On the importance of life history and age structure in biological invasions

The age specific patterns of reproduction and mortality dictated by the life history of an organism apply to potential invaders as well as resident species of an area, but whether certain life history traits are more invasive than others is an unresolved issue. We analyze a two-population system of an invading and a resident species and test the effects of age on the probability to invade when the organisms are iteroparous or semelparous. The life history characteristics of the populations are projected in Leslie matrices, and the probability that the invader exceeds different population sizes is calculated by Monte Carlo analysis. The simulations show that (a) the invasion probability of an iteroparous organism increases with age until the individuals introduced are mature for first reproduction, and then becomes independent of age; (b) the invasion probability is more age sensitive for iteroparous organisms with high juvenile mortality (Type III organisms) than for those with a lower (Type I); (c) invading semelparous organisms are most affected by competition from resident organisms; (d) variations in vital rates of semelparous residents have greater influence on the invasion probability of an iteroparous organism than variations in traits of the invader.     

Effect of stage-specific vital rates on population growth rates and effective population sizes in an endangered iteroparous plant

Effective population size (N(e)) determines the strength of genetic drift and can influence the level of genetic diversity a population can maintain. Assessing how changes in demographic rates associated with environmental variables and management actions affect N(e) thus can be crucial to the conservation of endangered species. Calculation of N(e) through demographic models makes it possible to use elasticity analyses to study this issue. The elasticity of N(e) to a given vital rate is the proportional change in N(e) associated with a proportional increase in that vital rate. In addition, demographic models can be used to study N(e) and population growth rate (λ) simultaneously. Simultaneous examination is important because some vital rates differ diametrically in their associations with λ and N(e). For example, in some cases increasing these vital rates increases λ and decreases N(e). We used elasticity analysis to study the effect of stage-specific survival and flowering rates on N(e), annual effective population size (N(a)), and λ in seven populations of the endangered plant Austrian dragonhead (Dracocephalum austriacum). In populations with λ ≥ 1, the elasticities of N(e) and N(a) were similar to those of λ. Survival rates of adults were associated with greater elasticities than survival rates of juveniles, flowering rates, or fecundity. In populations with λ < 1, N(e) and N(a) exhibited greater elasticities to juvenile than to adult vital rates. These patterns are similar to those observed in other species with similar life histories. We did not observe contrasting effects of any vital rate on λ and N(e); thus, management actions that increase the λ of populations of Austrian dragonhead will not increase genetic drift. Our results show that elasticity analyses of N(e) and N(a) can complement elasticity analysis of λ. Moreover, such analyses do not require more data than standard matrix models of population dynamics.     

Reliability of Conservation Actions Based on Elasticity Analysis of Matrix Models

Abstract: Matrix population models have entered the mainstream of conservation biology, with analysis of proportional sensitivities (elasticity analysis) of demographic rates becoming important components of conservation decision making. We identify areas where management applications using elasticity analysis potentially conflict with the mathematical basis of the technique, and we use a hypothetical example and three real data sets (Prairie Chicken [   Tympanuchus cupido ], desert tortoise [ Gopherus agassizii ], and killer whale [ Orcinus orca ]) to evaluate the extent to which conservation recommendations based on elasticities might be misleading. First, changes in one demographic rate can change the qualitative ranking of the elasticity values calculated from a population matrix, a result that dampens enthusiasm for ranking conservation actions based solely on which rates have the highest elasticity values. Second, although elasticities often provide accurate predictions of future changes in population growth rate under management perturbations that are large or that affect more than one rate concurrently, concordance frequently fails when different rates vary by different amounts. In particular, when vital rates change to their high or low values observed in nature, predictions of future growth rate based on elasticities of a mean matrix can be misleading, even predicting population increase when the population growth rate actually declines following a perturbation. Elasticity measures will continue to be useful tools for applied ecologists, but they should be interpreted with considerable care. We suggest that studies using analytical elasticity analysis explicitly consider the range of variation possible for different rates and that simulation methods are a useful tool to this end.     

