Survivorship of juvenile barnacles and mussels: spatial dependence and the origin of alternative communities

Mussels, barnacles, and rockweeds often form a distinct mosaic of patches on rocky intertidal shores, and it has been suggested that these communities may represent alternative community states. One way that alternative community states can arise is if early successional events are scale-dependent, but it is not known if juvenile survivorships of mussels and barnacles are, in fact, scale-dependent. Scale-dependence of barnacles (Semibalanus balanoides (L.)) and mussels (Mytilus edulis L.) was tested in the Gulf of Maine, USA. In winter 1997, clearings of 1, 2, 4 and 8 m in diameter and uncleared controls were made in stands of the rockweed Ascophyllum nodosum (L.) Le Jolis at 12 sites spread evenly across four bays on Swan's Island, ME. Summer and fall-winter survivorship of barnacles, which recruited in spring 1997, were estimated by tracking the 1997 cohort until late winter 1998. Survivorship of mussels was estimated from following the fate of transplanted juveniles over 4 days in late August 1997. Both barnacles and mussels showed better survival in 4 and 8 m clearings than in small clearings and controls. There was also significant variation in survivorship among sites. Densities of gastropods in the clearings did not reflect survivorship patterns of barnacles and mussels. Barnacle survivorship increased in fall and winter, and in large clearings was comparable to survivorship in barnacle-dominated habitats. Mussel survivorship was low in all clearing sizes suggesting that mussel beds develop slowly.     

The Impact of Desiccation on the Adhesion of Barnacles Attached to Non-stick Coatings

Fouling-release coatings prevent fouling of ships' hulls through hydrodynamic forces generated as the ship moves through the water. The effectiveness of such coatings may be evaluated by measuring the adhesion strength of settled organisms, e.g. barnacles. The influence of desiccation of the barnacle adhesive on such measurements was investigated. Shear forces required to remove barnacles of the genus Balanus increased during the course of desiccation up to the point when the barnacles suddenly self-detached. The increase was thought to be due to the rising cohesive strength of the adhesive. Growing tensile forces within the weakly cross-linked adhesive, however, are suggested to have led to self-detachment. The shear forces required to remove barnacles of the genus Elminius were generally low and did not differ significantly during the course of desiccation. The different results may be attributed to specific base morphologies. It was concluded that measuring the adhesion strength of members of the Balanidae on non-stick surfaces in air could produce flawed results due to the influence of desiccation of the barnacle adhesive. The investigations have also provided new insights into the characteristics of barnacle adhesive.     

Direct and indirect effects of predation,herbivory and surface rugosity on mussel recruitment

Summary The predatory gastropod Nucella lapillus, commonly preys upon the mussel, Mytilus edulis, and is thought to control the distribution and abundance of mussels on the rocky shores of New England, USA. In this study, done in Maine, USA, not only the presence of Nucella lapillus but also the roughness of the experimental surface and the presence of the herbivorous gastropod, Littorina littorea, were manipulated. Four types of surfaces were used as recruitment substrata for mussels: smooth bare granite, aggregations of the barnacle, Semibalanus balanoides, fiberglass resin castings of smooth bare granite and resin castings of aggregations of S. balanoides. To ensure that caged N. lapillus were not starving, barnacles were provided as alternative prey. Experiments showed no detectable effect of N. lapillus on the recruitment of M. edulis. Mussel recruitment was enhanced by surface rugosity and depressed by the activities of L. littorea. Analysis of covariance, using the number of algal species as the covariate, suggested that L. littorea reduced the number of newlyrecruited mussels by removing algae that provided recruitment sites, but no manipulations were done to test this conjecture. It is likely that previous reports of N. lapillus controlling mussel abundance are attributable to N. lapillus preying upon barnacles, which increase surface rugosity and enhance mussel recruitment. Review of literature on feeding preferences of N. lapillus supports this view. When handling times and prey availability are taken into account, Nucella shows a clear preference for barnacles over mussels.     

