Changes In Population Growth Rate Research Articles

Cumulative excess deaths in New Zealand in the COVID-19 era: biases from ignoring changes in population growth rates: reply

Cumulative excess deaths in New Zealand in the COVID-19 era: biases from ignoring changes in population growth rates: reply

Changes in reproduction mediate the effects of climate change and grassland management on plant population dynamics.

Climate change is one of the largest threats to grassland plant species, which can be modified by land management. Although climate change and land management are expected to separately and interactively influence plant demography, this has been rarely considered in climate change experiments. We used a large-scale experiment in central Germany to quantify the effects of grassland management, climate change, and their joint effect on the demography and population growth rate of 11 plant species all native to this temperate grassland ecosystem. We parameterized integral projection models with five years of demographic data to project population growth rate. We hypothesized that plant populations perform better in the ambient than in the future climate treatment that creates hotter and drier summer conditions. Further, we hypothesized that plant performance interactively responds to climate and land management in a species-specific manner based on the drought, mowing, and grazing tolerances as well as the flowering phenology of each species. Due to extreme drought events, over half of our study species went quasi extinct, which highlights how extreme climate events can influence long-term experimental results. We found no consistent support for our expectation that plants perform better in ambient compared with future climate conditions. However, several species showed interactive responses to the treatments, indicating that optimal management strategies for plant performance are expected to shift with climate change. Changes in population growth rates of these species across treatments were mostly due to changes in plant reproduction. Experiments combined with measuring plant demographic responses provide a way to isolate the effects of different drivers on the long-term persistence of species and to identify the demographic vital rates that are critical to manage in the future. Our study suggests that it will become increasingly difficult to maintain species with preferences for moister soil conditions, and that climate and land use can interactively alter demographic responses of the remaining grassland species.

Cumulative excess deaths in New Zealand in the COVID-19 era: biases from ignoring changes in population growth rates: comment

For measuring mortality and excess deaths, age standardised mortality has been an industry standard for centuries. Methods that acknowledge changes in population age structure should be used when assessing mortality.

Cumulative excess deaths in New Zealand in the COVID-19 era: biases from ignoring changes in population growth rates

Accurate health and economic data are needed to evaluate policy responses to COVID-19. A potentially comprehensive health indicator is excess deaths. Local commentators highlight an excess deaths series that suggests negative cumulative excess mortality for New Zealand in the first three years of COVID-19 – in other words, fewer deaths than expected. This flawed measure ignores changes in population growth. Deaths rose in New Zealand from 2015 to 2019 as population grew at two percent per annum. Population growth came almost to a standstill after the border closed in March 2020. Methods of extrapolating from the past to predict future deaths, to ascertain if actual deaths exceed projections, must account for this sharp change in population growth rates. Rather than New Zealand being unique, in having negative cumulative excess deaths in the COVID-19 era, cumulative deaths are about four percent above expected deaths (through 2022) once population growth rate changes are accounted for.

Aging of Copper Nanoparticles in the Marine Environment Regulates Toxicity for a Coastal Phytoplankton Species.

Environmental conditions in aquatic ecosystems transform toxic chemicals over time, influencing their bioavailability and toxicity. Using an environmentally relevant methodology, we tested how exposure to seawater for 1-15 weeks influenced the accumulation and toxicity of copper nanoparticles (nano-Cu) in a marine phytoplankton species. Nano-Cu rapidly agglomerated in seawater and then decreased in size due to Cu dissolution. Dissolution rates declined during weeks 1-4 and remained low until 15 weeks, when the large agglomerates that had formed began to rapidly dissolve again. Marine phytoplankton species were exposed for 5-day periods to nano-Cu aged from 1 to 15 weeks at concentrations from 0.01 to 20 ppm. Toxicity to phytoplankton, measured as change in population growth rate, decreased significantly with particle aging from 0 to 4 weeks but increased substantially in the 15-week treatment due apparently to elevated Cu dissolution of reagglomerated particles. Results indicate that the transformation, fate, and toxicity of nano-Cu in marine ecosystems are influenced by a highly dynamic physicochemical aging process.