Habitat Change and Plant Demography: Assessing the Extinction Risk of a Formerly Common Grassland Perennial

Abstract:  An important aim of conservation biology is to understand how habitat change affects the dynamics and extinction risk of populations. We used matrix models to analyze the effect of habitat degradation on the demography of the declining perennial plant Trifolium montanum in 9 calcareous grasslands in Germany over 4 years and experimentally tested the effect of grassland management. Finite population growth rates (λ) decreased with light competition, measured as leaf-area index above T. montanum plants. At unmanaged sites λ was <1 due to lower recruitment and lower survival and flowering probability of large plants. Nevertheless, in stochastic simulations, extinction of unmanaged populations of 100 flowering plants was delayed for several decades. Clipping as a management technique rapidly increased population growth because of higher survival and flowering probability of large plants in managed than in unmanaged plots. Transition-matrix simulations from these plots indicated grazing or mowing every second year would be sufficient to ensure a growth rate ≥1 if conditions stayed the same. At frequently grazed sites, the finite growth rate was approximately 1 in most populations of T. montanum . In stochastic simulations, the extinction risk of even relatively small grazed populations was low, but about half the extant populations of T. montanum in central Germany are smaller than would be sufficient for a probability of survival of >95% over 100 years. We conclude that habitat change after cessation of management strongly reduces recruitment and survival of established individuals of this perennial plant. Nevertheless, our results suggest extinction processes may take a long time in perennial plants, resulting in an extinction debt. Even if management is frequent, many remnant populations of T. montanum may be at risk because of their small size, but even a slight increase in size could considerably reduce their extinction risk.     

Fecundity and reproductive effort in Ophryotrocha labronica (Polychaeta: Dorvilleidae)

Reproductive effort in terms of fecundity and energy allocation was studied in Ophryotrocha labronica La Greca and Bacci, 1962 a small, semicontinuous iteroparous species. In O. labronica fecundity is more or less constant throughout life, and the total fecundity of the 64 couples examined in this experiment only declined by 12% from the first to the last spawning. No linear relationship was found between fecundity and body size.␣The energy content of germinal and somatic tissues was determined by differential scanning calorimeter. The␣reproductive effort and a reproductive index based on the fifth spawning were evaluated to compare the␣patterns of energy allocation of a semicontinuous iteroparous species with semelparous and annual iteroparous species. The reproductive index ranged from 0.20 to 0.77, with an average of 0.480. The average reproductive effort was 0.840, ranging from 0.60 to 0.96. These figures highlight the enormous amount of energy that O. labronica allocates to reproductive tissues. Received: 23 July 1997 / Accepted: 20 October 1998     

The Relationship between the Income Elasticities of Demand and Willingness to Pay

The relationship between income and willingness to pay for collectively provided public/environmental goods is investigated. We show that while the income elasticity of willingness to pay and the ordinary income elasticity of demand are related, knowledge of one is insufficient to determine the magnitude or even the sign of the other. The income elasticity of willingness to pay is influenced by additional factors which are generally unobservable. Examples are provided to illustrate the degree to which the two income elasticities may diverge. Our results indicate that even when goods are demand luxuries they may or may not have income elasticities of willingness to pay which are greater than one.     

Reproductive habits and life cycle of the small clam Kingiella chilenica (Bivalvia: Cyamiidae) in an estuarine sand flat from the south of Chile

A population of the small clam Kingiella chilenica Soot Ryen 1959 was studied from March 1986 to December 1988 in an intertidal flat at Queule River, in the south of Chile. The life cycle and life history pattern of the bivalve were established contrasting population structure and dynamics to its reproductive habits. Individuals are gonochoristic and semelparous, presenting a typical annual life cycle. The species is a sequential brooder whose embryos undergo direct development. After the brooding season (summer through autumn), the adults disappear gradually (autumn through winter). Recruited juveniles overwinter during a relatively long period, undergoing rapid growth during the spring to attain the adult stage during the summer. The number of brooded embryos increases in proportion to adult length cubed. Life history traits of this bivalve are compared to those reported for other small brooder clams. Some basic tendencies become apparent when traits for semelparous versus iteroparous species are contrasted. As in other semelparous sequential brooders (Transennella tantilla, Gaimardia bahamondei), the relationship between brood size and shell length obseved in K. chilenica does not fit the allometry hypothesis for marine brooding invertebrtes that allometric constraints on the brooding space limit the fecundity of larger individuals. Also contrary to theoretical predictions, small body size does not limit the diversity of a clam's developmental patterns. Possible explanations for this finding are discussed.     

