Population Multiplication Rate Research Articles

Combined influences of transgenerational effects, temperature and insecticide on the moth Spodoptera littoralis

Climate warming is expected to impact the response of species to insecticides. Recent studies show that this interaction between insecticides and temperature can depend on other factors. Here, we tested for the influence of transgenerational effects on the Insecticide × Temperature interaction in the crop pest moth Spodoptera littoralis. Specifically, we analysed reaction norms among experimental clutches based on a split-plot design crossing the factors temperature, insecticide and clutch. The study was performed on 2280 larvae reared at four temperatures (23, 25, 27 and 29 °C), and their response to the insecticide deltamethrin (three concentrations and a control group) was tested. Temperature had a global influence with effects on larval survival, duration of development, pupal body mass, and significant reaction norms of the clutches for temperature variations of only 2 °C. In addition to the expected effect of deltamethrin on mortality, the insecticide slightly delayed the development of S. littoralis, and the effects on mortality and development differed among the clutches. Projection models integrating all the observed responses illustrated the additive effects of deltamethrin and temperature on the population multiplication rate. Variation in the response of the clutches showed that transgenerational effects influenced the impact of insecticide and temperature. Although no evidence indicated that the Insecticide × Temperature interaction depended on transgenerational effects, the studies on the dependence of the Insecticide × Temperature interaction on other factors continue to be crucial to confidently predict the combined effects of insecticides and climate warming.

Life tables and elasticity analyses of Yangtze River fish species with implications for conservation and management

In conservation biology, knowing the life history strategy of a species is crucial for deciding which stage needs the greatest protection: reproduction, juveniles, or adults. Here, we compiled biological data for 83 Yangtze River fish species, constructed their life tables and conducted elasticity analyses, which calculates the proportional contribution of each vital rate (age-specific survival and fecundity) to the annual population multiplication rate. Life tables showed that, for our analyzed species, the survival rate of age 0 fish was extremely low, ranging from 1.25 × 10−6 to 1.74 × 10−2 year−1. All species showed a type III survivorship curve. Elasticity analysis revealed slow–fast continuum life histories ranging from species with a high contribution of fertility elasticity to the population growth rate, but lower juvenile and adult survival elasticity, as well as early maturation and short generation time to species that mature late, possess a long generation time, exhibit low fertility elasticity, and a high adult or juvenile survival elasticity. In addition, the analysis of all the species showed that fertility elasticity was significantly smaller than the sum of juvenile and adult survival elasticity, indicating the need to increase the juvenile and adult survival rates of Yangtze fish rather than focusing only on protecting reproduction and age 0 survival via a breeding season fishing ban which is the current conservation practice. Since overfishing has been proposed as the major factor responsible for the decline of Yangtze fish populations, we further suggest that controlling fishing pressure on juvenile and adult fish is another key conservation requirement.

Status of the Northern Silvery Grebe Podiceps juninensis in the northern Andes: recent changes in distribution, population trends and conservation needs

SummaryThe Northern Silvery Grebe Podiceps juninensis is an aquatic bird species considered Vulnerable to extinction in Ecuador and Endangered in Colombia. Globally the species is considered Near Threatened. Herein we report recent changes in distribution and population trends for the subpopulation in the northern Andes (Ecuador, Colombia), which is small and isolated. The Northern Silvery Grebe has undergone two documented extirpations, and at least one colonisation event, over the last three decades. More than 90% of the subpopulation relies on two wetlands, La Mica and Colta lakes in Ecuador, with the latter facing an accelerated sedimentation process which jeopardizes the fate of the wetland, and the species, in the mid-term. The population of La Mica lost 80% of its birds in 2000 after the damming of the lake, but then remained stable over our census period (2004–2013). In contrast, the Colta population results from a recent colonisation in the early 2000s, and has experienced an exponential growth since then (annual population multiplication rate: 1.464). The growth of this population is so fast that it must include an important contribution of immigrants. The temporal coincidence between the drop in population size at lake La Mica and the colonisation of lake Colta suggests that the Colta population was founded by immigrants from La Mica. Finally, the absence of current reports of the species at wetlands in Colombia suggests that the species remains at very low numbers, bolstering the shrinkage in northern distribution.

