Potential effect of enclosure size on field experiments with herbivorous intertidal gastropods

Many experimental studies on the ecology of benthic marine herbivores, particularly those on intraand inter-specific competition, have used enclosures to manipulate animal densities. We examined a potential problem in the interpretation of these experiments, i.e. the effects of different enclosure sizes on mortality and body weights of a rocky intertidal limpet fenced at natural densities. There was no mortality during the experiment but body weights of limpets were significantly different between fence sizes varying 4fold. Interpretations of enclosure experiments generally, and comparisons of the magnitude of treatment effects in competition experiments specifically, may be equivocal if enclosure sizes are d~fferent. Experimental studies on the ecology of intertidal gastropods have conlmonly used artificial enclosures (cages and/or fences) to exclude and enclose animals, thereby manipulating densities. In particular, intraand inter-specific competitive interactions between herbivorous gastropods have been assessed by such manipulations within enclosures (Branch 1984, Underwood 1985). In southeastern Australia, the common large limpet Cellana tramoserica has been used in numerous field experiments manipulating its densities on different shores and in a number of years; these experiments have consistently demonstrated that C. tramoserica is very susceptible to increased mortality and reduced body weights at high densities (Underwood 1978, Creese & Underwood 1982, Parry 1982, Fletcher 1984, Underwood 1984, Fletcher & Creese 1985). Underwood (1984) showed that reduced body weights of this species were directly correlated with available microalgae. Although the presence of competitive effects in each of these studies was unequivocal, comparisons of the magnitude of any treatment effects ( i .e . changes in mortality or body weight) between studies is always difficult because of seasonal, geographical and other differences. Such comparisons are now being attempted, however, particularly as the approach of meta-analysis, which includes identifying effect sizes in different experiments, becomes more generally accepted (Gurevitch et al. 1992). Difficulties in interpreting field experiments where densities of herbivorous intertidal gastropods are manipulated, due to cage or fence effects, have been considered. With Cellana tramoserjca, for example, Underwood (1978) found no difference in weight between limpets enclosed at average densities and non-enclosed limpets. Parry (1982) found C. tramoserica climbing on top of each other to form stacks of up to 10 limpets in cages with high densities, a phenomenon rarely seen in uncaged limpets. Underwood (1984) reported that chlorophyll a concentrations (as a measure of food availability) outside cages were within the range of values inside cages with control densities of C. tramoserica. He also pointed out that comparisons between inside and outside enclosures were confounded by the different and uncontrolled densities and mixtures of grazers outside his cages. This confounding will apply to any comparison of animals or food availability inside and outside enclosures as a check for 'caging' effects. In this study, we identify another factor that can influence the interpretation of enclosure experiments on herbivorous gastropods but which has not been previously considered the size of the experimental enclosures. For example, the size of the enclosures used in previous experiments on C. tramoserica varied considerably, with the maximum linear dimension ranging from 20 cm (Creese 0 Inter-Research 1993 Mar. Ecol. Prog. Ser. 98: 199-201, 1993 & Underwood 1982) to 40 cm (Parry 1982), although Fletcher (1984) used unfenced areas of 50 cm2. Materials and methods. We report here the results of a simple experiment to test for effects of enclosure size on mortality and body weights of Cellana tramoserica. The study site was the exposed rocky shore at Cheviot Beach, inside Point Nepean National Park, at the entrance to Port Phillip Bay, Victoria, Australia (Povey & Keough 1991). Plastic mesh fences (mesh size 1 cm) were used as enclosures. These fences had walls 10 cm high, with a horizontal flange folding outward from the base. Strips of medium density foam rubber (3 cm wide) were placed on these flanges, followed by a 2 cm wide strip of PVC. These strips were screwed into plastic plugs inserted into the rock, using stainless steel screws. The resulting compression of the foam-filled gaps between the fence material and the rock surface prevented movement of limpets in and out of fenced areas. Because the foam was qulte porous and highly compressed, there was no retention of water within the fences at low tide and the rock surfaces within enclosures appeared to dry at similar times as those of nearby unfenced rock. Limpets (shell length 20 to 25 cm) were enclosed at natural densities in fences of 3 different sizes (based on the 'standard' fence size used in other experiments with this species at this site; authors' unpubl. data): 1 limpet per 306 cm2 fence (quarter size), 4 limpets per 1225 cm2 fence (standard size) and 16 limpets per 4900 cm2 fence (4-times size). The limpets were collected, and then assigned randomly to fences. There were 5 replicate fences of each size. The experiment started on 11 February 1992, and missing limpets were replaced with marked individuals of similar size at fortnightly intervals. On 13 April 1992, all original limpets from the 2 smaller fence sizes, and a subsample of 5 original limpets from the each of the large fences, were collected, their shell lengths measured to the nearest mm, and their soft tissue dned at 50 "C and weighed. We estimated the amount of available food for the limpets by collecting 2 rock chips, each ca 1 cm2, from the rock surface within each fence, and 5 chips haphazardly from outside the fences. Total chlorophyll a was determined using the hot methanol extraction method recommended by Hill & Hawkins (1990). These values were converted to chlorophyll a concentrations by digitizing the rock chips to determine their area, and . using this area to standardize the chlorophyll values as l g Results and discussion. There was no mortality of limpets in any fence size during the experiment. Shell lengths and dry tissue weights of limpets from the different fence sizes were compared by analyses of variance; boxplots and plots of residuals revealed no major violations of the assumptions of normality and homoqc,