Life Table Response Experiment Research Articles

Comparative demographic analysis of threeNeobuxbaumiaspecies (Cactaceae) with differing degree of rarity

AbstractIn this study we use a demographic approach to analyse the differing abundance of three congeneric columnar cacti:Neobuxbaumia macrocephala(the rarest),Neobuxbaumia tetetzo(intermediate), andNeobuxbaumia mezcalaensis(the most common). Populations of these species were studied in the Tehuacan Valley (Central Mexico) over a 3‐year period. We employed traditional models and life table response experiments (LTRE) to explore the association between particular demographic traits and the degree of rarity of each species. Most matrices showed population growth rate (λ) values close to unity; the only exception wasN. mezcalaensisin 2001–2002 (λ=1.091±0.088). In the three species the highest elasticity values corresponded to entries referring to the stasis of pre‐reproductive plant and small adults. However, the LTRE indicated that most important differences in the λ values between years and species were associated with variation in fruit production and seedling recruitment.N. mezcalaensisyielded the highest mean λ due to its high fecundity in 2001–2002. This result suggests that the variation in demographic behaviour observed between species is sufficient to explain the higher abundance ofN. mezcalaensiscompared to the rareN. macrocephala. The high fecundity values and recruitment success observed in the former, even if they occur only seldom, may result in higher average λ values accounting for its high population densities.

DEMOGRAPHIC EFFECTS OF HABITAT FRAGMENTATION ON A TROPICAL HERB: LIFE-TABLE RESPONSE EXPERIMENTS

Habitat fragmentation is a leading cause of extinction, with effects that may be particularly pronounced in tropical ecosystems. However, little is known regarding the demographic mechanisms underlying changes in abundance in fragmented landscapes. Us- ing six years of demographic data collected from .6600 individuals of the Amazonian understory herb Heliconia acuminata, we calculated population growth rate (l) in exper- imentally isolated 10-ha forest fragments, 1-ha forest fragments, and continuous forest. We then used life-table response experiment analyses to elucidate the mechanisms responsible for observed differences in l. On average, l 1.05 in continuous forest, while l 1i n both 1-ha and 10-ha fragments. However, while the differences in l between 10-ha fragments and continuous forest were largely attributable to the negative contribution of stage-specific fertility rates, reduced l in 1-ha fragments was due to both reductions in reproductive rates and changes in the rate of plant growth. Our results show that similar reductions in l in fragments of different sizes can be driven by distinct demographic mechanisms. Without comprehensive demographic data, attempts to mitigate the decline of populations in frag- mented landscapes could be unsuccessful because they might be focusing on inappropriate demographic targets.

Space versus time variation in the population dynamics of three co‐occurring perennial herbs

1 Many plant species are currently restricted to small and isolated populations as a result of habitat destruction and fragmentation. Lack of sufficient data often leads to spatial variation being substituted for temporal variation in models to evaluate management options, but variation in population growth rate () between sites or over years may be caused by variation in different life history components. 2 We studied the demography of three coexisting and related perennial herbs (the longlived perennial Succisa pratensis, the shorter-lived Hypochaeris radicata and the longlived, but more clonal Centaurea jacea ) in the same plots over 4 years (1999?2003). 3 Life table response experiment analysis revealed that temporal and spatial variation in the life history components of S. pratensis and H. radicata were qualitatively different: variation in fecundity contributed most to variation in between sites, but variation in growth contributed most to variation between years. 4 In years with a below average, most life history components of S. pratensis and H. radicata had lower transition probabilities, whereas the reasons for low population growth differed between the three poor sites. All major life history components of S. pratensis had lower probabilities in the most productive of these sites, but in the other two, negative contributions from some components (fecundity or growth) were partly compensated for by positive contributions from others (growth or stasis). 5 Centaurea jacea showed a different pattern: in poor years, negative contributions from some life history components were buffered by positive contributions from other components; however, this buffering did not occur in poor sites. 6 Our analysis shows that the population dynamics of perennial plants may respond differently to temporal and spatial variation in environmental conditions. Moreover, co-occurring species with similar life histories responded differently to the same spatiotemporal variation. We conclude that temporal and spatial dynamics cannot readily be interchanged in population viability analyses and management studies, as such substitution may lead to incorrect projections of a species? population dynamics in time. Key-words : buffering of variation, demography, life history components, population growth rate, variation decomposition.

