Size Refuge Research Articles

Shifts in fish communities along the productivity gradient of temperate lakes—patterns and the importance of size‐structured interactions

Species biomass and size composition of fish faunas along a productivity gradient were studied in south Swedish lakes. Generally, with increasing productivity (measured as chlorophyll content), Salmoniformes were replaced by percids, which in turn were replaced by cyprinids, as suggested in previous studies. However, percids showed two peaks in biomass, one in medium productive lakes due to perch and one in highly productive lakes due to zander. Benthic piscivores were present in all lakes, whereas pelagic piscivores were absent in the least productive lakes. The proportion of piscivores in the total fish biomass showed a peak in medium productive lakes, largely reflecting the importance of piscivorous perch. The median size of the dominant cyprinids (roach) in the systems studied increased as the importance of piscivores increased, which was interpreted as a size refuge response to increased predation pressure. The same pattern was present for the dominant planktivore, vendace, in low productive systems. Although a predictable pattern of change in the fish fauna was found along the productivity gradient, other environmental factors such as structural complexity may be the ultimate cause of the observed succession pattern.

The eelgrass canopy an above-bottom refuge from benthic predators for juvenile bay scallops Argopecten irradians

Juvenile bay scallops Argopecten irradians commonly attach to shoots of eelgrass Zostera marina using byssal threads. Although this behavior has long been recognized, its adaptive value is poorly understood. This study examined (1) the size-specif~c nature of scallop attachment on eelgrass, and (2) the possible role of vertical attachment in providing refuge from benthic predators. Laboratory experiments using artificial eelgrass showed strong, inverse relationships between scallop size (over the range 6 to 20 mm) and several measures of attachment performance (percent attachment, rate of attachment, and height-above-bottom attained). Field experiments in which 10 to 15 mm scallops were tethered to natural eelgrass in Lake Montauk, Long Island, New York (USA), demonstrated a dramatic, highly significant enhancement of scallop survival at greater heights of attachment. Scallops tethered at 20 to 35 cm above bottom experienced > 59 O/ O survival over 4 d , compared to < l 1 O/O sunrival near the sediment surface. A similar pattern was observed in laboratory tethering experiments using transplanted natural eelgrass and 3 crab predators common in mid-Atlantic embayments: Carcinus maenas, Libinia dubia, and Dyspanopeus sayi. The refuge value of vertical attachment was found, however, to be less with D. sayi than with the other predators tested, since individuals of t h s species climbed eelgrass to feed on scallops in the upper canopy. These data demonstrate that vertical attachment on eelgrass functions as an effective predator-avoidance mechanism for juvenile bay scallops by placing them out of reach of many benthic predators. It has also been previously shown that scallop predatory risk decreases with scallop size, with a partial size refuge attained at about 40 mm. It is therefore suggested that scallops undergo an ontogenetic shift from a spatial to a size refuge from predation a s they move from the eelgrass canopy to the sediment surface over the course of their post-settlement life history. We hypothesize that this shift may result in a critical window of high predatory risk between ca 15 and 40 mm, when scallops are expected to be most susceptible to benthic predators.

Growth rate and substrate‐related mortality of a benthic brachiopod population

Collins, M. 1991 01 15: Growth rate and substrate-related mortality of a benthic brachiopod population. Lethaia, Vol. 24, pp. 1–11. Oslo. ISSN 0024–1164. Vital staining and careful examination of potential substrates enabled an accurate census of Terebratulina retusa to be made and prompted a study of their population dynamics. Seasonal samples of T. retusa from a deep water Modiolus-brachiopod assemblage were always dominated by small (>1 mm) individuals as growth rate of the post-larvae, estimated from changes in mean cohort length. was unexpectedly slow. Six months after settlement the animals had barely doubled in length, a rate of increase consistent with laboratory studies, but an order of magnitude less than conventional estimates. As the attainment of a size refuge is the only documented strategy by which articulate brachiopods counter overgrowth or disturbance this observation has profound implications for survival. Mortality of T. retusa in the Firth of Lorn, from different substrates, followed an unexpected pattern. Virtually the only substrate on which adult T. rehusa were recorded was the surface of M. modiolus shells, although juvenile T. retusa attached to this substrate suffered enhanced levels of mortality. Grazing pressures and spatial competition, believed to be reduced on complex surfaces, may account for the elevated mortality levels of M. modiolus-attached post-larvae prior to the apparent size refuge at a length of 2 mm. *Brachiopoda, Terebratulina, growth rate, disturbance, ecology, population structure.