冀北山区森林群落草本多样性及其与地形关系研究

通过野外调查取样,应用Menhinick丰富度指数、Simpson指数、Shannon-Wiener指数和Pielou均匀度指数研究了冀北山区12种典型森林群落内的草本植物多样性规律,并采用典范对应分析（CCA）方法研究其分布与地形的关系。结果表明：不同的群落类型草本植物的组成和多样性指数不同,山杨（Populus davidiana）纯林的种数最多为34种,黑桦（Betula dahurica Pall）纯林的种数最少为19种,其Simpson指数、Shannon-Wiener指数最高,分别为0.927 3和2.879 6;在1 200～1 700 m之间,随海拔升高物种多样性指数、均匀度指数和丰富度指数呈正相关性,但没有线性上升的趋势关系;选取的地形因子对样点草本的影响程度大小为：坡向〉海拔〉坡度〉坡位〉坡形,坡向是样点草本植物空间差异的最主要地形制约因子,环境解释率为98.7%,证明排序可信。     

Elasticity analyses of size-based red and white abalone matrix models: management and conservation.

Prospective elasticity analyses have been used to aid in the management of fished species and the conservation of endangered species. Elasticities were examined for deterministic size-based matrix models of red abalone, Haliotis rufescens, and white abalone, H. sorenseni, to evaluate which size classes influenced population growth (lambda) the most. In the red abalone matrix, growth transitions were determined from a tag recapture study and grouped into nine size classes. In the white abalone matrix, abalone growth was determined from a laboratory study and grouped into five size classes. Survivorship was estimated from tag recapture data for red abalone using a Jolly-Seber model with size as a covariate and used for both red and white abalone. Reproduction estimates for both models used averages of the number of mature eggs produced by female red and white abalone in each size class from four-year reproduction studies. Population growth rate (lambda) was set to 1.0, and the first-year survival (larval survival through to the first size class) was estimated by iteration. Survival elasticities were higher than fecundity elasticities in both the red and white matrix models. The sizes classes with the greatest survival elasticities, and therefore the most influence on population growth in the model, were the sublegal red abalone (150-178 mm) and the largest white abalone size class (140-175 mm). For red abalone, the existing minimum legal size (178 mm) protects the size class the model suggests is critical to population growth. Implementation of education programs for novice divers coupled with renewed enforcement may serve to minimize incidental mortality of the critical size class. For white abalone, conservation efforts directed at restoring adults may have more of an impact on population growth than efforts focusing on juveniles. Our work is an example of how prospective elasticity analyses of size-structured matrix models can be used to quantitatively evaluate research priorities, fishery management strategies, and conservation options.     

Robustness in life history of the brown seaweed Ascophyllum nodosum (Fucales, Phaeophyceae) across large scales: effects of spatially and temporally induced variability on population growth

