Intertidal Boulder Field Research Articles

Patterns of spatial and temporal variation of macrofauna under boulders in a sheltered boulder field

Abstract Spatial and temporal patterns of abundance of animals and plants must be quantified before models to explain distributions can be developed. These patterns also provide essential data for measuring potential effects of environmental disturbances. Studies in many different habitats have shown that most organisms, particularly invertebrates, have highly variable and interactive patterns of abundance, with much variability at the smallest temporal and spatial scales. Intertidal boulder fields in New South Wales, Australia, support a diverse fauna, many species of which are relatively rare. These habitats are commonly found near rock‐platforms and in sheltered estuaries and are subjected to many human disturbances. Although there have been a few studies on the fauna in boulder fields, none has documented variability of the assemblage using multivariate and univariate techniques and most studies have not incorporated different spatial and temporal scales. This study quantifies spatial variation at three scales (metres, tens of metres alongshore and tens of metres upshore) and temporal variation at two scales (3 months and 2 years) of the assemblage of molluscs and echinoderms in a sheltered boulder field subjected to little natural or human disturbance. Multivariate analyses revealed that each site contained a distinct assemblage, mainly due to the relative abundances of a few species. Most species, those generally only found under boulders and common, widespread species, had considerable spatial variability in abundances, with more than 90% measured at the smallest scale, that is metre to metre within a site. Changes in abundances over 3 months or 2 years varied among species and sites in unpredictable ways. These data show that sampling designs to measure impacts on these fauna will need to be complex and must incorporate a number of spatial and temporal scales if they are to be able to detect impact against such a variable background.

Body size and spatial variation of community structure in subarctic intertidal boulder fields

The influence of body size of organisms examined on the spatial variation in the structure of mussel communities (evaluated as Shannon-Wiener diversity, taxonomic richness, and density) in boulder fields was evaluated for 4 spatial scales (<1 m, 10 m, 10 km, and 50 km). A number of factors (orientation of mussel communities on boulders, water flux and intertidal height) that potentially influence community structure at the smallest scale examined were also investigated. Overall, although body size usually influenced the significance of the different spatial scales examined in parametric analyses, the proportion of the total variance explained by each spatial scale was relatively constant for each size class and index of community structure calculated. Community structure was typically most variable at the smallest spatial scale examined, with progressively larger spatial scales explaining less and less of the total variation among samples. The multivariate ANOSIM tests, which did not examine variation at the smallest spatial scale, always found significant differences among communities at the 10 m and 50 km scales but, depending on the body size examined, did not always detect significant differences among communities at the 10 km scale. The mean Bray-Curtis similarity among samples decreased with decreasing body size and with increasing spatial scale. As the absolute difference in mean similarities among the different spatial scales examined decreased with increasing body size, we suggest that the ability to distinguish groups using ANOSIM should be greatest for the smallest macro-organisms studied. Variation at the smallest scale was not due to the compass orientation of the communities on boulders. Small-scale variation in water flux and intertidal height had a variable influence on the indices of community structure, and they were also a function of body size. Potential mechanisms responsible for the observed spatial variation are discussed.

Morphological variation along intertidal gradients in a population of black abalone Haliotis cracherodii Leach 1814

Temporal patterns of morphological variation in multivariate size and shape in a population of black abalone Haliotis cracherodii Leach 1814 in an intertidal boulder field in southern California was examined. From July 1983 to May 1985, 707 individuals were tagged and measured for six shell traits. The extent and pattern of morphological variation in allometric size, tremata (= respiratory pore) number, and relative tremata size was similar among surveys. Shore-level gradients in morphology varied temporally and involved independent variation in both shell size and shape. In the summer and fall, individuals with relatively fewer smaller tremata occurred higher on the shore than individuals with relatively more larger tremata. Seasonal variation in intertidal distribution and the incidence of Octopus-drilled abalone shells indicated that declines in abalone abundance were due to predation by Octopus in the summer and fall. Shore-level gradients were thus associated with opposing selective regimes, predation in the low intertidal and desiccation in the high intertidal. These results support the hypothesis that shore-level gradients in morphology result from movement and differential growth and are a dynamic response of individuals to temporal changes in physical stresses and predation which vary along intertidal gradients.

Early successional mechanisms in the rocky intertidal: the rôle of direct and indirect interactions

Indirect interactions during succession have received relatively little attention compared with direct interactions. The direct and indirect interactions among barnacles, algae, and herbivores in a rocky intertidal boulder field were evaluated. A factorial experimental design manipulating barnacle and limpet densities on cleared plots was employed. Barnacle density and percent cover of alage were monitored fortnightly for 5 months. The removal of a fine algal mat by herbivores led to higher recruitment of barnacles. The herbivores also prevented the establishment of algae in the absence of barnacles. The barnacles provided a refuge from herbivores for the algae by inhibiting the grazing activities of the limpets. In the absence of herbivores, the barnacles did not have a detectable effect on the algae which grew in thick mats. Through direct and indirect interactions, both barnacles and herbivores may have negatively affected their own performance. This suggests that indirect interactions may be important ecologically, but they need not always feed back positively for a species or group of species.