A monocarpic tree species in a polycarpic world: how can Tachigali vasquezii maintain itself so successfully in a tropical rain forest community?

Summary Although monocarpy is rare among long‐lived plant species that grow in stable habitats, one monocarpic species, Tachigali vasquezii, is extremely abundant in the rain forests of the Bolivian Amazon. We analyse how T. vasquezii is able to maintain itself successfully by comparing its life‐history traits with those of polycarpic tree species of the same community. We then evaluate the relative importance of such traits using population matrix models. Monocarpic species are expected to have a high fecundity. Seed production per basal area for T. vasquezii is indeed nearly twice that of an average polycarpic species, but this is not sufficient to maintain stable populations. Life‐history theory predicts that a monocarpic strategy is advantageous if juvenile survival rates are high compared with adult survival. Although seedlings of T. vasquezii have a lower mortality rate than polycarpic species, its sapling have higher mortality. We found that the success of T. vasquezii is due to its very high diameter growth rates in the larger size classes, which are four times higher than that of an average, co‐occurring polycarpic species. Fast diameter growth is enabled by investment in large, leafy crowns and by a low wood density. Applying this high diameter growth in a population model yielded a population growth rate close to that of polycarpic species. Life table response experiment analysis of Tachigali and polycarp models showed that higher growth compensated for the negative demographic effect of 100% adult mortality following reproduction. Rapid growth enables T. vasquezii to reach reproductive maturity in only 49 years, compared with 79 years for an average polycarpic species. It also reduces the risk of dying before reproducing, and 50% more seedlings survive to maturity for T. vasquezii compared with polycarpic species. The dramatic negative demographic consequences of one‐time flowering can therefore be completely balanced by taking a shorter time to reach maturity. Taking a time perspective, it appears that Tachigali vasquezii has an advanced, rather than a delayed, reproduction.

Population demographics and trade-offs to reproduction of an invasive and noninvasive species of Rubus

Do trade-offs between growth and reproduction differ between an invasive and noninvasive plant species and how do such trade-offs relate to population demographics? To help address these questions, we compared demographics for an invasive plant species, Rubus discolor, with a noninvasive congener, R. ursinus, in several populations of varying density. Removal of floral buds from reproductive canes increased the size of juvenile canes that arose from clonal sprouting in R. ursinus, suggesting a trade-off between current reproduction and growth. Removal of floral buds had no effect on growth of R. discolor. R. ursinus displayed trade-offs between reproduction (sexual and vegetative) and future growth based on negative correlations between leaf area production and both clonal sprouting and seedling production during the previous year. R. discolor did not exhibit these trade-offs. Both species had high population growth rates in low-density populations, but exhibited little or no growth in high-density populations. A life table response experiment was used to determine the underlying cause for the effect of density on population growth. For R. ursinus, lack of population growth in high-density populations was due primarily to increased mortality of clonally sprouting canes, while for R. discolor, it was due to decreased clonal cane production. Elasticity analysis revealed that clonal growth was more important than sexual reproduction for population growth of both species. However, elasticity values for sexual reproduction in R. discolor were greater in high- than low-density populations. This suggests an increased reliance on sexual reproduction in populations that had reached stable sizes, which could increase the capacity of R. discolor to disperse to new sites. Elasticity analyses were also used to simulate the efficacy of various control strategies for R. discolor. Control of this species could be attained by reducing clonal production within existing populations while reducing seed production to limit establishment of new populations.