The eelgrass canopy an above-bottom refuge from benthic predators for juvenile bay scallops Argopecten irradians

Juvenile bay scallops Argopecten irradians commonly attach to shoots of eelgrass Zostera marina using byssal threads. Although this behavior has long been recognized, its adaptive value is poorly understood. This study examined (1) the size-specif~c nature of scallop attachment on eelgrass, and (2) the possible role of vertical attachment in providing refuge from benthic predators. Laboratory experiments using artificial eelgrass showed strong, inverse relationships between scallop size (over the range 6 to 20 mm) and several measures of attachment performance (percent attachment, rate of attachment, and height-above-bottom attained). Field experiments in which 10 to 15 mm scallops were tethered to natural eelgrass in Lake Montauk, Long Island, New York (USA), demonstrated a dramatic, highly significant enhancement of scallop survival at greater heights of attachment. Scallops tethered at 20 to 35 cm above bottom experienced > 59 O/ O survival over 4 d , compared to < l 1 O/O sunrival near the sediment surface. A similar pattern was observed in laboratory tethering experiments using transplanted natural eelgrass and 3 crab predators common in mid-Atlantic embayments: Carcinus maenas, Libinia dubia, and Dyspanopeus sayi. The refuge value of vertical attachment was found, however, to be less with D. sayi than with the other predators tested, since individuals of t h s species climbed eelgrass to feed on scallops in the upper canopy. These data demonstrate that vertical attachment on eelgrass functions as an effective predator-avoidance mechanism for juvenile bay scallops by placing them out of reach of many benthic predators. It has also been previously shown that scallop predatory risk decreases with scallop size, with a partial size refuge attained at about 40 mm. It is therefore suggested that scallops undergo an ontogenetic shift from a spatial to a size refuge from predation a s they move from the eelgrass canopy to the sediment surface over the course of their post-settlement life history. We hypothesize that this shift may result in a critical window of high predatory risk between ca 15 and 40 mm, when scallops are expected to be most susceptible to benthic predators.

Functional responses of blue crabs Callinectes sapidus Rathbun feeding on juvenile oysters Crassostrea virginica (Gmelin): effects of predator sex and size, and prey size

Laboratory experiments assessed functional responses (prey consumption-predator −1 as a function of prey density) of male and female blue crabs Callinectes sapidus Rathbun to six densities of juvenile American oysters Crassostrea virginica (Gmelin) as a function of predator sex and size, and prey size. Crusher chela heights of large male and female crabs (135–165 mm carapace width; CW) were compared to identify sex-related differences in cheliped morphology that might account for the different functional responses. Initial identification of the form of the functional response (positively density-dependent, inversely densitydependent) was based on an ANOVA of the pattern of proportional mortality rates with prey density. The fit of several functional response models was compared statistically. Large female blue crabs exhibited a positively density-dependent functional response to large juvenile oysters (35 mm shell-height; SH), whereas large male crabs demonstrated an inversely density-dependent response. These different functional responses were due to specific features of both the predator and prey: (l) the higher crushing strength associated with a larger chela height to carapace width ratio of large male crabs compared to females, and (2) density-dependent prey features involving a reduction in the number of oysters attached to the outside edges of the oyster-shell cultch at low densities, which are easily crushed. Large crabs feeding on small oysters (25 mm SH) exhibited inversely density-dependent functional responses. Large juvenile oysters attained an absolute prey refuge in size from small blue crabs (60–85 mm CW) of both sexes. These results suggest that (1) sex-related differences in claw morphology can lead to different functional responses, and (2) juvenile oysters gradually attain a partial prey refuge at low densities (< 16.7 oysters · m −2) and large shell-heights (> 35 mm SH) prior to attainment of an absolute prey refuge from all crabs at ≈ 45–50 mm SH.