Marine invertebrates in an algal succession. III. Mechanisms linking habitat complexity with diversity

A diverse community of marine invertebrates is associated with suceessional algal mats in an intertidal boulder field in southern California. Species richness and abundance of invertebrates increased from early to middle suceessional stages, then remained similar into the later stage. These changes are paralleled by those in the physical structure (biomass and surface area) of the algae. Increased complexity of the algal physical structure probably influenced the associated invertebrate community through several mechanisms. (1) It decreased mortality caused by predation from fish and crabs. (2) It reduced the severity of physical stresses, primarily wave shock. (3) It increased the accumulation of those individuals and species transported passively by wave action. (4) In mobile species, selection of algal substrates was largely based on physical aspects of algal structure. While no species tested appeared to select particular algal species. all actively chose algal clumps of high biomass. All four mechanisms result in the accumulation of greater numbers of individuals in algal mats with higher biomass and surface area. Because a larger sample of the available pool of individuals is collected, more species are found in a given area of algal mat when the structure is more complex. The suceessional patterns of change in species richness of the invertebrate community seem to result from this sampling phenomenon. Laboratory experiments and correlations in abundance between pairs of species gave no evidence of any competitive interactions. Any apparent “structure” in this assemblage seems to be the result of the summation of individual responses to changing algal physical structure during succession.

Marine invertebrates in an algal succession. I. Variations in abundance and diversity with succession

This is a study of the effects of successional algal changes on the community of small invertebrate animals living in the algae at a marine intertidal boulder field. At each of 14 sampling times over a 2-yr period, replicate samples were taken from all stages of succession. By sampling all successional stages simultaneously at each date over an extended period, we could analyze spatial and temporal variations within and among stages. Species richness, diversity, and abundance of the whole invertebrate community increased from early to middle successional stages, while remaining similar from middle to late stages. Evenness remained about constant throughout succession. The increases occur primarily through the addition of species to the community. Some taxonomic groups in this study showed different patterns. Unlike patterns seen in birds, insects, and spiders associated with terrestrial plant succession, species replacements are rare, and there are few early and middle successional specialists. Trophic structure changed markedly during succession. Spatial variation did not change with succession but temporal variation declined. Many of these results are different from the predictions of equilibrium-based theory. Succession in this marine animal community is very different from that in terrestrial animal groups, probably because of the higher rate of exchange of matter in this open coast system.

Ecology of the limpet Nacella delesserti (Philippi) at Marion Island in the sub-Antarctic southern ocean

The ecology of Nacella delesserti (Philippi) is described from studies of its biomass, habitats, feeding, growth and its rôle as a central prey species at sub-Antarctic Marion Island. N. delesserti is the most abundant macro-invertebrate on the shores of Marion Island. Mean densities in the intertidal boulder fields and in the subtidal zone are 936 and 158·m −2, respectively, with an overall mean of 209·m −2 and peak densities of 3866 and 1700·m −2 in the intertidal and subtidal zones. In subtidal areas where it exceeds ≈500·m −2 N. delesserti forms stacks or towers throughout the year, up to 42 being found one on top of the other. Mean biomass was 86.7g·m −2 and production 96.5g·m −2·yr −1, both an order of magnitude higher than previously described for any Antarctic or sub-Antarctic limpet. The main diet of N. delesserti is algal spores and sporelings, although it also grazes on the fronds of attached macroalgae. Loose fronds of the kelp Durvillaea antarctica (Chamisso in Choris) Hariot and other algae are also eaten and are probably important in sustaining the dense limpet populations. Both diet and habitat are very broad and unspecialized, probably related to the fact that Nacella delesserti is the only large invertebrate grazer on the shores of the island. Limpets held in cages and fed kelp fronds provided data on grazing rates, growth, and assimilation efficiency. Assimilation efficiency ranged from 0.14 in small limpets to 0.06 in large animals. N. delesserti is estimated to consume 779 g dry algal material·m −2·yr −1, and plays a major rôle in controlling algal distribution and abundance. Growth of free-living animals was calculated from the Von Bertalanffy equation. N. delesserti reaches ≈35 mm in the first year of growth, attaining 60 mm after ≈5 yr and living for a maximum of 8 to 10 yr. In the nearshore region mortality is ≈75% per annum in the first 1.5 yr, dropping to 40% in animals of 1.5 to 5 yr old and 15% in older limpets. The rolling of boulders is a major but unquantified source of mortality in the intertidal and shallow inshore areas. Size-specific predation by the fish Notothenia coriiceps (Richardson), starfish Anasterias rupicola (Verrill) and kelp gull Larus dominicus (Lichtenstein) accounts for, respectively, 15 to 30, 40, and 20% mortality per annum, and Nacella delesserti constitutes 27, 90, and 45% of their respective diets. Limpets with shell lengths between 25 and 50 mm are particularly vulnerable, those 〉50 mm attaining a relative refuge from predation. As a herbivore and prey species, N. delesserti is clearly a key species in the nearshore and intertidal zone of Marion Island.