Understanding the demography and function of biotope-forming seaweed species is of great importance for the conservation of the target species itself, as well as its associated organisms. The brown seaweed Ascophyllum nodosum is fundamental for the functioning of coastal marine ecosystems in the North Atlantic. In this study, we use a data-based size-classified matrix model to investigate the temporal and spatial variability in demography, and the environment-specific stochastic sensitivity and elasticity, of two A. nodosum populations, one in western Sweden and one on the Isle of Man in the Irish Sea. A significant difference between the two populations was that the Swedish population had comparably low and more variable stochastic population growth rate (λ _s). This pattern was partly explained by the relatively high and varying mortality rates during extreme ice-years in Sweden, and by the lower survival of small individuals during all years. There were also fewer large individuals in Sweden due to lower transitions to the larger size-classes and higher probability of shrinkage. Sensitivities were analogous in the two populations, and showed a high selection pressure for increased individual growth. Elasticities were also similar, with the exception that survival of the smallest individuals (i.e., transition a _1,1), had a higher elasticity on the Isle of Man. Overall, the stochastic growth rate (λ _s) was most sensitive to proportional changes in loop- (i.e., survival within size-class) and, to some extent, growth-transitions in both study areas. These results show that structurally and demographically diverging A. nodosum populations may be similarly sensitive to changes in vital rates. This, in turn, indicates a plastic life history of A. nodosum that may cope with large environmental variability. The results further suggest that environmental change affecting the survival or growth of the larger, reproductive A. nodosum individuals could have severe and regional effects on the abundance and biomass of this species, with potential negative effects on the biodiversity of the associated communities.     

南亚热带中幼龄针阔混交林生态化学计量特征

为了解南亚热带中幼龄针阔混交林植物、凋落物和土壤生态化学计量特征,本研究以10-11 a、7-9 a和3-5 a林龄人工针阔混交林为研究对象,通过对植物叶片（乔木、灌木和草本）、凋落物及土壤的碳（C）、氮（N）和磷（P）含量及计量比分析,探讨了中幼龄针阔混交林生态化学计量特征、相互关系及其N、P养分限制。结果表明,1）针阔混交林乔木、灌木和草本叶片碳含量均值分别为502.88、472.18和438.31 mg·g-1,其叶片碳含量表现为乔木〉灌木〉草本;叶片全氮含量均值分别为15.87、19.61和15.72 mg·g-1,叶片全磷含量均值为1-09、1.24和0.91 mg·g-1,其叶片氮和磷含量均表现为灌木〉乔木〉草本;凋落物碳、氮和磷含量均值分别为497-07、11-36和0.45 mg·g-1,凋落物氮和磷含量均低于植物。2）针阔混交林乔木叶片C/N、C/P和N/P均值分别为34.43、517-06和15.63,灌木和草本叶片C/N、C/P和N/P均值分别为26.60和28.55、438.77和507.59、16.52和17.95,而凋落物C/N、C/P和N/P为46.50、1193.26和26.17;不同林龄杉木叶片N/P均低于14,表明杉木生长受N限制;10-11 a林龄阔叶树生长受N的限制,7-9 a和3-5 a林龄阔叶树生长受P的限制,灌木和草本生长基本受P限制。3）植物叶片全氮和全磷含量呈极显著正相关（P〈0.01）,C/N与C/P呈极显著正相关（P〈0.01）,而全磷含量与C/N、C/P、N/P呈极显著和显著负相关（P〈0.01,P〈0-05）;土壤有机碳含量与土壤全氮含量、C/P、N/P呈极显著和显著正相关（P〈0.01,P〈0-05）。本研究为中幼龄人工林抚育及可持续经营提供科学参考。     

Genetic Variation and Outcrossing Rate in Relation to Population Size in Gentiana pneumonanthe L

The amount of genetic variation in the rare perennial herb Gentiana pneumonanthe L. was determined to explore its relation to population size. Differences in isozyme variation between maternal plants and their offspring were used to investigate the relationship between population size and outcrossing rate. In 25 populations in The Netherlands, differing in size from 1 to more than 50,000 flowering individuals, 16 allozyme loci were analyzed on leaves of maternal plants and offspring grown in a greenhouse. Population size was significantly positively correlated with the proportion of polymorphic loci, but only marginally with heterozygosity and the mean effective number of alleles. Most of the studied populations were characterized by a complete absence of rare alleles, and F -statistics suggest relatively high levels of genetic differentiation among populations and thus a low level of gene flow. Leaf samples (maternal) were mostly in Hardy-Weinberg equilibrium, while several offspring samples showed an excess of homozygotes, which suggests selection favoring heterozygotes. Because most small populations consist only of adult survivors from formerly larger populations, this may partly explain the absence of a clear relationship between genetic variation of the maternal plants and population size. A significant positive correlation was found between the level of cross-fertilization and population size. From these results, we conclude that, to some degree, small populations have a reduced level of genetic variation, while their present isolation in nature reserves has resulted in a very limited interpopulational gene flow level. At present a higher level of inbreeding in small populations contributes to a further loss of genetic variation and may also result in reduced offspring fitness.     

