The influence of desiccation and predation on vertical size gradients in populations of the gastropod Oxystele variegata (Anton) on an exposed rocky shore

Summary Oxystele variegata (Anton.) exhibits a vertical size gradient contrary to the model proposed by Vermeij (1972) for low/mid intertidal species, as shell size increases in an upshore direction. Settlement occurs in the lowest zones and juveniles are restricted to the lower shore by conditions of desiccation higher up the beach. Juveniles suffer rapid water loss due to a relatively large opercular surface area and circumference and have a much lower resistance to water loss than adults. This leads to high mortality under conditions of low humidities, and juveniles caged at the top of the balanoid zone, where adults normally occur, die within a few days. As animals increase in size their resistance to desiccation rises allowing them to migrate upshore. This is a response to high rates of predation by the whelk Burnupena delalandii in the lower balanoid zone. Predation is so intense as to override the advantages of higher food availability which lead to a greater body weight for adults protected by cages on the lower shore.     

The adaptive significance of aggregation behaviour in the dogwhelk Nucella lapillus (L.)

Summary Three kinds of aggregation behaviour were observed in an exposed shore population of Nucella lapillus. Aggregations on the open rock surface during the summer protected dogwhelks from water movement, and were not found on shores where the topography conferred protection. Feeding occurred mainly within these aggregations, probably because individuals experienced less disturbance there than when isolated. Physical contact was important in holding animals together.Winter and pre-breeding aggregations were usually found in clefts or pools. All age groups formed winter aggregations, but those of immature dogwhelks were not as permanent as those of adults, and the latter merged with the prebreeding aggregations. Winter aggregations protected dogwhelks from dislodgment when their ability to regain a foothold was reduced by low temperatures, while pre-breeding aggregations brought the sexes together for fertilization, but the permanence of adult winter aggregations suggested that reproductive activity may have been occurring within them.     

Enhanced condition in dogwhelks, Nucella lapillus (L.) living under mussel hummocks

Dogwhelks, Nucella lapillus eat both barnacles and mussels, Mytilus edulis. However, mussels are dangerous prey because they can snare and immobilize dogwhelks with their byssi. Despite this danger, adult dogwhelks are common inside mussel hummocks or stacks found on mature mussel beds. The whelks live in debris-filled chambers within the hummocks, unsnared by byssi. Evidence is presented to show that this is a highly favourable microhabitat for dogwhelks. Condition index (CI) measurements showed that mean CI for “hummock” dogwhelks was 20.99 in early spring, 29.65 in midsummer and 34.86 in early winter. In comparison, CI values for dogwhelks from nearby barnacle zones were 20.13, 25.03 and 24.66 respectively. The summer and winter CI values of “hummock” dogwhelks were enhanced to a highly significant extent. Data are also presented to show that shore elevation or dogwhelk shell height (over the size range of animals studied) had no confounding effect on this result. Predicted flesh and shell mass values for a standard 25 mm high whelk showed that occupation of hummocks was associated with a thin-walled shell morphology, and that the shell mass became proportionately lower in summer and early winter. A feeding experiment showed that CI rises slowly, even in dogwhelks fed to satiation on mussels. Dogwhelks were taken from the barnacle zone in March 1997 and either fed to satiation on Mytilus or starved, whilst being held in seawater at ambient seawater temperature. The CI of fed dogwhelks rose from 19.7 to 27.4 in 3 months, while that of starved Nucella fell from 19.7 to 16.3. In the meantime, the CI of dogwhelks in situ in the barnacle zone had risen from 19.7 to 23.8. Hence, 3 months' satiation ration had led to a nett 3.6 unit rise in CI; starvation to a nett 7.5 unit fall. All of the changes in CI were due to changes in flesh mass; shell mass was stable. Since “hummock” dogwhelks collected in early winter have a CI value about 10.2 units greater than barnacle zone dogwhelks, this indicates that “hummock” dogwhelks are long-term inhabitants of this microhabitat.     

