Drivers of plant species’ potential to spread: the importance of demography versus seed dispersal

Understanding the ability of plants to spread is important for assessing conservation strategies, landscape dynamics, invasiveness and ability to cope with climate change. While long‐distance seed dispersal is often viewed as a key process in population spread, the importance of inter‐specific variation in demography is less explored. Indeed, the relative importance of demography vs seed dispersal in determining population spread is still little understood. We modelled species’ potential for population spread in terms of annual migration rates for a set of species inhabiting dry grasslands of central Europe. Simultaneously, we estimated the importance of demographic (population growth rate) versus long‐distance dispersal (99th percentile dispersal distance) characteristics for among‐species differences in modelled population spread. In addition, we assessed how well simple proxy measures related to demography (the number and survival of seedlings, the survival of flowering individuals) and dispersal (plant height, terminal velocity and wind speed during dispersal) predicted modelled spread rates. We found that species’ demographic rates were the more powerful predictors of species’ modelled potential to spread than dispersal. Furthermore, our simple proxies were correlated with modelled species spread rates and together their predictive power was high. Our findings highlight that for understanding variation among species in their potential for population spread, detailed information on local demography and dispersal might not always be necessary. Simple proxies or assumptions that are based primarily on species demography could be sufficient.     

Possible Age-Dependent Variation in Egg-Loaded Host Selectivity of the Pierid Butterfly, Anthocharis scolymus (Lepidoptera: Pieridae): A Field Observation

To understand better the effects of age on host selection through conspecific egg detection by Anthocharis scolymus females, field observations were performed at a graveyard where the insect population was isolated and the host plant was limited to the cruciferous plant, Turritis glabra. We chased females and recorded their oviposition behaviors and the conditions of plants which females approached. Older females tended to avoid ovipositing on egg-loaded host plants and selectively laid eggs on nonloaded host plants. This result was not confounded by other factors such as seasonality, air temperature, plant height, plant phenological stage, surrounding vegetation, host plant density, and extent of plant damage. We discuss the possibility that females make an oviposition decision on how to lay their limited number of eggs during their remaining lifetime.     

Effects of local density on insect visitation and fertilization success in the narrow-endemic Centaurea corymbosa (Asteraceae)

Plant population size and density can influence the interactions between plants and pollinators, and affect plant reproductive success. We investigated the effects of local conspecific density on insect visitation and fertilization success in the rare, cliff-dwelling, self-incompatible Centaurea corymbosa , in which fecundity plays a key role in variation in population growth rate among years and among the six extant populations. Plant size, capitulum size, the abundance of co-flowering species, and the weather conditions were added as covariates in the analyses. Over three populations and two years (1995 and 2002), fertilization rate was positively related to the number of flowering conspecifics within 10 m. Fertilization rate varied among populations, but this variation over all six populations in 2002 could not be attributed to differences in population size. Observations in one population in 2003 showed that there was no difference in insect visitation per capitulum between plants in sparse vs dense patches whilst plants from sparse patches suffered reduced fertilization rate. Visitation and fertilization rates were not affected by plant size and the abundance of co-flowering species, but weather conditions at the time of flowering had a strong effect on both variables. Capitulum size had a positive effect on visitation rate, but an effect on fertilization rate only in 1995 and 2002 and not in 2003. Our results suggest that pollen limitation affects fertilization rate in C. corymbosa due to limited compatible mate availability rather than pollinator limitation. They agree with previous genetic results derived from paternity analysis. Whether or not the benefits of local spatial agregation to reproductive success result in increased individual fitness will depend on the relative reduction in survival of vegetative stages due to intra-specific competition.     

Rotifer population spread in relation to food, density and predation risk in an experimental system

1. Despite the popular use of diffusion models to predict the spatial spread of populations over time, we currently know little about how diffusion rates change with the state of the environment or the internal condition of individuals. To address this gap in our understanding, we measured rates of spread for many populations of the rotifer Brachionus calyciflorus in a suite of well-replicated experiments. 2. In one set of experiments, we manipulated food availability and population density along a continuous range of densities. In a second set, we manipulated the internal state of entire populations via food deprivation and exposure to predator kairomones. 3. Across replicate populations, diffusion rates were positively correlated with conspecific density. Diffusion rates were negatively correlated with food availability, especially when conspecific density was high. Diffusion rates of food-deprived populations or those exposed to predation risk were lower than controls. 4. Our results provide direct experimental evidence that rates of population spread are conditional on population density, food availability, body condition and predation risk.     