Effects of an Exotic Plant Invasion on Native Understory Plants in a Tropical Dry Forest

Abstract: The dry forests of southern India, which are endangered tropical ecosystems and among the world's most important tiger (Panthera tigris) habitats, are extensively invaded by exotic plants. Yet, experimental studies exploring the impacts of these invasions on native plants in these forests are scarce. Consequently, little is known about associated implications for the long‐term conservation of tigers and other biodiversity in these habitats. I studied the impacts of the exotic plant Lantana camara on understory vegetation in a dry‐forest tiger habitat in southern India. I compared the richness, composition, and abundance of tree seedlings, herbs, and shrubs and the abundance of grass among plots in which Lantana was cleared or left standing. These plots were distributed across two blocks—livestock free and livestock grazed. Removal of Lantana had an immediate positive effect on herb–shrub richness in the livestock‐free block, but had no effect on that of tree seedlings in either livestock block. Tree‐seedling and herb–shrub composition differed significantly between Lantana treatment and livestock block, and Lantana removal significantly decreased survival of tree seedlings. Nevertheless, the absence of trees, in any stage between seedling and adult, indicates that Lantana may stall tree regeneration. Lantana removal decreased the abundance of all understory strata, probably because forage plants beneath Lantana are less accessible to herbivores, and plants in Lantana‐free open plots experienced greater herbivory. Reduced access to forage in invaded habitats could negatively affect ungulate populations and ultimately compromise the ability of these forests to sustain prey‐dependent large carnivores. Additional research focused on understanding and mitigating threats posed by exotic plants may be crucial to the long‐term protection of these forests as viable tiger habitats.     

Patterns of chemical defences in plants: an analysis of the vascular flora of Chile

Summary. The plant apparency hypothesis predicts that apparent plants invest in broadly effective defences such as tannins while unapparent plants invest in specific toxins such as alkaloids. The stress hypothesis states that plants invest in cheaper defences if they have evolved in habitats that impose abiotic limitations to plant fitness. We tested these hypotheses by determining the concentrations of alkaloids and tannins in a representative sample of the vascular plants of continental Chile (with exclusion of Pteridophyta, Cactaceae, and Poaceae) consisting of 396 species. In a subsample of 166 species which contained both alkaloids and tannins, we constructed the A/T index (A/T = [alkaloids]/ [tannins]). We discarded the presumed effect of phylogeny (as estimated by taxonomy) on the variation observed in the data because no correlation of A/T with taxonomic relationships among species either at family or genus levels was found in a nested ANOVA with genera nested in families. Concentration of alkaloids was negatively correlated with that of tannins. We compared the value of A/T among species differing in life form (herbs, shrubs or trees), herb longevity (annual or perennial), leaf-shedding manner of woody plants (deciduous or evergreen), latitudinal range, and level of water stress typical in their natural habitat. Unapparent plants (herbs, annual) exhibited higher mean A/T index than apparent plants (shrubs and trees, perennial). A/T did not correlate with latitudinal range. Mean A/T values decreased from deserts to deciduous forests. The comparisons were not always significant due to the inevitable unbalance of the data set which lowers the power of the statistical tests employed. The results suggest that chemical defences are indeed distributed in a non-random manner among plants, and that to a large extent the predictions derived from the apparency and stress hypotheses are sustained.     