Density dependence in northern ungulates: interactions with predation and resources

Variation in the abundance of animals has traditionally been explained as the outcome of endogenous forcing from density dependence and exogenous forcing arising from variation in weather and predation. Emerging evidence suggests that the effects of density dependence interact with external influences on population dynamics. In particular, spatial heterogeneity in resources and the presence of capable predators may weaken feedbacks from density dependence to growth of populations. We used the Kalman filter to analyze 23 time series of estimates of abundance of northern ungulate populations arrayed along a latitudinal gradient (latitude range of 40°–70°N) to evaluate the influence of spatial heterogeneity in resources and predation on density dependence. We also used contingency tables to test whether density dependence was independent of the presence of carnivores (our estimate of predation) and multiple regressions to determine the effects of spatial heterogeneity in resources, predation, and latitude on the strength of density dependence. Our results showed that the strength of density dependence of ungulate populations was low in the presence of large carnivores, particularly at northern latitudes with low primary productivity. We found that heterogeneity in elevation, which we assume acted as a surrogate for spatial heterogeneity in plant phenology, also reduced effects of density dependence. Thus, we show that external forces created by heterogeneity in resources and predation interact with internal feedbacks from population density to shape dynamics of populations of northern ungulates.     

Scorpions,spiders and solpugids: predation and competition among distantly related taxa

Summary Scorpions, spiders and solpugids are generalist predators on the same types of arthropod prey. However, these potential competitors also frequently eat one another (=intraguild predation). In a 29 mo. experiment, >6,000 scorpions were removed from 300 (10x10m) quadrats. Significantly more spiders (but not solpugids) occurred in removal versus control quadrats. Two alternate hypotheses potentially explain this result: exploitation competition for jointly exploited prey or intraguild predation. There was no evidence of exploitation competition: no differences existed between removal and control quadrats in prey abundance or spider size and reproductive characteristics. It appears that the release from predation pressure in areas from which scorpions were removed produced the observed increase in spider abundance. Current ecological theory does not fully apply to situations whereby species at the same trophic level interact as both predators and potential competitors.     

Aquatic arthropod communities in Nepenthes pitchers: the role of niche differentiation,aggregation, predation and competition in community organization

Summary The structure and organization of aquatic arthropod communities in Nepenthes ampullaria pitchers were studied at two sites (M in Malacca and K in Kuching) in Malaysia. The communities consisted mainly of aquatic dipteran larvae. Community M was dominated by a filter feeder, Tripteroides tenax, which reached a high density despite a strongly aggregated distribution. Community K had five trophic groups: carrion feeders, filter feeders, detritus feeders, nipping predators and hooking predators, each including multiple species. The summed density of filter feeders in Community K remained much below the level attained by filter feeders in Community M. Niche differentiation within each trophic group with regard to pitcher age and feeding behaviour was not sufficient to allow species coexistence through niche separation alone. Aggregated distributions directly reduced interspecific encounters. Nevertheless, species belonging to the same trophic group commonly shared the same pitcher, because of high occurrence probabilities of dominant species and positive associations between some taxa (due mainly to similar occupancies by pitcher age). Predator coexistence in Community K may have been facilitated by self-limitation of the large predators through intraspecific cannibalism strengthened by aggregation. Prey coexistence, on the other hand, may have relied more on population suppression by predation, especially the selective removal of old instar Tripteroides.     

Predation on barnacles of intertidal and subtidal mussel beds in the Wadden Sea

Balanids are the numerically dominant epibionts on mussel beds in the Wadden Sea. Near the island of Sylt (German Bight, North Sea), Semibalanus balanoides dominated intertidally and Balanus crenatus subtidally. Field experiments were conducted to test the effects of predation on the density of barnacle recruits. Subtidally, predator exclusion resulted in significantly increased abundances of B. crenatus, while predator exclusion had no significant effects on the density of S. balanoides intertidally. It is suggested that recruitment of B. crenatus to subtidal mussel beds is strongly affected by adult shore crabs (Carcinus maenas) and juvenile starfish (Asterias rubens), whereas recruits of S. balanoides in the intertidal zone are mainly influenced by grazing and bulldozing of the very abundant periwinkle Littorina littorea, which is rare subtidally. Thus, not only do the barnacle species differ between intertidal and subtidal mussel beds, but the biotic control factors do so as well. Electronic Publication     

