Sexual Recruitment in Zostera marina: Progress toward a Predictive Model

Ecophysiological stress and physical disturbance are capable of structuring meadows through a combination of direct biomass removal and recruitment limitation; however, predicting these effects at landscape scales has rarely been successful. To model environmental influence on sexual recruitment in perennial Zostera marina, we selected a sub-tidal, light-replete study site with seasonal extremes in temperature and wave energy. During an 8-year observation period, areal coverage increased from 4.8 to 42.7%. Gains were stepwise in pattern, attributable to annual recruitment of patches followed by centrifugal growth and coalescence. Recruitment varied from 13 to 4,894 patches per year. Using a multiple linear regression approach, we examined the association between patch appearance and relative wave energy, atmospheric condition and water temperature. Two models were developed, one appropriate for the dispersal of naked seeds, and another for rafted flowers. Results indicated that both modes of sexual recruitment varied as functions of wind, temperature, rainfall and wave energy, with a regime shift in wind-wave energy corresponding to periods of rapid colonization within our site. Temporal correlations between sexual recruitment and time-lagged climatic summaries highlighted floral induction, seed bank and small patch development as periods of vulnerability. Given global losses in seagrass coverage, regions of recovery and re-colonization will become increasingly important. Lacking landscape-scale process models for seagrass recruitment, temporally explicit statistical approaches presented here could be used to forecast colonization trajectories and to provide managers with real-time estimates of future meadow performance; i.e., when to expect a good year in terms of seagrass expansion. To facilitate use as forecasting tools, we did not use statistical composites or normalized variables as our predictors. This study, therefore, represents a first step toward linking remotely acquired environmental data to sexual recruitment, an important measure of seagrass performance that translates directly into landscape-scale coverage change.     

双岛湾大叶藻的有性生殖特征

对威海双岛湾大叶藻(Zostera marina L.)种群的有性生殖特性进行了研究。结果表明:该水域中,大叶藻种群于3月下旬或4月上旬进入生殖枝始现期,5月上旬进入始花期,6月下旬进入种子散落高峰期,7月上旬进入生殖枝死亡始期,7月下旬生殖期结束,其有性生殖物候进程与水温密切相关,15℃是其开花适宜水温;生殖枝的侧枝数和扇状聚伞花序数在进入始花期时达到最大,分别为4.4和4.9个·株-1,佛焰苞数和生殖枝高度均在种子散落始期最大,分别为18.6个·株-1和96.2 cm,种子败育率较高,平均为40%,成熟种子产量为44.8粒·株-1;生殖枝密度在始花期时达到最大,为64株·m-2,生殖枝分化率平均仅有12.6%,有性生殖期间营养枝密度和种群总密度的下降幅度超过40%;生殖枝生物量在初果期达到最大(27.9 g·m-2),从始花期至种子散落高峰期平均种群生物量分配格局为地下部38.4%,营养枝52.4%,生殖枝9.2%,生殖分配较低,成熟种子产量为1859粒·m-2。     

Sexual reproduction of Daphnia pulex in a temporary habitat

Species of Daphnia (Crustacea: Cladocera) typically reproduce by cyclical parthenogenesis, in which a period of all-female parthenogenetic reproduction is followed by sexual reproduction. Sex in Daphnia is determined by the environment, with factors such as temperature, photoperiod and crowding stimulating the production of males and sexual females. Previous studies on Daphnia pulex from temporary pond habitats demonstrated the coexistence of male-producing and non-male-producing (NMP) females, as determined under crowding in the laboratory. A strong genetic component to this sex allocation variation suggested that sex expression in D. pulex is better described as a result of genotype-environment interaction. The present study examined the switch from parthenogenetic to sexual reproduction in two temporary-pond populations of D. pulex. Both populations showed a very early investment in sexual reproduction, independent of population density, by producing males very soon after the populations were reestablished from resting eggs in the early spring. Approximately 40% of the initial broods were male. Additional evidence for gender specialization was obtained by observing the sex of two or three successive broods for 85 individual females. Fifty-eight females produced successive broods of females, 13 females produced successive broods of males and 14 females produced successive broods which included both male and female broods. Females that produced successive female broods under natural conditions included a higher frequency of NMP females compared to a random sample of females, confirming the existence of NMP females. Sexual females were observed in both populations after the first appearence of males, suggesting that the presence of males may stimulate the production of sexual females. For D. pulex populations in a temporary environment, there appears to be an increased emphasis on sexual reproduction and a decreased influence of the environment on sex determination, compared to Daphnia populations in more permanent habitats. Received: 19 February 1996 / Accepted: 20 January 1997     