Life history and viability of a long-lived marine invertebrate: the octocoral Paramuricea clavata

The red gorgonian Paramuricea clavata is a long-lived, slow-growing sessile invertebrate of ecological and conservation importance in the northwestern Mediterranean Sea. We develop a series of size-based matrix models for two Paramuricea clavata populations. These models were used to estimate basic life history traits for this species and to evaluate the viability of the red gorgonian populations we studied. As for many other slow-growing species, sensitivity and elasticity analysis demonstrate that gorgonian population growth is far more sensitive to changes in survival rates than to growth, shrinkage, or reproductive rates. The slow growth and low mortality of red gorgonians results in low damping ratios, indicating slow convergence to stable size structures (at least 50 years). The stable distributions predicted by the model did not differ from the observed ones. However, our simulations point out the fragility of this species, showing both populations in decline and high risk of extinction over moderate time horizons. These declines appear to be related to a recent increase in anthropogenic disturbances. Relative to their life span, the values of recruitment elasticity for Paramuricea clavata are lower than those reported for other marine organisms but are similar to those reported for some long-lived plants. These values and the delayed age of sexual maturity, in combination with the longevity of the species, show a clear fecundity/mortality trade-off. Full demographic studies of sessile marine species are quite scarce but can provide insight into population dynamics and life history patterns for these difficult and under-studied species. While our work shows clear results for the red gorgonian, the variability in some of our estimates suggest that future work should include data collection over longer temporal and spatial scales to better understand the long-term effects of natural and anthropogenic disturbances on red gorgonian populations.     

Importance of individual and environmental variation for invasive species spread: a spatial integral projection model

Plant survival, growth, and flowering are size dependent in many plant populations but also vary among individuals of the same size. This individual variation, along with variation in dispersal caused by differences in, e.g., seed release height, seed characteristics, and wind speed, is a key determinant of the spread rate of species through homogeneous landscapes. Here we develop spatial integral projection models (SIPMs) that include both demography and dispersal with continuous state variables. The advantage of this novel approach over discrete-stage spread models is that the effect of variation in plant size and size-dependent vital rates can be studied at much higher resolution. Comparing Neubert-Caswell matrix models to SIPMs allowed us to assess the importance of including individual variation in the models. As a test case we parameterized a SIPM with previously published data on the invasive monocarpic thistle Carduus nutans in New Zealand. Spread rate (c*) estimates were 34% lower than for standard spatial matrix models and stabilized with as few as seven evenly distributed size classes. The SIPM allowed us to calculate spread rate elasticities over the range of plant sizes, showing the size range of seedlings that contributed most to c* through their survival, growth and reproduction. The annual transitions of these seedlings were also the most important ones for local population growth (lambda). However, seedlings that reproduced within a year contributed relatively more to c* than to lambda. In contrast, plants that grow over several years to reach a large size and produce many more seeds, contributed relatively more to lambda than to c*. We show that matrix models pick up some of these details, while other details disappear within wide size classes. Our results show that SIPMs integrate various sources of variation much better than discrete-stage matrix models. Simpler, heuristic models, however, remain very valuable in studies where the main goal is to investigate the general impact of a life history stage on population dynamics. We conclude with a discussion of future extensions of SIPMs, including incorporation of continuous time and environmental drivers.     

Forecasting extinction risk with nonstationary matrix models.

Matrix population growth models are standard tools for forecasting population change and for managing rare species, but they are less useful for predicting extinction risk in the face of changing environmental conditions. Deterministic models provide point estimates of lambda, the finite rate of increase, as well as measures of matrix sensitivity and elasticity. Stationary matrix models can be used to estimate extinction risk in a variable environment, but they assume that the matrix elements are randomly sampled from a stationary (i.e., non-changing) distribution. Here we outline a method for using nonstationary matrix models to construct realistic forecasts of population fluctuation in changing environments. Our method requires three pieces of data: (1) field estimates of transition matrix elements, (2) experimental data on the demographic responses of populations to altered environmental conditions, and (3) forecasting data on environmental drivers. These three pieces of data are combined to generate a series of sequential transition matrices that emulate a pattern of long-term change in environmental drivers. Realistic estimates of population persistence and extinction risk can be derived from stochastic permutations of such a model. We illustrate the steps of this analysis with data from two populations of Sarracenia purpurea growing in northern New England. Sarracenia purpurea is a perennial carnivorous plant that is potentially at risk of local extinction because of increased nitrogen deposition. Long-term monitoring records or models of environmental change can be used to generate time series of driver variables under different scenarios of changing environments. Both manipulative and natural experiments can be used to construct a linking function that describes how matrix parameters change as a function of the environmental driver. This synthetic modeling approach provides quantitative estimates of extinction probability that have an explicit mechanistic basis.     