Predation of Atlantic Petrel chicks by house mice on Gough Island

The impacts of predation by invasive mammals on island fauna are a major driver of insular biodiversity loss. Devastating, hitherto unsuspected impacts of predatory house mice on breeding seabirds have been described recently. We studied the fate of 178 Atlantic Petrel Pterodroma incerta nests at Gough Island, over four seasons, from October 2003 to January 2008. Introduced house mice Mus musculus were found in all study burrows checked for mouse visits. From October 2003 to September 2004, we video-recorded attacks by mice on six (of 13) live, healthy Atlantic Petrel chicks and on one (of three) great shearwater Puffinus gravis chicks. In all years, chicks died from mouse attacks. Stage-specific daily nest survival rates were modelled, from which estimates of breeding success were derived that accounted for the variable exposure periods studied among years. Average daily survival rate of eggs was 0.998, and hatching success through the entire incubation period (55.5 days) was 0.924 [95% confidence interval (CI) 0.903–0.940]. Daily chick survival rates were 0.990, which gave a modelled fledging success of 0.247 (CI 0.165–0.338) over the 138-day chick period, and average annual breeding success (chicks fledged per breeding attempt) of 0.228 (CI 0.150–0.318), which is low compared with congeners. Productivity estimates were used as a parameter in a population simulation model, which predicted a population multiplication rate (λ) of 0.993 (CI = 0.966–1.021). However, in the one season studied from laying to fledging (2007), from 58 nests, only one chick fledged (1.7%). This suggests the wide errors on the model results may obscure a more severe reality. More than 60% of model simulations resulted in an International Union for Conservation of Nature classification of Endangered. Our results add support to calls to eradicate mice from Gough Island. More generally, mice cannot be ignored as a potential threat to island fauna, and island restoration and management plans should routinely include eradication of introduced mice.

From organisms to populations: Modeling aquatic toxicity data across two levels of biological organization

A critical step in estimating the ecological effects of a toxicant is extrapolating organism-level response data across higher levels of biological organization. In the present study, the organism-to-population link is made for the mysid, Americamysis bahia, exposed to a range of concentrations of six toxicants. Organism-level responses observed were categorized as no effect, delayed reproduction, reduced overall reproduction, or both reduced overall reproduction and survival. Population multiplication rates of each toxicant concentration were obtained from matrix models developed from organism-level endpoints and placed into the four categories of organism-level responses. Rates within each category were compared with growth rates modeled for control populations. Population multiplication rates were significantly less than control growth rates only for concentrations at which overall reproduction and both reproduction and survival were significantly less than the control values on the organism level. Decomposition analysis of the significant population-level effects identified reduced reproduction as the primary contributor to a reduced population multiplication rate at all sublethal concentrations and most lethal concentrations. Mortality was the primary contributor to reduced population growth rate only when survival was less than 25% of control survival. These results suggest the importance of altered reproduction in population-level risk assessment and emphasizes the need for complete life-cycle test data to make an explicit link between the organism and population levels.

Modeling vital rates improves estimation of population projection matrices

AbstractPopulation projection matrices are commonly used by ecologists and managers to analyze the dynamics of stage‐structured populations. Building projection matrices from data requires estimating transition rates among stages, a task that often entails estimating many parameters with few data. Consequently, large sampling variability in the estimated transition rates increases the uncertainty in the estimated matrix and quantities derived from it, such as the population multiplication rate and sensitivities of matrix elements. Here, we propose a strategy to avoid overparameterized matrix models. This strategy involves fitting models to the vital rates that determine matrix elements, evaluating both these models and ones that estimate matrix elements individually with model selection via information criteria, and averaging competing models with multimodel averaging. We illustrate this idea with data from a population of Silene acaulis (Caryophyllaceae), and conduct a simulation to investigate the statistical properties of the matrices estimated in this way. The simulation shows that compared with estimating matrix elements individually, building population projection matrices by fitting and averaging models of vital‐rate estimates can reduce the statistical error in the population projection matrix and quantities derived from it.