Possibilities for dispersal in annual and perennial grasses in a savanna in Botswana

Dispersal of 11 dominant grass species in the savanna of southern Botswana was investigated. The dispersal is autochorous, anemochorous and epizoochorous independent of the life-cycle (annuals, perennials). Dispersal distances were estimated experimentally. Anemochorous species with a plume-like spikelet such as Chloris virgata and Enneapogon cenchroides have a low rate of descent (<1 m s^-1) and a low Reynold number (100–110). Nevertheless they can only be transported up to 13 m from the originating infructescence at a wind velocity of 10 m s^-1. Therefore, the majority of the disseminules remained near the parent plant. By analysing the seed pool under the canopy of trees of Dichrostachys cinerea epizoochorous species such as Tragus berteronianus were dominant at the cattle resting sites under trees. The results are discussed in relation to the three seed dispersal hypotheses and the model of selective interaction of dispersal, dormancy, and seed site as adaptions to variable environments.     

A stochastic model of seed dispersal pattern to assess seed predation by ants in annual dry grasslands

A combined field experiment and modelling approach has been used to provide evidence that ants may be responsible for an observed lower patchiness and higher plant diversity in the neighbourhood of ant nests, within Mediterranean dry grasslands belonging to the phytosociological class Tuberarietea guttatae. The hypothesis was that seeds occurring in clumps may have a higher probability to be harvested than seeds having a scattered distribution. In order to test this hypothesis, four analysis steps were performed. First, pattern of seed production and dispersal of four species was recorded; two of them were more abundant next to ant nests (Tuberaria guttata, Euphorbia exigua), whereas the other two were more abundant away from ant nests (Bromus scoparius and Plantago bellardi). Second, a stochastic model was developed to simulate the observed dispersal patterns of each studied species. Third, 10 seed spatial arrangements in accordance to the distribution patterns created by the model were offered to ants and the location of predated seeds was recorded. Finally, the observed pattern of seed predation was matched to models performed by different distributions of probability. Results showed that the probability of being predated decreased as distance among seeds increased. This preference of ants for high concentration of food items holds down the dominant species sufficiently to allow the subordinates to survive, thus increasing diversity near nests. The observed higher frequency of small-seeded, small-sized, or creeping therophytes close to the ant nests can be therefore seen as an example of indirect myrmecophily.     

Endozoochorous seed dispersal by cattle and horse in a spatially heterogeneous landscape

Seed dispersal has become an important issue in plant ecology and restoration management. In this paper we examined dung germinating seed content and seed deposition patterns of horses (Shetland and Konik breeds) and Scottish Highland cattle grazing two coastal dune nature reserves. Two times 2.5 l of fresh dung from each type of herbivore were collected during seven sessions in the main fruiting season. Dung samples were placed under greenhouse conditions after drying and cooling. Animal defecation patterns were derived from a study of herbivore activities during 6 h observation sessions 8 times a month. One hundred and seventeen plant species i.e. 27% of all species occurring in the study area, were recorded as seedlings emerging from the dung samples. The most abundantly and frequently recorded plant species were Urtica dioica, Juncus spp. and different species of Poaceae and Caryophyllaceae. In general seedling density is high (1158 seedlings/dung sample). Seedling density and species richness were further analysed in order to detect temporal variability and possible animal and site related characteristics. Dung deposition patterns reflect a non-random use of habitats and hence a non-random seed deposition among habitats. Calculated seed deposition per square meter ranged from a few (<10 germinating seeds) to more than 100 in the most frequently selected habitats. From the herbivores’ selective habitat use and their estimated mean retention time we can further assume their ability for inter-habitat endozoochorous seed dispersal. This characteristic of large herbivores is further discussed in the light of nature management and restoration.Indra Lamoot is an aspirant of FWO-Flanders.     