SENSITIVITY ANALYSIS OF THE STOCHASTIC GROWTH RATE: THREE EXTENSIONS†

SummaryThe perturbation analysis of population growth rate plays an important role in population biology. The sensitivity and/or elasticity (proportional sensitivity) of population growth rate to changes in the vital rates are regularly used (i) to predict the effects of environmental perturbations, (ii) to characterize selection gradients on life‐history traits, (iii) to evaluate management tactics, (iv) to analyse life table response experiments, and (v) to calculate the sampling variance in population growth rate. In a stochastic environment, population growth is described by the stochastic growth rate, which gives, with probability 1, the asymptotic time‐averaged growth rate of any realization. Tuljapurkar derived the sensitivity and elasticity of the stochastic growth rate to changes in the entries of the stochastic matrices. This paper extends his result to cover three cases, each of which has arisen recently in applications. The first gives the response of the stochastic growth rate to environment‐specific perturbations, applied only in a specified subset of the possible environments. The second gives the sensitivity and elasticity of the stochastic growth rate to changes in lower‐level parameters. The third applies to stochastic seasonal models, in which the projection matrix for each year is a periodic product of matrices describing seasonal transitions. In this case interest focuses on the sensitivity of the stochastic growth rate to changes in the entries of the seasonal matrices, not entries in the annual matrices. The paper describes examples of problems where each of these extensions is needed, and the algorithms for each of the new calculations.

Analysis of interspecific competition in perennial plants using Life Table Response Experiments

The impact of interspecific competition is usually measured by its effect upon plant growth, neglecting impacts upon other stages of the life cycle such as fecundity which have a direct influence upon individual fitness and the asymptotic population growth rate of a population (λ). We used parameterized matrix models for three perennial plant species grown with and without interspecific competition to illustrate how the methodology of Life Table Response Experiments (LTRE) can be used to link any change in population dynamics to changes in any part of the life cycle. Plants were herbaceous grassland species grown for two years in a field experiment at Rothamsted Experimental Station, England. Interspecific competition reduced λ by over 90% in all species. Survival and growth were slightly affected by competition whereas plant fecundity was greatly reduced. Nearly all of the observed difference in λ between the competition treatments was explained by the fecundity terms, and more precisely by a large difference in the number of seeds, and a high sensitivity of λ to the germination rate. Whereas most competition studies focus on the measurement of change in individual fitness, our study illustrates how informative it is to take account not only of the effect of competition upon vital rates but also of how different vital rates affect population growth rate.

POPULATION MATRIX MODELS OF AESCHYNOMENE VIRGINICA, A RARE ANNUAL PLANT: IMPLICATIONS FOR CONSERVATION

Matrix models are valuable tools for comparing population growth rates (λ) in different environments. Life table response experiments (LTRE) can then be used to identify those life history stages that are most important to observed population growth rate differences. We used matrix models of population growth and LTREs to analyze the effects of standing vegetation and seed dispersal on the population growth rate of Aeschynomene virginica, a rare, tidal, wetland annual. We combined seedling establishment, seedling survival to adulthood, adult fecundity, winter seed survival, probability of seeds entering a seed bank, and seed dispersal rates to calculate the matrix elements in a matrix model. Experimental removal of other plant species from field plots increased A. virginica fecundity, seedling establishment, and survival. The magnitude of these changes doubled between 1998 and 1999. Removal of interspecific competitors from population patches of A. virginica explained 54% of the variability of λ, while environmental variation between 1998 and 1999 explained only 1%. An additional 9% of the variability in λ was explained by adding the interaction of interspecific removal and year to the regression. Sensitivities of λ to changes in lower-level parameters varied over several orders of magnitude. Fecundity had the smallest sensitivity, while seedling establishment made the largest contribution to λ in 1998, and seedling survival to adulthood made the largest contribution in 1999. Simulations indicated that changes in seed dispersal would have small effects on λ. Eighty percent (1998) to 90% (1999) of the seeds produced in local patches growing without interspecific vegetation would have to disperse before patch population growth rates would become negative. Although seed dispersal had little impact on within-patch population dynamics, it may make seeds available to establish populations in nearby, open habitat patches. In population patches with interspecific vegetation, 30 seeds per plant (1998) and 80 seeds per plant (1999) would have to be added before patch population growth rates were positive. The most efficacious treatment to increase population sizes of this rare plant would be creating disturbed patches and not adding seeds.