Demographic mechanisms of the population decline of the song thrush Turdus philomelos in Britain

Summary In Britain, the song thrush Turdus philomelos is categorized as a species of high national conservation concern because of a large population decline during the last three decades. We calculated a series of annual national population estimates for woodland and farmland habitats combined for the period 1964–2000. We then used turning points analysis to identify seven blocks of years within the period of decline (1968–2000) with uniform rates of population change in the smoothed trend. We used recoveries of song thrushes ringed as nestlings, juveniles and adults during April–September to estimate survival rates separately for the post‐fledging period, the remainder of the first year and for adults. Daily survival probability was lower during the post‐fledging period than during the remainder of the first year or for older birds. There was evidence for variation in survival rates among blocks of years with different rates of population change, particularly for first‐year survival. There were significant positive correlations across blocks between mean population multiplication rate (PMR) and both post‐fledging and first‐year survival. Survival of first‐year birds was correlated negatively with the duration of the longest run of frost days and the survival of adults was correlated negatively with the duration of the longest summer drought. Variation among blocks in mean PMR was correlated with block means of the duration of runs of frost days and drought days, but significant correlations between PMR and both post‐fledging and first‐year survival remained after allowing for the influence of weather on survival. Changes in survival in the first winter, and perhaps also the post‐fledging period, are sufficient to have caused the song thrush population decline. The environmental causes of these changes are not known, but changes in farming practices, land drainage, pesticides and predators are all candidates. Adverse weather conditions contributed to the decline, but were not the primary driver.

BAYESIAN ESTIMATION OF A DEMOGRAPHIC MATRIX MODEL FROM STAGE-FREQUENCY DATA

Demographic matrix models are standard tools for analyzing the dynamics of age- or stage-structured populations. Here, we present a method for estimating the average vital rates that parameterize a demographic matrix using a series of measurements of population size and structure (an “inverse problem” of demographic analysis). We join a deterministic, density-independent demographic matrix model with a stochastic observation model to write a likelihood function for the matrix parameters given the data. Adopting a Bayesian perspective, we combine this likelihood function with prior distributions for the model parameters to produce a joint posterior distribution for the parameters. We use a numerical technique (Markov chain Monte Carlo) to estimate and analyze the posterior distribution, and from this we calculate posterior distributions for functions of the demographic matrix, such as the population multiplication rate, stable stage distribution, and matrix sensitivities. Although measurements of population size and structure rarely contain enough information to estimate all the parameters in a matrix precisely, our analysis sheds light on the information that the data do contain about the vital rates by quantifying the precision of the parameter estimates and the correlations among them. Moreover, we show that matrix functions such as the population multiplication rate and matrix sensitivities can still be estimated precisely despite sizable uncertainty in the estimates of individual parameters, permitting biologically meaningful inference. We illustrate our approach for three populations of pea aphids (Acyrthosiphon pisum).

EFFICIENT DATA COLLECTION FOR ESTIMATING GROWTH RATES OF STRUCTURED POPULATIONS

Demographic matrix models are used to estimate the population multiplication rate (finite rate of increase) of structured populations. When rates of transitions among age classes or stages (e.g., probabilities of growth or survival, fecundities) are estimated by tagging individuals, the number of individuals tagged from each stage determines the precision of the estimated transition rates, and hence the precision of the estimated population multiplication rate, λ. When there is a limit on the total number of individuals that can be tagged, tagging individuals at random does not produce the most precise estimate of λ possible. Here, using a linear approximation to the sampling variance of λ, I show how to calculate an allocation of tags among stages that provides a more precise estimate of λ than tagging individuals randomly. I illustrate this calculation with two examples, and show by simulation that stratified sampling based on this efficient sample composition can result in sizable increases in the precision of λ. The methods presented can also be used to compare the statistical efficiency of alternative sampling designs.

