Raziel Davison - Academia.edu (original) (raw)

Papers by Raziel Davison

Proceedings of the National Academy of Sciences

In classical evolutionary models, the force of natural selection diminishes with age toward zero ... more In classical evolutionary models, the force of natural selection diminishes with age toward zero by last reproduction. However, intergenerational resource transfers and other late-life contributions in social species may select for postreproductive longevity. We present a formal framework for estimating indirect fitness contributions via production transfers in a skills-intensive foraging niche, reflecting kinship and cooperation among group members. Among contemporary human hunter-gatherers and horticulturalists, indirect fitness contributions from transfers exceed direct reproductive contributions from before menopause until ages when surpluses end, around the modal age of adult death (∼70 y). Under reasonable assumptions, these benefits are the equivalent to having up to several more offspring after age 50. Despite early independence, minimal production surplus, and a shorter lifespan, chimpanzees could theoretically make indirect contributions if they adopted reliable food-shari...

Journal of Ecology, 2010

1. Life table response experiment (LTRE) analyses have been widely used to examine the sources of... more 1. Life table response experiment (LTRE) analyses have been widely used to examine the sources of differences in the long-term deterministic growth rate (r = log k) of stage-structured populations that live in spatially distinct habitats or under distinct experimental conditions. However, existing methods for LTRE analysis ignore the fact that persistent temporal variation in matrix elements results in a long-term stochastic growth rate (a = log k s) that is different from the deterministic growth rate (r) and thus do not take into account environmental stochasticity. 2. Here, we develop a stochastic extension of LTRE methods that can be used to compare stochastic growth rates among populations that differ in the observed variability of their matrix elements over time. We illustrate our method with actual data and explore a range of questions that may be addressed with these new tools. Specifically, we investigate how variability in weather conditions affected the population dynamics of the short-lived perennial plant species Anthyllis vulneraria and examine how differences in stochastic growth rates (a) are determined by contributions of mean matrix elements and variability in matrix elements. 3. We find that, consistent with the life history of the species, differences in mean fertility and growth made the largest contribution to differences in a, whereas in terms of variability fertility made the largest contribution in most populations. However, we also find that in all populations, the magnitude of the total contribution of mean matrix elements outweighed that of variability. Finally, increasing soil depth significantly lowered contributions of variability in matrix elements, but it was not related to contributions of differences in mean matrix elements. 4. Synthesis. Stochastic life table response experiment analysis described here provides the first systematic way of incorporating observed differences in temporal variability into the comparison of natural populations. A key finding from this study is that populations occurring on relatively deeper soils were better buffered against climatic variation than populations occurring on shallow soils. We expect this new approach to analyse temporal variability to prove especially useful in the analysis of natural populations experiencing environmental change.

Extra Methods & Supplemental Figures

PLoS ONE, 2021

Background Humans life histories have been described as “slow”, patterned by slow growth, delayed... more 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. Re...

The evolutionary biologist W. D. Hamilton (Hamilton 1966 <i>J. Theor. Biol.</i> <b... more The evolutionary biologist W. D. Hamilton (Hamilton 1966 <i>J. Theor. Biol.</i> <b>12</b>, 12–45. (doi:10.1016/0022-5193(66)90184-6)) famously showed that the force of natural selection declines with age, and reaches zero by the age of reproductive cessation. However, in social species, the transfer of fitness-enhancing resources by postreproductive adults increases the value of survival to late ages. While most research has focused on intergenerational food transfers in social animals, here we consider the potential fitness benefits of information transfer, and investigate the ecological contexts where pedagogy is likely to occur. Although the evolution of teaching is an important topic in behavioural biology and in studies of human cultural evolution, few formal models of teaching exist. Here, we present a modelling framework for predicting the timing of both information transfer and learning across the life course, and find that under a broad range of conditions, optimal patterns of information transfer in a skills-intensive ecology often involve postreproductive aged teachers. We explore several implications among human subsistence populations, evaluating the cost of hunting pedagogy and the relationship between activity skill complexity and the timing of pedagogy. Long lifespan and extended juvenility that characterize the human life history likely evolved in the context of a skills-intensive ecological niche with multi-stage pedagogy and multigenerational cooperation.This article is part of the theme issue 'Life history and learning: how childhood, caring and old age shape cognition and culture in humans and other animals'.