PHYSIOLOGICAL STRESS AND COLOR POLYMORPHISM IN THE INTERTIDAL SNAIL NUCELLA LAPILLUS

The intertidal snail Nucella lapillus exhibits considerable variation in shell color both within and between populations differentially exposed to wave action. Populations from high-wave-energy shores tended to be highly polymorphic and were dominated by pigmented morphs (especially brown), while those at more sheltered locations exhibited less polymorphism and were predominantly white. Field and laboratory experiments were conducted to determine the role of physiological stress and selective predation in maintaining the observed distribution of color morphs. The results demonstrated that 1) physiological stress from high temperature and desiccation during periods of tidal emersion was greater on protected shores, 2) under similar natural conditions, brown morphs heated up faster, attained higher temperatures, desiccated more rapidly, and suffered greater mortality than did white morphs, and 3) when pairs of brown and white morphs were tethered intertidally there was virtually no mortality of either morph on the exposed shore or in shaded microhabitats on the protected shore, but brown morphs suffered much greater mortality in sunny microhabitats on the protected shore. These findings demonstrate that the interpopulation variation in shell color of N. lapillus is in part a response to a selective gradient in physiological stress. Selection for crypsis by visually hunting predators did not appear to play a prominent role; however, only adults were considered, and the predation experiments were conducted in the fall before shorebirds that prey on whelks had arrived from their summer feeding grounds. Further experimentation to quantify the effects of visual predators such as birds and fish, particularly on juvenile snails, is necessary to assess adequately the importance of predation.     

Density-dependent mortality of wintering Dunlins Calidris alpina through predation by Eurasian Sparrowhawks Accipiter nisus

Density-dependent mortality in wintering shorebirds is critical to shorebird population dynamics, but is difficult to study and demonstrate. Most winter mortality of small shorebirds probably results from predation by raptors. Over 13 winters, I studied Eurasian Sparrowhawk Accipiter nisus predation on wintering Dunlins Calidris alpina at a rocky shore in south-east Scotland, a site apparently of marginal suitability for wintering Dunlins. Most Dunlins on the study site were juveniles (first winter birds), and winter juvenile mortality rate due to Sparrowhawk predation was strongly and positively related to Dunlin density (maximum winter count). There was no indication of any marked effect of weather on Dunlin winter mortality rate, except that mortality was slightly greater in winters with lower temperatures. Dunlin mortality was unrelated to the mortality through Sparrowhawk predation of a preferred prey, the Redshank Tringa totanus , or to sighting rates of hawks, suggesting that increased hunting of Redshanks by hawks or a numerical response by hawks were not involved in density-dependent mortality of Dunlins.     

White-Tailed Deer,Weather and Predation: a New Understanding of Winter Severity for Predicting Deer Mortality

Variation in white-tailed deer (Odocoileus virginianus) mortality during winter affects population growth in cold climates. Across the northern extent of their range, mortality increases with colder temperatures and snow. Few studies have examined the relationships between winter conditions and deer mortality, and no studies have concurrently studied this relationship for different ages of deer across multiple years and landscapes. We used recently developed cause-specific mortality models to evaluate temporal and age-class variation in deer mortality in farmland areas and compared to published results from forest areas in Wisconsin, USA, from 2011–2014. We then used temporally varying snow and temperature covariates to predict mortality trends using telemetry information from 860 deer. Cause-specific mortality in the farmland varied by age and year, similar to results from previous research in the forest. Human-related mortality was the leading cause of mortality in the farmland during most years and ranged from 4.3% to 10.3% for juveniles and 3.6% to 9.1% for adults from 2011–2014. Very little predation occurred in the farmland, and this differed from previous research in the forest where predation was the leading cause of mortality. During more severe winters (2013 and 2014), other mortality, usually associated with starvation, was the leading cause of mortality for juveniles in the farmland but not adults. In the forest, we found support for saturating effects of accumulated snow depth days >30.5 cm and accumulated temperature days >0°C on mortality. We also found support for the relationship of mortality with accumulated temperature days >0°C in the farmland but no relationship with snow depth. Deer tolerate sustained cold temperatures, but the timing of winter to spring transition is more important for deer survival in both forested and agricultural areas. In the absence of empirical survival information, managers can use our model to predict annual winter effects on deer survival, which can provide improved inference compared to traditional winter severity indices. Our results suggest changes in predator abundance may have minor influence on overwinter survival compared to winter weather. Based on mortality estimates from previous research, the highest predation rates on juvenile deer in the forest occurred when wolf (Canis lupus) counts were lowest and when wolf abundance was highest, juvenile deer predation rates were lowest. © 2021 The Wildlife Society.     

