Seed size as an indicator of seed quality: a case study of Primula veris

Seed size is a widely accepted measure of seed quality, because many earlier studies have shown that large seeds have high seedling survival, growth and establishment. We tested whether ovule loss increases size of the remaining seeds and whether such size increase affects seedling establishment. We removed all except one flower from inflorescences of Primula veris L. (Primulaceae), a perennial hemicryptophyte herb, at a late stage of flowering. Flower removal (FR) increased seed size by 33% compared to the control plants. We then divided the seeds within each treatment to small, middle-sized and large seeds and carried out a sowing experiment in the field. Within each experimental group, seedling establishment was positively associated with seed size. However, despite size differences, seeds from the FR and control groups had the same seedling establishment probability. Seeds from FR plants had a higher seedling emergence in May than those from control plants, but the number of seedlings alive per sowing plot in the late summer was the same in both experimental groups. Increase in seed mass after partial FR thus did not enhance seedling performance, although seed size variation due to other causes was positively correlated with seedling establishment. Further studies are needed to show whether plastic changes of seed size are usually adaptive or not.     

The interplay between shifts in biomass allocation and costs of reproduction in four grassland perennials under simulated successional change

When perennial herbs face the risk of being outcompeted in the course of succession, they are hypothesized to either increase their biomass allocation to flowers and seeds or to invest more in vegetative growth. We tested these hypotheses in a 3-year garden experiment with four perennials (Hypochaeris radicata, Cirsium dissectum, Succisa pratensis and Centaurea jacea) by growing them in the midst of a tall tussock-forming grass (Molinia caerulea) that may successionally replace them in their natural habitat. In all species except for the short-lived H. radicata, costs of sexual reproduction were significant over the 3 years, since continuous bud removal enhanced total biomass or rosette number. To mimic succession we added nutrients, which resulted in a tripled grass biomass and higher death rates in the shorter-lived species. The simulated succession resulted also in a number of coupled growth responses in the survivors: enhanced plant size as well as elevated seed production. The latter was partly due to larger plant sizes, but mostly due to higher reproductive allocation, which in turn could be partly explained by lower relative somatic costs and by lower root–shoot ratios in the high-nutrient plots. Our results suggest that perennial plants can increase both their persistence and their colonization ability by simultaneously increasing their vegetative size and reproductive allocation in response to enhanced competition and nutrient influxes. These responses can be very important for the survival of a species in a metapopulation context. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Determinants of seed production in Geranium maculatum

Summary We experimentally examined factors limiting seed production in two populations of the perennial woodland herb Geranium maculatum in central Illinois, USA. To test the pollinator-limitation hypothesis, we compared the seed production of plants whose flowers were supplementarily pollinated with outcross pollen to that of control plants receiving natural pollination only. To test if fruit production by early flowers suppresses fruit and seed formation by late flowers, a third group of plants was prevented from producing seed from the first 50% of the flowers to open (stigmas were excised at flower opening). Finally, to test if seed maturation and flower initiation are correlated with photosynthetic capacity, we performed a defoliation experiment in which either the stem leaves within the inflorescence, the stem leaves below the inflorescence, or the rosette leaves were removed during late flowering. Plants that reccived supplemental pollination produced 1.5–1.6 times more seeds than control plants. We found no difference between hand-pollinated plants and controls in mortality, flowering frequency or number of flowers produced in the year following the experiment. In both control and hand-pollinated plants, the fruit set and total seed production of early flowers were more than twice as high as those of late flowers. In one of the two populations, plants whose early flowers were prevented from setting seed produced significantly more seeds from their late flowers than did control plants. Seed predation was low and did not differ between early and late flowers. Leaf removal did not affect seed number or size in the year of defoliation, nor did it reduce survival or flower production in the subsequent year. This suggests that the plants were able to compensate for a partial defoliation by using stored resources or by increasing photosynthetic rates in the remaining leaves. Taken together, the results demonstrate that both pollinator activity and resource levels influence patterns of seed production in G. maculatum. While seed production was pollinatorlimited in both populations, a seasonal decline in resource availability was apparently responsible for the low seed production by late flowers.     