Projecting Population-Level Responses of Mysids Exposed to an Endocrine Disrupting Chemical

To fully understand the implications of a chemical's effect on the conservation of a species, effects observed at the physiological or individual level must be expressed in terms of the population. Since long-term field experiments are typically not feasible, vital rates such as survival and reproduction of individual organisms are measured in life table response experiments (LTRE) and employed to extrapolate the effects of a pollutant on the population. The population-level response of the mysid, Americamysis bahia, to varying concentrations of methoprene (0, 4, 8, 16, 31, 62 μg/L) was determined using age-structured population models. Models were parameterized from the results of an LTRE conducted throughout the entire mysid life cycle. A density-independent matrix model with time invariant demographic parameters was developed to measure the change in population growth rate, λ, with change in methoprene concentration. The values of λ were greater than one for all methoprene concentrations, indicating that populations exposed to the concentrations reported here would not become extinct. However, a general decrease in λ occurred with increasing methoprene concentration and would result in reduced population sizes. Sensitivity and decomposition analyses were conducted to determined the relative roles of the vital rates on altered population growth rates and determined that impaired reproduction was the primary influence on the observed decrease in λ. The model constructed was a useful tool for linking the individual-level effects to the population-level consequences of methoprene exposure on mysids, as well as defining the mechanism (reduced reproduction) responsible for the observed effects on population.

The effects of grassland management on plant performance and demography in the perennial herb Primula veris

Summary Because of changing land‐use practices and abandonment, many European calcareous grasslands are under increasing threat. In order to protect those grasslands that remain, better insights into how plant populations respond to different management scenarios are needed. Using transition matrix models and life‐table response experiment (LTRE) analysis, the effects of different management strategies (grazing, summer and autumn mowing, and no management) on plant performance and population dynamics of the perennial herb Primula veris (Primulaceae) were experimentally studied. Data were collected between 1999 and 2003 in a species‐rich calcareous grassland. Early grazing (May) resulted in low population growth rates (λ < 0·860) and a mean annual population decline of 11%. Under these conditions, both the proportion of flowering individuals and flower and seed production per plant were low, resulting in seed limitation overall. However, when grazing started later in the growing season (early July) flowering probability and overall seed set increased, as did population growth rates (λ > 1). Mowing in autumn (October) was the most favourable management scenario (mean λ= 1·213), resulting in high proportions of flowering individuals and a large seed output. Furthermore, this management yielded optimal conditions for recruitment and seedling establishment during the next growing season. Summer mowing (mid‐July) resulted in a similar increase of flowering and overall seed shed to autumn mowing, but recruitment rates were lower because of a dense and tall vegetation structure at the time of germination. Consequently, population growth rates (mean λ= 1·045) were lower compared with the autumn mowing regime. No management of the grassland resulted in low growth rates (λ < 0·843) and a mean annual population decline of 35%, because of high mortality rates of each life stage and a lack of recruitment. Recruitment rates were strongly reduced by lowered flowering probabilities and limited germination possibilities. Synthesis and applications. This study may enable conservation managers to understand better the effects of time and type of management on population dynamics of P. veris. In order to preserve the remaining populations of this long‐lived species, management interventions can promote flowering and seed shed and reduce productivity of the vegetation by mowing in autumn. Finally, this study has clearly shown that the lack of any management, which is the fate of many abandoned calcareous grassland relicts, will seriously restrict the long‐term survival of P. veris.

YELLOW-BELLIED MARMOT POPULATION DYNAMICS: DEMOGRAPHIC MECHANISMS OF GROWTH AND DECLINE

Multiple environmental factors may act synergistically to influence demo- graphic characteristics, and ultimately the dynamics, of biological populations. Using pro- spective and retrospective analyses of demographic data from a 40-year study of individually marked animals, we investigated the demographic mechanisms of the temporal and spatial dynamics of a yellow-bellied marmot (Marmota flaviventris) population. Prospective elas- ticity analyses indicated juvenile survival ( Pj) would have the largest relative influence on the projected population growth rate (l). Relative magnitudes of elasticities did not differ between years characterized by positive ( l. 1.0) and negative ( l, 1.0) population growth. However, retrospective analyses of a life table response experiment (LTRE) revealed that changes in fertility (F), followed by age of first reproduction (a) made the largest contri- butions to observed annual changes in l. Changes in F and a made the largest contributions to annual declines in l, whereas changes in Pj also were important to cause increases in l. Population dynamic differences among marmot colonies were due primarily to spatial variations in a and Pj. Our results indicate that changes in reproductive parameters ( a and F) primarily drive the temporal dynamics of our study population, and that demographic mechanisms of population increases might differ from those of population declines. Studies of the regulation of yellow-bellied marmot populations should focus on the factors or processes influencing reproductive parameters.