Embryonic mortality and predation on egg capsules of the Port Jackson shark Heterodontus portusjacksoni (Meyer)

Egg capsules of the Port Jackson shark Heterodontus portusjacksoni were examined during underwater visual census surveys at two sites in New South Wales, Australia, during the 2002–2005 reproductive seasons (austral winter). Embryonic mortality was high (0·783–0·896 per annum) with the majority (99·2%) resulting from predation. The crested horn shark Heterodontus galeatus and common tent shell Astralium tentoriformis (Gastropoda, Turbinidae) were positively identified as egg predators. Gastropod predation was insignificant, accounting for only 2·0 and 2·1% of total mortality at Terrigal Haven and Dent Rock, respectively. Most capsules appeared to have been depredated by large predatory fishes with the eastern blue groper Achoerodus viridis (Labridae) and the black stingray Dasyatis thetidis (Dasyatidae) as possible candidate predators. The rate of embryonic mortality in H. portusjacksoni is higher than that reported for other oviparous elasmobranchs. This high level of mortality has significant consequences for the conservation and management of this species, especially when combined with an understanding of their low fecundity, late maturity and protracted reproductive life.     

Shell form and growth of the bivalveScapharca broughtoni

Linear growth rates and age-related changes in shell form are analyzed in the bivalveScapharca broughtoni from several areas of Peter the Great Bay, Sea of Japan. Mollusks grow fast during the first 4 to 5 years of life; at the age of 10–12 years, the annual growth increment in shell size does not usually exceed 1–3 mm. Irrespective of habitat, the shell valve convexity ofS. broughtoni increases with age, but the most significant changes in shell form occur during the first year of life. The peculiarities of growth and age-related changes in shell form ofS. broughtoni are discussed from the standpoint of the functional morphology of burrowing bivalve mollusks, which, at the postlarval stage, change from an attached to a free-living mode of life.     

Mountain hare populations on islands: effects of predation by red fox

Summary On islands off the west coast of Sweden the density of mountain hares (Lepus timidus L.) is very high. One of the main predators on hares, the red fox (Vulpes vulpes L.), is only present during short periods. Data on hare density and predation by red fox and eagle owl (Bubo bubo (L.)) has been analyzed from five islands over several years. Winter mortality in years with low predation pressure was independent of hare density. But when red fox or eagle owl were present on islands (i.e., high predation pressure) winter mortality became density dependent. Thus, at low density, winter mortality did not increase through red fox predation. But at densities up to two hares/ha, predation pressure was increasing and could be limiting for these populations. At still higher hare density predation pressure became less intensive. The functional response for foxes preying on hares showed a type II or a sigmoid type III response pattern. In normal summers, the population increase due to reproduction was at least two-fold. When a fox was present there was instead a sharp decrease in hare numbers. Fox predation had a stronger effect in summer than in winter. By switching between islands and mainland areas from winter to summer, a fox can stabilize fluctuations in hare numbers on the islands. This is dependent on how often the ice permits a fox to reach an island and the lack of numerical response by predators.     

The population dynamics and reproductive cycle of Septifer virgatus (Bivalvia: Mytilidae) on an exposed rocky shore in Hong Kong

Exposed rocky shores in Hong Kong are dominated at mid-tidal levels by a broad band of the mytilid Septifer virgatus. A 48-month study of a population inhabiting the shores at Cape d'Aguilar has been undertaken. The species matures at a shell length of ≅ 15 mm, about one year after recruitment. Septifer virgatus is dioecious with a slight, but insignificant, female bias overall. There is, however, a significant juvenile male bias. Generally, most adult individuals remain mature year round, although spawning is limited to two periods in spring (February to March) and autumn (September to December) with subsequent recruitment into the adult population. The species lives for ≅ 4–5 years, although older individuals, possibly up to 12 years of age and with a maximum recorded shell length of 65 mm, occur as solitary individuals lower down the shore. Mortality in winter mainly affects newly recruited juveniles. A heavy mortality of adults in summer is thought to be related to high rock temperatures at midday (≅ 50°C), concurrent with low spring tides.
The life-history tactics and sexual strategy of Septifer virgatus can be correlated with seasonal changes in hydrography. There are a number of similarities with other local mytilids, notably the dioecious condition and the bimodal pattern of spawning and recruitment. Such similarities are discussed in relation to the habitats occupied by these species.     

