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1. Microbial symbionts buffer hosts from the demographic costs of environmental stochasticity

   https://doi.org/10.1111/ele.14438  

   Fowler, Joshua_C; Ziegler, Shaun; Whitney, Kenneth_D; Rudgers, Jennifer_A; Miller, Tom_E_X (May 2024, Ecology Letters)

   Abstract Species' persistence in increasingly variable climates will depend on resilience against the fitness costs of environmental stochasticity. Most organisms host microbiota that shield against stressors. Here, we test the hypothesis that, by limiting exposure to temporally variable stressors, microbial symbionts reduce hosts' demographic variance. We parameterized stochastic population models using data from a 14‐year symbiont‐removal experiment including seven grass species that hostEpichloëfungal endophytes. Results provide novel evidence that symbiotic benefits arise not only through improved mean fitness, but also through dampened inter‐annual variance. Hosts with “fast” life‐history traits benefited most from symbiont‐mediated demographic buffering. Under current climate conditions, contributions of demographic buffering were modest compared to benefits to mean fitness. However, simulations of increased stochasticity amplified benefits of demographic buffering and made it the more important pathway of host–symbiont mutualism. Microbial‐mediated variance buffering is likely an important, yet cryptic, mechanism of resilience in an increasingly variable world. 

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2. Fundamental constraints on vertebrate life history are shaped by aquatic–terrestrial transitions and reproductive mode

   https://doi.org/10.1038/s41559-025-02663-1  

   Brooks, George C; Uyeda, Josef C; Bone, Nicholas J; Conrad, Hailey M; Mull, Christopher G; Kindsvater, Holly K (May 2025, Nature Ecology & Evolution)

   Vertebrate life histories evolve in response to selection imposed by abiotic and biotic environmental conditions while being limited by genetic, developmental, physiological, demographic and phylogenetic processes that constrain adaptation. Despite the well-recognized shifts in selective pressures accompanying transitions among environments, the conditions driving innovation and the consequences for life-history evolution remain outstanding questions. Here we compare the traits of vertebrates that occupy aquatic or terrestrial environments as juveniles to infer shifts in evolutionary constraints that explain differences in their life-history traits and thus their fundamental demographic rates. Our results emphasize the reduced potential for life-history diversification on land, especially that of reproductive strategies, which limits the scope of viable life-history strategies. Moreover, our study reveals differences between the evolution of viviparity in aquatic and terrestrial realms. Transitions from egg laying to live birth represent a major shift across life-history space for aquatic organisms, whereas terrestrial egg-laying organisms evolve live birth without drastic changes in life-history strategy. Whilst trade-offs in the allocation of resources place fundamental constraints on the way life histories can vary, ecological setting influences the position of species within the viable phenotypic space available for adaptive evolution. 

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3. Individual life histories: neither slow nor fast, just diverse

   https://doi.org/10.1098/rspb.2023.0511  

   Van de Walle, Joanie; Fay, Rémi; Gaillard, Jean-Michel; Pelletier, Fanie; Hamel, Sandra; Gamelon, Marlène; Barbraud, Christophe; Blanchet, F. Guillaume; Blumstein, Daniel T.; Charmantier, Anne; et al (July 2023, Proceedings of the Royal Society B: Biological Sciences)

   The slow–fast continuum is a commonly used framework to describe variation in life-history strategies across species. Individual life histories have also been assumed to follow a similar pattern, especially in the pace-of-life syndrome literature. However, whether a slow–fast continuum commonly explains life-history variation among individuals within a population remains unclear. Here, we formally tested for the presence of a slow–fast continuum of life histories both within populations and across species using detailed long-term individual-based demographic data for 17 bird and mammal species with markedly different life histories. We estimated adult lifespan, age at first reproduction, annual breeding frequency, and annual fecundity, and identified the main axes of life-history variation using principal component analyses. Across species, we retrieved the slow–fast continuum as the main axis of life-history variation. However, within populations, the patterns of individual life-history variation did not align with a slow–fast continuum in any species. Thus, a continuum ranking individuals from slow to fast living is unlikely to shape individual differences in life histories within populations. Rather, individual life-history variation is likely idiosyncratic across species, potentially because of processes such as stochasticity, density dependence, and individual differences in resource acquisition that affect species differently and generate non-generalizable patterns across species. 

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4. Simulations of Life History Variation for Demographic Inference From Population Genomic Data

   https://doi.org/10.1111/mec.70155  

   Mascarenhas, Rilquer; Hickerson, Michael_J; Carnaval, Ana_Carolina (November 2025, Molecular Ecology)

   ABSTRACT Ecological differences among species, particularly dispersal capacity and life history strategies, influence population response to environmental changes. Genetic simulations now allow us to directly incorporate this variation into models of past demographic changes. However, the impact of life history strategies in demographic inference has been far less explored relative to that of dispersal capacity. Here, we utilise individual‐based simulations of a non‐Wright‐Fisher population to ask whether differences in life history traits (the average age of first reproduction of individuals, the average adult mortality and the average number of mates per reproductive season) lead to consistent and predictable differences in the summary statistics of genetic diversity commonly used for simulation‐based parameter estimation and demographic inference. Using a Random Forest model, we also estimate three population parameters (variance in reproductive success, generation time and effective population size) from genome‐wide SNP variation for two bird species known to have distinct life history strategies. The results demonstrate that life history variation leads to predictable differences in patterns of genetic diversity: higher values of life history traits, representing extreme polygamy, long adult longevity and later onset of reproduction, are associated with higher variance in reproductive success, longer generation times, smaller effective population sizes and overall lower genetic diversity. Parameter estimates from empirical datasets also agree with the general expectation that polygamic species with later onset of reproduction and long adult longevity exhibit higher variance in reproductive success, longer generation times and smaller effective population sizes. Since the signal of life history differences is observed in the genetic summary statistics, we argue that simulation‐ and model‐based multi‐species demographic inference will gain from the incorporation of life history information. 

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5. To Prosper, Live Long: Understanding the Sources of Reproductive Skew and Extreme Reproductive Success in Structured Populations

   https://doi.org/10.1086/730557  

   Snyder, Robin E; Ellner, Stephen P (August 2024, The American Naturalist)

   Kisdi, Éva; Akçay, Erol (Ed.)

   In many species, a few individuals produce most of the next generation. How much of this reproductive skew is driven by variation among individuals in fixed traits, how much by external factors, and how much by random chance? And what does it take to have truly exceptional lifetime reproductive output (LRO)? In the past, we and others have partitioned the variance of LRO as a proxy for reproductive skew. Here we explain how to partition LRO skewness itself into contributions from fixed trait variation, four forms of “demographic luck” (birth state, fecundity luck, survival trajectory luck, and growth trajectory luck), and two kinds of “environmental luck” (birth environment and environment trajectory). Each of these is further partitioned into contributions at different ages.We also determine what we can infer about individuals with exceptional LRO. We find that reproductive skew is largely driven by random variation in lifespan, and exceptional LRO generally results from exceptional lifespan. Other kinds of luck frequently bring skewness down rather than increasing it. In populations where fecundity varies greatly with environmental conditions, getting a good year at the right time can be an alternate route to exceptional LRO, so that LRO is less predictive of lifespan. 

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