ALBERT

Notes: Abstract This paper considers the adaptive significance of two different reproductive methods in two co-occurring, competing sea stars. The smaller (3 to 8 g mean wet weight) Leptasterias hexactis broods relatively few, large young in the winter, while the large (300 to 650 g mean wet weight) Pisaster ochraceus broadcasts relatively many, small eggs each spring. L. hexactis matures at a small size (2 g wet weight) in about 2 years, and P. ochraceus matures at a larger size (70 to 90 g wet weight) in about 5 years (Menge, 1974). As in many broadcasting asteroids, gonad and storage organ indices of P. ochraceus are inversely related over time, and maximum storage-organ index correlates with the summer feeding maximum (Mauzey, 1966). In contrast, both organ indices of L. hexactis and feeding increase and are positively correlated until early autumn, when feeding activity begins to decline. At this time the male gonad index continues to rise, and the storage-organ index drops. In contrast, both organ indices of females rise. Spawning occurs from November to January. Thereafter storage-organ indices decline in females, presumably because females draw upon energy reserves while brooding; storage-organ indices rise in males, presumably because males do not brood and can feed if food is available. The primary cause for the differences between annual reproductive cycles of P. ochraceus and L. hexactis is suggested to be patterns of food availability for the released young (planktonic food for the broadcasted young of P. ochraceus and benthic prey for the brooded young of L. hexactis. Estimates of pre-maturity survival and post-maturity longevity indicate that the probability of survival per individual of young P. ochraceus is vastly lower than that of L. hexactis. However, once mature, P. ochraceus has a much longer expected lifespan. Brooding is suggested to be a coadaptive consequence of competition-induced small size. Assuming planktonic mortality rates in this environment are roughly constant across broadcasting species, I suggest that a small broadcasting species could not produce enough offspring in its expected lifespan to replace itself. This hypothesis is partly supported by some simple simulations. Broadcasting is suggested to permit rapid location and utilization of spatially and temporally unpredictable, but highly desirable, resources by allowing rapid and widespread dispersal. Brooders presumably cannot disperse rapidly and must rely on more reliable, but perhaps less desirable, resources. Factors affecting reproductive patterns in marine invertebrates include (1) food availability for both adults and offspring, (2) planktonic mortality rates, (3) interactions between species and latitudinal changes in these factors, and (4) various physical factors. This paper suggests that competition and predation can have an important effect on the evolution of reproductive methods, a possibility heretofore largely ignored. Although several similar examples of co-occurring species' pairs which differ in reproductive method and size are available, the role of adult interactions is unknown in these examples.

Notes: Summary Menge and Sutherland (1976) predicted that in physically benign habitats: (1) community structure will be most strongly affected be predation, (2) the effect of predation will increase with a decrease in trophic position in the food web, (3) trophically intermediate species will be influenced by both predation and competition, and (4) competition will occur among prey species which successfully escape consumers. These predictions were tested in a tropical rocky intertidal community on the Pacific coast of Panama. The most abundant mobile species included fishes and crabs, which occupied the top trophic level, and predaceous gastropods and herbivorous molluscs, which occupied intermediate trophic levels. The most abundant sessile organisms were encrusting algae, foliose algae, barnacles, and bivalves. Diets were broad and overlapping, and 30.3% of the consumers were omnivorous. Each consumer group had strong effects on prey occurring at lower trophic levels: (1) Fishes and crabs reduced the abundance of predaceous snails, herbivorous molluscs, foliose algae, and sessile invertebrates. (2) Predaceous gastropods reduced the abundance of herbivorous molluscs and sessile invertebrates. (3) Herbivorous molluscs reduced the abundance of foliose algae and young stages of sessile invertebrates, and altered relative abundances of the encrusting algae. The encrusting algae, although normally the dominant space occupiers, proved to be inferior competitors for space with other sessile organisms when consumers were experimentally excluded. However, the crusts escaped consumers by virtue of superior anti-herbivore defenses and competed for space despite intense grazing. Observations do not support the hypothesis that the trophically intermediate species compete. Hence, with the exception of this last observation, the predictions of the Menge and Sutherland model were supported. Although further work is needed to evaluate other predictions of the model in this community, evidence from this study joins an increasing body of knowledge supporting the model. Contradictory evidence also exists, however, indicating that aspects of the model require revision.