Long-term variability in seed size and seedling establishment of <Emphasis Type="Italic">Maianthemum bifolium</Emphasis>

Relatively few studies conducted in natural plants populations focus on the relationship between seed size and their germination ability and seedling establishment. Maianthemum bifolium is a perennial herb that spreads vegetatively through rhizomatous growth and reproduces through seeds. However, this species is characterized as seed and microsite limited, and under undisturbed conditions seedlings are not noted. The studies were conducted in two populations of M. bifolium in six subsequent seasons. The mean seed mass was negatively correlated both per ramet as well as in the fruit with the number of seeds, and positively with its height and the number of flowers. The long-term mean annual production of seeds in the populations was 37 and 56 seeds per m². The seeds from both populations had similarly high germination abilities that were approximately 90% under laboratory conditions, 60% in garden, and 55% in the natural habitat. Seeds from four size classes were sown and a positive correlation was noted between seedling establishment and the mass of the seeds from which they grew (r_S = 0.64). Seedling survival was also significantly correlated with seed mass.     

The role of sexual and clonal reproduction in maintaining population in Fritillaria camtschatcensis (L.) Ker-Gawl. (Liliaceae)

Fritillaria camtschatcensis can reproduce by means of both sexual reproduction and clonal multiplication. Despite prolific seed production, no seedlings have been found for several years in natural populations on Mt Hakusan. The purposes of this paper are to clarify: (i) whether population size is maintained mainly by clonal multiplication; and (ii) if this is the case, to what extent occasional seedling establishment affects population growth rate and population structure. Two permanent quadrats were placed in subalpine meadows in 1992 on Mt Hakusan. Plant size, location and reproductive states for all ramets in the quadrats were recorded every year. Projection matrices were created based on field census, and computer simulation experiments were performed. Fritillaria camtschatcensis had two types of flower, male flower and cosexual flower, and they were changeable. This is the first report on sex lability in Fritillaria. Clonal growth was more closely correlated with life-history stages, especially with sexual states than with plant size. The population growth rate, , was 1.006 for the Mizuyajiri population and 1.047 for the Nanryu population, respectively. Seedlings were found in 1996 for the first time. These facts indicate that populations of F. camtschatcensis on Mt Hakusan can usually be maintained by clonal multiplication. However, it is not yet certain whether seeds germinate every year or whether a flush of seedling emergence occurs once in every few years in natural populations. Computer simulation revealed that: (i) there was a critical germination rate above which population growth rate suddenly increased; and (ii) occasional seedling establishment could provide almost the same contribution to population growth rate as that of annual seedling establishment. These results suggest that population size can be maintained mainly by clonal multiplication, and the role of sexual reproduction lies beyond maintaining the population size in F. camtschatcensis.     

Density-dependent flowering phenology,outcrossing, and reproduction in Impatiens capensis

Summary We investigated the effects of plant density on cleistogamous (CL) and chasmogamous (CH) flowering phenology and seed production in a natural Impatiens capensis population, by censusing individually marked plants at experimentally reduced and natural densities. CL flowering was earlier at natural density. This plastic density response may have resulted from a stress-related threshold for CL flowering; slower growing plants at natural density flowered earlier. Although apparently triggered by slow early growth, early CL flowering also involved an additional cost for later growth rate. In contrast, CH flowering was unrelated to relative growth rate, but apparently required a size threshold. Experimental density reduction resulted in earlier CH flowering and a dramatic increase in the percentage of plants producing CH flowers. Individual CL and CH flowering duration and flower production were greater at reduced density. These density-dependent effects caused differences between treatments in the shape and location of population flowering phenology curves. Moreover, the percentage of CH seeds produced per individual was much higher at reduced density. At natural density total seed production per plant was lower and more hierarchical than at lower density, suggesting that dominance and suppression shape jewelweed fitness distributions.     