Maternal Transfer and Long-Term Population Effects of PCBs in Baltic Grey Seals Using a New Toxicokinetic–Toxicodynamic Population Model

Empirical evidence has shown that historical exposure of polychlorinated biphenyls (PCBs) to Baltic grey seals not only severely affected individual fitness, but also population growth rates and most likely caused the retarded recovery rate of the depleted population for decades. We constructed a new model which we term a toxicokinetic–toxicodynamic (TKTD) population model to quantify these effects. The toxicokinetic sub-model describes in detail the bioaccumulation, elimination and vertical transfer from mother to offspring of PCBs and is linked to a toxicodynamic model for estimation of PCB-related damage, hazard and stress impacts on fertility and survival rates. Both sub-models were linked to a Leslie matrix population model to calculate changes in population growth rate and age structure, given different rates of PCB exposure. Toxicodynamic model parameters related to reproductive organ lesions were calibrated using published historical data on observed pregnancy rates in Baltic grey seal females. Compared to empirical data, the TKTD population model described well the age-specific bioaccumulation pattern of PCBs in Baltic grey seals, and thus, the toxicokinetic parameters, deduced from the literature, are believed to be reliable. The model also captured well the general effects of PCBs on historical population growth rates. The model showed that reduced fertility due to increased PCB exposure causes decreased vertical transfer from mother to offspring and in turn increased biomagnification in non-breeding females. The developed TKTD model can be used to perform population viability analyses of Baltic grey seals with multiple stressors, also including by-catches and different hunting regimes. The model can also be extended to other marine mammals and other contaminants by adjustments of model parameters and thus provides a test bed in silico for new substances.

The Components of Change in Population Growth Rates.

The demographic balance equation relates the population growth rate with crude rates of fertility, mortality, and net migration. All these rates refer to changes occurring between two time points, say, t and t + h. However, this fundamental balance equation overlooks the contribution of historical fertility, mortality, and migration in explaining these population counts. Because of this, the balance equation only partially explains a change in growth rate between time t and t + h as it does not include the contribution of historical population trends in shaping the population at time t. The overall population growth rate can also be expressed as the weighted average of age-specific growth rates. In this article, we develop a method to decompose the historical drivers of current population growth by recursively employing the variable-r method on the population's average age-specific growth rates. We illustrate our method by identifying the unique contributions of survival progress, migration change, and fertility decline for current population growth in Denmark, England and Wales, France, and the United States. Our results show that survival progress is mainly having an effect on population growth at older ages, although accounting for indirect historical effects illuminates additional contributions at younger ages. Migration is particularly important in Denmark and England and Wales. Finally, we find that across all populations studied, historical fertility decline plays the largest role in shaping recent reductions in population growth rates.

Host plant limitation of butterflies in highly fragmented landscapes

Insect herbivores can be limited by host plants in two ways: density-dependent competition for food resources or density-independent search time limitation. Our understanding of density-dependent host plant limitation is relatively well developed and well integrated into conservation plans for at-risk insects. Search time limitation, a density-independent process, is much less well developed. Here, we explore both mechanisms using empirically based models of monarch butterfly population dynamics. These mechanisms differ fundamentally in their predictions: resource competition leads to matching of herbivore densities to host plant densities, and visible competition via consumption of host plants. Search time limitation leads to changes in population growth rate that can cause herbivore numbers to decline when host plant densities are constant. Search time limitation also implies that host plants can limit herbivores, even when many individual plants are uneaten. For monarch butterflies, our calculations suggest that many parts of North America have host plant densities below the threshold for search time limitation, which contrasts with the typical assumption of resource competition. More generally, incorporating search time limitation into conservation plans is important for reframing our understanding of how host plants limit insect herbivores in highly fragmented landscapes.

