underwood, 1979; Branch, 1981;

    Creese, 1988). Growth rates and maximum sizes can be reduced where barnacles and

    other sessile organisms are abundant (Lewis and Bowman, 1975; Choat, 1977;

    Thompson, 1980). Reproduction and recruitment are usually highly seasonal but this

    varies considerably among species (Branch, 1981). Mortality rates also vary widely

    within and between species. They are related not only to species life histories (Choat

    and Black, 1979) but also to the characteristics of habitats, especially the presence of

    sessile organisms in conjunction with competitive interactions (Lewis and Bowman,

    1975; Branch, 1976; Choat, 1977; Underwood, 1978; Underwood and Jernakoff, 1981;

    Creese and Underwood, 1982; Marshall and Keough, 1994). The integration of life

    histories with habitat characteristics, therefore, determines demographic responses to the

    local environment.

    In New Zealand, there is a relatively diverse fauna of limpets, which are prominent on

    most rocky intertidal shores (Morton and Miller, 1973; Powell, 1979). However, the role

    they play in intertidal community structure and the demographic features of even the most

    common species have barely been investigated. As on rocky shores worldwide, limpets in

    R.A. Dunmore, D.R. Schiel / J. Exp. Mar. Biol. Ecol. 288 (2003) 173818

  • southern New Zealand are usually associated with algal films, crusts and filamentous algae,

    and there is considerable variation in species composition and abundances among sites

    (Raffaelli, 1979; Creese, 1988). The limpet fauna is especially rich in southern NewZealand,

    where several species of patellid limpets of the genus Cellana co-occur in the less-vegetated

    portions of the intertidal zone (Morton and Miller, 1973; Powell, 1979). The ornate limpet

    Cellana ornata is one of the most common and widely distributed limpets and is found along

    the length of the country. It is greatly abundant along the shores of the central South Island

    where it occurs on open reefs and boulder habitats and is the only abundant patellid limpet

    species commonly associated with barnacles (cf., Choat and Black, 1979).

    This work was prompted by the observations that C. ornata is the most abundant grazer

    in many intertidal areas, particularly in patches with few macroalgae, and that it tended to

    be smaller on boulders than on open reefs. We wished to understand its population

    dynamics and variability between sites, and how it affects wider community processes

    through maintenance of bare patches. To achieve this, the distribution, population

    structure, growth and mortality of C. ornata were described at two sites. We experimen-

    tally tested for variation in intraspecific competition and grazing ability of C. ornata by

    manipulating densities of limpets at the two sites. The effect of barnacle cover on limpets

    was also tested, using C. ornata and a co-occurring limpet, Cellana denticulata. It was

    hypothesized that the limpets were not able to feed effectively in barnacle-covered areas,

    resulting in decreased sizes.

    2. Methods

    2.1. Study sites

    Studies on the distribution, abundance, growth, mortality and conspecific grazing of C.

    ornata were done at two sites, First Bay and Blue Duck, situated near Kaikoura on the east

    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,

    where barnacle cover was dense. First Bay and Blue Duck are 20 km apart and differ in

    several characteristics. First Bay is a moderately exposed siltstone platform located on the

    Kaikoura Peninsula. It is generally protected from severe swells by rocky outcrops

    projecting from the peninsula. The platform is ca. 50 m from the low to high tide marks

    and ca. 70 m wide. Blue Duck is moderately exposed at most times but is subjected to

    strong wave action during southerly storms. It is composed mostly of large greywacke

    boulders, extends ca. 35 m from the low to high tide marks and is several hundred meters

    long. A major difference biologically between the sites was the presence of dense patches

    of barnacles on the boulder tops at the Blue Duck site, while few barnacles occurred at the

    First Bay reef site.

    2.2. Distribution and abundance

    To determine the abundance patterns seasonally of C. ornata, stratified random

    sampling was done in three tidal zones (0.20.7, 0.71.2 and 1.22.0 m above chart

    R.A. Dunmore, D.R. Schiel / J. Exp. Mar. Biol. Ecol. 288 (2003) 1738 19

  • datum), characterized by abrupt changes in habitat at similar tidal positions in both

    sites. At both sites, limpets were entirely confined to patches where rock surfaces were

    predominantly bare of foliose macroalgae, although barnacles, algal films and crust