Plasticity of reproductive components at different stages of development in the annual plant Thlaspi arvense L.

Summary This study examines the effect of different densities and the removal of all neighbours at different stages of development on all components of reproduction in the inbreeding annual Thlaspi arvense L. A 64-fold increase in density significantly reduced all repooductive components. The number of flower buds per plant was decreased most strongly; the order of decreasing plasticity among the other components was number of capsules per flower, individual seed weight, ovule number per capsule, flowers per flower bud and seeds per ovule. Removing neighbours at all stages of development increased seed yield of plants in comparison to the control without density reduction, but patterns of plasticity depended strongly on time of treatment. The main effect of the removal of neighbours at the vegetative stage was to increase the number of flowers per plant, but the number of ovules per capsule and seed weight increased also, and abortion of capsules decreased. Removing neighbours at the onset of flowering initially failed to affect flower number per plant, instead it resulted in a strong reduction of capsule abortion and an increase in seed weight. However, several weeks after flowering had initially ceased, fresh lateral inflorescences were produced, resulting in a second flush of reproduction. Removing neighbours at the stage of fruit ripening resulted at first only in an increase in seed witht, but later a second reproductive phase occurred. Fresh lateral branches were produced, but the apical meristem was also reactivated. The overall pattern of plasticity among all reproductive components in response to a removal of neighbours was the same as in response to density. The position of a capsule along the inflorescence influenced its number of ovules, the rate of seed abortion and the mean weight of seeds, with the type of effect depending on the developmental stage at which neighbours were removed. Significant negative correlations were found between the mean weight of seeds and the number of seeds in a capsule under all treatments.     

Regulation of reproduction in wild and cultivatedLycopersicon esculentum Mill. by vesicular-arbuscular mycorrhizal infection

Summary We examined how mycorrhizal infection byGlomus etunicatum Becker and Gerd. affected flowering phenology and components of reproduction in eight wild accessions and two cultivars ofLycopersicon esculentum Mill. We did this by performing a detailed demographic study of flower, fruit and seed production. Mycorrhizal infection had variable effects on the ten accessions. Infection significantly decreased the time taken to initiate flowering in some accessions. In addition, infection increased flowering duration in some accessions. In many accessions, infection significantly increased seed production, primarily by increasing the number of inflorescences and infructescences. In some accessions, mycorrhizal infection also increased the proportion of flowers that produced mature fruits or the number of seeds per fruit. Among accessions, shoot phosphorus content was correlated with seed productivity for both mycorrhizal and non-mycorrhizal plants. However, non-mycorrhizal plants produced more seed biomass per mg of shoot phosphorus than mycorrhizal plants.     

Consequences of variation in flowering phenology for seed head herbivory and reproductive success in Erigeron glaucus (Compositae)

Summary We examined the relationship between flowering phenology, reproductive success (seed production only), and seed head herbivory for 20 similarly sized clones of Erigeron glaucus growing at Bodega Bay Reserve, northern California, USA. Although clones tended to reach peak flowering on the same date, they differed in the proportion of their total flowers produced around that date (flowering synchrony). Clones also differed in the number and density of flower heads presented at any one time to pollinators and herbivores (floral display). Both of these characteristics had consequences for herbivory and plant reproductive success. The proportion of flower heads damaged by insect herbivores was greater for clones that concentrated flowering activity during the main flowering period for the population as a whole (high synchrony) compared to clones that spread flowering out temporally. The primary reason for this result was that clones with low flowering synchrony produced a significant proportion of their flower heads during the fall and therefore, escaped attack by the tephritid fly, Tephritis ovatipennis. Clones with intermediate synchrony had lower seed success (total number of viable seeds produced over the year) than clones with either low or high synchrony. The proportion of flower heads damaged by insect herbivores and number of tephritid flies reared from flower heads were both negatively correlated to floral display while seed head mass and germination rates were positively related to display. Thus, clones which produced dense floral displays were favored both in terms of reduced herbivory and increased successful seed production.     