Integrated population models poorly estimate the demographic contribution of immigration

Abstract Estimating the contribution of demographic parameters to changes in population growth is essential for understanding why populations fluctuate. Integrated population models (IPMs) offer a possibility to estimate the contributions of additional demographic parameters, for which no data have been explicitly collected—typically immigration. Such parameters are often subsequently highlighted as important drivers of population growth. Yet, accuracy in estimating their temporal variation, and consequently their contribution to changes in population growth rate, has not been investigated. To quantify the magnitude and cause of potential biases when estimating the contribution of immigration using IPMs, we simulated data (using northern wheatear Oenanthe oenanthe population estimates) from controlled scenarios to examine potential biases and how they depend on IPM parameterization, formulation of priors, the level of temporal variation in immigration and sample size. We also used empirical data on populations with known rates of immigration: Soay sheep Ovis aries and Mauritius kestrel Falco punctatus with zero immigration and grey wolf Canis lupus in Scandinavia with near‐zero immigration. IPMs strongly overestimated the contribution of immigration to changes in population growth in scenarios when immigration was simulated with zero temporal variation (proportion of variance attributed to immigration = 63% for the more constrained formulation and real sample size) and in the wild populations, where the true number of immigrants was zero or near‐zero (kestrel 19.1%–98.2%, sheep 4.2%–36.1% and wolf 84.0%–99.2%). Although the estimation of the contribution of immigration in the simulation study became more accurate with increasing temporal variation and sample size, it was often not possible to distinguish between an accurate estimation from data with high temporal variation versus an overestimation from data with low temporal variation. Unrealistically, large sample sizes may be required to estimate the contribution of immigration well. To minimize the risk of overestimating the contribution of immigration (or any additional parameter) in IPMs, we recommend to: (a) look for evidence of variation in immigration before investigating its contribution to population growth, (b) simulate and model data for comparison to the real data and (c) use explicit data on immigration when possible.

Optimal Strategies for Managing Wildlife Harvest Under System Change

ABSTRACTWildlife populations are experiencing shifting dynamics due to climate and landscape change. Management policies that fail to account for non‐stationary dynamics may fail to achieve management objectives. We establish a framework for understanding optimal strategies for managing a theoretical harvested population under non‐stationarity. Building from harvest theory, we develop scenarios representing changes in population growth rate () or carrying capacity () and derive time‐dependent optimal harvest policies using stochastic dynamic programming. We then evaluate the cost of falsely assuming stationarity by comparing the outcomes of forward projections in which either the optimal policy or a stationary policy is applied. Whendeclines over time, the stationary policy leads to an underharvest of the population, resulting in less harvest over the short term but leaving the population in a higher‐value state. Whendeclines over time, the stationary policy leads to overharvest, resulting in greater harvest returns in the short term but leaving the population in a lower and potentially more vulnerable state. This work demonstrates the basic properties of time‐dependent harvest management and provides a framework for evaluating the many outstanding questions about optimal management strategies under climate change. Published 2021. This article is a U.S. Government work and is in the public domain in the USA.

Using environmental variation to optimize aerial surveys of harbour seals

Abstract Harbour seals are surveyed aerially when they haul-out to moult in August. The proportion of the population hauled out throughout the year is related to temporal, environmental, and meteorological variables. Thus, monitoring is conducted under predefined ranges of conditions. Effects of variation within these ranges are rarely reviewed. We used linear models to assess effects of time, date and weather on the difference between counts predicted by a population growth model and observed counts, based on a 30-year time-series. Our top-ranked model explained 34.4% of the variance. Survey date and its interaction with survey year were the most important variables, with higher counts earlier in August, particularly early in the time series, where surveys may not have been timed optimally to capture the peak in the moult. Cloud cover, wind speed, temperature, and interactions between these were of lesser importance; there were fewer seals on land during cloudy, windy days and on clear, warm days. These effects of weather are likely related to temperature regulation. Power analyses suggested that correction for survey conditions would allow detection of a one percentage point annual change in population growth rate with 80% power 4 years sooner than without taking survey conditions into account.

Temperature-associated decreases in demographic rates of Afrotropical bird species over 30years.