On the distribution, diet and possible impact of the invasive European shore crabCarcinus maenas(L.) along the South African coast

The crab Carcinus maenas (L.) has an endemic distribution along the European Atlantic coastline but has been accidentally introduced into intertidal habitats in several parts of the world. Its discovery at the Cape Peninsula in South Africa is recorded here. The crab is well established in Table Bay harbour and the population is at least seven years old. From here it has spread 117 km north and 10 km south. Populations on the open coast are more recently established. The crab feeds on a wide range of intertidal animals, particularly the isopod Paridotea ungulata, limpets and polychaetes, but it is too early to make quantitative assessments of its impact on local intertidal communities. Initial predation experiments suggest that the barnacle Notomegabalanus algicola and the whelks Oxystele variegata and O. tigrina have no refuge in size or zonation from the crab. O. sinensis, of similar size to O. tigrina, is less vulnerable by virtue of its stronger shell. Its critical size (above which it is too large to be attacked) is correlated with crab size. Larger species, such as the mussels Choromytilus meridionalis and Aulacomya ater and the periwinkle Turbo sarmaticus, clearly achieve a refuge in size, but predation on their recruits may influence their abundance and size structure. C. maenas rejected only the urchin Parechinus angulosus as a source of food. While it is conceivable that shifts in community structure, selection for thicker shells in molluscs and localized extinction of certain species may occur at high crab densities, it is predicted that C. maenas will only have a significant impact in areas protected from wave action.

Predator-prey interactions in opisthobranch gastropods: effects of prey body size and habitat complexity

This study examines size-specific predation by navanax Aglaja inerxnis (Gastropoda: Opisthobranchia), upon the sea hare Aplysia californica (hereafter Aplysia) (Gastropoda: Opisthobranchia). Aplysia reach a refuge in size from navanax predation at ca 10 cm in length, but very few individuals reach this size. Most Aplysia in the field were 5 3 cm long. Navanax diet in the field was disproportionately composed of larger Aplysia (> 2 cm long). Laboratory experiments with 0.5, 1.0 and 2.0 cm Aplysia and 2 levels of habitat complexity revealed that encounter rates, which rose w t h Aplysia size, were overwhelmingly important in determining size-selectivities. Attack probabilities (given an encounter) were always equal to 1, and capture success probabhties (gven an attack) rose slightly with Aplysia size. Increasing habitat complexity reduced encounter rates disproportionately for 0.5 cm Aplysia as compared with 2.0 cm Aplysia. The results suggest that the effects of body size and habitat complexity on predation may interact strongly in a way which depends upon the specific foraging biology of the species involved and upon the predator-prey ratios used.

Resistance to shell breaking in two intertidal snails

The ability of shells to withstand shell breaking forces has been examined in two intertidal prosobranchs, Nucella lapillus and Littorina littorea, using four methods: measuring shell strength on a compressive testing machine, measuring the shell to body mass ratio, measuring the shell thickness and measuring the ability of crabs to break shells in aquarium experiments. Nucella lapillus consistently showed a relationship between shell vulnerability and environmental variables: the shells were easier to break at sites where rock and boulder movement was the least. Although some between-site differences were found in L. littorea shells, these were less than in N. lapillus and did not relate to environment variables: the shells were easier to break at sites where exposure to wave action was the least. Although some between-site differences were found in L. littorea shells, these were less than in N. lapillus and did not relate to environmental factors. However, both species appear to grow into a size refuge in which they are secure from predation by shore crabs at the sites where these crabs are commonest.