不同生境与花部特征对巫山淫羊藿结实特性的影响

对不同生境中巫山淫羊藿结实特性进行比较研究。结果表明:(1)三个种群在繁殖时期中不同花部特征的植株繁殖投资差异不大,如每株分枝数、每株花序数和每株开花数差异均不显著;(2)三个种群中每株结实数、每株结实率、每花序结实数在不同花部特征中差异显著,PLS和SAA要显著大于PESS;(3)三种花部特征在不同生境中单株分枝数和单株花序数均无显著差异。在不同花部特征中种群1开花数量相较少,因而在结实方面均值均要小于种群2和种群3;(4)不同花部特征的果实饱满种子数、败育数和败育率均具有显著差异,而种子数差异不大,但在种群间均无显著的差异。说明种子的多少更多地受到花部特征的影响。总之,巫山淫羊藿的结实特性受到生境和花部特征的双重影响,其中花型比生境对巫山淫羊藿结实特性的影响更为明显。     

The inheritance of partial self-compatibility inBrassica oleracea L.: results from a half diallel homozygous for a highly recessive S-allele

Summary In a study of partial self-compatbility inBrassica oleracea, flower number, seeded siliqua and seed production were recorded on self- and cross-pollinated inflorescences of the progenies of a half diallel between five unrelated inbred plants homozygous for the same recessiveS-allele.On cross-pollinated inflorescences significant amounts of additively controlled genetic variation were found for seed set per flowering site and its two components, seeded siliquae per flower and seeds per seeded siliquae. Considerable heterosis and gene interaction were also present and a simple additive dominance model did not fully explain the observed variation.On self-pollinated inflorescences, additive gene action was absent for the seed production variates although differences between progenies were highly significant and heterosis was present. Complex gene interactions were considered to be responsible. The characteristics of the method of assessment used and the relationship between self- and outcross seed production are discussed. It appears unlikely that the component lines could be selected for reduced self seed set, but selection for higher outcross seed set may be possible.     

Early in vitro flowering and seed production in culture in <Emphasis Type="Italic">Dendrobium</Emphasis> Chao Praya Smile (Orchidaceae)

Plantlets of Dendrobium Chao Praya Smile maintained in vitro were induced to flower, which produced viable seeds within about 11 months. A two-layer (Gelrite-solidified layer topped with a layer of liquid medium of the same volume and composition) culture system containing benzyladenine (BA) at 11.1 μM induced the highest percent of flowering (45%) in plantlets within 6 months from germination. The percentage of inflorescence induction was increased to 72% by pre-selecting morphologically normal seedlings prior to two-layer culture. Plantlets in culture produced both complete (developmentally normal but smaller than flowers of field grown plants) and incomplete flowers. Pollen and female reproductive organs of in vitro-developed complete flowers were morphologically and anatomically similar to flowers of field grown plants. In addition, 65% of the pollen grains derived from in vitro-developed flower were tetrad suggesting that regular meiosis occurred during microsporogenesis. The percentage of germination of pollen grains derived from in vitro-developed flowers and flowers of field grown plants, incubated on modified Knops’ medium for 8 days, were 18.2 and 52.8%, respectively. Despite a lower percentage of germination of the pollen grains derived from in vitro-developed flowers, flowers induced in culture could be self-pollinated and developed seedpods with viable seeds. Nearly 90% of these seeds developed into protocorms on germination in vitro. These seedlings were grown in culture and induced to flower in vitro again using the same procedure.     

