Effects of density versus size on grazing by a key exploited herbivore

– Fishing decreases the biomass of target species via reduction in the numbers and/or size of individuals. in natural systems, the strength of biological interactions, including predator-prey dynamics, are often density or size-dependent. Hence, changes in the numbers or size of key taxa may be expected to influence biological interactions but their effects do not need to be identical. Here we compare the effects of biomass reduction in populations of the exploited limpet Patella candei. Biomass removal was experimentally achieved by either removing individuals (density reduction) or by replacing large by small individuals (size reduction), while controlling for total limpet biomass in a laboratory-based experiment. At the experiment’s termination, biomass reduction led to proportional changes in area grazed. However, there was no difference whether this was achieved via changes in density or in size. Furthermore, no discernible effects of treatments were evident on different components of the algal assemblage. A field survey also revealed that P. candei biomass explained a greater proportion in variation in the area free of algae than density alone. Our results suggest that loss of biomass in populations of P. candei has quantitatively and qualitatively similar effects on algal cover regardless of whether it is caused by an equivalent (biomass) reduction in the numbers or size of individuals. LiMPET Patella Candei ALGAE inTErTiDAL LABOrATOry BiOMASS Versus DEnSiTy ExPLOiTATiOn 36 G. M. MArTinS, A. C.L. PrESTES, A. i. nETO Vie milieu, 2013, 63 (1) Here we conducted a laboratory experiment to investigate how the biomass removal, either via changes in the size or numbers of individuals, influences the short-term grazing effect by P. candei. Experimental populations were used to examine the effect of grazing by limpets in populations that have sustained a reduction in size similar to that found on natural shores as a consequence of limpet exploitation. This was compared to the effect of populations that sustained a reduction in density, while controlling for changes in total limpet biomass. in addition, we surveyed the shore for patches free of algae within the turf matrix and examined the relationship between the patch area and both the density and biomass of limpets. MATERIALS AND METHODS laboratory experiment: Ceramic tiles were submersed in the sea for 2 weeks (September 2010) to allow for algal recruitment. The tiles were secured to a plastic structure with fastening cables and suspended underwater inside the marina of Ponta Delgada. Plates were then retrieved and kept in tanks with natural seawater and under a continuous source of light during two more weeks. Plates were rearranged periodically. This was intended to stimulate algal growth and achieve a complete and homogenous cover of experimental plates. The unglazed face of ceramic tiles proved a good surface for the colonization of algae and did not appear to affect the grazing activity by limpets, which successfully grazed on the grown algae (Martins personal observation). Substratum microtopography is known to influence grazing in marine gastropods (e.g. Wahl & Hoppe 2002). Ceramic tiles were preferred over chips of natural rocks because they provide a standardised uniform substratum over which limpets can graze and hence avoid the potential effects of microtopographic differences among treatments. At the start of the experiment, plates were randomly assigned to one of 4 treatments: ‘normal’ condition (composed of 2 large and 2 small limpets, hereafter referred to as 2L2S), density reduction (composed of 1 large and 1 small limpet, hereafter referred to as 1L1S), size reduction (composed of 4 small limpets, hereafter referred to as 4S) and a control (no limpets). Large (mean ± 1SE: 17.06 ± 0.02 mm) and small (11.09 ± 0.02 mm) animals were carefully collected (on boulders at Lagoa, São Miguel) one week prior to the start of the experiment and kept under laboratory conditions for acclimation (fed on fucus spiralis ad libidum). At the start of the experiment, animals were randomly distributed among replicates as required by the experimental design. The size of limpet was chosen as a compromise between the requirements of the experimental design (see below) and the necessity to keep the experiment as realistic as possible. The selected animal size categories were similar to the modal size of limpet populations in the most and least exploited islands of the Azores, respectively (see Martins et al. 2008). Thus, the size reduction treatment corresponds to a reduction in size comparable to that observed in the field as a consequence of current rates of limpet exploitation. Treatments were designed so that the two treatments experiencing biomass reduction (1L1S, 4S) both had, on average, half the biomass of the ‘normal’ treatment (2L2S), but equivalent biomasses between them (mean ± 1SE shell free dry mass (mg) 2L2S = 120.7 ± 10.7, 1L1S = 69.1 ± 4.9, 4S = 54.0 ± 5.7). A total of 4 replicates per treatment were available and these were distributed between two 50 l tanks with natural seawater and a 12:12 h photoperiod. All treatments were split evenly between tanks so that 2 replicates per treatment were deployed in each tank. Limpet transplantation is not without difficulties and is usually associated with relatively high mortality rates (Martins personal observation). in order to reduce the influence of constantly replacing dead animals, the experiment was restricted to a 2-week period. During this period of time food supply never became a limiting factor and differences in intraspecific competition among treatments was most likely low. in this experiment we thus examine the effects of a reduction in grazing biomass on algal assemblages beyond the effects of changes in population dynamics (e.g. competition, antagonistic effects). During the 2-week period of the experiment, tanks were maintained via daily water changes (~25 % of total volume) and a simulated ~6h low tide (plates with animals were emersed). Mortality was relatively low (7.5 %) and dead animals were immediately replaced. At termination, all animals were measured, oven-dried (48 h) and weighed. After removal of limpets, plates were photographed. The percentage cover of algae was estimated using 100 regularly distributed points in CPCev3.6 (kohler & Gill 2006). in addition, the area grazed by limpets was remarkably clear and this was measured using image J (Abràmoff et al. 2004). Data were analysed using a 2-way mixed model analysis of variance (AnOvA) with treatment as a fixed and orthogonal factor and tank as random and orthogonal factor. The Student-newmankeuls (Snk) test was used for a posteriori comparison of means for significant terms. field survey: We surveyed ~500 m stretch of the coast at Lagoa, São Miguel (37°44’n, 25°35’W), for patches free of algae embedded within a matrix of algal turfs (e.g. Gelidium microdon). Patches were located at mid shore levels where Patella candei is most abundant and were dominated by the barnacle Chthamalus stellatus (see supplementary material). in each of these patches, all P. candei were counted and measured to the nearest mm using a vernier calliper. Each patch was photographed with a reference scale and the area of the patch was later estimated using image J (Abramoff et al. 2004). Using the length-mass relationship for P. candei (Martins et al. 2008) we estimated the biomass of each individual and calculated the total limpet biomass in each patch. Standard regression analysis was used to examine the relationship between patch areas and the density and biomass of limpets. Furthermore, since limpet density and biomass were naturally correlated (see Fig. 2C), we also examined the relationship between limpet biomass and patch area after calculating the density-adjusted patch area. That is, using the residuals of the relationship between limDEnSiTy vS. SizE EFFECTS On GrAzinG 37 Vie milieu, 2013, 63 (1) pet density and patch area as the dependent variable against limpet biomass (see Fig. 2D).