Consequences of limited seed dispersal within simulated annual populations

Summary Theoretical analyses of the consequences of dispersal have been almost entirely limited to studies of colonization. A simulation model is constructed and analyzed to study the effects of dispersal within local plant populations. Rules for the growth and reproduction of individuals are deterministic and related only to competition with other plants. Rules for the dispersal of individual seeds are stochastic; the generalized seed shadow is negatively exponential. The analysis has several interesting results. 1) In small, dense populations of annuals, mortality and growth behavior are significantly affected by small-scale changes in dispersal. 2) Stable population size, at any life stage, is not environmentally fixed (e.g. by the number of safe sites) but is also a function of reproductive capacity and dispersal. 3) There is a minimum productive capability which must be exceeded for a population to be able to respond to increased dispersal. The responsive range of the parameters may be related to the grain of safe site distribution. 4) When the total number of seeds per generation is large, high dispersal decreases the number of germlings that are able to establish. 5) For plants that reach the growing stage, however, expected reproduction can exceed that of genotypes with greater reproductive capacity but less dispersal. 6) An uneven distribution of seeds is essential to a spatial patterning of mortality which can deter competitive extinction.Present address until August 1977: c/o United States Embassy, B.P. 817, Yaounde, Cameroun, West Africa     

Anthesis and seed production in Zostera marina L. (eelgrass) from the chesepeak by

Anthesis and seed production in Zostera marina L. were studied in three areas of the Chesapeake Bay from January to June 1980. Inflorescence primordia with distinguishable anthers and pistils were first observed in February when water temperature was 3°C. Development of the reproductive shoots in the field continued after February as water temperature rose, with the first evidence of pollen release in mid-April (water temperature 14.3°C). Stigmata loss was first observed in samples taken in late April at all locations by as water temperatures averaged above 16°C. Pollination was complete at all locations by 19 May and anthers were no longer present. Few reproduction shoots were found on 3–5 June and seed release was assumed to be complete by this time (water temperature 25°C). The density of flowering shoots ranged from 11 to 19% of the total number of shoots, producing an estimated 8127 seeds m^?2.Comparison of flowering events with other areas along a latitudinal gradient from North Carolina to Canada indicated that reproductive events occurred earlier in the most southern locations and at successively later dates with increasing latitude.     

Seed germination for an annual form of Zostera marina from the sea of Cortez,Mexico

Seed of Zostera marina L. collected at Punta Chueca, Sonora, that was germinated in artificial seawater under growth chamber conditions was less affected by salinity than by temperature. Mean germination was higher for seed collected on reproductive shoots in situ (43%) than for seed collected in fresh beach debris (17%), but no germination was recorded for seed collected in dried beach debris. Mean germination for seed kept at either 15 or 35‰ salinity was approximately equal. Earliest germination was recorded in late April 1980, two weeks after collection, and germination continued through March 1981 at 18–20°C, conditions which are comparable to winter water temperatures in Canal del Infiernillo. Germination was inhibited at 28–32°C, temperatures that are near summer water conditions. The germination responses reflect the adaptive strategies of an annual population to habitat conditions near the southern limit of the species.     

Explosive seed dispersal in two perennial Mediterranean Euphorbia species (Euphorbiaceae)

The distance of explosive dispersal, its pattern in time, and the relative importance of autochory have been studied in two diplochorous species: Euphorbia boetica and E. nicaeensis. The seeds of E. boetica released by explosive dispersal reached a median distance of 156 cm and a maximum of almost 8 m, while the distances reached by the seeds of E. nicaeensis were lower: a median of 132 cm and a maximum of 5 m. The differences in explosive dispersal distance between species seem to depend on both seed mass and caruncle retention. The seeds of both species present a caruncle, but in E. boetica this is tiny, and in most cases is shed during the explosion of the capsules. The distances reached by the seeds of these species, dispersed just by capsule explosion, were similar to or greater than the distances to which ants disperse seeds in the Mediterranean sclerophyllous vegetation. Diplochorous plants may maximize either the distance of primary dispersal or that of secondary dispersal. Given that the seeds of E. boetica, that lose their caruncles, are not gathered by myrmecochorous ants, the results suggest that E. boetica maximizes its primary dispersal distance, whereas E. nicaeensis favors its secondary dispersal.     

Comparison of Zostera marina and maerl community metabolism

The photosynthetic and repiratory metabolism of Zostera marina and maerl communities was compared, in the same area of the Bay of Brest in March–April, using benthic chambers. P–E curves for both oxygen and carbon were established for bottom irradiances between 0 and 525 μmol m⁻² s⁻¹. An exponential function was fitted to calculate daily production. Community metabolic quotients did not differ for maerl and seagrass beds. Community photosynthetic quotients were significantly higher (1.19) whereas community respiratory quotients were lower (0.70) than 1. Maerl and seagrass bed P–E curves mainly differed by the minimum saturating irradiance (E_k). Net community production was estimated to 26.8 mmol C m⁻² d⁻¹ for Z. marina meadows and 8.6 mmol C m⁻² d⁻¹ for maerl beds. The two communities can, therefore, be considered as autotrophic during the March–April period. Community respiration did not differ between Z. marina meadows and maerl beds, with an average value of 53.8 mmol C m⁻² d⁻¹ during a day. In similar environmental conditions, the production of maerl beds corresponds to approximately one third that of seagrass meadows. The maerl communities, therefore, form productive ecosystems, relevant to temperate coastal ecosystems functioning.     