苔藓植物耐旱机制研究进展

耐旱藓类快速脱水并存活的能力可由快速建立起来的对环境变化的耐受机制来反映,保护细胞完整性的组成型机制与修复细胞损伤的诱导机制协同作用使苔藓植物渡过干旱胁迫.再水化时光合系统原初恢复非常迅速;ABA处理可显著改变PSⅡ的生理特征;基因表达的变化主要由翻译调控引起;脱水组织中贮存mRNPs既保护了mRNAs,又加快了再水化修复速度.山墙藓(Tortula ruralis)是耐旱研究较多的一个种,已建立了表达序列文库(EST),将会成为耐旱研究的重要模式植物.     

Influence of puffer predation on vertical distribution of mangrove littorinids in the Gulf of Nicoya, Costa Rica

The eastern Pacific puffer fish Sphoeroides rosenblatti (family Tetraodontidae) preys on the mangrove periwinkles Littorariafasciata and L. varia up to 13 cm above the water surface. Puffers capture snails by jumping partially out of the water and removing them by mouth from Rhizophora spp. prop roots. Field predation trials suggested puffer predation is intense, averaging 40.6% h⁻¹ on L. fasciata tethered at the water surface. L.fasciata <∼8 mm in shell length and L. varia <∼12 mm remained near or below the water surface and were easily crushed by puffers; gut content analyses showed that these snails were commonly eaten by puffers. L.fasciata >∼8 mm and L. varia∼12–16 mm were also crushed by puffers, but were generally located above their reach; these snails were much less abundant in puffer guts. L.varia >∼16 mm were abundant and some were submerged at high tide; however, none were found in puffer guts presumably because puffers encounter difficulty crushing these snails. S.rosenblatti is an important predator of L. fasciata and L. varia and appears to affect their vertical distributions. Received: 21 April 1998 / Accepted: 10 August 1998     

Trends in abundance of rocky intertidal seaweeds and filter feeders across gradients of elevation,wave exposure,and ice scour in eastern Canada

On NW Atlantic rocky shores, the main basal organisms in intertidal communities are seaweeds (Ascophyllum nodosum, Fucus spp. and Chondrus crispus) and filter feeders (barnacles, Semibalanus balanoides, and mussels, Mytilus spp.). Their ecology has been extensively studied in New England (United States), but knowledge gaps exist for northern shores, which are subjected to stronger environmental stress. Therefore, we studied the above organisms on Canadian shores. We quantified the summer abundance of these seaweeds and filter feeders across full vertical (intertidal elevation) and horizontal (wave exposure and winter ice scour) environmental gradients on the Gulf of St. Lawrence and open Atlantic coasts of Nova Scotia. At the regional scale along the open Atlantic coast, seaweeds showed similar abundances in Nova Scotia than values reported for New England. However, both filter feeders were considerably less abundant in Nova Scotia. At the local scale in Nova Scotia, intense winter ice scour (which only occurs on the Gulf of St. Lawrence coast) was associated with a very low abundance of all species except barnacles. Spatial trends in Nova Scotia were similar to patterns known for certain species elsewhere, such as A. nodosum being almost restricted to sheltered habitats, regardless of elevation, and C. crispus being almost restricted to low elevations, regardless of exposure. Other trends were, however, characteristic of Nova Scotia, such as C. crispus being frequent at low elevations in exposed habitats, unlike in New England, where mussels often predominate there because of competitive advantages. In Nova Scotia, mussels were always restricted to cracks and crevices, unlike in New England, where they form extensive intertidal beds on exposed shores. The direct effects of increased environmental stress and indirect effects through altered interspecific interactions might explain the regional differences in local species distribution, which will require experimental validation. Handling editor: K. Martens     

Aspects of the ecology of the prickly sculpin,Cottus asper Richardson,a persistent native species in Clear Lake,Lake County,California