Life history characteristics of the clam<Emphasis Type="Italic"> Mya arenaria</Emphasis> in the White Sea

Dynamics of Mya arenaria beds in two bights of the Chupa Inlet (Kandalaksha Bay, White Sea) were studied on a long-term basis. Observations were carried out at 1– to 3-year intervals from 1979 up to 1999. The studied soft-shell clam beds were characterised by a substantial instability of age structure. Since 1988, only one year-class has dominated in the beds while other generations have been scarce and recruitment was not observed. This pattern of Mya bed dynamics was related neither to interannual environmental changes nor to differential reproduction success or predation effects in the benthic assemblages. Favourable conditions for spat formation in 1988 (low abundance of other M. arenaria generations), as well as for juvenile survival during the following winter, resulted in high abundance of juveniles in both investigated locations in 1989. The mortality rate (μ) in this 1988 generation varied throughout the period of investigation and was related to age. The mortality level decreased for the first 2–4 years of the life cycle, then stabilised for the next 3–4 years, and eventually increased in subsequent years. Overall μ values ranged from 0 to 1.68 year^–1. The oldest specimens observed were 17 years old and had a maximum shell length of 79 mm. Significant differences in average growth rates were observed between molluscs of different locations. Communicated by H.-D. Franke     

Seasonal population changes of five parasitoids attacking the scale insect Nipponaclerda biwakoensis on the common reed, with special reference to predation by wintering birds

Seasonal changes in the abundance of five species of hymenopterous parasitoids (four species of Encyrtidae and one species of Eulophidae) attacking the scale insect Nipponaclerda biwakoensis on the common reed were investigated for 2 years in Lake Biwa, with special reference to predation by the reed bunting, Emberiza schoeniclus, during winter. The scales settled on reed shoot stems under sheath leaves, passing through three discrete generations per year. The abundance of adult female scales increased exponentially from July (first generation) to December (third generation). Adult female scales of the third generation overwintered on reed shoots. During winter, female scale abundance dramatically declined, whereas the number of predation marks made by reed buntings using their bills on reed sheath leaves increased. The generations of all five parasitoids were synchronized with the host scale generations, and the five parasitoids overwintered as larvae inside the scale bodies. The abundance of parasitized scales and parasitoid adults emerging from the scales also increased from July to December, but greatly decreased during winter. The overall parasitism rate of the female scales remained at relatively low levels (less than 40%) throughout the year, including before and after winter. A bird exclusion experiment revealed that the dramatic winter decrease of the abundance of the scale and its five parasitoids was due to intensive and non‐selective predation by the buntings on unparasitized and parasitized scales. Additionally, the proportion of immature parasitoids removed by birds varied between the five parasitoid species. Thus, seasonal population changes of the five scale parasitoids are considerably affected by bird predation on overwintering immature parasitoids.     

The population dynamics of pike,Esox lucius,and perch,Perca fluviatilis,in a simple predator-prey system

Synopsis The population dynamics and predator-prey relationship of pike, Esox lucius, and perch, Perca fluviatilis, were examined in simple fish communities in two adjacent shallow lakes, Lochs Kinord and Davan, Deeside, Scotland. Few perch survive to age 3 but Z is low for fish > 3 years and perch live up to 17 years. Population fecundity remained relatively high and constant in perch because of the multi-age spawning stock and the presence of older more fecund perch. Growth rates of perch in both lochs are relatively high as a consequence of low stock abundance. The N, B, and P of adult perch were unusually low. The age range of pike, and N, B, P, and growth were in the range of values reported elsewhere. There was little variation in the strength of pike year classes and the importance of cannibalism and low occurrence of alternative prey in the lochs suggest that the populations were self-regulating. Cannibalism by adults was responsible for most of mortality in perch larvae, and predation by pike and adult perch was responsible for the majority of juvenile losses. This conclusion is supported by the high biomass ratios of pike:juvenile perch of 1.0–30.8. While the number of adult fish was almost equal, the biomass of adult pike was 2–3 × that of perch in Kinord and 6 × in Davan. In L. Kinord, where year class strength was stable, high predation pressure from perch and pike reduced perch abundance rather than eliminated year classes. Perch year classes fluctuated in abundance in L. Davan and were eliminated in the first summer in two sampling years. The pike, and particularly the perch populations, have features characteristic of fish communities in unperturbed ecosystems: namely, a wide range of age classes, stability in biomass with variation dampened by longevity, and low production.     

