A functional group approach to the structure of algal-dominated communties.-Oikos 69: 476498. We suggest that relatively few species attributes are of overriding importance to the structure of benthic marine algal communities and that these are often shared among taxonomically distant species. Data from the western North Atlantic, eastern North Pacific and Caribbean suggest that patterns in algal biomass, diversity and dominance are strikingly convergent when examined at a functional group level relative to the productivity and herbivore-induced disturbance potentials of the environment. We present a simple graphical model that provides a way to predict algal community composition based on these two environmental axes. This predictability stems from algal functional groups having characteristic rates of mass-specific productivity, thallus longevity and canopy height that cause them to "behave" in similar ways. Further, herbivore-induced disturbances have functionally similar impacts on most morphologically and anatomically similar algae regardless of their taxonomic or geographic affinities. Strategies identified for marine algae parallel those of a terrestrial scheme with the addition of disturbance-tolerant plants that characteristically coexist with and even thrive under high levels of disturbance. Algal-dominated communities, when examined at the functional group level, appear to be much more temporally stable and predictable than when examined at the species level.

We compared the accuracy and repeatability of 2 common methods of estimating percentage covers of sessile organisms: visual estimates and random-point-quadrats (RPQ). Comparisons of estimates were made using both quadrats in the rocky intertidal zone and simulated quadrats drawn on a computer, where estimates could be compared with true, digitized percent cover values. In each case, visual estimates were found to be more repeatable (less within-and among-observer variation) and more accurate (closer to the true value as determined by digitizing) than the RPQ method. RPQs using 100 points were more accurate and less variable than those using 50 points, but were still less accurate (and much slower to carry out) than visual estimates. The RPQ method often missed rare species (<2 % cover) altogether, but when it 'hit' them, values were usually overestunated. Visual estimates also tended to overestimate percent covers of species (although less than the RPQ method), especially uncommon ones. Thus although the probabilistic RPQ method is supposedly more objective and is statistically valid, visual estimates may give a more accurate representation of relahve coverage of sesslle organisms, and can reduce overall s a m p h g error because they make increased sample sizes possible. Use of small subdvisions in quadrats, pre-field observer training, and a conscious effort to avoid bias are necessary to make the visual method valid and accurate.

Competitive superiority among encrusting species is established when the margin of one species consistently overgrows another. By measuring overgrowth patterns for the two most abundant tidepool species of crustose coralline algae (Lithophyllum impressum and Pseudolithophyllum whidbeyense) in the San Juan Islands of Washington State, we documented a reversal in competitive dominance that occurs at about the +1 m level; L. impressum wins in upper zones and P. whidbeyense in lower zones. Regardless of tidepool elevation or species, thicker crusts overgrow thinner ones and crusts of equal thickness are competitively equal. Both crust species are grazed by limpets (primarily Lottia pelta and Tectura scutum), which are the dominant herbivores in tidepools. Laboratory experiments show that limpet grazing can reduce crust thickness. Laboratory and field analyses of limpet grazing indicate that corallines in tidepools in the upper intertidal zone are subject to high frequency (bite rate per unit area) and low intensity (penetration depth per bite) limpet grazing, whereas those in low tidepools are subjected to opposite grazing characteristics. L. impressum is a thick crust, which has a multicellular covering over its meristem. This covering protects the growing portion of the plant from the frequent, low—intensity herbivory that occurs in the higher tidepools. In contrast, P. whidbeyense lacks a multilayered epithallus and is more susceptible to injury in high tidepools. In low pools, meristems of both crusts are injured. L. impressum heals these deep wounds by regenerating vertically, but the net result is a thinner plant that is easily overgrown. P. whidbeyense dominates the low zone because it is capable of much more rapid lateral growth including over its own deep wounds; the crust overgrows itself, thereby developing a thicker thallus and improving its competitive success over L. impressum. We suggest that a continuum of defensive and regenerative characteristics exists among organisms; the adaptive nature of these depends on the levels of production and the frequency and intensity of disturbance in their environment. It is evident that herbivory is important in this system, not by the traditional mechanism of releasing limiting resources (e.g., primary space), but by mediating competitive abilities through the relative susceptibility to, and recovery from, herbivory—induced disturbance.

The species composition of pools in the intertidal zone on the coast of Washington State varies greatly from pool to pool and from time to time. While assemblages change somewhat predictably from the low— to the high—intertidal zone (presumably owing to different stress tolerances of the species), the variance among pools at a given tidal height cannot be ascribed to such physical factors. Some pools at each height are dominated by one species that monopolizes space on the rock or in the water column and modifies the pool environment. Each dominant species, once established, can spread rapidly through a pool (either by vegetative growth or by enhanced recruitment of its conspecifics) and is thus potentially self—perpetuating. When abundant, most dominants appear to prevent potential competitors from settling and surviving by monopolization of resources, abrasion of the substratum, and/or collection of sediment. Six such dominants were identified for Washington tidepools: from low to high pools, these are (1) the surfgrass Phyllospadix scouleri, (2) articulated coralline algae, (3) the mussel Mytilus californianus (exposed shores), (4) the cloning anemone Anthopleura elegantissima (more protected shores), (5) the red alga Rhodomela larix, and (6) the green alga Cladophora sp. Colonial diatoms also appear capable of dominating low pools in the absence of wave disturbance. However, each dominant monopolizes only 20—50% of the pools at any height. Disturbances, defined here as a loss of biomass exceeding 10% cover of a sessile species within 6 mo and caused by extrinsic forces, were observed frequently in regularly censused tidepools. Disturbance agents included waves, excessive heat, wave—driven logs or rocks, and unusual influxes of predators and herbivores. Severe disturbances (those affecting a large proportion of the organisms in a pool) tended to occur in high pools in the summer (due to heat stress) and low pools in the winter (due to wave damage). Overall, a disturbance occurred in every pool studied an average of every 1.6 yr. About half of the 231 observed disturbances affected one of the six dominant species. The frequencies of these disturbances ranged from one every 2—5 yr, and recovery of the species to its original level required 3 mo to > 2 yr. Some species (e.g., Rhodomela) were disturbed frequently bu recovered quickly because of rapid vegetative growth. However if asexual propagation was not possible, such as when the entire population of a species was removed from a pool, the slowness and irregularity of recruitment of sexual propagules greatly impeded recovery. Experimental manipulations involving the total removal of dominant species from pools showed that such large disturbances often require > 3 yr for recovery. The irregularity of planktonic recruitment can be compounded by the presence of herbivores, which can remove most settling organisms from the substratum, or by the absence of other organisms that are necessary for the settlement of a dominant (e.g., seed—attachment sites for Phyllospadi...

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Resale or republication not permitted without written consent of the publisher Biomarker signatures in intertidal and subtidal macroalgae and seagrasses may vary considerably in space and time.

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