BackgroundWe compare life histories and selection forces among chimpanzees and human subsistence ... more BackgroundWe compare life histories and selection forces among chimpanzees and human subsistence societies in order to identify the age-specific vital rates that best explain fitness variation, selection pressures and species divergence.MethodsWe employ Life Table Response Experiments that quantify vital rate contributions to population growth rate differences. Although widespread in ecology, these methods have not been applied to human populations or to look at species differences among humans and chimpanzees. We also estimate correlations between vital rate elasticities and life history traits to investigate differences in selection pressures and test predictions of life history theory.ResultsChimpanzees’ earlier maturity and higher adult mortality drive species differences, whereas infant mortality and fertility variation drive differences among humans. Human fitness is decoupled from longevity by postreproductive survival, while chimpanzees forfeit higher potential lifetime fert...

Philosophical Transactions of the Royal Society B: Biological Sciences

The evolutionary biologist W. D. Hamilton (Hamilton 1966 J. Theor. Biol. 12 , 12–45. ( doi:10.101... more The evolutionary biologist W. D. Hamilton (Hamilton 1966 J. Theor. Biol. 12 , 12–45. ( doi:10.1016/0022-5193(66)90184-6 )) famously showed that the force of natural selection declines with age, and reaches zero by the age of reproductive cessation. However, in social species, the transfer of fitness-enhancing resources by postreproductive adults increases the value of survival to late ages. While most research has focused on intergenerational food transfers in social animals, here we consider the potential fitness benefits of information transfer, and investigate the ecological contexts where pedagogy is likely to occur. Although the evolution of teaching is an important topic in behavioural biology and in studies of human cultural evolution, few formal models of teaching exist. Here, we present a modelling framework for predicting the timing of both information transfer and learning across the life course, and find that under a broad range of conditions, optimal patterns of informa...

Ecological Modelling

Abstract Demographic rates differ between populations and also fluctuate over time, sometimes dri... more Abstract Demographic rates differ between populations and also fluctuate over time, sometimes driving large fitness differences, but the strength of stochastic effects remain heretofore unresolved. We demonstrate the importance of stochastic processes by comparing the drivers of long-term population growth. We quantify stochastic contributions to differences in population growth rates among 218 plant and two animal populations representing 62 species (all records from the COMPADRE and COMADRE matrix databases suitable for our analyses) using the Small Noise Approximation Life Table Response Experiment (SNA-LTRE), a recently developed matrix model tool for decomposing the stochastic contributions of elasticities, variability and correlations. Stochastic influences comprise over a quarter of all contributions to population growth variation among populations (mean ± SD = 28 ± 14%). The relative importance of stochastic effects decreases with generation time and lifespan, confirming predictions that longevity buffers populations against the negative effects of variability. Stochastic effects are larger when populations differ widely in growth rates, suggesting that stochasticity is likely to be important where ecological conditions vary greatly, and are larger among herbaceous perennials than among woody plants, ferns and succulents, possibly reflecting phenotypic plasticity in response to fluctuating environments. Overall, we show that stochastic effects are often strong enough to warrant the additional effort required to characterize their contributions to population growth.

Proceedings of the National Academy of Sciences

The rapid growth of contemporary human foragers and steady decline of chimpanzees represent puzzl... more 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 m...

Canadian Journal of Fisheries and Aquatic Sciences

Hatcheries can support salmon fisheries but also impact natural populations. We model the proport... more Hatcheries can support salmon fisheries but also impact natural populations. We model the proportional hatchery contributions to ocean catch, natural-area spawning, and egg production based on hatchery production, maturation, fecundity, and straying. We develop indices of hatchery-origin catch per stray spawner measuring the trade-off between supplementing harvest and limiting natural-area impacts; higher values indicate success in increasing hatchery ocean harvest contributions relative to strays spawning in natural areas. Hatchery fish maturing early lowers catch per stray (and proportion hatchery-origin catch) by shifting the age distributions of both catch and spawners toward younger ages. Age-dependent fecundity may complicate predicted effects of changing maturation schedules. Increased straying does not affect catch but increases hatchery-origin spawning and decreases catch per stray. Differences in hatchery production affect hatchery contributions to both catch and spawning,...