Synopsis The biology of the prickly sculpin was investigated in Clear Lake, Lake County, California in order to determine how it has persisted in the face of introductions of numerous exotic species when most other native species have declined in abundance or have become extinct. Sculpins over 15 mm SL inhabited all types of benthic habitats in the lake, while post larval sculpins were pelagic when the postlarvae of exotic species were absent. The feeding ecology of sculpins was distinct from the other fishes in the lake in that they fed largely on amphipods and chironomid midge larvae regardless of the time of year, time of day, or habitat. Sculpins were uncommon in the stomachs of piscivorous fishes, except juvenile largemouth bass (Micropterus salmoides). It is concluded that prickly sculpins have persisted in Clear Lake in part because they are ecologically distinct from the exotic species and are not preyed upon by them to any great extent, and in part because they have managed to survive other man-related perturbations of this ecosystem.     

Trophic relationships of nonnative brown trout, Salmo trutta, and native Bonneville cutthroat trout, Oncorhynchus clarkii utah, in a northern Utah, USA river

Nonnative trout invasions have caused the widespread decline of cutthroat trout populations in western North America. In contrast to other nonnative salmonids, the role of nonnative brown trout in native cutthroat trout decline is poorly understood. Specifically, the level of ecological similarity that occurs between these species and the importance of other trophic mechanisms (e.g., predation) in their interactions are key uncertainties. We evaluated the trophic relationships of brown trout and cutthroat trout in a northern Utah river using a combination of diet and stable isotope analyses. We compared the dietary habits of these two species using multiple and complementary measures. Based on both stomach contents and δ¹³C signatures, we found that these species consumed a similar and opportunistic diet (i.e., they were nonselective in their foraging patterns). However, at most sizes, brown trout ingested larger prey—including fishes—and occupied a higher relative trophic position (i.e., δ¹⁵N) than cutthroat trout. Overall, these results demonstrate a high degree of dietary similarity and therefore strengthen earlier conclusions regarding interspecific competition between these two species. Our study, when considered alongside the work of others, suggests there is potential for predatory interactions between these species (i.e., brown trout preying on small cutthroat trout). We believe that future research on brown trout–cutthroat trout interactions should consider predatory effects in greater detail.

The dual importance of competition and predation as regulatory forces in terrestrial ecosystems: evidence from decomposer food-webs

The relative importance of predation and competition (resource limitation) in influencing the components of a below-ground food-web consisting of three trophic levels (bacteria and fungi; bacterial-feeding and fungal-feeding nematodes; and top predatory nematodes) was estimated using microbial biomass and nematode frequency data collected throughout a 1-year period in two agro-ecosystems. The study suggested that bacterial and fungal biomass were likely to be regulated by grazing and competition respectively, and that these differences were likely to be attributed to the biological (probably morphological) differences between bacteria and fungi, in contrast to the predictions of the hypothesis of Hairston et al. (1960). Top predatory nematodes were sometimes strongly related to the microbial but not microbial-feeding trophic levels, indicating that microbial biomass may directly influence top predator numbers, and that the intermediate level may simply serve as a conduit by which resources pass from the bottom to top trophic levels. This study also suggests that the detritus food-web acts as two distinct (bacterial-and fungal-based) compartments.     

Impact of predation on early stages of the armoured catfish Hoplosternum thoracatum (Siluriformes-Callichthyidae) and implications for the syntopic occurrence with other related catfishes in a neotropical multi-predator swamp

This study investigated the role of predators in preventing competitive exclusion among three closely related armoured catfishes (Callichthys callichthys, Hoplosternum littorale and H. thoracatum) that occur synthopically in multi-predator freshwater swamps of Suriname, South America. The potential impact of predation on armoured catfish was determined by combining laboratory measurements of predation rates on five early developmental stages of the armoured catfish H. thoracatum for 24 aquatic predators with field studies of the density of the predators in the swamps. The contribution of a particular predator to the total predation pressure on its prey was determined to a large extent by the density of the predator in the swamp. Seemingly innocuous predators with low or moderate predation rates in the laboratory may be extremely important in the swamps due to their high abundance. Small-sized omnivorous fishes and aquatic invertebrates were major predators of early developmental stages of armoured catfish. Both qualitative and quantitative ontogenetic changes in the predation pressure on armoured catfish were observed. Major predation on eggs, larvae and juveniles of H. thoracatum resulted from a different set of predators in each developmental stage of the prey. In all developmental stages of H. thoracatum the predation pressure involved several predator species and not a single, dominant predator. The potential predation pressure of the 24 predators taken together and the number of predators that were able to prey on H. thoracatum decreased sharply with increasing age (size) of the prey. Even if egg (nest) predation is prevented by the guarding male, the potential impact of the 24 predators on the populations of armoured catfish is large. Predation may account for the high mortality of H. thoracatum observed in the swamps. The high predation pressure on callichthyid catfishes may help to explain the coexistence of three closely related and morphologically quite similar armoured catfishes in Surinamese swamps.     