Effects of Flow Regime and Cyprinid Predation on a Headwater Stream

We used descriptive and experimental approaches on Gould Creek, a first—order tributary of the Mississippi River near Lake Itasca, Minnesota to assess the influence of (1) flow regime on the colonization dynamics and abundance of invertebrates and cyprinids, and (2) cyprinid predation on invertebrates and fishes. Stream flow varied annually during the 3—yr (1984—1986) period. One dry year (1984), with few periods of elevated flow, was followed by two wet years (1985—1986), with prolonged elevated (nonscouring) discharge. The density of benthic and drifting invertebrates increased dramatically with elevated flow. Benthic riffle invertebrates in particular increased, from a maximum of 9000 individuals/m2 in 1984 to 91 000 individuals/m2 in 1985 and 51 000 individuals/m2 in 1986. Larval Hydropsychidae and Simuliidae were the primary groups increasing in abundance during elevated flow. To assess directly the influence of flow regime on benthic insect densities, flow was manipulated in six subsections of a riffle and colonization of natural rock substrates monitored. Total insect abundance was higher under elevated (nonscouring) vs. low flow within 6—8 d; after 24 d insect abundance was three times as high under elevated flow. Insect families responded differently to elevated flow, with the largest increase resulting from a pulse of colonization by larval Hydropsychidae in 6—8 d. Cyprinid density in Gould Creek also increased with elevated flow from 0.1—0.3 cyprinids/m2 in 1984 to 1—2 cyprinids/m2 in 1985 and 1986. The increased fish density was most pronounced during spawning periods in spring (May—June) and consisted primarily of older individuals. All cyprinids, except the creek chub (Semotilus atromaculatus), decreased in abundance 4—5 wk after spring colonization, regardless of flow conditions and invertebrate abundance. All cyprinids in Gould Creek selected pool habitats. Experiments in an artificial stream on Gould Creek indicated cyprinids influenced invertebrate abundance, but the effect of cyprinid predation was variable among habitats. Invertebrate abundance decreased most in structurally complex pools but exhibited little response to cyprinid predation in shallow riffle and raceway habitats. Because predation intensity varied among habitats, pool—dwelling invertebrates such as Chironomidae and Crustacea decreased more in the presence of cyprinid predation than riffle—dwelling Hydropsychidae and Simuliidae. However, if Simuliidae occurred in pool habitats, they were strongly selected by cyprinids, resulting in a significant depression in prey size in pools. Experiments in the artificial stream indicated creek chubs preyed on adult cyprinids, but larger species (adults 70—80 mm) were less susceptible to predation than smaller species (adults 50—60 mm). However, even taxa with small adult size were preyed on at a low rate, and all cyprinids strongly selected pools, with creek chubs having minimal effect on habitat use. These results suggest that (1) the hydrologic regime has broad and pronounced effects on the colonization dynamics and abundance of invertebrates and fishes in headwater streams, and (2) cyprinid predation has weaker but variable effects on the abundance of stream organisms. Predation intensity varies (a) over short temporal scales, because of the dynamic nature of flow regime and the rapid colonizing ability but short post—spawning persistence of cyprinids, (b) over small spatial scales, because of increased abundance of cyprinids in pool vs. riffle habitats, (c) between invertebrate and vertebrate trophic levels because the creek chub is a relatively ineffective piscivore, and (d) between small and large fish because many minnows have a size refuge from creek chubs.

Polinices Predation Patterns and Mercenaria Morphology Models

DeAngelis et al. (1985) have described a model of evolutionary response of bivalve prey to size-selective naticid snail predation. The model analyzed the allocation of bivalve energy among reproduction, overall growth in size, and supplementary growth in shell (p. 818). Using parameter values for Polinices duplicatus as predator and Mercenaria mercenaria as prey, model predicted three optimal strategies for bivalves faced with naticid predation: (1) delayed reproduction with energy diverted into rapid growth in order to reach a size refuge; (2) early reproduction, possibly with some extra shell thickness; and (3) greatly increased shell thickness for deterring predator attacks. This model and an earlier one (DeAngelis et al. 1984) are elaborations on a general qualitative model for bivalve prey (Seed and Brown 1978). [excerpt]

Ultilization of a seagrass meadow and tidal marsh creek by blue crabs Callinectes sapidus. I. Seasonal and annual variations in abundance with emphasis on post-settlement juveniles

