Demographic factors and genetic variation influence population persistence under environmental change

Population persistence has been studied in a conservation context to predict the fate of small or declining populations. Persistence models have explored effects on extinction of random demographic and environmental fluctuations, but in the face of directional environmental change they should also integrate factors affecting whether a population can adapt. Here, we examine the population‐size dependence of demographic and genetic factors and their likely contributions to extinction time under scenarios of environmental change. Parameter estimates were derived from experimental populations of the rainforest species, Drosophila birchii, held in the lab for 10 generations at census sizes of 20, 100 and 1000, and later exposed to five generations of heat‐knockdown selection. Under a model of directional change in the thermal environment, rapid extinction of populations of size 20 was caused by a combination of low growth rate (r) and high stochasticity in r. Populations of 100 had significantly higher reproductive output, lower stochasticity in r and more additive genetic variance (V_A) than populations of 20, but they were predicted to persist less well than the largest size class. Even populations of 1000 persisted only a few hundred generations under realistic estimates of environmental change because of low V_A for heat‐knockdown resistance. The experimental results document population‐size dependence of demographic and adaptability factors. The simulations illustrate a threshold influence of demographic factors on population persistence, while genetic variance has a more elastic impact on persistence under environmental change.     

Demography_Lab,an educational application to evaluate population growth: Unstructured and matrix models

Training in Population Ecology asks for scalable applications capable of embarking students on a trip from basic concepts to the projection of populations under the various effects of density dependence and stochasticity. Demography_Lab is an educational tool for teaching Population Ecology aspiring to cover such a wide range of objectives. The application uses stochastic models to evaluate the future of populations. Demography_Lab may accommodate a wide range of life cycles and can construct models for populations with and without an age or stage structure. Difference equations are used for unstructured populations and matrix models for structured populations. Both types of models operate in discrete time. Models can be very simple, constructed with very limited demographic information or parameter‐rich, with a complex density‐dependence structure and detailed effects of the different sources of stochasticity. Demography_Lab allows for deterministic projections, asymptotic analysis, the extraction of confidence intervals for demographic parameters, and stochastic projections. Stochastic population growth is evaluated using up to three sources of stochasticity: environmental and demographic stochasticity and sampling error in obtaining the projection matrix. The user has full control on the effect of stochasticity on vital rates. The effect of the three sources of stochasticity may be evaluated independently for each vital rate. The user has also full control on density dependence. It may be included as a ceiling population size controlling the number of individuals in the population or it may be evaluated independently for each vital rate. Sensitivity analysis can be done for the asymptotic population growth rate or for the probability of extinction. Elasticity of the probability of extinction may be evaluated in response to changes in vital rates, and in response to changes in the intensity of density dependence and environmental stochasticity.     

Sensitivity analysis of effective population size to demographic parameters in house sparrow populations

The ratio between the effective and the census population size, , is an important measure of the long‐term viability and sustainability of a population. Understanding which demographic processes that affect most will improve our understanding of how genetic drift and the probability of fixation of alleles is affected by demography. This knowledge may also be of vital importance in management of endangered populations and species. Here, we use data from 13 natural populations of house sparrow (Passer domesticus) in Norway to calculate the demographic parameters that determine . Using the global variance‐based Sobol’ method for the sensitivity analyses, we found that was most sensitive to demographic variance, especially among older individuals. Furthermore, the individual reproductive values (that determine the demographic variance) were most sensitive to variation in fecundity. Our results draw attention to the applicability of sensitivity analyses in population management and conservation. For population management aiming to reduce the loss of genetic variation, a sensitivity analysis may indicate the demographic parameters towards which resources should be focused. The result of such an analysis may depend on the life history and mating system of the population or species under consideration, because the vital rates and sex–age classes that is most sensitive to may change accordingly.     

Demographic mechanism of a historical bird population collapse reconstructed using museum specimens

Long-term studies of demographic rates provide clues about the external causes of animal population declines, but systematic monitoring is rarely in place until after the decline has occurred. This study evaluates alternative hypotheses about the demographic mechanisms underlying the historical collapse of corncrake (Crexcrex) populations in Britain and Ireland in the late nineteenth and early twentieth centuries using characteristics of museum specimens. The proportion of adult corncrakes that are 1-year old was estimated from feather characteristics of birds collected before, during and after the population decline and showed a marked transitory reduction during the decline. This pattern would be expected if the decline was caused by a large reduction in the recruitment of young birds to the breeding population and is the opposite of what would be expected if a change in adult survival had caused the decline. These results are consistent with previous suggestions that the corncrake population decline was caused by adverse effects on breeding productivity caused by the mechanization of the harvesting of hay crops.     