Trade-offs between seed dispersal and dormancy in an amphi-basicarpic cold desert annual

Background and Aims

Several studies have demonstrated trade-offs between depth of seed dormancy and dispersal ability for diaspore-dimorphic species. However, relatively little is known about trade-offs between these two life history traits for a species that produces more than two diaspore morphs. The aim of this study was to investigate the relationship between seed dormancy and dispersal in Ceratocarpus arenarius, an amphi-basicarpic cold desert annual that produces a continuum of dispersal unit morphs.

Methods

A comparison was made of dispersal and dormancy breaking/germination responses of dispersal units from ground level (a), the middle of the plant canopy (c) and the top of the plant canopy (f). Various features of the morphology and mass of dispersal units and fruits (utricles) were measured. The role of bracteoles in diaspore dispersal by wind, settlement onto the soil surface and dormancy/germination was determined by comparing responses of intact dispersal units and fruits. Movement of dispersal units by wind and animals, seed after-ripening, germination phenology and the presence of water-soluble germination inhibitors in bracteoles were tested using standard procedures.

Key Results

Dispersal units a, c and f differed in morphology and mass; in the majority of cases, extremes were exhibited by a and f, with c being intermediate. Overall, relative dispersal ability was f > c > a, whereas relative intensity of dormancy was a > c > f. Bracteoles increased dispersal distance by wind, enhanced settlement of diaspores onto the soil surface and mechanically inhibited germination.

Conclusions

The results provide evidence for a model in which there is a continuous inverse-linear relationship between diaspore dispersal ability and depth of dormancy. Thus, dispersal unit heteromorphism of C. arenarius results in a continuum, from no dispersal ability/high dormancy (dispersal unit a) to high dispersal ability/low dormancy (unit f), which may be a bet-hedging strategy in the cold desert environment.     

Consequences of differential utilization of meristems in the annual Spergularia marina and the perennial S. media

The differences in vegetative expansion and in reproduction associated with the annual Spergularia marina and the perennial S. media were investigated and explained as a function of their differential utilization of meristems. Plant size and the lime of first reproduction depended on at which node transition of the apical meristems from growth to reproduction took place. Leaf birth-rate and number of flowers depended on the time of activation and the proportion of axillary meristems used in producing shoots.     

Differentiation of perennial and annual types due to habitat conditions in the wild riceOryza perennis

Populations of the common wild rice of AsiaO. rufipogon Griff. (=Oryza perennis Moench) were studied with regard to interrelations between life-history traits and habitat conditions. They showed a perennial-annual continuum and differed in reproductive allocation and many other traits. Perennial populations were found in deep swamps, while annual populations were in shallower, temporary swamps which were parched in the dry season. The perennial and annual types tended to be in association with other perennial and annual plants, respectively, suggesting their niche differentiation. Intermediate perennial-annual populations were in communities with a high species diversity. In one population, plants growing on the periphery of the swamp were of annual and those in deeper center were of perennial type. Another population seemed to be differentiated into an annual and an intermediate type in accordance with different degrees of habitat disturbance. The small genetic distances found between the sub-populations suggested their differentiation within a gene pool.Contribution from National Institute of Genetics, Japan, No. 1415.     

Effects of edge/interior and patch structure on reproduction in Zostera marina L. in Chesapeake Bay,USA

We examined the effects of location (patch edge vs. interior) and shoot density (individual, patchy, continuous) on reproduction in three natural and two transplanted Chesapeake Bay (USA) stands of the submerged marine angiosperm Zostera marina L. (eelgrass; Zosteraceae). There were no edge effects on demographic-based reproductive effort or reproductive output (propagule production), and patch structure (individual, patchy, continuous) alone never accounted for the majority of variability in any metric. Transplant site was the most important predictor of eelgrass reproduction response, and relationships among metrics were predictable within sites. Our results suggest that, in Chesapeake Bay eelgrass, environmental factors acting at a regional scale (km) have a stronger impact on reproductive investment than those at a patch scale (1–10 m). Since tradeoffs between clonal and sexual production are mediated primarily by exogenous environmental factors, site selection may be more critical than transplant configuration for producing self sustaining stands, and achieving long-term restoration success.     