Natural Resource Modeling

Many studies of semelparous salmon populations use Leslie matrices that classify individuals on t... more Many studies of semelparous salmon populations use Leslie matrices that classify individuals on the basis of age alone and do not explicitly impose death upon reproduction. Although these models may suffice for studying long-term population dynamics (like asymptotic growth rate), they do not accurately represent the diversity of individual life history outcomes in semelparous populations. Cohorts breeding at different ages have different life history traits (e.g., age at first reproduction and remaining life expectancy) that are obscured in Leslie models and this distorts our understanding of life history diversity and its importance for semelparous population dynamics. We present a simple transformation that uses age-specific breeding probabilities to reconfigure Leslie matrices as explicitly semelparous models. Explicitly semelparous models conserve asymptotic measures like population growth rate, vital rate elasticities, life expectancy at birth, and generation time but also better predict life history schedules and reproductive values. Strictly age-classified Leslie models underestimate ages at first reproduction and mean ages at death for older breeders but overestimate mean ages at death for early breeders. Leslie models also slightly overestimate variance in lifetime reproductive success, and underestimate entropy exhibited by life history outcomes.

Background/Question/Methods Natural populations employ different strategies for persisting in dis... more Background/Question/Methods Natural populations employ different strategies for persisting in disturbed environments. Given prevalent disturbance in both natural and managed settings, it is important to understand life history adaptations that mediate population persistence and species coexistence in variable environments. How do we distinguish strategies for persisting under frequent fire disturbance? What can we learn about disturbance-adaptation from the responses of populations to frequent burning? We examine demographic data on five sympatric woody plant species growing in the Brazilian cerradoand analyze demographic responses to fire frequency as indicators of life-history adaptations to frequent fire disturbance. We apply Markov chain theory to investigate how fire frequency affects life expectancy and age schedules of development, and we use these responses to as demographic diagnostics to characterize species as either ‘demographically resistant’ or ‘demographically resilie...

ABSTRACT Background/Question/Methods When we observe population dynamics from a life history pers... more ABSTRACT Background/Question/Methods When we observe population dynamics from a life history perspective we see that average rates of survival and fertility change with age. Actuarial senescence, or the decline in survival probabilities with age, is observed in many species and its underlying causes are the subject of great interest. We investigate the role of optimal resource allocation and reproductive tradeoffs in driving senescence of a model organism with a life history similar to the butterfly Speyeria mormonia. We examine the effects of larval provisioning, adult resource availability and reproductive energetic requirements on predictions of optimal foraging and reproductive schedules across the adult life cycle. Given strong limitations on lifespan and lifetime fecundity, existing theory predicts that reproductive output should increase toward the end of reproductive lifespans. We investigate how different behavioral tradeoffs and resource constraints affect the optimal timing of terminal reproductive investment. We then look at how optimal foraging and reproduction schedules drive mortality through nutritional consequences of resource acquisition and investment. We obtain exact results numerically by calculating the fitness of every possible permutation of a binomial decision tree, in which every day represents a choice to either forage or reproduce. We then compare predictions under different tradeoffs and constraints. Results/Conclusions We found that, based on the tradeoffs associated with foraging and reproduction, the life cycle was divided into as many as four phases of optimal behavior and mortality: I. Concerted foraging and resource accumulation leading to ontogenescence, a pre-reproductive decrease in mortality due to developing robustness. II. Income breeding, with alternating foraging and reproduction leading to stable mortality; III. Terminal reproduction, in which foraging ceases and the costs of repeated reproduction produce a signal of senescence. IV. Post-reproductive lifespans are an incidental outcome as some individuals lay all their eggs without starving to death. An interesting and possibly counter-intuitive result is that individuals with higher resource provisions do not necessarily have higher life expectancies; they have no incentive to defer reproduction so terminal investment begins earlier, along with its associated survival costs. Finally, without any imposed age-dependence of the rates in our model, we found a signal of actuarial senescence near the end of reproductive lifespans as individuals bear the survival costs of terminal investment. We conclude that physiological, behavioral and environmental tradeoffs can have strong effects on optimal allocation schedules, and that these in turn can produce qualitatively different shapes of mortality.