Demographic parameters of a free-ranging deep-diving cetacean,the long-finned pilot whale

Demographic parameters provide baselines to estimate future population trajectories which can then be used in management decisions. The aim here was to estimate demographic parameters of long-finned pilot whale (Globicephala melas) from the Strait of Gibraltar by fitting mark-recapture models to photo-identification data of primary and secondary marked individuals. These parameters were used to forecast the future population trajectories in a population viability analysis (PVA) given different scenarios of demographic rates. Survival rate increased with age from 0.629, 95% CI [0.409, 0.805] for calves, 0.869, 95% CI [0.758, 0.934] for juveniles, to 0.972, 95% CI [0.953, 0.983] for adults. A preliminary mean observed interval of viable calves was 4.5 years. The PVA estimated the population would persist over 100 years with a 100% probability for all scenarios except those with lower 95% CI survival values, for which the probability of extinction reached 100%. Population growth rate was negative in all scenarios except those with 95% CI upper survival values. Interbirth interval and juvenile survival were found most influential and depended on the correct identification of secondary marked (e.g., calves and juveniles) individuals on a long-term basis. This population was found in a precarious state prior to a morbillivirus outbreak that might even more endanger its long-term viability.     

Integral projection models perform better for small demographic data sets than matrix population models: a case study of two perennial herbs

1. Matrix population models are widely used to describe population dynamics, conduct population viability analyses and derive management recommendations for plant populations. For endangered or invasive species, management decisions are often based on small demographic data sets. Hence, there is a need for population models which accurately assess population performance from such small data sets.
2. We used demographic data on two perennial herbs with different life histories to compare the accuracy and precision of the traditional matrix population model and the recently developed integral projection model (IPM) in relation to the amount of data.
3. For large data sets both matrix models and IPMs produced identical estimates of population growth rate (λ). However, for small data sets containing fewer than 300 individuals, IPMs often produced smaller bias and variance for λ than matrix models despite different matrix structures and sampling techniques used to construct the matrix population models.
4. Synthesis and applications . Our results suggest that the smaller bias and variance of λ estimates make IPMs preferable to matrix population models for small demographic data sets with a few hundred individuals. These results are likely to be applicable to a wide range of herbaceous, perennial plant species where demographic fate can be modelled as a function of a continuous state variable such as size. We recommend the use of IPMs to assess population performance and management strategies particularly for endangered or invasive perennial herbs where little demographic data are available.     

Demographic analysis of continuous-time life-history models

I present a computational approach to calculate the population growth rate, its sensitivity to life-history parameters and associated statistics like the stable population distribution and the reproductive value for exponentially growing populations, in which individual life history is described as a continuous development through time. The method is generally applicable to analyse population growth and performance for a wide range of individual life-history models, including cases in which the population consists of different types of individuals or in which the environment is fluctuating periodically. It complements comparable methods developed for discrete-time dynamics modelled with matrix or integral projection models. The basic idea behind the method is to use Lotka's integral equation for the population growth rate and compute the integral occurring in that equation by integrating an ordinary differential equation, analogous to recently derived methods to compute steady-states of physiologically structured population models. I illustrate application of the method using a number of published life-history models.     

兰科植物种群动态研究进展

兰科植物种群动态研究中,种群统计学分析能够很好地揭示植物个体在时空上的变化,是研究种群动态的核心.在自然生境中,许多附生兰科植物更倾向于离散或斑块状分布,可以通过集合种群研究分析斑块之间个体的基因流动,判断物种种群保护的规模.长期的种群动态研究能够获得兰科植物生活史和种群动态方面的可靠信息,以及一定环境条件下其时空波动及与种群功能之间的关系;短期的研究能够更好地理解具有结构性的独立植株与其所处的群落间的关系.本文根据种群生态学原理以及兰科植物的生态特点,从种群的密度及分布、种群统计学、种群的调节、集合种群和种群生存力分析（PVA）模型等方面阐述了国内外兰科植物种群动态研究进展.     