Evolution of a unique seed maturity pattern in Croton bonplandianum Baill strengthens ant-plant mutualism for seed dispersal

Summary The female flowers of Croton bonplandianum bear nectar glands which become active during fruit maturation and attain peak activity just prior to the splitting of fruits. This temporal specificity of nectar gland activity is shown to facilitate seed dispersal by ants, which are attracted to the plant only during the fruit maturation period. The nectar glands establish a nectar influence zone with a radius of 60 cm around the plant within which seed dispersal by ants is effective. Seed dispersal by ants is facilitated only if the seeds are placed within this nectar influence zone. This is accomplished by an intriguing evolutionary shift in the maturation pattern of the fruits. Unlike the usual acropetal development, fruit maturation in Croton is temporally asymmetrical, with the fruits nearer the parental axis maturing early. This unique pattern of fruit development together with the polychasial branching system leads to a concentration of seeds within the nectar influence zone and enhances seed dispersal by ants. The proximate factors responsible for this asynchronous fruit maturity were investigated.     

Migration load in plants: role of pollen and seed dispersal in heterogeneous landscapes

Evolution of local adaptation depends critically on the level of gene flow, which, in plants, can be due to either pollen or seed dispersal. Using analytical predictions and individual-centred simulations, we investigate the specific influence of seed and pollen dispersal on local adaptation in plant populations growing in patchy heterogeneous landscapes. We study the evolution of a polygenic trait subject to stabilizing selection within populations, but divergent selection between populations. Deviations from linkage equilibrium and Hardy-Weinberg equilibrium make different contributions to genotypic variance depending on the dispersal mode. Local genotypic variance, differentiation between populations and genetic load vary with the rate of gene flow but are similar for seed and pollen dispersal, unless the landscape is very heterogeneous. In this case, genetic load is higher in the case of pollen dispersal, which appears to be due to differences in the distribution of genotypic values before selection.     

Breeding dispersal in a heterogeneous landscape: the influence of habitat and nesting success in greater snow geese

Despite numerous studies on breeding dispersal, it is still unclear how habitat heterogeneity and previous nesting success interact to determine nest-site fidelity at various spatial scales. In this context, we investigated factors affecting breeding dispersal in greater snow geese (Anser caerulescens atlanticus), an Arctic breeding species nesting in two contrasting habitats (wetlands and mesic tundra) with variable pattern of snowmelt at the time of settlement in spring. From 1994 to 2005, we monitored the nesting success and breeding dispersal of individually marked females. We found that snow geese showed a moderate amount of nest-site fidelity and considerable individual variability in dispersal distance over consecutive nesting attempts. This variability can be partly accounted for by the annual timing of snowmelt. Despite this environmental constraint, habitat differences at the colony level consistently affected nesting success and settlement patterns. Females nesting in wetlands had higher nesting success than those nesting in mesic tundra. Moreover, geese responded adaptively to spatial heterogeneity by showing fidelity to their nesting habitat, independently of snowmelt pattern. From year to year, geese were more likely to move from mesic to high-quality wetland habitat, regardless of previous nesting success and without cost on their subsequent nesting performance. The unpredictability of snowmelt and the low cost of changing site apparently favour breeding-site dispersal although habitat quality promotes fidelity at the scale of habitat patches.     

Land-use and edge effects unbalance seed dispersal and predation interactions under habitat fragmentation

The process of fragmentation can greatly influence plant–animal interactions. To assess the degree to which it affects the balance between two interactions of opposite sign, namely seed dispersal and post-dispersal seed predation, we selected 16 patches of chestnut forest in O Courel and El Bierzo, northwestern Spain. We assessed the effect of fragmentation over two different seed dispersal–predation systems using Helleborus foetidus and Ilex aquifolium as model species. In the first case, field experiments consisted of seed-offering trays with selective exclusion of rodents and ants in a two-way orthogonal design. In the second experiment, we placed experimental branches and trays on the floor to assess seed dispersal and predation. The interactions between several fragment traits and the relative contribution of rodents, ants and birds to seed removal were analyzed by means of generalized linear mixed models. Results show that for H. foetidus, differences in seed dispersal–predation were accounted for by patch shape, which affected mainly the dispersal phase. Major seed dispersal took place in patches with a smaller edge to core ratio and high plant cover (abandoned patches), whilst the latter also showed maximum seed predation. For I. aquifolium, fragmentation effects were significant only for seed predation, which was increased in abandoned patches. This shows that the effects of habitat fragmentation can emerge at different phases depending on specific traits of the interacting animals. It also highlights the importance of traditional land-use practices in species interactions.