Effects of predation pressure on species packing on a resource gradient: insights from nonlinear dynamics

The classical case of three competitors arranged on a resource gradient such that the central competitor will be excluded due to competition from the other two is studied from the point of view of the effects of added predators. The basic formulation is motivated by a desire to understand the effects of asymmetries in multidimensional Lotka-Volterra systems. We first study the effects of perfectly specialist predators and find a rich collection of possible behaviors of the system including (1) extinction of all predators and subsequent extinction of the subordinate competitor, (2) dominant competitors and their predators coexist but the subdominant competitor goes extinct, (3) all species except the predator of the subordinate competitor coexist in coordinated phase-reversed chaos, (4) exclusion of one or more species occurs through an expanding heteroclinic cycle, and (5) all species coexist in an uncoordinated chaos. We then study the effects of five qualitatively distinct forms of polyphagy. In one case, corresponding to the well-known vulnerability to predation versus competitive ability trade-off, it is possible to have the subordinate competitor be the only survivor in the system. The other three cases of polyphagy lead to distortions in the basic pattern seen in the previously analyzed specialist case. Studying this case of ecologically motivated asymmetries in the basic Lotka-Volterra formulation is a step in the direction of fully understanding interacting populations.     

Stream drift,size-specific predation,and the evolution of ovum size in an amphibian

Summary A stream-breeding race of small-mouthed salamanders (Ambystoma texanum) in central Kentucky produces ova that are twice as large as those of a pond-breeding race found nearby. Embryos of stream-breeders also hatch at a more advanced developmental stage than those of pond-breeders. Morphological evidence indicates that stream-breeders were derived from pond-breeding stock. Assuming that differences between stream and pond-breeders reflect evolutionary change, and that the ancestral pond stock that invaded streams was similar to extant pond-breeders, we examined three hypotheses that might explain changes in ovum size and stage at hatching following the invasion of streams. (1) Larger ovum size evolved indirectly as a consequence of selection for rapid development which minimizes mortality risk from stream drying. (2) Increased ovum (hatchling) size and stage at hatching of stream-breeders are adaptations to resist stream current. (3) Increased ovum (hatchling) size and stage at hatching are adaptations to reduce predation on hatchlings from stream invertebrates. The results of field and laboratory studies only support hypotheses (2) and (3). Hatchlings that were relatively large or at a more advanced developmental stage had slower drift rates and were less vulnerable to predation by Phagocata gracilis, a flatworm abundant in streams in central Kentucky. Developmental and growth parameters were not correlated significantly with ovum size in populations of either geographic race. Differences in degree of parental care among races also cannot explain variation in ovum size since both races abandon their eggs immediately after oviposition.     

种子的脱水行为及其分子机制

成熟脱水是种子发育的末端事件。根据种子的脱水行为,可以把种子分为正常性、顽拗性和中间性种子。许多过程或者机制授予或者提高种子的脱水耐性,不同的过程可能在不同的水合水平上对水分丧失起保护作用,这些过程的缺乏或者无效表达可能决定个别物种的种子的脱水敏感性程度。到目前为止,涉及种子脱水耐性的过程或者机制有：细胞内脱分化;代谢的‘关闭’;抗氧化系统的存在和有效运转;保护性分子(包括胚胎发育后期高丰度表达蛋白,蔗糖、寡糖或者半乳糖苷环多醇,亲水脂分子,油素)的存在;以及在重新水合过程中修复机制的存在和运转。     

The impact of predation on the viburnum whitefly, (Aleurotrachelus jelinekii)

Summary In association with a detailed study of the population dynamics of the viburnum whitefly, Aleurotrachelus jelinekii, observations were made on the predators including spiders and five species of insect, notably the mirid Campyloneura virgula and the neuropteran Conwentzia psociformis. Although the species richness of the predator guild was positively correlated with prey population density, at the individual level there was little evidence over twelve years of a density dependent relationship. In these populations natural enemies do not seem to play an important role in the dynamics.     