Notes: Summary Previous studies indicated that at Taboguilla Island (Gulf of Panama), persistence of many intertidal organisms depended on holes and crevices in the rock as refuges from both vertebrate (fishes) and invertebrate (crabs, gastropods, chitons) consumers. Here, we evaluate the influences of substratum heterogeneity and consumers on patterns of diversity of sessile organisms in this habitat. Local substratum topography is highly variable, ranging from smooth to irregular surfaces. Algal crusts typically dominate all low zone rock surfaces, and most other sessile spegies (invertebrates and foliose algae) occur in holes and crevices. Number (S) and diversity (H′) of sessile species is lower on homogeneous surfaces than on heterogeneous surfaces. Rate of increase in S with area sampled is positively correlated with substratum heterogeneity; number of species sampled per transect at a homogeneous site would be about 10 vs 30 to 60 on a heterogeneous site. Large fishes and crabs forage intensively over both substratum types, but cannot enter holes and crevices to eat prey. Gastropods, chitons, limpets, and small crabs feed on both substrata but vary in abundance from hole to hole. Prey mortality is thus intense and constant on open surfaces, but variable in space and time in holes and crevices. When consumers are excluded from the general rock surface, algal crusts are settled upon and overgrown by foliose algae, hydrozoans, and sessile invertebrates, particularly bivalves. Both S and H′ first increase, as sessile species invade and become more abundant, and then decrease as the rock oyster Chama echinata begins to outcompete other species and dominate primary space. Hence, consumers normally keep local diversity low by removing most sessile prey from open surfaces. In these experiments, a consumer pressure gradient was established by removing 0, 1, 2, 3, and all of 4 distinct groups of consumers. As predicted by the intermediate disturbance hypothesis, lowest diversity occurred at lowest (total exclusion) and highest consumer pressure (normal condition). Highest diversity occurred at intermediate consumer pressure. Unexplained variation in this relationship is probably due to quantitative and qualitative differences in consumer regime, variation among plots in substratum heterogeneity, and insufficient time for competitive dominance by Chama to be fully expressed. On a small (0.25 m2) spatial scale, consumers maintain low diversity by keeping prey scarce and causing local extinctions. On larger spatial scales, they may maintain and even produce high diversity through their interaction with substratum heterogeneity and possibly low dispersal rates of sessile species.

Notes: Summary Petraitis (1990) recently critized previous generalizations regarding the effects of predation in the New England rocky intertidal region (e.g., Menge 1976; Lubchenco and Menge 1978). Contrary to Lubchenco's and my conclusions, Petraitis concluded that (1) barnacles and not mussels are the favored prey of dogwhelks and (2) barnacles and not dogwhelks control mussel abundances in the mid and low rocky intertidal region. I provide evidence that these criticisms are unwarranted. First, Lubchenco and I never claimed that diet composition reflected prey preference. Moreover, predators can influence prey abundance without preferring the prey. Hence, claims regarding preferences have no bearing on our conclusions. Second, Petraitis' experiments do not invalidate Lubchenco's and my experimental results. Reanalyses of our experimental data support the earlier conclusion that at wave-sheltered sites, whelks reduce the abundance of mussels independently of barnacle abundances. Further, at all but one of Lubchenco's and my study sites, predator densities were higher than at Petraitis' site. Thus, the absence of a predator effect in Petraitis's study was most likely due to low predator density rather than a lack of generality of our earlier results. This reevaluation therefore suggests that within a broader conceptual framework, Petraitis' apparently divergent results are actually consistent with ours.

Notes: Summary The predation intensity exerted by populations of the gastropod Thais lapillus at different study areas in the rocky intertidal community of New England is unrelated to predator density. Specifically, very similar intensities are exerted by populations differing in density by at least an order of magnitude. Predation intensity is, in part, a joint function of individual rates of prey consumption and various environmental characteristics. Major factors potentially affecting the individual feeding rates of Thais are (1) prey abundance and productivity, (2) other predators, (3) canopy-forming algae, (4) wave shock, (5) desiccation and (6) snail phenotype and/or history. The effects of the first two of these factors seem unimportant. The effects of the latter 4 on prey consumption rates were studied by estimating field feeding rates of snails held in cages with prey in microhabitats which were characterized by one of two alternative states of each factor. For example, microhabitats could be exposed or protected, at higher or lower levels in the mid intertidal, or under a canopy or not. In addition, exposed-phenotype or protected-phenotype snails were used in each experiment. All of factors (3) to (6) had statistically significant effects except wave shock. The latter would probably also have had a significant effect if the experiments had been performed in the stormier part of the year as well as late summer. The results indicate that sparse populations of Thais can exert intense predation pressure on their prey if they are in protected sites covered with a dense canopy (i.e. in cool, moist habitats in calm waters). Areas with sparser canopy (i.e. greater desiccation stress) and more severe wave shock or both apparently reduce average feeding rates of snails. This appears to explain the paradoxical lack of correlation between predation intensity and snail density. An unexpected result with potentially major implications is the nonlinear response of Thais feeding rates to combinations of factors (3) to (6). Four-way analyses of variance on experiments at exposed and protected sites indicate that 7 of 14 1st-order interactions, 2 of 8 2nd-order interactions, and even 1 of 2 3rd-order interactions are statistically significant. These results suggest that individual predators cannot be assumed to be identical, and that socalled “higher order” interactions cannot be safely ignored in models of interacting multi-species systems. Hence, it appears that to obtain a thorough understanding of the organization of natural communities, both field and theoretical ecologists alike should begin to grapple with such complexities of nature rather than ignore them.