Demography of Machaerium cuspidatum, a shade-tolerant neotropical liana

The population dynamics of the liana Machaerium cuspidatum (Fabaceae) were studied in an Amazonian forest in Ecuador using a stage-classified matrix model. Growth, mortality and number of new seedlings were recorded over two 1.5-y periods in eight 20 × 250-m transects. The population growth rate (λ) was 1.032, which was significantly higher than 1. The dynamics of the species were most influenced by survival of large plants, which is typical for slow-growing woody species. λ varied little among years or transects in spite of the high seedling production in one period. The importance of canopy openness and crown height for population growth was studied using bootstraps on λ for different subsets of the whole population. The results suggest that the population growth rate was lower in tall forest (height > 10 m) than in the population in general. The population growth rates in areas with shaded understorey and in low forest (≤ 20 m) were the same as for the whole population. A life table response experiment (LTRE) revealed that the lower λ in tall forest resulted from lower growth and survival of large plants in tall forest. These results indicate that the species is shade-tolerant but that it is sensitive to variations in forest dynamics. The dominance of the species and the increasing population size suggest that the forest has had a low disturbance rate for a long time.

Demography of American chestnut populations: effects of a pathogen and a hyperparasite

Summary 1 Matrix models were used to evaluate the effect of chestnut blight infection on transition probabilities and population growth rates for American chestnuts. Disease-free, epidemic and recovering (i.e. pathogen infected with a double-stranded (ds) RNA hypovirus) populations were compared. 2 Population growth rates (λ) did not differ significantly over time or with disease status. However, predicted stable stage distributions differed between population types, with disease-free and recovering populations more similar to each other than either was to epidemic populations. 3 Survival had the highest proportional contribution to population growth rates as revealed by elasticity analyses. However, reductions in stasis of the largest trees contributed most to reductions in population growth rate when comparing diseased with disease-free populations using LTRE. 4 The presence of hypovirus reduces pathogen virulence, allowing individual American chestnut trees to increase in size. Where dsRNA has spread, chestnut populations in Michigan have attained population dynamics similar to those found in disease-free populations. 5 Matrix models and life table response experiments can be used to detect important pathogen-mediated changes in the dynamics of host populations.

THE EFFECTS OF HERBIVORY AND POLLEN LIMITATION ON A DECLINING POPULATION OF TRILLIUM GRANDIFLORUM

In long‐lived organisms, it is often difficult to determine which environmental factors will have the largest effects on population dynamics. In this study, I incorporated the results of short‐term experiments and observations into a demographic matrix model to determine the effects of both herbivory and pollen limitation on the dynamics of a declining population of the perennial herb Trillium grandiflorum. While pollen supplementation experiments in both 1999 and 2000 revealed that plants produce fewer seeds as a result of pollen limitation, this pollen limitation had almost no effect on the growth rate of the population. White‐tailed deer (Odocoileus virginianus) consumed nearly half of the reproductive plants in this population in 2000 and 2001. Herbivory causes reproductive and large nonreproductive plants to regress in stage and have lower fecundity. In the absence of herbivory, demographic projections suggest that the population would shift from declining to growing, the proportional representation of new recruits would rise, and plants in larger stage classes would have higher reproductive values. Such shifts in the reproductive values and stable stage distribution result in shifts in elasticities. A life table response experiment demonstrated that, while herbivory affected six vital rates, two of these contributed disproportionately to the change in the population growth rate: reproductive plants remaining reproductive and large nonreproductive plants remaining nonreproductive. In this population, herbivores contribute more to its decline than pollinators. Thus, active long‐term management of deer populations appears necessary for the conservation of understory herbs such as Trillium grandiflorum.

Dynamics of harvested populations of the tropical understory herb Aechmea magdalenae in old-growth versus secondary forests

The prevalence of secondary forests is increasing across tropical landscapes, but the effects of these environments on the population dynamics of understory species and on their capacity to regenerate after harvest remains poorly documented. We compare the population dynamics of the understory bromeliad, Aechmea magdalenae, between old-growth and secondary rainforests in Southeast Mexico. A. magdalenae is a non-timber forest product (NTFP), the ramets of which are harvested from wild populations to make forest plantations. The fates of 600 rosettes in the two forest types were monitored over 3 years to build annual transition matrices. Population projections using the matrices show that secondary forest populations have finite rates of growth significantly higher than those of old-growth populations under both ramet-harvest and no-harvest conditions. Life table response experiments indicate that differences in the capacity for re-growth after harvest was due to greater vegetative reproduction of the largest secondary forest rosettes and faster growth of their ramets. For understory NTFP such as A. magdalenae, secondary forests may offer more economic incentive and greater potential for forest conservation than old-growth forests.