Intra versus interspecific interactions of ladybeetles (Coleoptera: Coccinellidae) attacking aphids

Summary The nature and relative strengths of intra versus interspecific interactions among foraging ladybeetle larvae were studied experimentally by measuring short-term growth rates of predators and reductions in population sizes of prey in laboratory microcosms. In these microcosms, ladybeetle larvae foraged singly or as conspecific or heterospecific pairs, for pea aphids on bean plants over a two-day period. Similarly sized third instar larvae ofHippodamia convergens andH. tredecimpunctata, H. convergens andH. sinuata, andH. convergens andCoccinella septempunctata, were tested in experiments designed to ensure that paired larvae experienced moderate competition. Interspecific competition in these experiments did not differ significantly from intraspecific competition, in that an individual's weight gain did not depend on whether its competitor was heterospecific or conspecific. Furthermore, aphid populations were reduced equally by heterospecific and conspecific pairs. These results suggest that there is little or no difference between intra and interspecific interactions among larvae of these ladybeetles when two similarly sized individuals co-occur on a host plant. Thus, the species diversityper se of assemblages of ladybeetle larvae may have little influence over the short term on the reduction of aphid populations by ladybeetle predation.     

First observations of predation by New Zealand Greenshell mussels (Perna canaliculus) on zooplankton

Observations made overseas of predation by blue mussels and zebra mussels on mesozooplankton (>200 μm) have raised concern within New Zealand that the Greenshell mussel, Perna canaliculus, which is cultured in large tonnages throughout hundreds of marine farms within the New Zealand coastal zone, could exert ecologically detrimental effects by preying on zooplankton. We conducted experiments at Clova Bay, Pelorus Sound in May 2002 to determine the rates that P. canaliculus ingests prey, up to and including the mesozooplankton size range. Single mussels from farms were incubated with seawater enriched with zooplankton (>60 μm) in gently circulated 15-l pails. Depletion of chlorophyll-a (chl-a), ciliate microzooplankton, and nauplii, copepodites, and adults of copepods was determined over 5 h, relative to controls with no mussels. Two experiments were made over consecutive days. Gut contents of these experimental mussels, and of mussels examined soon after collection from a farm, were described.Gut contents of experimental and of freshly collected mussels (standard shell length ∼90 mm) had numerous copepod parts, whole copepods and larval bivalves present. Experimental mussels cleared chl-a and ciliates from 59- to 137-l individual⁻¹ day⁻¹, respectively, averaged across the two experiments. Faster ciliate than chl-a clearance was probably caused by the high proportion (56%) of phytoplankton below the retention size for P. canaliculus (ca. 5 μm) and by faster ciliate grazing in controls than treatments. The average clearance rates of adult, copepodite, and naupliar copepod stages by mussels were 20, 31, and 49 l individual⁻¹ day⁻¹, respectively. The clearance rates of each copepod stage were not significantly different between the two experiments. Clearance of nauplii was significantly greater than of adults and copepodites, while adult and copepodite clearance rates were nearly significantly different. The mean lengths of the adult, copepodite, and naupliar copepods were 430, 265, and 165 μm, respectively. The decreasing clearance rates with increasing size and development of prey (from ciliates, through naupliar, copepodite to adult copepods), suggested that prey escape ability, related to body size and/or morphology, affected capture rates. Mussel faecal samples indicated complete digestion of the gut contents. Pseudofaecal samples showed very low rejection rates of mesozooplankton by mussels. The results are considered in context of current biophysical modelling studies of impacts of large mussel farms in New Zealand. Designs of future experiments to improve accuracy of estimates of mesozooplankton clearance rates by P. canaliculus are considered.