Raptor predation on wintering waders in southeast Scotland

Predation on waders was investigated over the winters of 1982-83 and 1983-84 at a rocky shore and a small estuary in southeast Scotland. Probably all the waders which were found after being eaten were killed by raptors; mammalian predation and death due to severe weather were of minimal importance in comparison to the effects of raptor predation. Sparrowhawks were probably the major predators of waders at both sites, but owls also took some. Differences were found between the rocky shore and the estuary in the species of raptors which were seen (more Peregrines and Merlins seen on the estuary) and in the numbers of wader kills which had been plucked and eaten on open ground (more open ground pluckings, indicative of Peregrines and Merlins, found on the estuary). It is suggested that Peregrines and Merlins took more waders at the estuary due to their preference for hunting over large open expanses. Winter mortality rates due to predation varied in different species of waders: at the rocky shore the Redshank suffered the heaviest winter mortality (20% and 16%) whilst on the estuary the Ringed Plover had the highest winter mortality (19%). At both sites no large waders such as Curlews and Oystercatchers died as a result of predators and for these species other factors, such as severe weather or starvation, appeared to be a greater source of mortality. Significantly more juvenile Redshanks, Turnstones and, probably, Dunlin and Ringed Plovers were found eaten than was expected from the age-composition of their respective populations. In the Redshank and Turnstone several differences in the behaviour of juveniles and adults were discovered which may have made juveniles more vulnerable to predation.     

The ecology of Mytilus edulis L. (Lamellibranchiata) on exposed rocky shores

Summary Growth studies in M. edulis L. have shown that rates vary considerably according to age, size and environmental conditions. This may in part be attributed to its sessile habit, being unable to move away from the variable external conditions.The use of modal length frequency distributions is somewhat limited, since with three or more year groups represented, the growth of the majority of the population is so slow that individual year classes lose their identity. Growth boxes, containing marked animals of different ages, and set out in a variety of habitats gave information regarding local and seasonal growth rates. Disturbance rings were shown to be annual, and from them growth curves have been constructed.Growth is particularly seasonal, little or none occurring during the winter. Growth rates varied considerably with a variety of environmental factors (both biotic and physical) and some of these are discussed. Variable individual growth rates, together with slow growth of the majority of animals in mixed populations, are perhaps the major factors in producing population structures typical of this species on open shores.Survivorship curves for mussels in a variety of habitats have been constructed by following the survival of groups of marked animals. Whilst high mortalities occurred in the mid and low shore, survival in the upper shore in the absence of major predators, was greatly enhanced, resulting in established populations of considerable age. Periods of maximum mortality during spring and summer could be correlated with the abundance of major predators.The almost cosmopolitan distribution of M. edulis in the N. hemisphere has been made possible by virtue of its high reproductive capacity, successful larval dispersion and wide tolerance of environmental conditions, Its patchy and apparently erratic distribution both from one shore to annother and even on the same shore, is greatly influenced by the local and seasonal abundance of major predators. Whilst the upward extension of mussels is prevented, ultimately, by physical factors (e.g. temperature, dessication), its lower limits (and absence from the shallow sub littoral on many shores) are governed essentially by predators. The upward extension of many predators such as crabs or starfish, resulting in locally intense predation, may in turn be partly influenced by the actual topography of the shore itself.     