Chronic herbivory negatively impacts cone and seed production,seed quality and seedling growth of susceptible pinyon pines

Although herbivory often reduces the reproduction of attacked trees, few studies have examined how naturally occurring insect-resistant and susceptible trees differ in their reproduction, nor have these effects been experimentally examined through long-term herbivore removals. In addition, few studies have examined the effects of herbivory on the quality of seeds produced and the implications of reduced seed quality on seedling establishment. We evaluated the impact of chronic herbivory by the stem-boring moth, Dioryctria albovittella, on cone and seed production of the pinyon pine (Pinus edulis) during two mast years. Three patterns emerged. First, moth herbivory was associated with reductions in cone production, viable seed production and seed mass. Specifically, pinyons susceptible to moth attack had 93–95% lower cone production, and surviving cones produced 31–37% fewer viable seeds, resulting in a 96–97% reduction in whole tree viable seed production. In addition, surviving seeds from susceptible trees had 18% lower mass than resistant trees. Second, long-term experimental removal of the herbivore resulted in increased rates of cone and seed production and quality, indicating that moth herbivory was the driver of these reductions. Third, seed size was positively associated with seed germination and seedling biomass and height, suggesting that trees suffering chronic herbivory produce poorer quality offspring. Thus, the resistance traits of pinyons can affect the quality of offspring, which in turn may affect subsequent seedling establishment and population dynamics.     

Seed size variation in the palm Euterpe edulis and the effects of seed predators on germination and seedling survival

Intraspecific variation in seed size is common in wild plant populations and has important consequences for the reproductive success of individual plants. Multiple, often conflicting evolutionary forces mediated by biotic as well as abiotic agents may maintain such a variation. In this paper we assessed seed size variation in a population of the threatened, commercially important palm Euterpe edulis in southeast Brazil. We investigated (i) how this variation affects the probability of attack by vertebrate and invertebrate post-dispersal seed predators, and (ii) if seed size influences the outcome of seeds damaged by beetles in terms of seed germination and early survival of seedlings. Euterpe edulis seeds varied in diameter from 8.3 to 14.1 mm. Neither insects nor rodents selected the seeds they preyed upon based on seed size. Seed germination and total, shoot and root biomasses of one-year seedlings were significantly and positively affected by seed size. Root biomass and seedling survival were negatively affected by seed damage caused by a scolytid beetle (Coccotrypes palmarum) whose adults bore into seeds to consume part of the endosperm, but do not oviposit on them. Seed size had a marginally significant effect on seedling survival. Therefore, if any advantage is accrued by E. edulis individuals producing large seeds, this is because of greater seed germination success and seedling vigor. If this is so, even a relatively narrow range of variation in seed size as observed in the E. edulis population studied may translate into differential success of individual plants.     

Origins of variance in seed number and mass: interaction of sex expression and herbivory in Lomatium salmoniflorum

Summary As in many plant species, Lomatium salmoniflorum (Umbelliferae) individuals produce many flowers, only a subset of which produce mature seeds that escape seed parasitism and enter the seed bank. The interrelationships between the timing and number of flowers produced, sex expression, seed set, and seed parasitism were studied for their direct and indirect effects on the numbers and masses of viable seeds produced by individual plants. In a sample population of 369 plants that produced 161 386 flowers, 76% of the plants produced some hermaphroditic flowers. The percentage of hermaphroditic flowers increased significantly with the total number of flowers produced by a plant. Seed set was 65–90% in plants producing >600 flowers, but was highly variable in plants producing fewer flowers. Hand-pollinated plants showed the same pattern of seed set, suggesting that variable seed set in small plants may result from insufficient resources for seed development. The majority of schizocarps was produced by only 12% of the plants. Parasites killed 24.5% of the seeds prior to dispersal. Another 14.5% of the seeds lacked endosperm. Hence, the initial 161 386 flowers, which included 25874 hermaphroditic flowers each capable of producing two seeds, produced 42 468 seeds of which an estimated 25906 entered the seed bank as undamaged seeds with fully developed endosperm. Path analysis indicated that the number of hermaphroditic flowers on a plant and the percentage of seeds attacked by seed parasites had the greatest direct effects on the number of viable seeds entering the seed bank. The date at which a plant began flowering and the percentage of flowers setting seed had smaller or only indirect effects on viable seed production. Mean seed mass for plants was not significantly related to any of the factors that affected seed number, but little of the variance in seed mass occurred among plants. Masses of intact seeds in the population ranged 9-fold in both 1987 and 1988. Thirty-five percent of the variance was among seeds within umbels, 46% was among umbels within plants, and only 19% was among plants. The large variation among umbels within plants resulted from a seasonal pattern in which seeds from umbels produced late in the spring had lower mean seed masses than seeds from umbels produced early in the spring. Overall, the results indicate that both direct and indirect interactions between number of flowers, the date of initiation of flowering, seed set, and seed parasitism affect the number of viable seeds entering the seed bank. These interactions strongly bias viable seed output to a small minority of plants that produce many seeds with a wide range of masses over the growing season.     