The effect of a rosette-crown fly, Botanophila turcica, on growth,biomass allocation and reproduction of the thistle Carthamus lanatus

Plant growth and reproductive output of the winter annual invasive thistle, Carthamus lanatus was characterised in relation to plant size in three native populations in southern France. The effects of the rosette-crown feeding fly Botanophila turcica on these plant characteristics were assessed by comparing unattacked with naturally attacked plants at each site and by a field experiment. Indirect effects of B. turcica on plant seed production were also compared with direct seed loss caused by a guild of capitulum-feeding insects that incidentally attacked the marked plants at these sites. C. lanatus showed no size or weight requirement for flowering, but larger flowering plants produced less total receptacle surface and less seed production (female reproductive potential) in proportion to plant weight than smaller flowering plants. B. turcica did not select hosts on the basis of size or density. B. turcica reduced plant relative growth rate (RGR) in all situations, but attacked plants compensated fully at two of three sites as attack failed to halt rosette growth. Attacked plants suffered 12 % mortality, and 71 % lower seed production than unattacked plants at the site with the lowest RGR. This corresponded to 9 % lower seed production for the whole thistle population compared to 8.6–19.5 % direct seed loss to capitulum insects across all sites.     

巴丹吉林沙漠东缘天然梭梭种群空间分布异质性

梭梭(Haloxylon ammodendron(C.A.Mey.)Bunge)是生长于沙漠地区的一种特有灌木,在维持荒漠生态系统平衡和地区经济发展中发挥着不可替代的作用。为掌握天然梭梭种群空间分布规律,以巴丹吉林沙漠东缘塔木素地区的"野生肉苁蓉及梭梭产籽基地"为试验区,采用样线法测定梭梭株高、冠幅直径,统计梭梭分布密度,分析梭梭种群数量特征空间分布特征及其与生境之间的关系。结果表明:梭梭林密度、株高、冠幅直径均符合正态分布,表现为强度变异;株高、密度、冠幅直径的半方差函数理论模型均为高斯理论模型,其相应的变程为1249、909、1035 m;空间自相关比例均超过70%,受随机因素影响较小,保持着较好的天然分布和生长状态。梭梭种群数量特征的分形维数均大于1.5,空间依赖性强,空间结构性好。长期风蚀作用,小地形海拔高度体现为不同沙层厚度,梭梭株高、冠幅直径和密度在空间上的分布呈西高东低,北高南低的趋势,与海拔高度存在极显著相关;梭梭群落的空间异质性表现出一定的适度沙埋效应。     

Warming increases the spread of an invasive thistle

Background

Global warming and shifted precipitation regimes increasingly affect species abundances and distributions worldwide. Despite a large literature on species'' physiological, phenological, growth, and reproductive responses to such climate change, dispersal is rarely examined. Our study aims to test whether the dispersal ability of a non-native, wind-dispersed plant species is affected by climate change, and to quantify the ramifications for future invasion spread rates.

Methodology/Principal Findings

We experimentally increased temperature and precipitation in a two-cohort, factorial field study (n = 80). We found an overwhelming warming effect on plant life history: warming not only improved emergence, survival, and reproduction of the thistle Carduus nutans, but also elevated plant height, which increased seed dispersal distances. Using spatial population models, we demonstrate that these empirical warming effects on demographic vital rates, and dispersal parameters, greatly exacerbate spatial spread. Predicted levels of elevated winter precipitation decreased seed production per capitulum, but this only slightly offset the warming effect on spread. Using a spread rate decomposition technique (c*-LTRE), we also found that plant height-mediated changes in dispersal contribute most to increased spread rate under climate change.

Conclusions/Significance

We found that both dispersal and spread of this wind-dispersed plant species were strongly impacted by climate change. Dispersal responses to climate change can improve, or diminish, a species'' ability to track climate change spatially, and should not be overlooked. Methods that combine both demographic and dispersal responses thus will be an invaluable complement to projections of suitable habitat under climate change.     

