American Naturalist最新文献

AbstractVegetation-free space, or "halos," surrounding habitat patches are visually striking spatial phenomena observed in various ecosystems. These halos are linked to the landscape of fear hypothesis, where risk-averse herbivores concentrate grazing near safe shelters within their habitat. We develop theory demonstrating how habitat distribution shapes trophic interactions, leading to alternative stable states in spatial patterns. Using coral reefs as a model system, we investigate the relationship between halo patterns and predator populations. Specifically, we address the inconsistency between theoretical predictions and empirical observations, where halos are absent in some protected reefs and their sizes are uncorrelated with predator abundance. Our findings reveal that long-term coral distribution patterns influence trophic interactions, supporting the landscape of fear hypothesis. When coral patches are dispersed, herbivore shelter from predators is more evenly distributed across the seascape, facilitating overgrazing and halo oscillation. When coral patches are clustered, limited shelter stabilizes halos, but reduced herbivore limitation can also drive critical transitions to cycles with low vegetation that are difficult to reverse. Discordance between theory and observations may therefore arise from differences in underlying spatial shelter distribution, with broad implications for how landscapes of fear emerge from patchy ecosystems to signal resilience.

AbstractHabitat fragmentation poses a significant risk to population survival, causing both demographic stochasticity and genetic drift within local populations to increase, thereby increasing genetic load. Higher load causes population numbers to decline, which reduces the efficiency of selection and further increases load, resulting in a positive feedback that may drive entire populations to extinction. Here, we investigate this eco-evolutionary feedback in a metapopulation consisting of local demes connected via migration, with individuals subject to deleterious mutation at a large number of loci. We first analyze the determinants of load under soft selection, where population sizes are fixed, and then build on this to understand hard selection, where population sizes and load coevolve. We show that under soft selection, very little gene flow (less than one migrant per generation) is enough to prevent fixation of deleterious alleles. By contrast, much higher levels of migration are required to mitigate load and prevent extinction when selection is hard, with critical migration thresholds for metapopulation persistence increasing sharply as the genome-wide deleterious mutation rate becomes comparable to the baseline population growth rate. Moreover, critical migration thresholds are highest if deleterious mutations have intermediate selection coefficients but lower if alleles are predominantly recessive rather than additive (due to more efficient purging of recessive load within local populations). Our analysis is based on a combination of analytical approximations and simulations, allowing for a more comprehensive understanding of the factors influencing load and extinction in fragmented populations.

AbstractA pattern of allometry in which the degree of male-biased sexual size dimorphism (SSD) increases with species body size is known as "Rensch's rule." Over the past decades, a growing amount of Rensch's rule studies has advanced our understanding of SSD, our knowledge of its prevalence in nature, and our comprehension of the mechanisms underlying its evolution. However, Bernhard Rensch, when describing the pattern for the first time, considered the allometry of SSD only as a special case of a more general pattern in which dimorphism in any relative sexual difference increased with body size. In this perspective I revisit the history of Rensch's rule, starting with its popularization in recent decades, then diving into the original works by Rensch to rediscover his original observations, and finally discussing the implications of studying Rensch's pattern beyond its applications to SSD. The strong bias toward body size in the study of Rensch's rule has proven valuable regarding our understanding of the evolution of SSD. Using empirical examples, I propose, however, that expanding the study of the pattern to other traits might prove insightful for the general study of sexual dimorphism and phenotypic diversity.

AbstractAllee effects are common to diverse taxa, but their consequences for coexistence are rarely considered by ecologists. Recent research has suggested that Allee effects are incompatible with modern coexistence theory or that their impacts on coexistence are no different from other sources of positive density dependence that generate alternate stable states through priority effects. We use a graphical approach that builds on mathematically robust theory to develop simple conditions for coexistence and alternate stable states when an Allee effect is present. We show that weak Allee effects (those that do not depress population growth rates below zero in the absence of competition) can be integrated with modern coexistence theory but often produce outcomes distinct from other priority effects. This integration allows us to determine how Allee effects alter stabilizing and fitness differences. Importantly, we characterize a high-density criterion for a third alternate stable state that indicates species coexistence even when mutual invasibility is not met. Strong Allee effects (those that preclude invasibility even in the absence of competitors) permit coexistence only when the high-density criterion is satisfied. Our model offers an intuitive extension of modern coexistence theory that accounts for more than two alternative stable states and provides a guide for empirical research on how Allee effects structure ecological diversity.

AbstractUnderstanding whether and why microevolutionary patterns of trait covariation match macroevolutionary divergence is essential for linking evolution at different timescales. However, recent work has focused on developmental constraints for alignment between intraspecific variation and divergence, neglecting a potential role of natural selection on function to connect these scales. Here, we compare the support for the selection and constraint hypotheses to explain both phenotypic trait covariation and species divergence. To test these hypotheses, we collected data on hindlimb and jumping performance traits within and across species of two frog genera. We compared patterns of within-species phenotypic variation (the P matrix) with divergence and selective covariance matrices, from which we could extract the major axes of the realized adaptive landscape (AL), the directions in which adaptive peaks shifted the most over evolutionary time. We also tested whether the major axes of the AL were related to selection on jumping performance. We found high alignment between patterns of variation across scales. Most divergence occurred in allometric size, defined as the first eigenvector of the P matrix. However, jumping performance gradients were unaligned with the major axes of the AL and the P matrix. Across species, however, evolution of maximum acceleration showed a strong negative relationship with changes in allometric size. We infer that the jumping peak evolved under fluctuating selection, and species have tracked the peak along the direction of most within-species variation, allometric size. We conclude that long-term hindlimb divergence was constrained by developmental interactions among traits associated with growth and not net directional selection. Nonetheless, divergence on size indirectly influenced jumping evolution.

