American Naturalist最新文献

AbstractDemographic stochasticity and Allee effects are two common mechanisms that increase extinction risk in small populations. High demographic stochasticity produces population fluctuations that cause extinction in small populations. Meanwhile, strong Allee effects create low-density thresholds, where growth rates are negative below the threshold and positive above. We hypothesized that stochastic fluctuations may drive populations over these thresholds, increasing the probability that a population establishes in a habitat. To test this hypothesis, we utilized properties of discrete-time Markov processes and a Ricker model with an Allee effect to quantify colonization and extinction rates. We show that demographic stochasticity can increase colonization rates over a range of carrying capacities in populations with strong Allee effects. In contrast, while higher demographic stochasticity always increases extinction rates of established populations, waiting times to extinction due to demographic stochasticity often exceed thousands of generations, even at relatively small carrying capacities ($K=50$). Given the frequency of catastrophic disturbances such as fires, extinction rates from demographic stochasticity are near negligible even in small populations with strong Allee effects. Thus, the net effect of demographic stochasticity is often positive. Overall, our study provides novel insights into a mechanism through which demographic stochasticity promotes species persistence.

AbstractGenomic conflicts arise when different genes in a genome are selected for opposite phenotypic effects. One well-known conflict occurs in plants, between mitochondrial genes causing cytoplasmic male sterility (CMS) and their nuclear suppressors, called restorers of male fertility. The evolution of CMS-restorer polymorphisms has been modeled many times, but empirical validations remain indirect. Here we use a new biological model, a freshwater snail, to directly observe evolutionary trajectories. In this species, CMS-associated mitogenomes coexist with male-fertile ones in populations. Models predict such a coexistence when nuclear restorers make CMS mitogenomes less fit than male-fertile ones, thus preventing the fixation of CMS. During 11 generations of experimental evolution, we observed rapid decreases in the frequency of CMS mitogenomes in a restorer-rich nuclear background, with an estimated ∼20% fitness disadvantage, consistent with theoretical conditions for the maintenance of cytonuclear polymorphism. In parallel, in an ancillary experiment, eggs laid by isolated snails carrying CMS showed a reduced hatching rate. Although significant, this reduction did not reach 20%, suggesting that fitness differentials in populations are enhanced by competition or rely on unmeasured traits. Our study illustrates the speed at which evolution can proceed in the context of cytonuclear conflicts over sex allocation.

AbstractFemale ornamentation is common in birds but usually resembles that of males. In contrast to this general pattern, here we show that the red bill of wild adult common waxbills (Estrilda astrild) often becomes mottled with black when females breed. This color change is not explained by reallocation of red carotenoid pigments away from the bill but requires deposition of melanin pigments. The change is very noticeable and makes female bills resemble the black bill of nestlings and fledglings. Perhaps this color change exploits useful innate responses of males toward nestlings, such as ceasing mating-related behavior and initiating parental care. Unlike the vast majority of female signals and ornaments, black-mottled bills are not derived from a male trait, but they are derived from a nestling trait, in accordance with the idea that color signals often evolve using preexisting developmental paths.

AbstractBamboo is a unique, dynamic, and diverse group of plants shown to harbor communities of obligate specialists. Such phenomena, however, have primarily been investigated in the Neotropics. Additionally, mechanisms underlying specialist bamboo communities are generally poorly understood. By studying bird and arthropod communities across two seasons in bamboo and adjacent rainforest in the Eastern Himalaya, we provide some of the first systematic evidence of bamboo-specialist communities. We show that arthropod communities differ significantly between habitats and across seasons and that bamboo-specialist birds likely feed on distinct arthropods within bamboo sheaths using specialized bills and unique foraging behaviors in this part of the world. We hypothesize that this bird-bamboo association could be driven by a dietary specialization to the unique arthropods in bamboo. These results contribute to our understanding of how species can specialize on temporally dynamic resources and highlight the need for more research on such lesser-known habitats.

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.