American Naturalist最新文献

AbstractHow biodiversity and ecosystem functions change with succession has proven to be difficult to predict. Generally, it is thought that species accumulation over time should increase function, yet other successional trajectories can have alternative effects on diversity and function. We hypothesize that community diversity and function may respond in opposite ways to successional drivers such as nutrient availability, species interactions, or abiotic stress. The microbial communities within Sarracenia purpurea leaves perform degradation functions, providing essential nutrients to the plant, but we know little about how succession within the leaf influences bacterial diversity and degradation. We collected pitcher plant fluid from leaves aged 2-24 weeks to use in microcosm experiments. We used amplicon sequencing and a degradation assay to quantify diversity and ecosystem function. Because bacterivore activity increases with leaf age, we hypothesize that bacterial diversity will decrease over time, enhancing function if functionally important species are tolerant to predation. We thus added a common bacterivore to half of the replicated microcosms. We found that succession had opposite effects on diversity and function in pitcher plant bacteria but was unrelated to predator activity. As the leaves aged, bacterial degradation increased while diversity declined, with no significant effects from predator addition. This negative relationship between biodiversity and function likely results from functional traits associated with low nutrient availability or poor competitive ability. By broadening the landscape of successional scenarios and identifying their underlying mechanisms, we can advance our ability to predict diversity and functional dynamics in natural communities.

AbstractCoevolution requires reciprocal genotype-by-genotype (G × G) interactions for fitness, which occur when the fitness of a genotype in one species depends on the genotype it interacts with in another species and vice versa. However, in mutualisms, when G × G interactions are mutually beneficial, simple models predict that the resulting positive feedbacks will erode genetic variation. Here, we explore how genotype-by-environment (G × E) interactions, which occur when the fitnesses of different genotypes respond differently to different environments, and G × G × E interactions, which occur when the environment changes the outcome of G × G interactions, maintain variation in mutualisms. We build a spatial population genetic model in which the fitnesses of two partners depend on mutually beneficial G × G, G × E, and G × G × E interactions. Our analysis shows that variation will always be maintained via migration-selection balance with stronger G × E than G × G interactions. However, strong G × G interactions can erode variation by allowing genotypically matched partners to fix, and, more surprisingly, weak G × G interactions can erode variation by allowing genotypically mismatched partners to fix at high dispersal rates, leading to apparent maladaptation between partners. We parameterize our model using data from three published reciprocal transplant experiments, infer the relative strengths of G × E and G × G, and discuss the implications for the maintenance of genetic variation.

AbstractHumans provide massive inputs of food to wildlife, with profound ecological and evolutionary consequences. By potentially altering wildlife host immunity, density, and behavior, provisioning can influence transmission of wildlife pathogens and thus may impose strong selection pressure on pathogens. But surprisingly we lack theory on the eco-evolutionary consequences of provisioning for host-pathogen dynamics. Here we develop a mathematical model of the eco-evolutionary dynamics of a wildlife pathogen under provisioning, motivated by Mycoplasma gallisepticum, a bacterial pathogen that emerged, spread, and evolved higher virulence in provisioned house finches. We model how provisioning influences the evolution of pathogen virulence, defined here as the mortality increase associated with infection in identical background hosts. In our model, house finches recover from infection and acquire incomplete immunity; this incomplete immunity is stronger if their initial infection was with a more virulent pathogen strain (as previously found empirically). We find that even when provisioning improves individual host fitness (via survival, fecundity, or immune defenses), it should still select for higher pathogen virulence and thus may actually lead to declines in host populations. These negative effects arise because provisioning magnifies the impact of incomplete immunity, selecting for higher virulence and driving host populations down. Our results highlight that food provisioning can select for more virulent pathogens, with potentially far-reaching implications for conservation.

AbstractUnder radical environmental change, populations may need to adapt quickly to avoid substantial declines in abundance and threats to their persistence. The outcome of this race between evolution and demography depends on the genetic architecture of adaptation, which determines how fast evolution can proceed. In particular, adaptation may require coordinated evolution at multiple loci (e.g., cooperating ion transporters for ion uptake), with single-locus changes being deleterious. Such selection on coadapted genes leads to a fitness landscape with a valley, which can in turn favor the evolution of structural variants that link beneficial alleles at different loci. Here, we investigate how epistasis and recombination jointly affect population dynamics under such a fitness valley. We assume that adaptation occurs from standing genetic variation and model the eco-evolutionary dynamics deterministically. We show that recombination has strong impacts on population decline and recovery in this context. Higher recombination rates cause evolutionary trajectories to be pulled toward unfit states, leading to prolonged evolutionary plateaus, during which the population can decline precipitously. In highly detrimental cases where coadapted mutations are located on different chromosomes, chromosomal fusions that are preexisting at low frequency can lead to faster population recovery by allowing the genetic system to escape the attraction to unfit intermediate states. Our results provide insights into eco-evolutionary dynamics in systems where chromosome number varies drastically among sibling species, such as the copepod Eurytemora affinis species complex, and offer new perspectives on the impacts of genome architecture on population dynamics in stressful environments.