Life style and genetic variation in teleosts: the case of pelagic (<Emphasis Type="Italic">Aphia minuta</Emphasis>) and benthic (<Emphasis Type="Italic">Gobius niger</Emphasis>) gobies (Perciformes: Gobiidae)

The pattern of genetic variability of two species of Mediterranean gobiids was compared, with reference to their different life history traits (Aphia minuta paedomorphic and pelagic; Gobius niger metamorphosed and benthic). The aim was to evaluate how different life histories can affect the genetic structure in these marine teleosts. The study was carried out on populations of both species sampled in the western Mediterranean and in the Adriatic Sea. Seven restriction endonucleases were used for the RFLP analysis of a mitochondrial DNA segment comprising the NADH dehydrogenase subunits 3, 4L and 4. The results highlighted two different patterns of genetic variation, a weak genetic structure in A. minuta and population subdivision in G. niger. These observations may be explained not only in terms of the different dispersal capabilities of these species, but also considering that A. minuta is an abbreviate iteroparous spawner while G. niger is a protracted iteroparous spawner. Because abbreviate iteroparity is a reproductive strategy selected in stable environments with high resource availability, Pliocene and Pleistocene climate oscillations may have represented factors that negatively influenced the reproductive success of A. minuta, producing demographic fluctuations and bottlenecks, as suggested by the mismatch distribution analysis. The weak genetic structure of A. minuta populations seems to be therefore due to a more recent re-colonization of the Mediterranean basin after a severe population decline, rather than to the high vagility of this pelagic goby.     

Longevity can buffer plant and animal populations against changing climatic variability

Both means and year-to-year variances of climate variables such as temperature and precipitation are predicted to change. However, the potential impact of changing climatic variability on the fate of populations has been largely unexamined. We analyzed multiyear demographic data for 36 plant and animal species with a broad range of life histories and types of environment to ask how sensitive their long-term stochastic population growth rates are likely to be to changes in the means and standard deviations of vital rates (survival, reproduction, growth) in response to changing climate. We quantified responsiveness using elasticities of the long-term population growth rate predicted by stochastic projection matrix models. Short-lived species (insects and annual plants and algae) are predicted to be more strongly (and negatively) affected by increasing vital rate variability relative to longer-lived species (perennial plants, birds, ungulates). Taxonomic affiliation has little power to explain sensitivity to increasing variability once longevity has been taken into account. Our results highlight the potential vulnerability of short-lived species to an increasingly variable climate, but also suggest that problems associated with short-lived undesirable species (agricultural pests, disease vectors, invasive weedy plants) may be exacerbated in regions where climate variability decreases.     

Life-history patterns of two Southern CaliforniaNebalia species (Crustacea: Leptostraca): the failure of form to predict function

Laboratory and field studies onNebalia hessleri andN. daytoni off the toast of San Diego, Southern California, showed that although morphologically similar, the habitats, behavior, and natural history of these two species are surprisingly different. In laboratory experiments, Bach species avoided the other's habitat (sand and mats of macrophyte detritus), and in the field, transplanted individuals failed to survive in the other species' habitat.N. hessleri, which inhabits subtidal mats of macrophyte detritus, survived and reproduced well in the laboratory, was iteroparous, and a large percentage of adults were male. This species occurred at very high densities in the field, and ate essentially everything offered in the laboratory, with a diet in the field consisting largely of plant detritus and carrion. The other species,N. daytoni, differed in nearly every way, it inhabited organically impoverished sands, survived poorly in the laboratory, was apparently semelparous, and a small percentage of the adult population was male.