Demography and population dynamics of Drosera anglica and D. rotundifolia

Summary 1 We studied demography and population dynamics of the sympatric perennial herbs Drosera anglica and D. rotundifolia on a boreal bog in SE Norway. Dry mass of 2872 D. anglica plants and 2467 D. rotundifolia plants (estimated from field morphological measurements) was used to classify plants into five species-specific size classes. Demographic changes within these two populations were followed from 1995 to 1999, and within segments (quartiles) along the water table gradient and the peat productivity gradient. 2 Mortality was strongly size dependent, and varied between years, for both species; it was high for seedlings, low for the smallest mature rosettes and increased again for the largest mature rosettes. The proportion of fertile rosettes increased with increasing rosette size. Fecundity varied considerably between years, but little relative to gradient position. 3 Growth rate (λ) was > 1, except in the second year, when it fell to 0.572 for D. anglica and 0.627 for D. rotundifolia . For D. anglica small, but significant, differences were found between the two extremities of the water table gradient, and for D. rotundifolia between the second and the uppermost quartile. There was a tendency for D. anglica populations to have a lower growth rate in the most productive sites, whereas D. rotundifolia grew less on both low and high peat productivity. Elasticity analysis showed that stasis and size increase (primarily within mature stages) made major contributions to λ for D. anglica in all years. 4 The variance in population growth rate (var λ) was high between years, and higher for D. anglica than for D. rotundifolia , while the variance between quartiles along the two main gradients was low. Life-table response experiment (LTRE) analyses revealed that for both species, differences in probabilities of transitions within mature stages, and in growth to larger stages, contributed most to var λ. 5 The effects of global warming are uncertain: drier growing seasons would affect Drosera populations negatively, while initially positive responses to a wetter climate may be balanced by competition from increased Sphagnum growth.

Cropping system effects on giant foxtail (Setaria faberi) demography: II. Retrospective perturbation analysis

Cropping system characteristics affect weed management by altering key demographic rates of weeds, including recruitment, seedling survival, fecundity, and seed survival. To facilitate the design and improvement of cropping systems that limit weed population growth, analytical methods are needed to identify weed management “choke points” (weed life stages that vary in response to management and whose variation strongly affects weed population growth rate). The objectives of this study were to (1) determine whether wheat–red clover green manure can limit giant foxtail population growth rate (λ) in a wheat–corn–soybean crop sequence and (2) identify choke points in the giant foxtail life cycle with respect to the green manure treatment. Demographic data were used to construct a periodic matrix model of giant foxtail population growth in a wheat–corn–soybean crop sequence, with either a wheat sole crop (W) or a wheat–red clover intercrop (R) in the wheat phase. Identification of choke points was accomplished by adapting the life-table response experiment (LTRE) design for retrospective perturbation analysis of the periodic matrix model. The difference inλ(Δλ) between the two treatments was decomposed into contributions from each parameter in each rotation phase of the periodic model. Each LTRE contribution was equal to the product of the sensitivity ofλto changes in a given parameter by the treatment difference in that parameter. Those parameters making large contributions to Δλrepresented weed management choke points. Giant foxtail population growth rate in the simulation was more than twice as great in the W treatment (λ= 2.54) than in the R treatment (λ= 1.16). Retrospective perturbation analysis indicated that the proportion of seeds surviving predation in the wheat phase made the largest LTRE contribution (0.55) to Δλ, followed by seedling recruitment in the soybean (0.41) and corn (0.20) phases. By identifying weed management choke points within a given system, retrospective perturbation analysis can target research and management efforts for greater reductions in weed population growth.

