American Naturalist最新文献

AbstractThe ability to secure food for offspring and withstand the cost of reproduction favors high-quality mothers that overproduce the larger sex, typically sons, only if they will receive adequate food, as this should enhance these sons' fitness returns. However, high-quality daughters ensure that grandoffspring receive quality parental care and may possess greater reproductive value than their brothers, favoring daughters also from high-quality mothers. Using a mixed cross-fostering approach, we investigated effects of early rearing conditions, covariance between breeders and their genetic parents in parental quality, and primary offspring sex ratios in Carolina wrens. In this socially and genetically monogamous bird, sons grew larger than daughters, paternal food provisioning impacted the condition and recruitment of sons but not daughters, and females overproduced sons when paired with males that provisioned at a high rate, reflecting females' anticipation of the quality of the rearing environment. Components of reproductive potential, including total fecundity, were inherited matrilineally, and all else being equal, females in better condition produced larger-than-average clutches biased toward daughters, who eventually produced larger-than-average clutches themselves. Sex ratios therefore varied with components of parental investment but in opposing directions for matrilineally inherited and environmental effects, suggesting that multiple countervailing selective forces shape sex ratio variation.

AbstractTheoretical studies from diverse areas of population biology have shown that demographic stochasticity can substantially impact evolutionary dynamics in finite populations, including scenarios where traits that are disfavored by natural selection can nevertheless increase in frequency through the course of evolution. Here, we analytically describe the eco-evolutionary dynamics of finite populations from demographic first principles. We investigate how noise-induced effects can alter the evolutionary fate of populations in which total population size may vary stochastically over time. Starting from a generic birth-death process, we derive a set of stochastic differential equations (SDEs) that describe the eco-evolutionary dynamics of a finite population of individuals bearing discrete traits. Our equations recover well-known descriptions of evolutionary dynamics, such as the replicator-mutator equation, the Price equation, and Fisher's fundamental theorem in the infinite population limit. For finite populations, our SDEs reveal how stochasticity can predictably bias evolutionary trajectories to favor certain traits, a phenomenon we call "noise-induced biasing." We show that noise-induced biasing acts through two distinct mechanisms, which we call the "direct" and "indirect" mechanisms. While the direct mechanism can be identified with classic bet-hedging theory, the indirect mechanism is a more subtle consequence of frequency- and density-dependent demographic stochasticity. Our equations reveal that noise-induced biasing may lead to evolution proceeding in a direction opposite to that predicted by natural selection in the infinite population limit. By extending and generalizing some standard equations of population genetics, we thus describe how demographic stochasticity appears alongside, and interacts with, the more well-understood forces of natural selection and neutral drift to determine the eco-evolutionary dynamics of finite populations of nonconstant size.

AbstractInducible defenses can affect the persistence, structure, and stability of consumer-resource systems. Theory shows that these effects depend on characteristics of the inducible defense, including timing, costs, efficacy, and sensitivity to consumer density. However, the expression and costs of inducible defenses often vary among life stages, which has not been captured in previous unstructured models. To explore how inducible defenses expressed in stage-structured populations affect consumer-resource dynamics, we developed a model based on the biology of plant-herbivore interactions, with the plant (resource) population structured into juvenile and mature stages. We then investigated the joint effects of inducible defenses and resource life history (i.e., patterns of fecundity, maturation, and mortality) by simulating dynamics for plant populations occurring along a fast-slow pace-of-life continuum. In general, high inducible defense costs, or a slow pace of life coupled with high herbivore growth rates, promoted persistent cycles. However, these cycles fundamentally differed, with either the plant or the herbivore population peaking first. Additionally, plant population pace of life influenced the relative effects of stage-specific induction strength on equilibrium densities and the extent to which inducible defenses enabled persistence. Our work illustrates how life history modifies the population-level effects of trait-mediated interactions, with implications for conservation and pest management.

AbstractIn many species, individuals are embedded in a network of kin with whom they interact. Interactions between kin can affect survival and fertility rates and thus the life history of individuals. These interactions indirectly affect both the network of kin and the dynamics of the population. In this way, a nonlinear feedback between the kin network and individual vital rates emerges. We describe a framework for integrating these kin interactions into a matrix model by linking the individual kin network to a matrix model. We demonstrate the use of this framework for African elephant populations under varying poaching pressure. For this example, we incorporate effects of the maternal presence and matriarchal age on juvenile survival and effects of the presence of a sister on young female fecundity. We find that the feedback resulting from the interactions between family members shifts and reduces the expected kin network. The reduction in family size and structure severely reduces the positive effects of family interactions, leading to an additional decrease in population growth rate on top of the direct decrease due to the additional mortality. Our analysis provides a framework that can be applied to a wide range of social species.

AbstractClimate change will affect both the mean and the variability in environmental conditions and may have major negative impacts on population densities in the future. For annual plants that already live in an extreme environment like the Sonoran Desert, keeping a fraction of their seeds dormant underground (for possibly years at a time) is critical to survive. Here, we consider how this form of bet hedging (i.e., delayed germination) for 10 Sonoran Desert annuals mediates responses to precipitation shifts. We use a demographic model parameterized with long-term field and precipitation data to explore how forecasted changes in precipitation impact annual plant species' population densities. We then examine how instantaneous evolution of optimal germination fractions in the shifted precipitation regimes bolsters population densities. Our results indicate that overall less rainfall and, to a lesser extent, increased variance in rainfall drive population levels down. Instantaneous evolution of optimal germination fractions in new regimes benefited species' populations only marginally, and only for small to moderate shifts in precipitation. Thus, even rapid evolution is unlikely to save populations experiencing larger shifts in precipitation. Finally, we predict that specialists that can capitalize on wet-year bonanzas or are water use efficient will be the most resilient to precipitation shifts as long as their seed survivorships are sufficiently high.