AbstractNavigation to and from a familiar site is a common animal behavior called homing. Many species use olfactory environmental landmarks or scents deposited in the environment by themselves or their conspecifics for homing. Birds regularly commute to and from their nest, and olfaction, an underappreciated sense in birds, may facilitate this behavior. Burrow-nesting seabirds, for example, rely on olfaction to locate their breeding colony and to identify their burrow, but the specific chemical information they use is unclear. We examined the chemical profiles of the colony landscape and its avian occupants at a breeding island of Leach's storm petrels (Hydrobates leucorhous) to determine whether place-specific chemicals, bird-produced chemicals, or both enable homeward navigation in this burrow-nesting species. We found that the colony contains spatial gradients of chemicals that may facilitate multiple stages of homing. We also show that burrows possess unique odors owing to chemicals deposited by their occupants. Moreover, the burrow shapes the odor of the birds such that individuals carry the scent of their nest and mated pairs possess similar chemical profiles. The bidirectional transfer of compounds between burrows and birds may enable burrow recognition in this species and potentially functions as a means of communication between conspecifics.

AbstractMilk is not cheap. The energetic cost for mammalian mothers to provide and sustain milk production makes it a finite resource. Offspring are therefore expected to wean before sexual maturity and reproductive activity, whether on their own or through termination by their mothers. Weaning delays should result in a reproductive trade-off for the mother: the possibility of begetting a fitter pup at the cost of a longer interbirth interval. Using 20 years of data, we show the occurrence of repeated suckling events between female Galápagos sea lions (GSLs; Zalophus wollebaeki) and their adult (≥5 years) biological offspring well beyond the average age of independence and when the offspring are themselves already reproductively active. This behavior, "supersuckling," suggests that GSL mother-offspring relationships are more complex and longer lasting than previously thought. To our knowledge, this is the first long-term documentation of known mother-offspring pairs repeatedly performing this behavior in any marine mammal species.

AbstractAccording to optimal foraging theory, mesopredators should forage in areas where their prey is abundant while avoiding high predation risk. Here, we investigate how environmental factors influence mesopredators' abilities to minimize spatiotemporal overlap with predators while increasing spatiotemporal overlap with prey. We paired 30 western diamond-backed rattlesnake (Crotalus atrox) 3D printed replicas with game cameras in West Texas for 2 years to quantify several spatiotemporal factors affecting prey availability and predation risk. Concurrently, 25 C. atrox were radiotracked at the same site to gather activity and microhabitat selection data regarding free-ranging individuals. Random forest algorithms were trained using data obtained from the game camera and applied to predict the probability of predation and the probability of prey encounter for each radiotracking event. Time of day, month, vegetation structure, and concealment percentage all had a significant association with the probability of predation and the probability of prey encounter. Our results suggest that rattlesnakes choose to be active when and where the probability of prey encounter was significantly higher than the probability of predator encounter, thus following optimal foraging theory. Our results demonstrate that mesopredators increase chances of prey capture while reducing predator detection in natural settings.

AbstractUnderstanding how temperature affects adaptation of cell size is challenging because cell size mediates numerous physiological and ecological trade-offs. While physiological mechanisms can lead to decreases in cell size with warming (the temperature-size rule [TSR]), it is unclear how ecological processes (competition, predation) combine to modify the TSR. Here, we evaluate how ecological interactions affect thermal adaptation of phytoplankton cell size. We perform an eco-evolutionary analysis of a nutrient-phytoplankton-zooplankton model. The model assumes that phytoplankton experience size-dependent constraints on resource allocation that cause small cells to sacrifice investment in growth machinery, thereby reducing maximum growth rate but increasing competitive ability. We find that trophic interactions strongly impact the evolutionarily stable cell size across temperatures. Without zooplankton, cell size declines monotonically with temperature, consistent with the TSR. With zooplankton, cell size varies unimodally with temperature, due to temperature-dependent shifts in the grazer's capacity to ease nutrient competition by controlling phytoplankton biomass. Size-selective grazing does not qualitatively alter this result but can facilitate coexistence of phytoplankton via a competition-predation resistance trade-off. Trophic interactions therefore can produce temperature-size responses in phytoplankton that differ qualitatively from the canonical TSR, and an understanding of how temperature affects cell size is incomplete without this ecological component.

AbstractEvaluating the evolutionary impacts of anthropogenic activity on populations is key to understanding species resiliency and to designing effective conservation strategies. Sequencing DNA from historical specimens provides the opportunity to establish a historical baseline and empirically assess changes in genetic diversity, changes in effective population size, and selection over time. Here, we sequenced historical and contemporary samples of the cardinalfish Taeniamia zosterophora collected in 1908 and in 2021-2022 across two sites with differing human impact in the Philippines. At both sites, genetic diversity increased over time, with contemporary samples having significantly higher Watterson's θ than historical samples. This diversity increase was primarily attributable to positive selection on low-frequency alleles such that they increased toward intermediate frequencies through time. For the putatively neutral fraction of the genome, in contrast, there was a slight but significant decline in Watterson's θ at both low and high human impact sites, suggesting that drift strengthened and effective population sizes declined through time. There was more evidence for selection and greater loss of neutral diversity at the site with higher human impact. Our results provide empirical evidence for the surprising preservation of genetic diversity through the action of natural selection in the face of anthropogenic impacts.

AbstractRensch's rule posits that sexual size dimorphism increases with overall body size among animal species with larger males and that it decreases with body size among species with larger females. The rule, originally based on patterns observed in a limited dataset rather than theory, has attracted much attention in the sexual selection literature. However, evidence for the rule has been equivocal. We test Rensch's rule with a recently published dataset on sexual size dimorphism in mammals using linear regressions with phylogenetic controls. We find that neither male-biased nor female-biased dimorphic species conform to Rensch's rule across mammals. When the analysis is restricted to within-family comparisons, as Rensch originally intended, the rule applies only to three of the 21 mammalian groups tested. We find very limited support for the "rule" in mammals and suggest that it is unlikely to be the general phenomenon that Rensch proposed.