DEMOGRAPHY AND DISPERSAL: LIFE TABLE RESPONSE EXPERIMENTS FOR INVASION SPEED

Long-distance dispersal has many consequences. One of these is the speed with which a population can invade a new habitat. This invasion wave speed can be calculated from information on demography (in the form of a stage-structured matrix population model) and dispersal (in the form of a distribution of stage-specific dispersal distances). Wave speed complements population growth rate (λ) as an index of population performance in conservation biology. Here we present a new way to calculate the sensitivity of wave speed to changes in short- and long-distance dispersal—indeed, to dispersal at any distance—using the order statistics of the dispersal distribution. This makes it possible to examine the effects of changing the distance dispersed by the longest-dispersing individual, the shortest, the median individual, etc. We apply the method to data on the Pied Flycatcher in the Netherlands, the Starling in the United States, and the Sparrowhawk in England and the Netherlands. We find that wave speed is vastly more sensitive to changes in the uppermost percentiles of the dispersal distribution than to changes elsewhere. The sensitivity results make it possible to carry out life table response experiment (LTRE) calculations, which quantify the contribution of demography, short-distance dispersal, and long-distance dispersal to interpopulation differences in wave speed. We find that 30% of the difference in wave speed between the Pied Flycatcher and the Starling is due to demography and about 70% to dispersal. Most of the dispersal effects come from the top 10% of the dispersal distribution. In contrast, 86% of the difference in wave speed between the two populations of Sparrowhawks is due to differences in demography, and about 14% to differences in dispersal. Again, most of the dispersal effects are due to the upper percentiles of the dispersal distribution. The patterns revealed in the analysis of the four populations are very similar, suggesting that the dependence of wave speed on long-distance dispersal may exhibit common patterns of sensitivity and elasticity.

Effects of zinc exposure on the polychaete Dinophilus gyrociliatus: a life-table response experiment

A life-table response experiment (LTRE) was performed in order to study the effects of enhanced zinc on the survival, fecundity and demographic indices of Dinophilus gyrociliatus (Polychaeta). A control group and three treated cohorts kept in artificial seawater enriched with nominal concentrations of zinc at 0.25, 0.50 and 1.00 μg Zn/ml, respectively, were considered. Zinc marginally affected survival but had a strong impact on the reproduction of D. gyrociliatus. Zinc enrichment had no effect on age at maturity, whereas it induced a slight increase in fecundity during the early reproductive phases at 0.50 and 1.00 μg Zn/ml, and a sharp decrease in fecundity from the 20th day of life at all levels of metal enrichment tested. Treated groups were characterised by a marked reduction of the net growth rate ( R 0 ) and a decrease in generation time ( T) with respect to the control groups, but no effect on life expectancy ( e 0 ) or on population growth rates ( λ) was observed. R 0 and λ, which are the most commonly used indices of fitness, were not equally susceptible to zinc. λ appears less sensitive to stress induced by metal enrichment with respect to R 0 and other life history traits like age-specific fecundity. D. gyrociliatus appears to be an appropriate bioindicator species in LTRE to assess the consequences of environmental heavy metals on individual life-history and population dynamics.

Population‐level responses to long‐term cadmium exposure in two strains of the freshwater gastropod Biomphalaria glabrata: Results from a life‐table response experiment

A life-table response experiment was conducted to ascertain the demographic effects of low-level cadmium exposure on two strains, BS90 and NMRI, of the freshwater gastropod Biomphalaria glabrata. Snails were exposed to cadmium continuously from the embryonic stage through adulthood. Results indicated that cadmium significantly affected a number of individual-based parameters, including %hatch, juvenile survival, and adult survival in both strains. Also, fecundity and time to maturity were significantly affected in the NMRI strain. A stage-based, deterministic, population model indicated that population growth rate (lambda) was significantly affected by cadmium. Elasticity analysis indicated that juvenile survival, in general, had the greatest contribution to lambda. Decomposition analysis indicated that the effects of cadmium on the juvenile stage in BS90 and the embryonic stage in NMRI contributed most to cadmium-induced changes in lambda. The BS90 strain was more sensitive to cadmium toxicity than NMRI. Moreover, the two strains differed in pattern of response with different aspects of their respective life histories contributing to cadmium-induced changes in lambda. Comparisons were also made between the main model, based on a Z transformation of the life-cycle graph, and more commonly used matrix models.