Tropical mountains harbor globally significant levels of biodiversity and endemism. Climate change threatens many tropical montane species, yet little research has assessed the effects of climate change on the demographic rates of tropical species, particularly in the Afrotropics. Here, we report on the demographic rates of 21 Afrotropical bird species over 30years in montane forests in Tanzania. We used mark-recapture analyses to model rates of population growth, recruitment, and apparent survival as functions of annual mean temperature and annual precipitation. For over one-half of focal species, decreasing population growth rates were associated with increasing temperature. Due to the trend in temperature over time, we substituted a time covariate for the temperature covariate in top-ranked population growth rate models. Temperature was a better explanatory covariate than time for 6 of the 12 species, or 29% of all focal species. Population growth rates were also lower for species found further below their elevational midpoint and for smaller-bodied species. Changes in population growth rates were more closely tied to changes in recruitment than to changes in apparent survival. There were no consistent associations between demographic rates and precipitation. This study demonstrates temperature-associated demographic impacts for 6 (29%) of 21 focal species in an Afrotropical understory bird community and highlights the need to incorporate the impacts of climate change on demographic rates into conservation planning across the tropics.

Climate change impacts on population growth across a species' range differ due to nonlinear responses of populations to climate and variation in rates of climate change.

Impacts of climate change can differ substantially across species’ geographic ranges, and impacts on a given population can be difficult to predict accurately. A commonly used approximation for the impacts of climate change on the population growth rate is the product of local changes in each climate variable (which may differ among populations) and the sensitivity (the derivative of the population growth rate with respect to that climate variable), summed across climate variables. However, this approximation may not be accurate for predicting changes in population growth rate across geographic ranges, because the sensitivities to climate variables or the rate of climate change may differ among populations. In addition, while this approximation assumes a linear response of population growth rate to climate, population growth rate is typically a nonlinear function of climate variables. Here, we use climate-driven integral projection models combined with projections of future climate to predict changes in population growth rate from 2008 to 2099 for an uncommon alpine plant species, Douglasia alaskana, in a rapidly warming location, southcentral Alaska USA. We dissect the causes of among-population variation in climate change impacts, including magnitude of climate change in each population and nonlinearities in population response to climate change. We show that much of the variation in climate change impacts across D. alaskana’s range arises from nonlinearities in population response to climate. Our results highlight the critical role of nonlinear responses to climate change impacts, suggesting that current responses to increases in temperature or changes in precipitation may not continue indefinitely under continued changes in climate. Further, our results suggest the degree of nonlinearity in climate responses and the shape of responses (e.g., convex or concave) can differ substantially across populations, such that populations may differ dramatically in responses to future climate even when their current responses are quite similar.

Bridging gaps in demographic analysis with phylogenetic imputation.

Phylogenetically informed imputation methods have rarely been applied to estimate missing values in demographic data but may be a powerful tool for reconstructing vital rates of survival, maturation, and fecundity for species of conservation concern. Imputed vital rates could be used to parameterize demographic models to explore how populations respond when vital rates are perturbed. We used standardized vital rate estimates for 50 bird species to assess the use of phylogenetic imputation to fill gaps in demographic data. We calculated imputation accuracy for vital rates of focal species excluded from the data set either singly or in combination and with and without phylogeny, body mass, and life-history trait data. We used imputed vital rates to calculate demographic metrics, including generation time, to validate the use of imputation in demographic analyses. Covariance among vital rates and other trait data provided a strong basis to guide imputation of missing vital rates in birds, even in the absence of phylogenetic information. Mean NRMSE for null and phylogenetic models differed by <0.01 except when no vital rates were available or for vital rates with high phylogenetic signal (Pagel's λ > 0.8). In these cases, including body mass and life-history trait data compensated for lack of phylogenetic information: mean normalized root mean square error (NRMSE) for null and phylogenetic models differed by <0.01 for adult survival and <0.04 for maturation rate. Estimates of demographic metrics were sensitive to the accuracy of imputed vital rates. For example, mean error in generation time doubled in response to inaccurate estimates of maturation time. Accurate demographic data and metrics, such as generation time, are needed to inform conservation planning processes, for example through International Union for Conservation of Nature Red List assessments and population viability analysis. Imputed vital rates could be useful in this context but, as for any estimated model parameters, awareness of the sensitivities of demographic model outputs to the imputed vital rates is essential.

Is Orius sauteri Poppius a Promising Biological Control Agent for Walnut Aphids? An Assessment from the Laboratory to Field.