We investigate the effects of optimal time and resource allocation on age patterns of fertility a... more We investigate the effects of optimal time and resource allocation on age patterns of fertility and mortality for a model organism with (1) fixed maximum lifespan, (2) distinct juvenile and adult diets, and (3) reliance on nonrenewable resources for reproduction. We ask when it is optimal to tolerate starvation vs. conserve resources and then examine the effects of these decisions on adult mortality rates. We find that (1) age-related changes in tradeoffs partition the life cycle into as many as four discrete phases with different optimal behavior and mortality patterns, and (2) given a cost of reproduction, terminal investment can produce a signal of actuarial senescence. Also, given limitations imposed by non-replenishable resources, individuals beginning adult life with more replenishable resources do not necessarily live longer, since they can engage in capital breeding and need not defer reproduction to forage; low reproductive overheads and low costs of starvation also encourage capital breeding and may lead to earlier terminal investment and earlier senescence. We conclude that, even for species with qualitatively similar life histories, differences in physiological, behavioral and environmental tradeoffs or constraints may strongly influence optimal allocation schedules and produce variation in mortality patterns and life expectancy.

Journal of Ecology, 2014

1. Schedules of survival, growth and reproduction are key life-history traits. Data on how these ... more 1. Schedules of survival, growth and reproduction are key life-history traits. Data on how these traits vary among species and populations are fundamental to our understanding of the ecological conditions that have shaped plant evolution. Because these demographic schedules determine population

The American Naturalist, 2013

Ecology Letters, 2013

How populations respond to climate change depends on the interplay between life history, resource... more How populations respond to climate change depends on the interplay between life history, resource availability, and the intensity of the change. Roe deer are income breeders, with high levels of allocation to reproduction, and are hence strongly constrained by the availability of high quality resources during spring. We investigated how recent climate change has influenced demographic processes in two populations of this widespread species. Spring began increasingly earlier over the study, allowing us to identify 2 periods with contrasting onset of spring. Both populations grew more slowly when spring was early. As expected for a long-lived and iteroparous species, adult survival had the greatest potential impact on population growth. Using perturbation analyses, we measured the relative contribution of the demographic parameters to observed variation in population growth, both within and between periods and populations. Within periods, the identity of the critical parameter depended on the variance in growth rate, but variation in recruitment was the main driver of observed demographic change between periods of contrasting spring earliness. Our results indicate that roe deer in forest habitats cannot currently cope with increasingly early springs. We hypothesise that they should shift their distribution to richer, more heterogeneous landscapes to offset energetic requirements during the critical rearing stage.

The American Naturalist, 2013

Proceedings of the National Academy of Sciences

In classical evolutionary models, the force of natural selection diminishes with age toward zero ... more In classical evolutionary models, the force of natural selection diminishes with age toward zero by last reproduction. However, intergenerational resource transfers and other late-life contributions in social species may select for postreproductive longevity. We present a formal framework for estimating indirect fitness contributions via production transfers in a skills-intensive foraging niche, reflecting kinship and cooperation among group members. Among contemporary human hunter-gatherers and horticulturalists, indirect fitness contributions from transfers exceed direct reproductive contributions from before menopause until ages when surpluses end, around the modal age of adult death (∼70 y). Under reasonable assumptions, these benefits are the equivalent to having up to several more offspring after age 50. Despite early independence, minimal production surplus, and a shorter lifespan, chimpanzees could theoretically make indirect contributions if they adopted reliable food-shari...

Journal of Ecology, 2010

1. Life table response experiment (LTRE) analyses have been widely used to examine the sources of... more 1. Life table response experiment (LTRE) analyses have been widely used to examine the sources of differences in the long-term deterministic growth rate (r = log k) of stage-structured populations that live in spatially distinct habitats or under distinct experimental conditions. However, existing methods for LTRE analysis ignore the fact that persistent temporal variation in matrix elements results in a long-term stochastic growth rate (a = log k s) that is different from the deterministic growth rate (r) and thus do not take into account environmental stochasticity. 2. Here, we develop a stochastic extension of LTRE methods that can be used to compare stochastic growth rates among populations that differ in the observed variability of their matrix elements over time. We illustrate our method with actual data and explore a range of questions that may be addressed with these new tools. Specifically, we investigate how variability in weather conditions affected the population dynamics of the short-lived perennial plant species Anthyllis vulneraria and examine how differences in stochastic growth rates (a) are determined by contributions of mean matrix elements and variability in matrix elements. 3. We find that, consistent with the life history of the species, differences in mean fertility and growth made the largest contribution to differences in a, whereas in terms of variability fertility made the largest contribution in most populations. However, we also find that in all populations, the magnitude of the total contribution of mean matrix elements outweighed that of variability. Finally, increasing soil depth significantly lowered contributions of variability in matrix elements, but it was not related to contributions of differences in mean matrix elements. 4. Synthesis. Stochastic life table response experiment analysis described here provides the first systematic way of incorporating observed differences in temporal variability into the comparison of natural populations. A key finding from this study is that populations occurring on relatively deeper soils were better buffered against climatic variation than populations occurring on shallow soils. We expect this new approach to analyse temporal variability to prove especially useful in the analysis of natural populations experiencing environmental change.