Establishment and reproduction of Aeschynomene virginica (L.) (Fabaceae) a rare,annual, wetland species in relation to vegetation removal and water level

Aeschynomene virginica is a rare annual plant found in freshwater tidal wetlands of the eastern United States. We hypothesized that standing vegetation and water inundation were two important environmental factors in its population dynamics. To test these hypotheses, we sowed seeds into plots with undisturbed vegetation or plots with all aboveground vegetation removed in 1998 and 1999. Presence/absence of seedlings was noted and seedling survival to reproduction, final size, and seed set were measured throughout both growing seasons. Seedling establishment from germination to the first true leaf stage increased with decreasing water depth. Vegetation removal plots had greater seedling establishment, higher seedling survival, and higher seed set per plant than non-removal plots. In a greenhouse study designed to test the effects of water level on seed germination and seedling establishment, no seedlings established in submerged soils, and seed germination and seedling establishment were lower in waterlogged soil than in wet soil. Physical stress associated with deeper water likely limits the distribution of A. virginica to higher elevations, where seeds that colonize patches with low vegetative cover are more likely to produce reproductive adults that produce more seeds relative to patches with established vegetation. A. virginica appears to be a fugitive species specializing on open habitat patches in tidal wetlands. This species may be dependent on disturbances for population establishment and maintenance.     

The inheritance of partial self-compatibility in Brassica oleracea L. inbreds homozygous for different S-alleles

Summary In a study of partial self-compatibility in Brassica oleracea, flower number, seeded siliqua and seed production were recorded on self and cross-pollinated inflorescences of 32 progenies obtained by inter-crossing and selfing 8 plants homozygous for the incompatibility alleles S₂, S₅, S₁₅ and S₄₅.Progeny differences for both self-and out-cross seed production could be largely attributed to G.C.A. effects which were essentially uncorrelated. For cross-pollinated inflorescences heterosis was also important. Significant differences were found for selfed seed set and its two components, the proportion of flowers producing seeded siliquae and the numbers of seed per seeded siliqua, between parents with the same S-allele which could not be attributed to S-genotype alone. No evidence of increased self-compatibility in particular S-allele heterozygotes (mutual weakening) could be found.Outcross seed production depended primarily on the numbers of seeds set per seeded siliqua while self seed production was largely determined by the proportion of flowers which produced seeded siliqua. It is suggested that this is a key character for the production of inbred lines with reduced partial self-compatibility.     

Effects of seed dispersal on Juniperus communis recruitment on a Mediterranean mountain