Temperature impact on the midsummer decline of Daphnia galeata: an analysis of long-term data from the biomanipulated Bautzen Reservoir (Germany)

1. The influence of water temperature on occurrence and duration of a midsummer decline (MSD) of Daphnia galeata was studied in the biomanipulated Bautzen Reservoir in Germany. The proportion of piscivores in the fish community of the reservoir has been enhanced experimentally since 1981. As a consequence, Daphnia galeata has dominated the zooplankton. Over 18 years of study (1981–1998), a long‐lasting MSD (longer than 30 days) occurred in 7 years, whereas a short MSD (shorter than 30 days) was observed in 6 years. During the remaining 5 years, an MSD was not observed.
2. Two hypotheses were examined to explain the observed patterns. First, we postulated that high water temperature during winter and early spring (January–April) leads to an MSD after an early and high spring peak of daphnids. On the other hand, low temperature during winter and early spring should not cause an MSD owing to a slower increase of the population, resulting in a later peak of daphnids. Second, we hypothesized that the mean water temperature during early summer (May and June) influences the occurrence of an MSD (by controlling young‐of‐the‐year (YOY) fish predation on daphnids).
3. The water temperature during winter and early spring explains 83%, and the early summer water temperature 55%, of interannual variation in the occurrence of an MSD.
4. The interannual variation in duration of an MSD was neither explained by temperature during winter and early spring nor by early summer temperature alone, but in 14 of the 18 years (78%) by a combination of both.
5. We conclude that water temperature during winter and early spring had a strong impact on Daphnia mortality by influencing height and timing of the spring peak which, in turn, influenced the extent of overexploitation of their food resources. By contrast, the water temperature during early summer probably influenced the mortality of daphnids caused by predation of YOY fish. The relative timing of both sources of mortality, which depends on the temperature regime during the first 6 months of the year, is the key process in controlling the occurrence and duration of an MSD. A long‐lasting MSD, therefore, is likely in Bautzen Reservoir only if temperatures are high during winter and early spring, as well as during early summer.
6. As a consequence of climate warming, recent climate records reveal warming during winter, spring and early summer in middle Europe, rather than an increase in mean annual temperatures. If our findings and conclusions are related to this regional and temporal pattern of climate warming, an increasing frequency of years with a long‐lasting MSD and, consequently, a decreasing efficiency of biomanipulation can be predicted.     

Population dynamics,growth and reproductive rates of Littorina nigrolineata Gray from a moderately sheltered locality in North Wales

The population dynamics of Littorina nigrolineata Gray on a moderately sheltered boulder shore was characterized by an almost constant mortality rate of post-juvenile snails both within and between years. Cohorts of post-juvenile snails had a “half life” of about 5 months. Juvenile snails had higher mortality rates than post-juveniles. Mortality rate was independent of size in post-juveniles. The population of post-juvenile snails fluctuated only about two-fold during the 3 yr of study. Maxima of population size occurred each September as a result of an autumnal pulse of recruitment into the post-juvenile size classes. These recruits originated from eggs laid in the summer of the previous year. The size-frequency structure of the population lacked well defined modes except during the spring and summer when the pulse of recruits formed a distinct mode, which later disappeared due to growth and mortality of the recruits. Oviposition continued throughout the year with a pulse in summer. Newly laid eggs took 4–7 wk to hatch. The hatchlings had a mean shell height of 0.5 mm and were estimated to take 6–9 months to grow to a shell height of about 4 mm. Growth increments over periods of 3 months. estimated from tagged cohorts of snails ranging from 3 to 16 mm, released on a stone jetty, revealed seasonal and size-specific changes in growth rate. Growth rate was significantly lower between December and March than at other times of the year. Seasonal differences in growth rate did not, however, significantly affect the yearly size increments of snails hatching in summer or winter. Growth increments did not decline linearly with increasing body size, as is assumed by the von Bertalanffy growth model, but increased to a maximum at about 6–7 mm shell height. Beyond the maximum, the growth increments decreased linearly with increasing shell height. This non-linearity caused an inflexion in the growth curve, so that the shell height. This non-linearity caused an inflexion in the and 12.4 mm by the von Bertalanffy growth curve. The deviation from the von Bertalanffy growth model was possibly due to departures from the assumptions that the assimilation rate is proportional to the square of a linear size dimension and that the metabolic rate is proportional to the cube of the same dimension. A von Bertalanffy growth curve, fitted to growth increments measured from “growth checks” on tagged snails on the boulder shore, predicted slightly faster growth rates than the von Bertalanffy growth curve fitted to increments of mean cohort size on the stone jetty. The closeness of the two curves, however, suggested that both methods of measuring growth increments were tolerably accurate.     