AbstractBet hedging is a ubiquitous strategy for risk reduction in environments that change unpredictably, where a lineage lowers its variance in fitness across environments at the expense of also lowering its arithmetic mean fitness. Classically, the benefit of bet hedging has been quantified using geometric mean fitness (GMF); bet hedging is expected to evolve if and only if it has a higher GMF than the wild type. We build on previous research on the effect of incorporating stochasticity in phenotypic distribution, environment, and reproduction to investigate the extent to which these sources of stochasticity impact the evolution of real-world bet-hedging traits. We demonstrate that modeling stochasticity can alter the sign of selection for bet hedging compared with deterministic predictions. Bet hedging can be deleterious at small population sizes and beneficial at larger population sizes. This phenomenon occurs across parameter space for conservative and diversified bet hedgers. We apply our model to published data to show that incorporating stochasticity is necessary to explain the evolution of real-world bet-hedging traits, including Papaver dubium variable germination phenology, Salmonella typhimurium antibiotic persistence, and seed banking in Clarkia xantiana. Our results suggest that GMF is not enough to predict when bet hedging is adaptive in a wide range of scenarios.

AbstractIt has been 40 years since T. P. Hughes put forward the idea that the size of modular corals is a better predictor of demographic fates than age. However, colonies of similar size may exhibit different shapes, and shape holds great ecological and evolutionary significance. This study used orthomosaics of coral reefs to track changes in 796 Pocillopora acuta colonies in Kenting National Park, Taiwan, over 2 years. We quantified relationships between coral demographic fates and three morphological traits: planar area (size), circularity (shape), and perimeter-to-area ratio, which integrates size and shape. Together, area and circularity consistently explained the most variation for all modular processes except shrinkage, which was explained best by area alone. Including circularity with area significantly improved the capacity to predict survival and fission, with large and circular colonies surviving better and with large and irregular colonies more prone to fission. Circularity also improved predictions of proportional area change, with smaller circular colonies experiencing higher rates of change. Fusion was unrelated to any single morphological trait, presumably because it relies on proximity in space. Perimeter-to-area ratio is the best single trait for survival prediction. Our results highlight that size and shape should both be considered for the demographic modeling of modular organisms.

AbstractThe timing of early life cycle events has cascading effects on phenology and fitness. These effects may be critical for climate resilience of plant populations, especially in Mediterranean environments, where delayed rainfall onset causes delayed germination. To examine impacts of germination timing on 10 species of the Streptanthus/Caulanthus clade, we induced germination across a range of dates in ambient seasonal conditions and recorded phenological and fitness traits. Later-germinating cohorts accelerated flowering, partially stabilizing flowering date, but the degree of this compensatory plasticity differed across species. Fitness declined with later germination; the magnitude of this decline depended on the balance between direct negative effects of later germination and compensatory positive effects of accelerated flowering. The resulting species' differences in fitness responses suggest differential vulnerability to climate change. Species from wetter, cooler, less variable habitats exhibited greater phenological plasticity, accelerating flowering more and declining less in seed set with later germination than desert species. However, other fitness responses to germination timing, such as first-year fitness, were evolutionarily labile across the clade and unrelated to climate. Although compensatory phenological plasticity may buffer the impacts of delayed germination, it cannot prevent long-term declines in population fitness as fall rains come later with climate change.

AbstractDehnel's phenomenon describes a seasonal and reversible winter decrease in body size, which is a trait that predicts total energy demand. However, the phenomenon remains less well studied than common energy-saving or energy-seeking strategies of mammals. Here, we explore the generality of Dehnel's phenomenon in Sorex shrews on three continents. First, we use new field sampling to document seasonal phenotypic change in masked shrews (Sorex cinereus) in North America at the lowest latitude yet investigated for this species (35.7°). This includes the first documentation of appendicular skeleton remodification in Sorex. Summer-to-winter decreases in S. cinereus body mass, braincase height, and femur length were 13%, 11.5%, and 8.7%, respectively, with subsequent increases of each in second-year individuals. Second, we compile a comprehensive dataset of studies relevant to Dehnel's phenomenon to test whether seasonal plasticity in Sorex globally is related to climate, demonstrating that body and braincase plasticity are functions of cold season temperatures. Meta-analytical models for both of these traits generalized by (a) applying at both inter- and intraspecific scales and (b) predicting the seasonal change newly observed for S. cinereus. Our results support body size plasticity as an environmentally responsive innovation in these very small homeothermic mammals.