Blue crabs Callinectes sapidus Rathbun were sampled in a lower Chesapeake Bay seagrass bed and adjacent tidal marsh creek from October 1982 through December 1986, using a drop net and suction sampler. Suction sampling was 88 % efficient and provided more accurate estimates of juvenile blue crab abundance, that were at least an order of magnitude greater than those obtained by conventional trawl gear For data analysis, C. sapidus were separated into 4 size classes by carapace width-54.25 mm (recently settled 1st and 2nd juveniles); 4.25 to 11.00 mm (3rd through 7th juvenile crabs); > 11.00 to 525.00 mm (8th through 12th juvenile crabs); and >25.00 mm (older juveniles and adults). Seasonal and annual cycles of C. sapidus abundance were observed in both habitats with annual differences most pronounced in the grassbed. Densities of C. sapidus were significantly greater in the grassbed in all but 2 of the 48 sampling dates. Settlement of the new year class appeared to be by megalopae and occurred from August through December A pulse of small C. sapidus C4.25 mm was observed in the grassbed from early to mid-September, whereas in the marsh creek this pulse occurred 2 to 4 wk later and consisted of 3rd stage and generally larger crabs (>4.25 mm). The small numbers of 1st and 2nd stage juvenile crabs ( 25 mm throughout the study period did not differ significantly among years (1983 to 1986) even though there were significant differences in the abundance of smaller sized individuals between 1983-84 and 1985-86. This suggests mortality within or emigration from grassbeds around this size. A possible ontogenetic shift in habitat use by crabs 11 to 25 mm may reflect a refuge in size from predation.

LABORATORY EVIDENCE FOR A SIZE REFUGE IN COMPETITIVE INTERACTIONS BETWEEN THE HYDROIDSHYDRACTINIA ECHINATA(FLEMMING) ANDPODOCORYNE CARNEA(SARS)

Size refugia from competition, whereby one organism may grow too large to subsequently be overgrown by a superior spatial competitor, have rarely been documented in marine benthic communities. Size-symmetrical and size-asymmetrical competitive interactions were established between colonies of two hermit crab-associated hydroids, Hydractinia echinata and Podocoryne carnea, to assess the outcome of competition for space between the two species and its possible size-dependence. In size-symmetrical interspecific contests, P. carnea overgrew and killed H. echinata in 100% of 74 observed encounters. In size-asymmetrical contests in which H. echinata was always the larger colony, P. carnea was able to overgrow H. echinata in only 55% of 76 contests. H. echinata reaches a size refuge from overgrowth by P. carnea, but this "safe" size depends on the position occupied by a colony of H. echinata on a substrate with respect to physical or biological barriers to growth.The outcome of intraspecific competition for space between P. carnea colonies depends on the relative growth rates of the competitors. In 23 intraspecific contests, the P. carnea colony with the highest rate of stolonal growth was always competitively dominant, and also overgrew H. echinata most rapidly in the size-symmetrical interspecific encounters. The ability of P. carnea to overgrow H. echinata in size-asymmetrical contests, however, did not depend on the growth rate of the P. carnea colony.Data on the distribution and abundance of these two species suggest that P. carnea recruits to hermit crab shells at a low frequency and is thus a rare member of the hermit crab epifaunal community. The observed differences in interspecific competitive ability may reflect asymmetry in the frequencies with which these species encounter one another. The probability that a colony of H. echinata will encounter P. carnea is low, hence there will be little selection for interspecific competitive ability in H. echinata. The probability that a colony of P. carnea will encounter the common H. echinata is high; P. carnea, therefore, should maintain a mechanism for recognizing and overgrowing this important spatial competitor.

Crab-resistant features of shells of burrowing bivalves: Decreasing vulnerability by increasing handling time

The vulnerability of burrowing bivalves to shell-breaking predation by crabs was found to be influenced strongly by shell features: size, shell thickness, degree of inflation, and the presence or absence of a gape. The relationship between the critical size of a bivalve (maximum size of vulnerability) and crab size was determined for four different morphotypes of bivalves. For the three bivalves where a “size refuge” was present, critical size increased with crab size. Nevertheless, when offered a choice crabs preferred clams well below the critical size and ate them in the order predicted by the critical-size experiment. Examination of the mechanics of shell crushing revealed how these shell features decreased vulnerability. Larger crabs could efficiently handle larger clams because both chela strength and degree of chela gape increased with crab size. Strain gauges attached to crab chelae showed that thick-shelled clams resisted a greater total number of force pulses than did thin-shelled clams of the same body weight. This may be related to the ability of thick-shelled clams to withstand greater loads than thin-shelled clams when loaded only once. This suggests that the reason for the increased resistance to crabs is prolongation of the shell-breaking time. Even though a large thick-shelled, tightly-closing, clam could eventually be opened, it will probably be rejected in favour of prey with shorter handling times.