A modeling exercise to show why population models should incorporate distinct life histories of dispersers

Dispersal is an important form of movement influencing population dynamics, species distribution and gene flow between populations. In population models, dispersal is often included in a simplified manner by removing a random proportion of the population. Many ecologists now argue that models should be formulated at the level of individuals instead of the population level. To fully understand the effects of dispersal on natural systems, it is therefore necessary to incorporate individual-level differences in dispersal behavior in population models. Here, we parameterized an integral projection model, which allows for studying how individual life histories determine population-level processes, using bulb mites, Rhizoglyphus robini, to assess to what extent dispersal expression (frequency of individuals in the dispersal stage) and dispersal probability affect the proportion of successful dispersers and natal population growth rate. We find that allowing for life-history differences between resident phenotypes and disperser phenotypes shows that multiple combinations of dispersal probability and dispersal expression can produce the same proportion of leaving individuals. Additionally, a given proportion of successful dispersing individuals result in different natal population growth rates. The results highlight that dispersal life histories, and the frequency with which disperser phenotypes occur in the natal population, significantly affect population-level processes. Thus, biological realism of dispersal population models can be increased by incorporating the typically observed life-history differences between resident phenotypes and disperser phenotypes, and we here present a methodology to do so.     

Environmental factors influence both abundance and genetic diversity in a widespread bird species

Genetic diversity is one of the key evolutionary variables that correlate with population size, being of critical importance for population viability and the persistence of species. Genetic diversity can also have important ecological consequences within populations, and in turn, ecological factors may drive patterns of genetic diversity. However, the relationship between the genetic diversity of a population and how this interacts with ecological processes has so far only been investigated in a few studies. Here, we investigate the link between ecological factors, local population size, and allelic diversity, using a field study of a common bird species, the house sparrow (Passer domesticus). We studied sparrows outside the breeding season in a confined small valley dominated by dispersed farms and small‐scale agriculture in southern France. Population surveys at 36 locations revealed that sparrows were more abundant in locations with high food availability. We then captured and genotyped 891 house sparrows at 10 microsatellite loci from a subset of these locations (N = 12). Population genetic analyses revealed weak genetic structure, where each locality represented a distinct substructure within the study area. We found that food availability was the main factor among others tested to influence the genetic structure between locations. These results suggest that ecological factors can have strong impacts on both population size per se and intrapopulation genetic variation even at a small scale. On a more general level, our data indicate that a patchy environment and low dispersal rate can result in fine‐scale patterns of genetic diversity. Given the importance of genetic diversity for population viability, combining ecological and genetic data can help to identify factors limiting population size and determine the conservation potential of populations.     

Demographic consequences of changing body size in a terrestrial salamander

Changes in climate can alter individual body size, and the resulting shifts in reproduction and survival are expected to impact population dynamics and viability. However, appropriate methods to account for size‐dependent demographic changes are needed, especially in understudied yet threatened groups such as amphibians. We investigated individual‐ and population‐level demographic effects of changes in body size for a terrestrial salamander using capture–mark–recapture data. For our analysis, we implemented an integral projection model parameterized with capture–recapture likelihood estimates from a Bayesian framework. Our study combines survival and growth data from a single dataset to quantify the influence of size on survival while including different sources of uncertainty around these parameters, demonstrating how selective forces can be studied in populations with limited data and incomplete recaptures. We found a strong dependency of the population growth rate on changes in individual size, mediated by potential changes in selection on mean body size and on maximum body size. Our approach of simultaneous parameter estimation can be extended across taxa to identify eco‐evolutionary mechanisms acting on size‐specific vital rates, and thus shaping population dynamics and viability.     

The population ecology of Rattus exulans on Tiritiri Matangi Island,and a model of comparative population dynamics in New Zealand

Abstract

On Tiritiri, a small predator-free island in northern New Zealand, kiore (Rattus exulans) were live and snap trapped in grassland and forest. In both habitats, kiore abundance peaked in late summer/autumn. The increase followed a 3 month breeding season during which females produced two to three litters, each averaging 7 young. During the population decline in autumn and winter, animals lost weight. Few bred in the breeding season of their birth and none lived to breed in a second breeding season, so the population consisted of distinct age cohorts. These patterns may relate to a highly seasonal food supply.

Kiore elsewhere in New Zealand show seasonal breeding, but the length of breeding, sexual maturation, and litter size vary. Other studies of kiore in the Pacific show less marked seasonal fluctuations, longer breeding seasons, and smaller litters. We propose a model to explain the variation in rodent demography in New Zealand. The model is based on the seasonal availability of food, along with the modifying influences of predation and dispersal.     