The impact of predation by Kelp GullsLarus dominicanus on the sub-Antarctic limpetNacella delesserti

The Southern Kelp GullLarus dominicanus derives about 45% of the mass of its marine diet from the limpetNacella delesserti at sub-Antarctic Marion Island. It selectively feeds on the largest limpets (35–65 mm) —a pattern most obvious in adult gulls — and eliminates about 19.6% of these limpets per year in the intertidal zone and in shallow waters, thus accounting for about 50% of the known annual mortality of limpets of this size. The gulls cannot forage for limpets in water more than 40 cm deep, and in the intertidal zone boulder-fields provide the limpets a refuge against the gulls. The gulls are also selective in feeding most often on limpets with pale shells. This alters the colour composition of limpet populations in habitats where predation is most intense.Larus dominicanus seldom feeds on limpets on the mainland of South Africa, and only on small limpets, probably because a greater range of food is available, but possibly also because the limpet species that occur on South African shores attach with forces that are three to six times those ofN. delesserti.

Effects of Winter Feeding on Limpets by Black Oystercatchers, Haematopus Bachmani

Black Oystercatchers fed on limpets in an intertidal boulder field at a rate of 2 limpets°m—2°yr—1. The birds exert a major influence on the microdistribution of these molluscs. Where accessible to oystercatcher predation, limpets are unlikely to survive over a winter once they reach moderate size (10—14 mm). Collisella pelta are exceptional in having refuge in large size. Oystercatchers thus produce and reinforce patchy limpet distributions. Adaptations of the limpets include crypsis and habitat selection of setting larvae. Spatial heterogeneity on local, regional, and broader geographic scales complicates these effects, especially since settlement behavior is constrained from making distinctions between all levels of heterogeneity, e.g., between cliffs where limpets will be relatively safe and boulders where they are at high risk. Locally the effects of bird predation are most notable for Notoacmea persona, but N. scutum, C. digitalis, and C. pelta are also consumed in significant numbers. The resulting microdistribution of limpets affects algal cover, which tends to be effectively removed by their grazing on steep and vertical walls, but which can maintain itself on large horizontal and gently sloping surfaces. Feeding in this locale was confined to the birds' nonbreeding season, fall and winter. By its end the limpets were reduced in number and the birds foraged elsewhere and on other prey. Limpet recruitment may be insufficient to allow year—round feeding in such areas by these birds, whose foraging may represent an adaptive seasonal adjustment to local conditions.

The responses of a community to disturbance: The importance of successional age and species' life histories

The responses of different successional stages of a temperate intertidal algal community to disturbance were investigated with a field experiment. The experiment was conducted in a low intertidal boulder field in southern California. In this habitat, the top surfaces of boulders are covered with algae. The composition of the assemblage on any particular boulder depends on the length of time since it was last overturned by wave action. When a boulder is overturned, the algae on what was formerly the top surface, are killed in whole or part by a combination of sea urchin grazing, anoxia, light levels below compensation intensity, and mechanical damage caused by crushing or abrasion. The length of time that a boulder remains overturned and the local abundance of sea urchins determines the intensity of the disturbance. When the boulder is righted, recolonization begins either by vegetative regrowth of survivors and/or by spores from outside.Using a three-factorial design, this natural form of disturbance was experimentally mimicked and the responses of three different successional stages of the algal community monitored. Boulders in each successional category were overturned for periods of 17, 27 and 54 days in areas with and without sea urchins, then righted. Two aspects of community response to perturbation were evaluated. These were (1) the assemblage's ability to resist change and (2) its ability, if altered, to adjust to some semblance of its original state. The resistance of each assemblage and of its component species to change was measured by the percent decrease in algal cover and by the decline in percent similarity of the community to its original composition. The recovery rate of each assemblage and of the cover lost by each species during the first 35 days following a disturbance was measured by the rate of increase in percent similarity to the original composition and the percent reestablishment of lost cover.The experimental evidence demonstrates that the successional stages of the producer level of an intertidal algal community differ significantly in their responses to disturbance. Early successional communities suffer more damage from a given level of perturbation but recover more quickly than either middle or late successional communities. Damage to any particular assemblage of algae, irrespective of successional age, is more extensive and recovery slower, the longer the boulder is overturned and/or sea urchins are present. Several thresholds in these responses were also identified.Differences in community responses and non-linearities in these responses were attributable to the life history characteristics of the component species rather than emergent properties of the assemblage. These characteristics have evolved in response to a variety of recurrent natural disturbances. This interpretation is in agreement with recent critical reevaluations of the trends and mechanisms of successional change in natural communities.