Extra Methods & Supplemental Figures

PLoS ONE, 2021

Background Humans life histories have been described as “slow”, patterned by slow growth, delayed... more 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. Re...

The evolutionary biologist W. D. Hamilton (Hamilton 1966 <i>J. Theor. Biol.</i> <b... more The evolutionary biologist W. D. Hamilton (Hamilton 1966 <i>J. Theor. Biol.</i> <b>12</b>, 12–45. (doi:10.1016/0022-5193(66)90184-6)) famously showed that the force of natural selection declines with age, and reaches zero by the age of reproductive cessation. However, in social species, the transfer of fitness-enhancing resources by postreproductive adults increases the value of survival to late ages. While most research has focused on intergenerational food transfers in social animals, here we consider the potential fitness benefits of information transfer, and investigate the ecological contexts where pedagogy is likely to occur. Although the evolution of teaching is an important topic in behavioural biology and in studies of human cultural evolution, few formal models of teaching exist. Here, we present a modelling framework for predicting the timing of both information transfer and learning across the life course, and find that under a broad range of conditions, optimal patterns of information transfer in a skills-intensive ecology often involve postreproductive aged teachers. We explore several implications among human subsistence populations, evaluating the cost of hunting pedagogy and the relationship between activity skill complexity and the timing of pedagogy. Long lifespan and extended juvenility that characterize the human life history likely evolved in the context of a skills-intensive ecological niche with multi-stage pedagogy and multigenerational cooperation.This article is part of the theme issue 'Life history and learning: how childhood, caring and old age shape cognition and culture in humans and other animals'.

BackgroundWe compare life histories and selection forces among chimpanzees and human subsistence ... more BackgroundWe compare life histories and selection forces among chimpanzees and human subsistence societies in order to identify the age-specific vital rates that best explain fitness variation, selection pressures and species divergence.MethodsWe employ Life Table Response Experiments that quantify vital rate contributions to population growth rate differences. Although widespread in ecology, these methods have not been applied to human populations or to look at species differences among humans and chimpanzees. We also estimate correlations between vital rate elasticities and life history traits to investigate differences in selection pressures and test predictions of life history theory.ResultsChimpanzees’ earlier maturity and higher adult mortality drive species differences, whereas infant mortality and fertility variation drive differences among humans. Human fitness is decoupled from longevity by postreproductive survival, while chimpanzees forfeit higher potential lifetime fert...

Philosophical Transactions of the Royal Society B: Biological Sciences

The evolutionary biologist W. D. Hamilton (Hamilton 1966 J. Theor. Biol. 12 , 12–45. ( doi:10.101... more The evolutionary biologist W. D. Hamilton (Hamilton 1966 J. Theor. Biol. 12 , 12–45. ( doi:10.1016/0022-5193(66)90184-6 )) famously showed that the force of natural selection declines with age, and reaches zero by the age of reproductive cessation. However, in social species, the transfer of fitness-enhancing resources by postreproductive adults increases the value of survival to late ages. While most research has focused on intergenerational food transfers in social animals, here we consider the potential fitness benefits of information transfer, and investigate the ecological contexts where pedagogy is likely to occur. Although the evolution of teaching is an important topic in behavioural biology and in studies of human cultural evolution, few formal models of teaching exist. Here, we present a modelling framework for predicting the timing of both information transfer and learning across the life course, and find that under a broad range of conditions, optimal patterns of informa...