Walnut aphids are major pests of walnut production with few commercially available natural enemies. We conducted laboratory and field experiments to evaluate the potential of Orius sauteri Poppius (Anthocoridae), a predatory bug, as a biological control agent against two walnut aphid species: the dusky-veined aphid (Panaphis juglandis Goeze) and the walnut aphid (Chromaphis juglandicola Kaltenbach). Both species co-occur on walnut trees; P. juglandis is distributed on the upper surface (adaxial) of leaves while C. juglandicola is found on the lower surface (abaxial) of leaves. Based on functional response experiments, O sauteri had a strong capacity for consuming both aphid species. Biocontrol efficacy of O. sauteri for each species in the laboratory and field experiments was high, 77% for P. juglandis and 80% for C. juglandicola, regardless if one or two predators being present. However, biocontrol efficacy declined 15-25% for C. juglandicola and 20-50% for P. juglandis when both aphid species were present on the same leaf. The efficacy of O. sauteri under (semi)-field conditions gave similar findings based on the percentage reduction of aphids and change in population growth rates of aphids. The reduced biocontrol efficacy of the predatory bug against mixed species populations of aphids can be explained by competition between the aphid species and differences in their preferred location on leaves. Our experiments showed that O. sauteri is a promising biocontrol agent, but biocontrol efficacy may decline when both aphid species are present on walnut trees. This should be considered in the commercial release of O. sauteri in walnut orchards to promote economic and environmental benefits of walnuts production.

Surface Water Potential Assessment and Water Demand Evaluation (A Case of Dabus Watershed, Blue Nile Basin)

Dabus sub-basin is one of the main tributaries of Blue Nile with a considerable surface water potential which can be used for hydro power, irrigation, and water supply. Lack of studies regarding surface water potential and demands at the sub-basin level is the reason why this potential was underutilized. The objective of this study is to assess the surface water potential and evaluate the current and future demand by using Water Evaluation and Planning (WEAP) model at Dabus sub-basin. The model was constructed on four different scenarios starting from the current account (2020) wherein all the data is filled into the model to estimate the surface water potential and demands for different sectors. The scenarios include Scenario 1: reference scenario; Scenario 2: Change in population growth rate; Scenario 3: Irrigation water demand projection; and Scenario 4: Increased domestic water demand. The scenario has helped in analyzing “what if” questions. For all the scenarios the overall demand, coverage and unmet demand were analyzed based on three-time horizon as (2020-2030, 2030-2040, and 2040-2050). The model estimated the average annual flow as 6.536 Billion Cubic Meter (BCM) which is generated from annual precipitation of 14.987 BCM. The model showed 100% demand coverage for all the scenarios except the irrigation demand projection scenario which have unmet demand on some of the months of the year.

Recent increase in annual survival of nesting female Common Scoter Melanitta nigra in Iceland

Despite their unfavourable conservation status, little is known about the population status, trends and demography of Common Scoter Melanitta nigra. Capture-mark-recapture (CMR) data from 154 recapture events of 88 individually marked breeding female Common Scoter Melanitta nigra in the Aðaldalur valley, northeast Iceland, during 2009–2018 generated an estimated annual apparent survival probability of 0.848 (95% CL 0.775–0.901). This exceeds a previous Icelandic CMR estimate (0.783, 95% CL 0.715–0.839) from nesting females during 1925–1958 at the nearby Mývatn (45 km to the SSE) when the population was declining at c. 2% per annum. Spring count data from Mývatn show that numbers of male and female Common Scoter at this important breeding site (thought to constitute over 80% of the Icelandic breeding national total) have increased by c. 1.8% per annum since 1974, and by 4–5% per annum since 2009. A population model showed that the observed change in survival corresponded well with the observed change in population growth rate. We reflect on causes of the increase in mean expected lifespan (from 4.1 to 6.1 years) between 1925–1958 and 2009–2018, speculating whether changes in food supply and/or reduction in gill-net fishing on Mývatn in the last 25 years could have been contributing factors to explain recent increases in breeding females there. An alternative, not necessarily mutually exclusive explanation could also be that the dramatic declines in chronic marine oil spill pollution in European waters could have contributed to higher recent female annual survival. While fully recognising the challenges of comparing survival rates from two sites in two different periods, survival rate has increased significantly between the two studies in a way that could potentially explain the increase in abundance at Mývatn. This could indicate that female survival has increased in the population as a whole. We strongly recommend demographic monitoring as a contribution to monitoring sea duck populations, species that are difficult to count and otherwise monitor annual changes in abundance in any meaningful way across large geographical ranges.

