Demographic Consequences of Phenological Shifts in Response to Climate Change

When a phenological shift affects a demographic vital rate such as survival or reproduction, the altered vital rate may or may not have population-level consequences. We review the evidence that climate change affects populations by shifting species’ phenologies, emphasizing the importance of demographic life-history theory. We find many examples of phenological shifts having both positive and negative consequences for vital rates. Yet, few studies link phenological shifts to changes in vital rates known to drive population dynamics, especially in plants. When this link is made, results are largely consistent with life-history theory: Phenological shifts have population-level consequences when they affect survival in longer-lived organisms and reproduction in shorter-lived organisms. However, there are just as many cases in which demographic mechanisms buffer population growth from phenologically induced changes in vital rates. We provide recommendations for future research aiming to understand the complex relationships among climate, phenology, and demography, which will help to elucidate the extent to which phenological shifts actually alter population persistence. Expected final online publication date for the Annual Review of Ecology, Evolution, and Systematics, Volume 52 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Beyond demographic buffering: Context dependence in demographic strategies across animals

Temporal variation in vital rates (e.g., survival, reproduction) can decrease the long-term mean performance of a population. Species are therefore expected to evolve demographic strategies that counteract the negative effects of vital rate variation on the population growth rate. One key strategy, demographic buffering, is reflected in a low temporal variation in vital rates critical to population dynamics. However, comparative studies in plants have found little evidence for demographic buffering, and little is known about the prevalence of buffering in animal populations. Here, we used vital rate estimates from 31 natural populations of 29 animal species to assess the prevalence of demographic buffering. We modeled the degree of demographic buffering using a standard measure of correlation between the standard deviation of vital rates and the sensitivity of the population growth rate to changes in such vital rates across populations. We also accounted for the effects of life-history traits, i.e., age at first reproduction and spread of reproduction across the life cycle, on these correlation measures. We found no strong or consistent evidence of demographic buffering across the study populations. Instead, key vital rates could vary substantially depending on the specific environmental context populations experience. We suggest that it is time to look beyond concepts of demographic buffering when studying natural populations towards a stronger focus on the environmental context-dependence of vital-rate variation.

Regional features of birth rate and mortality in the Lower Volga region in the famine of 1932-1933s

The paper is exploring the problem of the vital rate data in the Lower Volga region during the famine of 1932-1933. Despite the ample quantity of papers presenting this problem the estimations and indicators differ even in the papers of the same authors and valuation methods are not always reliable. The birth rate of the Lower Volga region was 76223 while the mortality was 184570 during the 1933 famine peak by our estimate. However, there are no vital rate data on the Kalmykia in the central statistical administration archives and the registration of 15,2 thousand deaths were not ascertained identically. The real losses from the famine of 1932-1933 in the Lower Volga region (excluding Kalmykia) are estimated at 175 thousand maximum and birth rate losses are 147 thousand in 1932-1934. The mortality of the Lower Volga region had clear geographical distribution and location. The high mortality regions were allocated on the Volga Upland and abutting the Oka-Don plain eastern frontier and on the Medium Syrt frontier in Saratov Krai. The allocation of high mortality regions to the Volga River is interpreted as associating with regions containing major cities and towns with high mortality neighborhoods to the Volga. Stalingrad Krai is defined as a region with lower mortality and gradual slow in its increase with a low peak displaced to July 1933. In 1933 the Lower Volga mortality dynamics was from north to south epidemic; whereas in the south there was time to assume the measures as opposed to northern regions. Some Lower Volga regions in 1933 were characterized by a catastrophic low birth rate and high mortality and at the same time by high birth rate and low mortality and positive vital rate data. The distribution of high mortality regions was determined by the character of local authorities activities and local conditions including geographical description (orthometric height), that requires background study.

Human uniqueness? Life history diversity among small-scale societies and chimpanzees

Background Humans life histories have been described as “slow”, patterned by slow growth, delayed maturity, and long life span. While it is known that human life history diverged from that of a recent common chimpanzee-human ancestor some ~4–8 mya, it is unclear how selection pressures led to these distinct traits. To provide insight, we compare wild chimpanzees and human subsistence societies in order to identify the age-specific vital rates that best explain fitness variation, selection pressures and species divergence. Methods We employ Life Table Response Experiments to quantify vital rate contributions to population growth rate differences. Although widespread in ecology, these methods have not been applied to human populations or to inform differences between humans and chimpanzees. We also estimate correlations between vital rate elasticities and life history traits to investigate differences in selection pressures and test several predictions based on life history theory. Results Chimpanzees’ earlier maturity and higher adult mortality drive species differences in population growth, whereas infant mortality and fertility variation explain differences between human populations. Human fitness is decoupled from longevity by postreproductive survival, while chimpanzees forfeit higher potential lifetime fertility due to adult mortality attrition. Infant survival is often lower among humans, but lost fitness is recouped via short birth spacing and high peak fertility, thereby reducing selection on infant survival. Lastly, longevity and delayed maturity reduce selection on child survival, but among humans, recruitment selection is unexpectedly highest in longer-lived populations, which are also faster-growing due to high fertility. Conclusion Humans differ from chimpanzees more because of delayed maturity and lower adult mortality than from differences in juvenile mortality or fertility. In both species, high child mortality reflects bet-hedging costs of quality/quantity tradeoffs borne by offspring, with high and variable child mortality likely regulating human population growth over evolutionary history. Positive correlations between survival and fertility among human subsistence populations leads to selection pressures in human subsistence societies that differ from those in modern populations undergoing demographic transition.