Ecological Modelling

Abstract Demographic rates differ between populations and also fluctuate over time, sometimes dri... more Abstract Demographic rates differ between populations and also fluctuate over time, sometimes driving large fitness differences, but the strength of stochastic effects remain heretofore unresolved. We demonstrate the importance of stochastic processes by comparing the drivers of long-term population growth. We quantify stochastic contributions to differences in population growth rates among 218 plant and two animal populations representing 62 species (all records from the COMPADRE and COMADRE matrix databases suitable for our analyses) using the Small Noise Approximation Life Table Response Experiment (SNA-LTRE), a recently developed matrix model tool for decomposing the stochastic contributions of elasticities, variability and correlations. Stochastic influences comprise over a quarter of all contributions to population growth variation among populations (mean ± SD = 28 ± 14%). The relative importance of stochastic effects decreases with generation time and lifespan, confirming predictions that longevity buffers populations against the negative effects of variability. Stochastic effects are larger when populations differ widely in growth rates, suggesting that stochasticity is likely to be important where ecological conditions vary greatly, and are larger among herbaceous perennials than among woody plants, ferns and succulents, possibly reflecting phenotypic plasticity in response to fluctuating environments. Overall, we show that stochastic effects are often strong enough to warrant the additional effort required to characterize their contributions to population growth.

Proceedings of the National Academy of Sciences

The rapid growth of contemporary human foragers and steady decline of chimpanzees represent puzzl... more 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 m...

Canadian Journal of Fisheries and Aquatic Sciences

Hatcheries can support salmon fisheries but also impact natural populations. We model the proport... more Hatcheries can support salmon fisheries but also impact natural populations. We model the proportional hatchery contributions to ocean catch, natural-area spawning, and egg production based on hatchery production, maturation, fecundity, and straying. We develop indices of hatchery-origin catch per stray spawner measuring the trade-off between supplementing harvest and limiting natural-area impacts; higher values indicate success in increasing hatchery ocean harvest contributions relative to strays spawning in natural areas. Hatchery fish maturing early lowers catch per stray (and proportion hatchery-origin catch) by shifting the age distributions of both catch and spawners toward younger ages. Age-dependent fecundity may complicate predicted effects of changing maturation schedules. Increased straying does not affect catch but increases hatchery-origin spawning and decreases catch per stray. Differences in hatchery production affect hatchery contributions to both catch and spawning,...

Natural Resource Modeling

Many studies of semelparous salmon populations use Leslie matrices that classify individuals on t... more Many studies of semelparous salmon populations use Leslie matrices that classify individuals on the basis of age alone and do not explicitly impose death upon reproduction. Although these models may suffice for studying long-term population dynamics (like asymptotic growth rate), they do not accurately represent the diversity of individual life history outcomes in semelparous populations. Cohorts breeding at different ages have different life history traits (e.g., age at first reproduction and remaining life expectancy) that are obscured in Leslie models and this distorts our understanding of life history diversity and its importance for semelparous population dynamics. We present a simple transformation that uses age-specific breeding probabilities to reconfigure Leslie matrices as explicitly semelparous models. Explicitly semelparous models conserve asymptotic measures like population growth rate, vital rate elasticities, life expectancy at birth, and generation time but also better predict life history schedules and reproductive values. Strictly age-classified Leslie models underestimate ages at first reproduction and mean ages at death for older breeders but overestimate mean ages at death for early breeders. Leslie models also slightly overestimate variance in lifetime reproductive success, and underestimate entropy exhibited by life history outcomes.

Background/Question/Methods Natural populations employ different strategies for persisting in dis... more Background/Question/Methods Natural populations employ different strategies for persisting in disturbed environments. Given prevalent disturbance in both natural and managed settings, it is important to understand life history adaptations that mediate population persistence and species coexistence in variable environments. How do we distinguish strategies for persisting under frequent fire disturbance? What can we learn about disturbance-adaptation from the responses of populations to frequent burning? We examine demographic data on five sympatric woody plant species growing in the Brazilian cerradoand analyze demographic responses to fire frequency as indicators of life-history adaptations to frequent fire disturbance. We apply Markov chain theory to investigate how fire frequency affects life expectancy and age schedules of development, and we use these responses to as demographic diagnostics to characterize species as either ‘demographically resistant’ or ‘demographically resilie...