Growth rate as a measure of food value in thaidid gastropods: Assumptions and implications for prey morphology and distribution

Rates of body growth were measured for three species of rocky intertidal gastropods: Thais (or Nucella) lamellosa (Gmelin), T. (or N.) canaliculata (Duclos), and T. (or N.) emarginata (Deshayes). Three size classes of each predator species were grown experimentally in cages at two tidal heights on four size classes of each prey species: Semibalanus cariosus (Pallas), Balanus glandula Darwin, and Mytilus edulis L. Growth was also assessed for Thais canaliculata feeding on Mytilus californianus Conrad, and Thais emarginata feeding on Chthamalus dalli Pilsbry. Rates of body growth varied as a function of predator size, prey size and prey species. With one exception (large Thais canaliculata) intermediate sized Balanus glandula promoted the most rapid growth for all sizes of all three species of Thais; thus these potentially competing predator species have the same highest ranked prey. Among prey promoting slower growth, those encountered commonly in the normal habitats of the predator promoted more rapid growth than those encountered rarely, suggesting that past evolutionary experience has influenced present food value (= growth potential) of prey. For some predators, the ranking of prey species changed with predator size; Chthamalus dalli and similar sized Balanus glandula promoted comparable growth in small Thais emarginata but Chthamalus dalli promoted much slower growth than similar sized Balanus glandula in larger Thais emarginata. Rank differences based on prey size in Balanus glandula became more pronounced and in some instances changed with increasing predator size; larger B. glandula promoted relatively faster growth for large snails than for small snails in all Thais species. Finally, growth rates were correlated with two important attributes of fitness in T. canaliculata and T. emarginata; in general, prey promoting more rapid growth also resulted in an earlier age of first reproduction for initially immature snails and higher rate of egg capsule production for mature individuals. Thus, these growth rates provide a basis from which to examine quantitatively patterns of prey selection from an energy- or growth-maximization perspective. These patterns of predator growth also permit inferences about the possible influence of predation on the evolution of prey morphology, life-history, and microhabitat distribution. Size refuges from Thais predation were confirmed for both Semibalanus cariosus and Mytilus californianus. Both Balanus glandula and Mytilus edulis, on the other hand, were vulnerable to predation by all sizes of Thais examined (> 10 mm) regardless of prey size; thus, neither of these species achieves a size refuge. In addition, on rocky shores, vertical distributions of the prey species reflect general preference patterns of their predators: higher value prey species, i.e. those with less well developed defensive morphologies ( Balanus glandula, Mytilus edulis), generally occur higher on the shore. This pattern would be expected if preferred prey are consumed first lower on the shore where they are more available.

Papers from the Echinoderm Conference. 11. A stable system of predation on a holothurian by four asteroids and their top predator

Seven species of asteroids feed on Cucumaria lubrica, but together they harvest only 3% of the population or 10% of the standing crop biomass per year at the locality of the study. The rates of predation by the asteroid Solaster dawsoni on the predators of C. lubrica are high enough and the rates of growth and successful recruitment into the area by the predators of C. lubrica are low enough to indicate that [he predators of C. lubrica are possibly kept low in abundance by the higher predator Solaster dawsoni. Solaster stimpsoni, the most abundant predator of C. lubrica in the area, has a behavioural escape mechanism which becomes increasingly effective as S. stimpsoni grows large and when it is on vertical rock surfaces. While S. dawsoni removes about 24–32% of the S. stimpsoni population each year, probably preventing a buildup in numbers, the refuge in size of a reproductive stock allows the persistence of the long-lived, slow-growing, S. stimpsoni. Dermasterias, a predator of C. lubrica with a refuge in size but with no behavioural escape mechanism to S. dawsoni, is 0.07 times as common as S. stimpsoni with a size-frequency distribution represented predominantly by large adults. Solaster endeca and Leptasterias, predators of C. lubrica with no known refuge to S. dawsoni, are 0.004 and 0.008 times as common as S. stimpsoni and may be considered strays from other habitats. No significant changes in abundance were observed in the 3 trophic levels of the association from 1965 to 1976: C. iubrica, 4400 m-2; S. stimpsoni, 0.5 m-2; S. dawsoni, 0.007 m-2. The stability of the system results from different control mechanisms and refuges at each trophic level.