The Importance of Environmental Variability and Transient Population Dynamics for a Northern Ungulate

The pathways through which environmental variability affects population dynamics remain poorly understood, limiting ecological inference and management actions. Here, we use matrix-based population models to examine the vital rate responses to environmental variability and individual traits, and subsequent transient dynamics of the population in response to the environment. Using Sitka black-tailed deer (Odocoileus hemionus sitkensis) in Southeast Alaska as a study system, we modeled effects of inter-annual process variance of covariates on female survival, pregnancy rate, and fetal rate, and summer and winter fawn survival. To examine the influence of environmental variance on population dynamics, we compared asymptotic and transient perturbation analysis (elasticity analysis, a life-table response experiment, and transience simulation). We found that summer fawn survival was primarily determined by black bear (Ursus americanus) predation and was positively influenced by mass at birth and female sex. Winter fawn survival was determined by malnutrition in deep-snow winters and was influenced by an interaction between date of birth and snow depth, with late-born fawns at greater risk in deep-snow winters. Adult female survival was the most influential vital rate based on classic elasticity analysis, however, elasticity analysis based on process variation indicated that winter and summer fawn survival were most variable and thus most influential to variability in population growth. Transient dynamics produced by non-stable stage distributions produced realized annual growth rates different from predicted asymptotic growth rates in all years, emphasizing the importance of winter perturbations to population dynamics of this species.

The demographic buffering strategy has a threshold of effectiveness to increases in environmental stochasticity

Animal populations have developed multiple strategies to deal with environmental change. Among them, the demographic buffering strategy consists on constraining the temporal variation of the vital rate(s) (e.g., survival, growth, reproduction) that most affect(s) the overall performance of the population. Given the increase in environmental stochasticity of the current global change scenario, identifying the thresholds beyond which populations are not able to remain viable -despite their potential buffering strategies- is of utmost importance.Tortoises are known to buffer the temporal variation in survival (i.e. this vital rate has the highest contribution to the population growth rate λ) at the expense of a high variability on reproductive rates (lowest contribution to λ). To identify the potential threshold in buffering ability, here we use field data collected across a decade on 15 locations of Testudo graeca along South-Eastern Spain. We analyse the effects of environmental variables (precipitation, temperature, and NDVI) on the probability of laying eggs and the number of eggs per clutch. Finally, we couple the demographic and environmental data to parametrise integral projection models (IPMs) to simulate the effects of different scenarios of drought recurrence on population growth rate.We find that droughts negatively affect the probability of laying eggs, but the overall effects on the population growth rates of T. graeca under the current drought frequencies (one per decade) are negligible. However, increasing the annual frequency of droughts decreases the buffering ability of T. graeca populations, with a threshold at three droughts per decade.Although some species may buffer current environmental regimes by carefully orchestrating how their vital rates vary through time, a demographic buffering strategy may alone not warrant population viability in extreme regimes. Our findings support the hypothesis that the buffering strategy indeed has a threshold of effectiveness. Our methodological approach also provides a useful pipeline for ecologists and managers to determine how effective the management of environmental drivers can be for demographically buffering populations, and which scenarios may not provide long-term species persistence.

Periodic catastrophes over human evolutionary history are necessary to explain the forager population paradox

The rapid growth of contemporary human foragers and steady decline of chimpanzees represent puzzling population paradoxes, as any species must exhibit near-stationary growth over much of their evolutionary history. We evaluate the conditions favoring zero population growth (ZPG) among 10 small-scale subsistence human populations and five wild chimpanzee groups according to four demographic scenarios: altered mean vital rates (i.e., fertility and mortality), vital rate stochasticity, vital rate covariance, and periodic catastrophes. Among most human populations, changing mean fertility or survivorship alone requires unprecedented alterations. Stochastic variance and covariance would similarly require major adjustment to achieve ZPG in most populations. Crashes could maintain ZPG in slow-growing populations but must be frequent and severe in fast-growing populations—more extreme than observed in the ethnographic record. A combination of vital rate alteration with catastrophes is the most realistic solution to the forager population paradox. ZPG in declining chimpanzees is more readily obtainable through reducing mortality and altering covariance. While some human populations may have hovered near ZPG under harsher conditions (e.g., violence or food shortage), modernHomo sapienswere equipped with the potential to rapidly colonize new habitats and likely experienced population fluctuations and local extinctions over evolutionary history.