ABSTRACT Background/Question/Methods When we observe population dynamics from a life history pers... more ABSTRACT Background/Question/Methods When we observe population dynamics from a life history perspective we see that average rates of survival and fertility change with age. Actuarial senescence, or the decline in survival probabilities with age, is observed in many species and its underlying causes are the subject of great interest. We investigate the role of optimal resource allocation and reproductive tradeoffs in driving senescence of a model organism with a life history similar to the butterfly Speyeria mormonia. We examine the effects of larval provisioning, adult resource availability and reproductive energetic requirements on predictions of optimal foraging and reproductive schedules across the adult life cycle. Given strong limitations on lifespan and lifetime fecundity, existing theory predicts that reproductive output should increase toward the end of reproductive lifespans. We investigate how different behavioral tradeoffs and resource constraints affect the optimal timing of terminal reproductive investment. We then look at how optimal foraging and reproduction schedules drive mortality through nutritional consequences of resource acquisition and investment. We obtain exact results numerically by calculating the fitness of every possible permutation of a binomial decision tree, in which every day represents a choice to either forage or reproduce. We then compare predictions under different tradeoffs and constraints. Results/Conclusions We found that, based on the tradeoffs associated with foraging and reproduction, the life cycle was divided into as many as four phases of optimal behavior and mortality: I. Concerted foraging and resource accumulation leading to ontogenescence, a pre-reproductive decrease in mortality due to developing robustness. II. Income breeding, with alternating foraging and reproduction leading to stable mortality; III. Terminal reproduction, in which foraging ceases and the costs of repeated reproduction produce a signal of senescence. IV. Post-reproductive lifespans are an incidental outcome as some individuals lay all their eggs without starving to death. An interesting and possibly counter-intuitive result is that individuals with higher resource provisions do not necessarily have higher life expectancies; they have no incentive to defer reproduction so terminal investment begins earlier, along with its associated survival costs. Finally, without any imposed age-dependence of the rates in our model, we found a signal of actuarial senescence near the end of reproductive lifespans as individuals bear the survival costs of terminal investment. We conclude that physiological, behavioral and environmental tradeoffs can have strong effects on optimal allocation schedules, and that these in turn can produce qualitatively different shapes of mortality.

We investigate the effects of optimal time and resource allocation on age patterns of fertility a... more We investigate the effects of optimal time and resource allocation on age patterns of fertility and mortality for a model organism with (1) fixed maximum lifespan, (2) distinct juvenile and adult diets, and (3) reliance on nonrenewable resources for reproduction. We ask when it is optimal to tolerate starvation vs. conserve resources and then examine the effects of these decisions on adult mortality rates. We find that (1) age-related changes in tradeoffs partition the life cycle into as many as four discrete phases with different optimal behavior and mortality patterns, and (2) given a cost of reproduction, terminal investment can produce a signal of actuarial senescence. Also, given limitations imposed by non-replenishable resources, individuals beginning adult life with more replenishable resources do not necessarily live longer, since they can engage in capital breeding and need not defer reproduction to forage; low reproductive overheads and low costs of starvation also encourage capital breeding and may lead to earlier terminal investment and earlier senescence. We conclude that, even for species with qualitatively similar life histories, differences in physiological, behavioral and environmental tradeoffs or constraints may strongly influence optimal allocation schedules and produce variation in mortality patterns and life expectancy.

Journal of Ecology, 2014

1. Schedules of survival, growth and reproduction are key life-history traits. Data on how these ... more 1. Schedules of survival, growth and reproduction are key life-history traits. Data on how these traits vary among species and populations are fundamental to our understanding of the ecological conditions that have shaped plant evolution. Because these demographic schedules determine population

The American Naturalist, 2013

Ecology Letters, 2013

How populations respond to climate change depends on the interplay between life history, resource... more How populations respond to climate change depends on the interplay between life history, resource availability, and the intensity of the change. Roe deer are income breeders, with high levels of allocation to reproduction, and are hence strongly constrained by the availability of high quality resources during spring. We investigated how recent climate change has influenced demographic processes in two populations of this widespread species. Spring began increasingly earlier over the study, allowing us to identify 2 periods with contrasting onset of spring. Both populations grew more slowly when spring was early. As expected for a long-lived and iteroparous species, adult survival had the greatest potential impact on population growth. Using perturbation analyses, we measured the relative contribution of the demographic parameters to observed variation in population growth, both within and between periods and populations. Within periods, the identity of the critical parameter depended on the variance in growth rate, but variation in recruitment was the main driver of observed demographic change between periods of contrasting spring earliness. Our results indicate that roe deer in forest habitats cannot currently cope with increasingly early springs. We hypothesise that they should shift their distribution to richer, more heterogeneous landscapes to offset energetic requirements during the critical rearing stage.

The American Naturalist, 2013