Interactions between a pollinating seed predator and its host plant: the role of environmental context within a population

Plant–insect interactions often are important for plant reproduction, but the outcome of these interactions may vary with environmental context. Pollinating seed predators have positive and negative effects on host plant reproduction, and the interaction outcome is predicted to vary with density or abundance of the partners. We studied the interaction between Silene stellata, an herbaceous perennial, and Hadena ectypa, its specialized pollinating seed predator. Silene stellata is only facultatively dependent upon H. ectypa for pollination because other nocturnal moth co‐pollinators are equally effective at pollen transfer. We hypothesized that for plants without conspecific neighbors, H. ectypa would have higher visitation rates compared to co‐pollinators, and the plants would experience lower levels of H. ectypa pollen deposition. We predicted similar oviposition throughout the study site but greater H. ectypa predation in the area without conspecific neighbors compared to plants embedded in a naturally high density area. We found that H. ectypa had consistently higher visitation than moth co‐pollinators in all host plant contexts. However, H. ectypa pollinator importance declined in areas with low conspecific density because of reduced pollen deposition, resulting in lower seed set. Conversely, oviposition was similar across the study site independent of host plant density. Greater likelihood of very high fruit predation combined with lower pollination by H. ectypa resulted in reduced S. stellata female reproductive success in areas with low conspecific density. Our results demonstrate local context dependency of the outcomes of pollinating seed predator interactions with conspecific host plant density within a population.     

Structure of the woody vegetation in disturbed and undisturbed arid savanna,Botswana

The structure of woody vegetation was studied in little disturbed arid savanna and in adjacent over-grazed vegetation. In the over-grazed areas density and cover of woody plants were higher than in the less disturbed vegetation. The difference was accounted for by one species, Acacia mellifera, which was strongly dominant in the overgrazed vegetation. In the open savanna, the woody species varied in height from small shrubs to trees, while the dense shrub vegetation was of uniformly low stature.It is suggested that, while the differences in total abundance of woody species depend on differences in the amount of soil water available for woody growth, differences in species composition and height distribution are governed by the spatial and temporal distribution of water in the soil profile.     

Direct and plant trait‐mediated effects of the local environmental context on butterfly oviposition patterns

Variation in the intensity of plant–animal interactions over different spatial scales is widespread and might strongly influence fitness and trait selection in plants. Differences in traits among plant individuals have been shown to influence variation in interaction intensities within populations, while differences in environmental factors and community composition are shown to be important for variation over larger scales. However, little is still known about the relative importance of the local environmental context vs. plant traits for the outcome of interactions within plant populations. We investigated how oviposition by the seed‐predator butterfly Phengaris alcon on its host plant Gentiana pneumonanthe was related to host plant traits and to local environmental variation, as well as how oviposition patterns translated into effects on host plant fruit set. We considered the local environmental context in terms of height of the surrounding vegetation and abundance of the butterfly's second host, Myrmica ants. The probability of oviposition was higher in plants that were surrounded by lower vegetation, and both the probability of oviposition and the number of eggs increased in early‐flowering and tall plants with many flowers in the three study populations. Flowering phenology, shoot height and flower production were, in turn, related to higher surrounding vegetation. Myrmica abundance was correlated with vegetation height, but had no effect on oviposition patterns. Oviposition and subsequent seed predation by the caterpillars strongly reduced host plant fruit set. Our results show that plant–animal interactions are context‐dependent not only because the context influences the abundance or the behavior of the animal interactor, but also because it influences the expression of plant traits that affect the outcome of the interaction. The results also demonstrate that heterogeneity in environmental conditions at a very local scale can be important for the outcomes of interactions.     