Habitat-specific size structure variations in periwinkle populations (<Emphasis Type="Italic">Littorina littorea)</Emphasis> caused by biotic factors

Shell size distribution patterns of marine gastropod populations may vary considerably across different environments. We investigated the size and density structure of genetically continuous periwinkle populations (Littorina littorea) on an exposed rocky and a sheltered sedimentary environment on two nearby islands in the south-eastern North Sea (German Bight). On the sedimentary shore, periwinkle density (917 ± 722 individuals m⁻²) was about three times higher than on the rocky shore (296 ± 168 individuals m⁻²). Mean (9.8 ± 3.9 mm) and maximum (22 mm) shell size of L. littorea on the sedimentary shore were smaller than on the rocky shore (21.5 ± 4.2 and 32 mm, respectively), where only few small snails were found. Additionally, periwinkle shells were thicker and stronger on the rocky than on the sedimentary shore. To ascertain mechanisms responsible for differences in population structures, we examined periwinkles in both environments for growth rate, predation pressure, infection with a shell boring polychaete (Polydora ciliata) and parasitic infestation by trematodes. A crosswise transplantation experiment revealed better growth conditions on the sedimentary than on the rocky shore. However, crab abundance and prevalence of parasites and P. ciliata in adult snails were higher on the sedimentary shore. Previous investigations showed that crabs prefer large periwinkles infested with P. ciliata. Thus, we suggest that parasites and shell boring P. ciliata in conjunction with an increased crab predation pressure are responsible for low abundances of large periwinkles on the sedimentary shore while high wave exposure may explain low densities of juvenile L. littorea on the rocky shore. We conclude that biotic factors may strongly contribute to observed differences in size structure of the L. littorea populations studied on rocky and sedimentary shores.     

Dispersion pattern and mortality of seeds and seedlings ofFagus crenata Blume in a cool temperate forest in western Japan

The seed and seedling mortality ofFagus crenata Blume after a mast year (1993) was examined in relation to density and distance from the nearest conspecific adult tree in a mixed conifer-hardwood forest in Ohdaigahara, western Japan. The mortality of fallen seeds during winter amounted to 93.7%, and 79.2% of the current-year seedlings died in the first growing season. The most important factor of death for both seeds and seedlings was predation by vertebrates. The mortality of seeds during winter was positively correlated with sound seed density. The mortality of seedlings was positively correlated with density but not significantly related to the distance from the nearest crown edge of a conspecific adult tree. Mortality patterns varied with stages and spatial scales due to the behavior of predators; it is thus important to investigate the spatial pattern of seeds and seedling mortality at various temporal and spatial scales. After the first growing season, the difference in seedling density between distance classes was not significant at <4m from the nearest adult trees due to density-dependent mortality. However, seedling density was significantly lower in the ≥4 m class than in the <4 m classes.     

Life history strategy and egg diapause in the intertidal collembolan Anurida maritima

ABSTRACT.

• 1 In a field survey of 1 year the intertidal collembolan Anurida maritima (Guérin) was found to be a univoltine species. Every year in early May a new generation emerges from eggs that overwinter. The first animals become adult in July. Almost all eggs laid in summer do not hatch, but undergo a period of diapause. Diapause is terminated in autumn by temperatures below 5°C. However, due to the low temperatures in winter further egg development is suppressed until spring.
• 2 The adult animals die, mainly in autumn. One of the causes of mortality may be starvation. In late autumn body size decreases and glycogen and lipid content are lower.
• 3 It is argued that in autumn due to low temperatures A.maritima, which forages during low tide on the open shore and seeks refuge underground before the incoming tide, is too sluggish to find food in the limited period of low water. This probably explains why this cosmopolitan species has developed a strategy to survive the winter in the temperate zone in the egg stage.