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.

Competition for Space: Growth Rate, Reproductive Output, and Escape in Size

The coexistence of two species on a homogeneous substratum has been modeled for the situation where one is the fugitive (faster recruit, poorer competitor) and the other is the dominant (slower recruit, better competitor; Levins and Culver 1971; Armstrong 1976). The consequences of distinct size (or age) classes in competitive encounters have received little theoretical attention, and there are few examples of size-dependent competitive success for real organisms. A system of three differential equations has been used in this study to examine potential coexistence between one species with two size classes, and a second species not divided into classes. The first species is outcompeted by the second when small but cannot be outcompeted (overgrown) when large. It thus has a size refuge from competition, and the three equation system is stable for realistic values of recruitment and growth rates. An example of a size refuge in space competition was found in the rocky subtidal marine community in northern Mas...

Population structure and foraging biology of the predaceous chilean asteroid Meyenaster gelatinosus and the escape biology of its prey

Observations of the feeding biology of Meyenaster gelatinosus (Meyen) were made between Horcon and the southern Golfo de Penas, Chile. Of 811 sea stars examined, 436 were feeding on individuals representing 30 prey species. M. gelatinosus preys upon almost all the echinoderms and molluscs in its habitat, yet most of the prey species have extremely effective running escape behaviour in which they eventually release their attachment to the substratum, usually assuring that they will be swept to safety. Many of the molluscs exaggerate this by dorsoventral flattening of their mantles, so that they glide even farther. The echinoid Loxechinus albus has a very effective pedicillariae defense. Even at a distance, prey species usually discern foraging M. gelatinosus from non-foraging individuals, and on several occasions were observed touching M. gelatinosus which were eating conspecifics. Comparisons of the sizes of individual M. gelatinosus and their L. albus prey items showed no correlation; none of the prey species except possibly Concholepas choncholepas and M. gelatinosus itself has a refuge in size from attacking M. gelatinosus. The density of M. gelatinosus in 3 of 4 widely separated study areas where such data were collected was 0.04/m2. The mean radius of M. gelatinosus ranged from 150 to 210 mm in five study areas.

Size‐Limited Predation: An Observational and Experimental Approach with the Mytilus‐Pisaster Interaction

In many space—limited systems, for instance forests, coral reefs, and mussel beds, the main biological component of the spatial matrix is provided by large—bodied species. At some point during their growth these matrix species surpass the ability of any single individual of a predator species to consume them, although they still may be overwhelmed by concentrated group attacks (i.e., by aphids, fungi, internal parasites). These species thus surpass through continued growth a size threshold limiting their consumers, and thereby attain a refuge in size. The implications of these refuges are discussed, with special reference to a predatory starfish, Pisaster ochraces, and its primary prey, the mussel, Mytilus californianus. The mean wet weight of Pisaster varies between localities from ° 150 g to 2,640g. Long—term (12—yr) observation indicates little annual variation and generally no trends within areas, yet consistent differences between areas. Five transplant experiments demonstrate that Pisaster is an indeterminate grower, andis capable of adjusting its body size through growth to local conditions. Limited recovery of individually marked starfish suggest that exchange between areas is restricted. Observations on the natural diet of Pisaster show a pronounced maximum body size limit to the mussels that can be consumed by intertidal starfish of a given size. This limit was explored experimentally in two ways. First, Pisaster was allowed to reinvade an area from which it had been manually excluded for 5 yr. In this interval those mussels comprising the matrix of the clump had attained shell lengths &gt; 8—10 cm. and could not be consumed by Piaster weighing 203 g. Second, artificial clumps of known—size mussels were protected by plastic screening, allowed to attach, and then exposed to predation. Survival was significantly greater in those mussels expected to be too large to be eaten. The significance of size—limited predation is that prey and predator can coexist in very intimate proximity, that the surviving prey often attain large sizes and probably make a reproductive contribution greater than their abundance, and that these same prey diversify the spatial structure of the environment, usually by adding a vertical dimension not otherwise present.