AbstractChanging climates are driving population declines in diverse animals worldwide. Winter conditions may play an important role in these declines but are often overlooked. Animals must not only survive winter but also preserve body condition, a key determinant of growing season success. We hypothesized that ectotherms overwintering in soil face a trade-off between risks of cold damage (including freezing) near the surface and elevated energy use at deeper depths. To test this hypothesis, we developed landscapes of mortality risk across depth for overwintering bumble bee queens. These critical pollinators are in decline in part because of climate change, but little is known about how climate affects overwintering mortality. We developed a mechanistic modeling approach combining measurements of freezing points and the temperature dependence of metabolic rates with soil temperatures from across the United States to estimate mortality risk across depth under historic conditions and under several climate change scenarios. Under current conditions, overwintering queens face a Goldilocks effect: temperatures can be too cold at shallow depths because of substantial freezing risk but too hot at deep depths where they risk prematurely exhausting lipid stores. Models suggest that increases in mean temperatures and in seasonal and daily temperature variation will increase risk of overwinter mortality. Better predictions of effects of changing climate on dormant ectotherms require more measurements of physiological responses to temperature during dormancy across diverse taxa.

AbstractLarge-scale temporal and spatial biodiversity patterns have traditionally been explained by multitudinous particular factors and a few theories. However, these theories lack sufficient generality and do not address fundamental interrelationships and coupled dynamics among resource availability, community abundance, and species richness. We propose the equilibrium theory of biodiversity dynamics (ETBD) to address these linkages. According to the theory, equilibrium levels of species richness and community abundance emerge at large spatial scales because of the population size dependence of speciation and/or extinction rates, modulated by resource availability and the species abundance distribution. In contrast to other theories, ETBD includes the effect of biodiversity on community abundance and thus addresses phenomena such as niche complementarity, facilitation, and ecosystem engineering. It reveals how alternative stable states in both diversity and community abundance emerge from these nonlinear biodiversity effects. The theory predicts how the strength of these effects alters scaling relationships among species richness, (meta)community abundance, and resource availability along different environmental gradients. Using data on global-scale variation in tree species richness, we show how the general framework is useful for clarifying the role of speciation, extinction, and resource availability in driving macroecological patterns in biodiversity and community abundance, such as the latitudinal diversity gradient.

AbstractSenescence is ubiquitous yet highly variable among species, populations, and individuals, for reasons that are poorly understood. It is not clear how environmental conditions affect senescence, especially in the wild. We explored the influence of environment on the degree of laying date age-specific variation and reproductive success senescence in wild blue tits. We disentangled the effects of age from those of previously encountered environmental conditions by introducing two complementary estimates of "relative environmental age." These estimates quantify the cumulative past environment experienced by an individual through two population-level metrics: average breeding failure and adult mortality. Results confirmed that laying date first advanced and annual reproductive success first increased with age up until about 3 years old, when these trends were reversed, consistent with a senescent decline. Both proxies for environmental conditions influenced laying date age-specific rates, such that females experiencing a more favorable environment had faster phenological decline. Conversely, environmental age did not affect reproductive success and its senescence. This study demonstrates that past environment can shape phenological age-specific change beyond the effects of chronological age and suggests that senescence will be best understood by investigating the deterioration of performances with accumulating exposure to detrimental conditions across a variety of traits.

AbstractExtreme cold events, which have become more frequent, can revert the direction of long-term responses to climate change. In 2021, record snowstorms swept the United States, causing wildlife die-offs that may have been associated with rapid natural selection. Our objective was to determine whether the snowstorms caused natural selection in Eastern Bluebirds (Sialia sialis). To test which mechanism most influenced their survival, we measured the morphology and coloration of fatalities and survivors at three sites. Survival was associated with a longer tarsus and with a wider, longer, and deeper beak, in support of the starvation and thermal endurance hypotheses. Additionally, bluebirds with more-ornamented plumage were less likely to have survived, perhaps because of an early energy investment in mate and site acquisition. As bluebirds encounter increasingly warm summer conditions, the longer extremities favored during the snowstorms may continue to be favored through their thermoregulatory benefits. However, the dull plumage coloration favored by natural selection during the snowstorms may be opposed by sexual selection benefits of more-ornamented plumage. Overall, responses to extreme events are difficult to predict from responses to long-term climate change, and responses to one event, such as the 2021 snowstorms, may not predict responses to a future extreme event.

AbstractPollen grains from different plants potentially compete for ovule access because flowers produce many more pollen grains than ovules. Pollen competition could occur on pollinators, where there is finite space for pollen placement. Here, we explore the explosive pollen deposition in Hypenia macrantha (Lamiaceae, a perennial flowering plant native to South America that is frequently visited by hummingbirds) and determine whether it can improve male performance by reducing pollen loads deposited by previously visited flowers. Through the simulation of floral visits utilizing a hummingbird skull, we showed that explosive pollen deposition by untriggered flowers dislodges almost twice as many pollen grains as already-triggered flowers. In addition, pollen removal increases with the amount of deposited pollen by the floral explosion, suggesting that the precision or the explosive force of pollen deposition plays a pivotal role in this pollen removal process. These results suggest that explosive pollen placement, a mechanism that has evolved in many unrelated angiosperm clades, may confer a prepollination male competition advantage to plants.