Density-dependent shoot-borer herbivory increases the age of first reproduction and mortality of neotropical tree saplings

Shoot herbivory by the sapling specialist shoot-borer Cromarcha stroudagnesia (Lepidoptera, Pyralidae, Chrysauginae) is shown to have large direct and indirect effects on the rates of height increment and mortality of saplings of its host tree, Tabebuia ochracea (Bignoniaceae), in the secondary successional tropical dry forests of the Area de Conservación Guanacaste, Costa Rica. Experiments and field observations over 3-4 years show a substantial reduction in sapling height increments due to C. stroudagnesia herbivory, of equivalent magnitude to the difference in height increments between undamaged saplings in canopy gaps and full understorey shade. Extrapolating this data at average amounts of C. stroudagnesia herbivory increases the duration of the pre-reproductive sapling life stage by about 40% relative to undamaged plants. This is an underestimate, as top shoot herbivory by C. stroudagnesia also increased the probability of canopy gap saplings being overtopped and shaded by surrounding vegetation. Sapling mortality was increased by C. stroudagnesia herbivory, with 11.8% of the most heavily damaged young saplings dying in 3 years while no undamaged saplings died. Cromarcha stroudagnesia herbivory strongly increases with the number of conspecific T. ochracea saplings and the number of conspecific shoots within 50 m of focal saplings. It is therefore likely to disproportionately decrease the number of saplings and rate of recruitment to reproductive age in areas of high conspecific sapling density. These results suggest that sapling herbivory, especially herbivory of terminal meristems, has an important but largely unexplored influence on the population dynamics of tropical tree species. They further demonstrate that sapling herbivory by insects, in addition to the well-studied insect predation and herbivory of seedlings, is likely to influence tree species coexistence in tropical forests.     

Host plant conspicuousness and the distribution of eggs and larvae in the butterfly,Anthocharis scolymus (Lepidoptera: Pieridae)

We investigated how the distribution pattern of eggs and larval on the host plant, Turritus glabra, was influenced by the oviposition behavior of the pierid butterfly Anthocharis scolymus. Females searched for the host plants visually and they frequently approached taller host plants with sparse surrounding vegetation. After encountering host plants, oviposition behavior of females was independent of host plant characteristics such as height, density, and type of surrounding vegetation. A female laid eggs singly on a host plants. Most females appeared to lay their eggs regardless of the presense of eggs on the host plant. Consequently egg and larva tended to be abundant on conspicuous host plants as measured by height or relative isolation from other plants. However, overcrowding of eggs on an individual host decreased the survival rate of larvae.     

INDIVIDUAL FLOWERING TIME IN A GOLDENROD (SOLIDAGO CANADENSIS): FIELD EXPERIMENT SHOWS GENOTYPE MORE IMPORTANT THAN ENVIRONMENT

Individual plants of an old field population of the clonal perennial goldenrod, Solidago canadensis L. are consistent in rank order of flowering time over years. In order to analyze whether this consistency is due more to environmental variations among microsites of individual clones or due to genetic variation among clones, similar-sized sections of rhizomes were transplanted to an experimental garden in a randomized block design with five replicates per clone. Three years later we examined the transplants and the original plants for their flowering phenology. In the experimental garden there was an unexpected soil moisture gradient across blocks, with associated gradients in height of goldenrods and height of surrounding vegetation. No gradient was significantly correlated with the flowering time of the transplants. That is, the time of flowering of transplants was consistent among replicates of individual clones across the soil moisture gradient and regardless of the size of the transplants themselves, or the height of surrounding vegetation. Further, the flowering times of the transplants were significantly positively correlated with the flowering times of the original plants in the same year. We conclude that the differences in flowering time among clones within a goldenrod population is largely determined by genetic variation, rather than by environmental factors.     

THE AERODYNAMICS OF POLLEN CAPTURE IN TWO SYMPATRIC EPHEDRA SPECIES

Wind-tunnel analyses of the behavior of airborne pollen around ovules of two Ephedra species (E. trifurca and E. nevadensis) indicate that at certain airflow speeds (0.5 m/sec and 1.0 m/sec) each species is capable of biasing pollination in favor of conspecific pollen. A computer procedure was designed to evaluate the physical basis for this aerodynamic discrimination. This procedure indicates that differences in size and density confer significantly different inertial properties to the two pollen species. Operating within the specific aerodynamic environments generated around ovules from each species, these differences are sufficient to account for the biases observed in the probability of pollination. Within natural populations, there exists significant variation in pollen size (and possibly in density). Accordingly, it is possible that, under certain ambient wind conditions, ovules from each species can select subsets of the entire airborne population of Ephedra pollen.     