Consequences of harvesting on age structure, sex ratio and population dynamics of red deer Cervus elaphus in central Norway

We explore some of the consequences of harvest on population growth rate, age and sex structure in a Norwegian population of red deer Cervus elaphus, using age‐structured demographic models. Survival rates were estimated from individuals marked and monitored annually during 1977–1995, and information about reproduction patterns were obtained from hunting material in the same region. The population had an actual doubling time of 14 years, corresponding to a multiplication rate of 1.051. Harvesting led to a reduction of about 10% of the potential multiplication rate that equalled 1.166. Including stochasticity had only a small effect on the population multiplication rate. Due to a high hunting pressure, males had less than a 10% chance of reaching 4.5 years of age and the male‐biased harvest strongly biased the sex ratio. Assuming that when the number of females per male increases above a given threshold some females would not manage to mate, we investigated at which level male harvesting could be maintained without having demographic consequences on the population growth rate. We concluded that the hunting pressure on males could probably be increased further but indirect consequences of a strongly biased sex ratio (e.g. on population genetic structure) remain to be studied. Variation in the multiplication rate mainly resulted from the variation in winter calf survival. In its present form the harvesting regime reduces the growth rate and biases the sex and age structure, but does not seem to threaten the population's viability and productivity.

Application of Life-History Theory and Population Model Analysis to Turtle Conservation

As more reptiles find their way onto endangered species lists, it is increasingly important to identify management alternatives that can be applied across taxa. I have compared life tables from several turtle populations using elasticity analysis, a method that calculates the proportional contribution of each vital rate (age-specific survival and fecundity) to the annual population multiplication rate, k [In (X) = r, the intrinsic rate of increase]. Most freshwater turtles share similar elasticity patterns across age classes, in spite of large variations in mean annual fecundity, annual survival, and age at maturity. High adult survival elasticity and low fecundity elasticity in these species suggests that conservation efforts that reduce mortality of adults are likely to stabilize declining populations. Desert tortoises and sea turtles had different elasticity patterns, with relatively higher juvenile elasticities when summed across age classes. Three different life tables for painted turtles also showed variation in elasticity patterns. Approximate elasticities can be generated for age-based matrices without a complete life table for each species, requiring only age at maturity, adult female annual survival, and population multiplication rate. This approximation may help identify sensitive life stages for poorly known species, thereby guiding research and management efforts and furthering our understanding of lifehistory patterns.

Long-Term Viability of the Re-Introduced Population of the White-Tailed Eagle Haliaeetus albicilla in Scotland

The white-tailed eagle became extinct in Scotland early in the twentieth century after persecution by humans. A population has been re-established in western Scotland by the release, mainly as juveniles, of 82 wild-bred birds imported from Norway between 1975 and 1985. Some released immatures wandered widely, but birds settled on fixed home ranges within 100 km of the release site on Rum, usually at 3-4 years old. In 1992 there were c. 19 individuals, including eight pairs, occupying fixed home ranges. There was also an unknown number of immature birds. Annual survival rates were estimated at 73% for young birds prior to settlement and 94% thereafter from the observed numbers of released and established birds. However, estimates were imprecise because of the small size of the population and the lack of reliable individual marks. By 1992 a total of 29 young had been fledged in the wild from five home ranges. None of these young have yet bred. The mean breeding success of released birds increased with age up to 0.465 fledged young/pair/year at about 6 years old. There was significant variation in breeding success among individuals and/or home ranges. 13% of bird-years of occupation of fixed home ranges involved either unmated birds or supernumeraries in the home ranges of pairs, creating trios. A simulation model indicated that the observed proportion of unmated birds was similar to that expected from differences in the numbers of males and females arising by chance. It was estimated from the available demographic data that the population would be more likely to grow slowly than decrease if stochastic effects due to small population size did not operate. However, the 95% confidence interval for the estimated population multiplication rate encompassed decline at 11% per year and growth at 11% per year. Simulation models indicated that, with realistic assumptions about variation among individual birds or ranges and demographic parameters set at their maximum-likelihood values, there was a probability of about 0.6 that the present population would become extinct within 100 years. Only conditions which would permit population growth at 2-3% per year in the absence of stochastic, small-population effects would reduce the probability of extinction within 100 years to less than 0.1. Simulation of supplementary releases of imported juveniles indicated a substantial reduction in extinction risk from the release of a further 60 juveniles. Variation in the rate of release, within a range considered feasible (10 birds per year -1 or 20 birds year -1 ), had little effect on this result.