Abstract. The recruitment of the relict shrub Juniperus communis on a mountain in SE Spain was studied during the period 1994–1998. The main objective was to determine both the quantitative and qualitative effects of bird dispersal on seedling establishment. Seed removal by birds, seed rain, post‐dispersal seed predation, germination, and seedling emergence and survival were analysed in different microhabitats. Birds removed 53 ‐ 89% of the seeds produced by plants. Seed rain was spatially irregular as most seeds accumulated near stones used by birds as perches and below mother plants while a few seeds were dropped in wet meadows and open ground areas. Post‐dispersal seed predation by rodents affected < 10% of dispersed seeds but varied significantly among microhabitats. Only 3.6 ‐ 5.5% of dispersed seeds appeared viable, as many seeds had aborted or showed wasp damage. Seeds germinated in the second and third springs after sowing, reaching a germination percentage of 36%. Seedling emergence was concentrated in wet meadows. Seedling mortality was high (75–80%), but significantly lower in wet meadows, the only microhabitat where seedlings could escape from summer drought, the main mortality cause. Seed abortion, germination and seedling mortality proved to be the main regeneration constraints of J. communis on Mediterranean mountains. Birds exerted a strong demographic effect, although their qualitative effect was limited by abiotic factors which caused the pattern of seed rain to differ from the final pattern of recruitment between microhabitats.     

Efficiency of endozoochorous seed dispersal in six dry-fruited species (Cistaceae): from seed ingestion to early seedling establishment

We combined laboratory and nursery experiments to analyse the effectiveness of sheep as endozoochorous seed dispersers of six native shrubby Cistaceae species collected in SE Spain (Helianthemum apenninum (L.) Mill., H. violaceum (Cav.) Pers., Fumana ericoides (Cav.) Grand., F. thymifolia (L.) Spach, Cistus monspeliensis L. and C. laurifolius L.), considering the main stages after seed ingestion, i.e. seed recovery, seed germination, seedling emergence and early seedling establishment. Seed recovery after gut passage was high (around 40%) for all the species, except F. thymifolia (12%). Most seeds (ca. 90%) were recovered within 48 h after ingestion for all the species, although seeds were still recovered up to 96 h after ingestion. Gut passage increased germination up to seven-fold compared to non-ingested seeds. Furthermore, seedling emergence from seeds contained in pellets was overall similar (intact pellets) to or higher (crumbled pellets) than emergence from seeds without dung. Survival of emerged seedlings and mass of seedlings after 20 days were not reduced by dung. Sheep act therefore as effective dispersers of these Cistaceae species by scattering seeds and promoting germination, while faeces do not hamper seedling establishment. We conclude that the interaction between herbivorous ungulates and these dry-fruited species may be considered a mutualism qualitatively similar to the mutualism between frugivorous vertebrates and fleshy-fruited plants.     

Differential seed and seedling predation by crabs: impacts on tropical coastal forest composition

Recently, the importance of seed predation by crabs on mangrove species distributions and densities has been established by several studies. In a tropical coastal terrestrial forest in Costa Rica, we investigated the relative importance of predation by land crabs, Gecarcinus quadratus, and hermit crabs, Coenobita compressus, on measured forest composition through a series of seed removal and seedling establishment experiments. We also used natural light-gaps and adjacent non-gap sites to test how canopy cover affects crab predation (seed removal) and seedling establishment. We found fewer tree species (S=18) and lower densities (seedlings, saplings, and adults) in the coastal zone within 100 m of coastline, than in the inland zone (S=59). Land crab densities were higher in the coastal zone (3.03±1.44 crabs m^–2) than in the inland zone (0.76±0.78 crabs m^–2), and hermit crabs were not present in the inland zone. Seed removal and seedling mortality also were higher in the coastal zone than in the inland zone, and in the open controls than in the crab exclosures. Mortality of seeds and seedlings was two to six times higher in the controls than exclosures for four of the five experiments. Crabs preferred seeds and younger seedlings over older seedlings but showed no species preferences in the seed (Anacardium excelsum, Enterolobium cyclocarpum, and Terminalia oblonga) and seedling (Pachira quinata and E. cyclocarpum) stages. We conclude that the observed differences in tree densities were caused by differential crab predation pressure along the coastal gradient, while the differences in species composition were due to predator escape (satiation) by seed quantity. Canopy cover did not affect seed removal rates, but did affect seedling survival with higher mortality in the non-gap versus gap environments. In summary, crab predation of seeds and seedlings, and secondarily canopy cover, are important factors affecting tree establishment in terrestrial coastal forests.