Rapid adaptation of Brachionus angularis (Rotifera) to invasion by Brachionus calyciflorus

To test the hypothesis that Brachionus angularis can rapidly develop local adaptation to invasion by other monogonont rotifers, replicate populations of B. angularis were exposed to two environments invaded by B. calyciflorus at different inoculation densities and an environment free from invaders. During the selection experiment, the density of each population was reduced to starting conditions every 3 days, and the changes in population growth rate and mictic ratio of B. angularis were continuously tracked. During the 81-day (69 asexual generations) period of selection, the B. angularis populations in the environments with invaders showed a significant increase in growth rate but a non-significant change in mictic ratio over time when compared with those in the environment without invaders. A common garden experiment revealed that when exposed to the environments with invaders, the B. angularis populations evolved in the environments with invaders had higher growth rates than those evolved in the environment without invaders, and the opposite was also true, indicating that local adaptation was rapidly formed. Info-chemicals released by invaders to the environment might increase the heritability of higher growth rates, and thus facilitate the local adaptation of B. angularis populations to invasion by B. calyciflorus.

On the importance of having a good mother: maternal investment affects duckling mortality risk in wood ducks

Most avian populations experience more variation in recruitment than adult survival, and this drives much of the change in population growth rates from year to year. In duck species, the probability of duckling survival is an important component of recruitment into the breeding population. We investigated how variation in maternal investment in offspring by nesting female wood duck Aix sponsa affected duckling mortality (1‐survival) to 60 days of age using capture‐mark‐recapture techniques. Our primary sample consisted of 3035 ducklings, from 545 nests, that were marked with uniquely coded webtags on the day of hatch over nine nesting seasons (2008–2016). We also included 4437 hatch‐year and after‐hatch‐year individuals to increase precision on parameter estimates because some ducklings were not recaptured until after fledging. Duckling mortality was greatest during the first week of life and approached zero by the second week after hatching. We observed a quadratic effect of hatch date on duckling mortality suggesting that successful females improve fitness by nesting early in the year, but not too early (15 days before mean hatch). Increased egg volume and duckling mass reduced the probability of ducking mortality. Our results suggest that risk of mortality for ducklings is influenced, at least in part, by maternal effects, which are manifested through energy allocation and behavior during the breeding season. As such, individual heterogeneity among breeding females affects duckling mortality risk and likely the recruitment process for wood ducks.

The devil is in the detail: Nonadditive and context-dependent plant population responses to increasing temperature and precipitation.

In climate change ecology, simplistic research approaches may yield unrealistically simplistic answers to often more complicated problems. In particular, the complexity of vegetation responses to global climate change begs a better understanding of the impacts of concomitant changes in several climatic drivers, how these impacts vary across different climatic contexts, and of the demographic processes underlying population changes. Using a replicated, factorial, whole-community transplant experiment, we investigated regional variation in demographic responses of plant populations to increased temperature and/or precipitation. Across four perennial forb species and 12 sites, we found strong responses to both temperature and precipitation change. Changes in population growth rates were mainly due to changes in survival and clonality. In three of the four study species, the combined increase in temperature and precipitation reflected nonadditive, antagonistic interactions of the single climatic changes for population growth rate and survival, while the interactions were additive and synergistic for clonality. This disparity affects the persistence of genotypes, but also suggests that the mechanisms behind the responses of the vital rates differ. In addition, survival effects varied systematically with climatic context, with wetter and warmer+wetter transplants showing less positive or more negative responses at warmer sites. The detailed demographic approach yields important mechanistic insights into how concomitant changes in temperature and precipitation affect plants, which makes our results generalizable beyond the four study species. Our comprehensive study design illustrates the power of replicated field experiments in disentangling the complex relationships and patterns that govern climate change impacts across real-world species and landscapes.