Disturbance in Marine Intertidal Boulder Fields: The Nonequilibrium Maintenance of Species Diversity

The effects of disturbance on local species diversity were investigated in an algaldominated intertidal boulder field in southern California. In this habitat, the major form of disturbance occurs when waves, generated by winter storms, overturn boulders. These natural physical disturbances open space, interrupt successional sequences, and determine local levels of species diversity. Because small boulders are more frequently overturned than larger ones, the plants and sessile animals of boulder fields are distributed in a patchwork of successional stages. Boulders which are subjected to intermediate disturbance frequencies are usually less dominated than those which are very frequently disturbed, and always less dominated than those which are seldon disturbed. In all seasons most small boulders have fewer species than those of intermediate size. Large boulders also usually have fewer species, except in the spring, when defoliation of the algal canopy during the previous winter has opened space for colonization. Species richness on these boulders declines during summer months, and is less than that on boulders of intermediate size in the fall. Small boulders, with a shorter disturbance interval, support only sparse early successional communities of the green alga, Ulva, and barnacles. Large, infrequently disturbed boulders are dominated by the late successional red alga, Gigartina canaliculata. Intermediate—sized boulders support the most diverse communities composed of Ulva, barnacles, several middle successional species of red algae, and Gigartina canaliculata. Comparison of the pattern of succession on experimentally stabilized boulders with that on unstable ones confirms that differences in the frequency of disturbances are responsible for the above patterns of species composition. The frequency of disturbance also determines the degree of between—boulder variation in species composition and diversity. Small boulders which are frequently overturned sample the available pool of spores and larvae more often. As a result, a greater number of different species occur as single dominants on these boulders. Boulders with an intermediate probability of being disturbed are most variable in species diversity. Assemblages on these boulders range from being dominated by a single species to being very diverse while most communities on boulders which are frequently or seldom disturbed are strongly dominated. Observations on the local densities of three species of middle successional red algae over two year—long periods indicate that most of these are variable in time. More local populations went extinct or became newly established on boulders than remained constant in size. These species persist globally in the boulder field mosaic by colonizing recent openings created by disturbances. These results lend support to a nonequilibrium view of community structure and, along with other studies suggest that disturbances which open space are necessary for the maintenance of diversity in most communities of sessile organisms.

Query: Testing the Effect of an Intervention in Sequential Ecological Data

My data are from an experiment that was conducted in an intertidal boulder field and designed to evaluate the effect of a starfish, through its activities as a predator, on other species in the community. I designated six 10 mi X 10in study areas in a fairly small and homogeneous region of the boulder field. Although these areas were chosen to be as similar as possible, they nevertheless exhibited some differences. I added large numbers of starfish to two areas, removed all the starfish I could find from two others, and left two unaltered to serve as controls. The experimental situations were maintained by periodically adding and removing more starfish. The areas were sampled over a period of about two years, with the manipulation of starfish abundance occurring after about one year of sampling. Unfortuantely, it was not possible to sample all the areas at the same time or at the same number of times. On each sampling date in an area, the abundances of the species expected to be affected were estimated by counting the number of each species on each of about 30 rocks. I have included a table showing the mean number per rock of Tegula eiseni, a snail that is the principal prey of the starfish, for all six areas over the course of the study. (Rock size did not differ between areas or between sampling dates within an area.) From the control data, there appears to have been a general downward trend in the abundance of this species, irrespective of the manipulation. I would like to be able to test for differences in trend before and after manipulation within an area, and for differences in trend after manipulation between experimental and control areas. Therefore, analysis of variance, any sort of before-and-after comparison of means, or nonparametric comparisons of central tendency, seem inappropriate. Comparisons of leastsquares regressions also seem a poor choice because the data are enumerative and are not normally distributed (the distribution of no/rock has considerable positive skew), and because some of the trends (Al and A2) are clearly not linear. I have been unsuccessful in finding a nonparametric test for differences between two trends, especially where the number of observations differ. Time-series analyses which evaluate intervention effects seem promising for comparing trends before and after manipulation within an area, but more than my modest mathematical ability is required to apply these methods. Also, I have found no timeseries technique for testing for differences in trend between two sets of data.