Population Structure and Growth Patterns of Opuntia echios var. gigantea along an Elevational Gradient in the Galípagos Islands1

Cacti growing in forests potentially experience growth limitation due to reduced light availability. To test this hypothesis, we studied the population structure of Opuntia echios var. gigantea at 15 sites on the south side of Isla Santa Cruz, Galípagos Islands, Ecuador. Populations were located in communities ranging from arid scrub at low elevations to closed‐canopy tropical dry forest at higher elevations. Ordination confirmed the existence of a strong elevation‐vegetation gradient. Opuntia abundance peaked at lower elevations (ca 30 m), with lower densities in closed‐canopy sites. For populations in scrub vegetation, density declined fairly regularly with plant height. Populations in forested sites had few plants of intermediate height, suggesting periodic recruitment. Scrub populations had random dispersion, while those in forests were aggregated. The change in spatial pattern may be related to a change in primary reproductive mode from asexual propagation via fallen fruits to propagation via fallen cladodes. Height was significantly correlated with stem diameter. Intercepts of these relationships increased toward higher elevations, probably in response to the increasing height of the surrounding canopy.     

The influence of plant density on the responses of Sinapis alba to CO2 and windspeed

Plants in nature live in populations of variable density, a characteristic which may influence individual plant responses to the environment. We investigated how the responses of Sinapis alba plants to different wind speeds and CO₂ concentrations could be modified by plant density. In our wind-density experiment the expectation that mechanical and physiological effects of wind will be ameliorated by growing in high density, as a result of positive plant interactions, was realised. Although individual plants were smaller at higher densities, the effect of increasing windspeed was much less than at lower plant densities. A similar reduced sensitivity of individual plant growth under high densities was also observed under CO₂ enrichment. When measured as a population or stand response, there was no effect of density on the CO₂ responses, with all stands showing very similar increases in total biomass with CO₂ enrichment. In the wind speed experiment, total biomass per stand increased significantly with density, although there was no effect of density on the wind speed response. Specific leaf area decreased with increasing wind speed and this response was significantly affected by the density at which the plants grew.     

Relationships between grazing and waterfowl production in the Canadian prairies

The survival of waterfowl nests is positively correlated with the amount of grassland on the landscape, and population growth rates of some waterfowl species (e.g., mallards [Anas platyrhynchos]) are sensitive to nest survival rates. Thus, the effect of actions that alter grassland vegetation physiognomy, such as grazing, on waterfowl production is of interest to waterfowl habitat managers. Additionally, grasslands contribute other ecological goods (e.g., forage for livestock and wildlife) and services (e.g., photosynthesis, carbon sequestration), which can be influenced by grazing practices. We address key uncertainties about the linkages between grazing, vegetation physiognomy, and the survival and density of duck nests at study-site, field, and nest-site spatial scales. Using data from 2,554 duck nests found in 434 grazed or idled fields (median field size = 48.0 ha) in the Canadian Prairie Pothole Region between 2002 and 2009, we found that vegetation physiognomy affected nest survival at both the field and nest-site scales, such that nest survival increased with nest-site vegetation density and late-season field vegetation density. Nest survival also responded to early-season within-field variation in vegetation height in a quadratic manner, such that survival was greatest in fields with moderate variation in vegetation height. Nest survival was negatively related to the intensity of grazing and to the amount of cropland in the surrounding landscape. Both the abundance of wetlands and the average vegetation height in the field had a positive influence on nest density. Fields idled during the breeding season had greater densities of nests than fields grazed either early or late in the breeding season. Leaving lands idled may be the most effective way to increase both waterfowl nest survival and nest density. When management of upland vegetation is required, we recommend grazing at moderate stocking rates (between 2 and 2.5 animal unit months [AUM]/ha) after the waterfowl breeding season is complete and monitoring vegetative characteristics to ensure they remain suitable to attract nesting waterfowl (e.g., leaving vegetation height >28 cm). Where grazing must be carried out during the breeding season, low to moderate stocking rates should be encouraged as these rates appear to have the least negative impact on both waterfowl nest survival and nest density. These stocking rates also will maintain rangeland in good condition to the long-term benefit of producers. © 2013 The Wildlife Society.     