A Demographic model for ring-necked ducks breeding in Minnesota

Population models can provide insights into how perturbations of demographic vital rates influence population growth rates (λ) and help prioritize efforts to achieve management goals. Although population models have been developed for numerous duck species, this tool has not been developed for ring-necked ducks (Aythya collaris). Ring-necked ducks are a late-nesting species that may not be surveyed well by the May Waterfowl Survey, which is timed optimally for earlier nesting ducks like the mallard (Anas platyrhynchos). Information specific to ring-necked ducks would help identify important influences on the rate of population growth and the direction of population change. We used data from our own research during 2008–2012, long-term (1975–2016) survey data from northcentral Minnesota, USA, vital rate estimates from the literature, and long-term (1987–2016) banding data to develop a population model for ring-necked ducks. We estimated annual survival and recovery rates of after-hatching-year and hatching-year female ring-necked ducks in Minnesota. Survey results and our matrix models indicated that ring-necked ducks are declining in the forested portion of Minnesota. Thus, we examined a boom-or-bust simulation scenario that might maintain a population through periodic occurrence of exceptional reproductive years in conjunction with high hatching-year survival. Our results indicated that long-term persistence is only expected in this population if reproductive output doubled at the same time that hatching-year survival was at its highest value, or if the population is maintained through immigration. Sensitivity analysis indicated that unit changes in 30-day brood survival will produce the most change in λ in the parameter space observed, although elasticity analysis indicated that proportional changes in annual survival of adult females will produce the largest proportional changes in λ. Management to improve brood survival to increase λ might include improving the habitat quality of brood-rearing lakes, especially those with more open water and less nesting habitat than those used for nesting. Our findings might also help explain disparities between annual breeding waterfowl surveys, which indicate stable or increasing populations, and hunter experiences in the fall. In Minnesota, hunter experiences have not matched expectations based on historical fall numbers and this could occur if regional production declined and fewer young birds were available for harvest. Our findings highlight the need for further study to inform management in this rapidly changing region. © 2019 The Wildlife Society.     

Population Viability Analysis of the Epiphytic Ghost Orchid (Dendrophylax lindenii) in Cuba

To design feasible conservation and management policies for wild species, it is critical to understand the effects of periodic disturbances, be they natural or anthropogenic. The Caribbean Basin is characterized by high cyclonic activity that has a strong impact on the demography and population dynamics of many taxa, including epiphytic orchids. We conducted a 5‐yr study of rare ghost orchid demography, Dendrophylax lindenii, to assess the stability of a protected population of this species in Cuba. Using both stochastic and deterministic integral projection models, we found that mean annual population growth rates are negative (λ = 0.975). However, we found both population growth rate and extinction risk are highly sensitive to survival rates and reproduction, a difficult to quantify rate for many orchids including our study species. While this species is fairly long‐lived, its relatively slow increase in annual survival with increasing size may reflect the lack of a protected (i.e., subterranean) storage organ—a life‐history trait that may typify other epiphytic species and increase susceptibility to disturbance events. Hurricanes, which are predicted to increase in frequency as a result of climate change, dramatically increase adult mortality. Simulations of these effects indicate that hurricanes and similar disturbances could result in near certain extinction in short time horizons (25 yr) if their annual probability of occurrence exceeds 14 percent. These results suggest a need to better quantify recruitment rates, as well as the sensitivity of population dynamics of this and other orchid species to hurricanes and other periodic disturbances.     

Impact factors of purse seine net and dip net fisheries on a chub mackerel population

There are two major kinds of fisheries for the chub mackerel in Japan. The purse seine net fishery exploits young and adult fish during the foraging season, while the dip net fishery exploits fish before and during the spawning season. To compare the damage to the chub mackerel stock caused by purse seine net and dip net fisheries, we introduced impact factors of the two kinds of fisheries on the two kinds of fisheries and estimated number of eggs spawned in 1982. We defined the impact factor as the ratio of the average reproductive value of individuals caught by each fishery to the reproductive value at maturity. The number of individuals caught by the purse seine net fishery was 66 times larger than the number caught by the dip net fishery, while the impact factor of the former was 84% of that of the latter. Thus, the total damage caused by the purse seine net fishery was much higher than the damage caused by the dip net fishery. We can evaluate the damage caused by mortality factors on endangered species using this method.