Notes: [Auszug] Seasonal development of dissolved-oxygen deficits (hypoxia) represents an acute system-level perturbation to ecological dynamics and fishery sustainability in coastal ecosystems around the globe. Whereas anthropogenic nutrient loading has increased the frequency and severity of hypoxia in ...

Notes: Summary Knowledge of predation intensity and how and why it varies among communities appears to be a key to understanding of community regulation. Along the rocky shores of New England, predation intensity in the mid intertidal zone appears to be low with exposure to severe wave shock, low desiccation stress, and a sparse cover of canopy algae, and high at areas protected from waves, with high desiccation potential and a dense cover of algae. As a result, predators at exposed headlands have no controlling influence on community structure, while at protected sites, they exert a strong and controlling effect on community structure. Experimental-observational studies of the effects of wave shock and desiccation on survival, foraging range and activity of the primary predator in this community (Thais lapillus) indicate that: (1) wave shock is a continuous and actual source of mortality at exposed sites but is relatively unimportant at protected sites; (2) mortality rates from desiccation at protected sites are potentially high and greater than at exposed sites; however, (3) actual desiccation stress is greatly reduced at protected sites by a dense algal canopy; (4) mortality from desiccation is greater in the higher mid intertidal than in the lower mid intertidal. Comparisons of activity patterns of Thais from April through November (these snails are usually active from May to early October) at an exposed and a protected site suggest snails at the former site restrict their active feeding to crevices while those at the latter site forage throughout the habitat. Field experiments support this hypothesis. Hence, differences in predator effectiveness at exposed and protected communities are probably due in part to the influence of wave shock. Exposed areas receive frequent severe wave shock in all seasons, even summer. Thus, the risk of being swept off the shore for snails foraging away from the shelter of a crevice at such areas is apparently great and exerts a strong selective force on foraging range. The importance of waves as a selective agent is further reinforced by the fact that crevices are nearly barren of prey, while just a few cm beyond the limits of the crevice, prey occur in great abundance. In contrast, at protected sites wave shock is never as severe as at exposed sites and is a relatively minor factor among several which might affect the foraging activity of a Thais. A major factor which varies among protected sites is the algal canopy. The influence of this factor is considered in a companion paper.

Notes: Summary The interaction between the sympatric, predaceous seastars, Asterias forbesi and A. vulgaris was studied for five years at eight study sites in northern New England. These species range in depth from the low intertidal to at least 50 m and cooccur over a broad geographic range from central Maine to Cape Hatteras. Both overlap greatly in times and intensity of feeding, body size, diet composition and size of prey consumed. Variations occur in these characteristics from site to site but are always positively correlated. Such similarity along resource dimensions is generally taken to indicate that species compete for resources. In this study, interspecific competition does not seem to occur. Though these seastars are generally smaller than their potential size, and food seems in short supply in some subhabitats, food seems unlimited in other subhabitats. Hence, exploitation competition probably occurs sporadically, not chronically, and is probably a weak selective agent. Laboratory experiments suggest that neither intra- nor interspecific aggression occurs between these seastars. Hence, interference competition seems non-existent in this case. Observations of massive mortality from disease and storms, large variations in seastar density, and a patchy food supply suggests that these populations are generally held below carrying capacity by a kaleidoscopic suite of selective agents. Under such conditions resource shortage would be unlikely to exert strong selective pressure. The high overlaps are thus most likely a reflection of the general absence of pressure to subdivide resources rather than an indication of severe competition. In studies of competition, ecological overlaps should be supplemented by other evidence, including experiments before they can be used as indications of competitive pressure.

Notes: Summary Predation intensity often increases along local gradients of decreasing environmental rigor. Assuming factors such as productivity and heterogeneity do not change along such gradients, potential factors responsible for this pattern include: (1) increased effectiveness per individual predator, (2) increased numbers of individual predators, (3) increased numbers of predator species of the same general morphological attributes, and (4) increased numbers of predator species of different general morphological attributes. I term these factors the components of predation intensity. In relatively protected low rocky intertidal regions of northern New England, community structure depends in part on the foraging activities of up to six species (three general types) of predators (Lubchenco and Menge 1978). These include three species of crab, two species of seastars, and one thaidid gastropod. This predator guild prevents mussel and barnacle populations from outcompeting Chondrus crispus. This red alga dominates space when predators are present, but is outcompeted by mussels when predators are excluded. Prey consumption rates (mg per predator individual per hr or mg g-1 hr-1) in field experiments indicate that the rank from most to least effective predator type is crabs, seastars, and the gastropod. Statistically significant variations occur between predator types (largest differences), species of a given type, and individuals of a species (smallest differences). Estimates of the relative contribution of each species to total predation intensity in the low zone at several sites indicate that each predator species is a major predator at one or more sites. Thus, if one predator species in this guild becomes scarce, the other predators may increase their effects and reduce variation in the total predation intensity exerted by the guild. Comparisons with other systems suggest that increased diversity of types of foraging characteristics in predatory guilds is an important component of increased predation intensity along gradients of decreased environmental rigor.