Effects of windspeed on the growth and biomass allocation of white mustard Sinapis alba L.

Summary We examined how different wind speeds and interactions between plant age and wind affect growth and biomass allocation of Sinapis alba L. (white mustard). Physiological and growth measurements were made on individuals of white mustard grown in controlled-environment wind tunnels at windspeeds of 0.3, 2.2 and 6.0 ms^–1 for 42 days. Plants were harvested at four different dates. Increasing wind speed slightly increased transpiration and stomatal conductance. We did not observe a significant decline in the photosynthetic rate per unit of leaf area. Number of leaves, stem length, leaf area and dry weights of total biomass and plant parts were significantly lower in plants exposed at high wind speed conditions. There were no significant differences in the unit leaf rate nor relative growth rates, although these were always lower in plants grown at high wind speed. Allocation and architectural parameters were also examined. After 42 days of exposure to wind, plants showed higher leaf area ratio, root and leaf weight ratios and root/shoot ratio than those grown at control treatment. Only specific leaf area declined significantly with wind speed, but stem and reproductive parts also decreased. The responses of plants to each wind speed treatment depended on the age of the plant for most of the variables. It is suggested that wind operates in logarithmic manner, with relatively small or no effect at lower wind speeds and a much greater effect at higher speeds. Since there is no evidence of a significant reduction in photosynthetic rate of Sinapis with increasing wind speed it is suggested that the effect of wind on plant growth was due to mechanical effects leading to changes in allocation and developmental patterns.     

Demographic change and microhabitat variability in a grassland endemic, Silene douglasii var. oraria (Caryophyllaceae)

Variable spatial and temporal environments are known to affect the population dynamics of plants, but studies of local scale variability and its relationship to demographic change within a population remain limited. Using mapped plants, we examined the population dynamics of a coastal grassland endemic, Silene douglasii var. oraria, in two habitats over 10 yr. We hypothesized that ecological differences between rocky and grassy habitats might influence demographic parameters, including adult survival, growth, and density. Soil pH, soil moisture. and other abiotic variables differed little between habitats, but microsite differences in light, soil depth. and vegetation height were related to variation in Silene density and plant circumference. We also found significantly higher population densities, lower adult mortality, and more juvenile recruitment in rocky areas. Finite rates of population growth varied across years and habitats (lambda = 0.82-1.12). with different patterns evident in the two habitats. In both, observed population sizes in 1992 were similar to matrix projections using 1982-1985 data. Populations declined in size in some years despite high adult survivorship and variable recruitment. More intensive study of seedlings is needed, including experimental evaluation of the role of light and competition. However, the habitat-specific differences we observed imply that ecological studies and conservation plans developed for rare plants should consider the effect of local scale variability on demography.     

Mating patterns and spatial distribution of conspecific neighbours in the Mediterranean shrub <Emphasis Type="Italic">Myrtus communis</Emphasis> (Myrtaceae)

Mate abundance is one of the most important sources of variation in plant mating systems. We examined within-population heterogeneity in the pollen pool at two spatial scales (sites and plants), and investigated the mating pattern variation in Myrtus communis under diverse situations of conspecific neighbourhood, using allozyme electrophoresis of naturally pollinated progeny arrays. For mating analyses, plants sampled were classified into four neighbourhood groups (from high to low) based on the local density around them and the distance to their nearest neighbour. The pollen pool was much more genetically heterogeneous among mother plants (~21%) than among sites (~2%), probably because of the high levels of selfing found (average s = 0.65). Outcrossing rates differed significantly among neighbourhood groups and showed a marked trend towards higher values from the lowest (t _m = 0.26) to the highest (t _m = 0.45) degree of conspecific aggregation. However, the lowest levels of biparental inbreeding and correlated paternity were found in the most isolated group of plants, indicating that these plants crossed with more and less genetically related fathers. Our study provides a clear demonstration of positive correlation between conspecific aggregation and the outcrossing rates. We discuss the ecological implications of these results in the context of Mediterranean ecosystems.