Ecological Monographs最新文献

Size-structured differences in resource use stabilize species coexistence in animal communities, but what behavioral mechanisms underpin these niche differences? Behavior is constrained by morphological and physiological traits that scale allometrically with body size, yet the degree to which behaviors exhibit allometric scaling remains unclear; empirical datasets often encompass broad variation in environmental context and phylogenetic history, which complicates the detection and interpretation of scaling relationships between size and behavior. We studied the movement and foraging behaviors of three sympatric, congeneric spiral-horned antelope species (Tragelaphus spp.) that differ in body mass—bushbuck (26–40 kg), nyala (57–83 kg), and kudu (80–142 kg)—in an African savanna ecosystem where (i) food was patchily distributed due to ecosystem engineering by fungus-farming termites and (ii) predation risk was low due to the extirpation of several large carnivores. Because foraging behavior is directly linked to traits that scale allometrically with size (e.g., metabolic rate, locomotion), we hypothesized that habitat use and diet selection would likewise exhibit nonlinear scaling relationships. All three antelope species selected habitat near termitaria, which are hotspots of abundant, high-quality forage. Experimental removal of forage from termite mounds sharply reduced use of those mounds by bushbuck, confirming that habitat selection was resource driven. Strength of selection for termite mounds scaled negatively and nonlinearly with body mass, as did recursion (frequency with which individuals revisited locations), whereas home-range area and mean step length scaled positively and nonlinearly with body mass. All species disproportionately ate mound-associated plant taxa; nonetheless, forage selectivity and dietary composition, richness, and quality all differed among species, reflecting the partitioning of shared food resources. Dietary protein exhibited the theoretically predicted negative allometric relationship with body mass, whereas digestible-energy content scaled positively. Our results demonstrate cryptic size-based separation along spatial and dietary niche axes—despite superficial similarities among species—consistent with the idea that body-size differentiation is driven by selection for divergent resource-acquisition strategies, which in turn underpin coexistence. Foraging and space-use behaviors were nonlinearly related to body mass, supporting the hypothesis that behavior scales allometrically with size. However, explaining the variable functional forms of these relationships is a challenge for future research.

The usual theoretical condition for coexistence is that each species in a community can increase when it is rare (mutual invasibility). Traditional coexistence theory implicitly assumes that the invading species is common enough that we can ignore demographic stochasticity but rare enough that it does not compete with itself, even after it has reached a stationary spatial distribution. However, short-distance dispersal of discrete individuals leads to locally dense population clusters, and existing theory breaks down. We have an intuition that when we account for invader–invader competition, shorter-range dispersal should reduce the invader's ability to escape competition, but exactly how does this translate into lower population growth? And how will invader discreteness affect outcomes? We need a way of partitioning the contributions to coexistence, but current modern coexistence theory (MCT) does not apply under these conditions. Here we present a computationally based partitioning method to quantify the contributions to coexistence from different mechanisms, as in MCT. We also build up an intuition for how invader clumping and discreteness will affect these contributions by analyzing a case study, a lattice-based spatial lottery model. We first consider fluctuation-dependent coexistence, partitioning the contributions of variable environment, variable competition, demographic stochasticity, and their correlations and interactions. Our second example examines fluctuation-independent coexistence maintained by a fecundity–survival trade-off, and partitions the contributions to coexistence from interspecific differences in fecundity, in mortality, and in dispersal. We find that demographic stochasticity harms an invader, but only slightly. Localized invader dispersal, on the other hand, can have a strong effect. When invaders are more clumped, they compete with each other more intensely when rare, so they too become limited by environment-competition covariance. More invader clumping also means that variation in competition changes from helping the invader to harming it. More broadly, invader clumping is likely to weaken any coexistence mechanism that relies on the invader escaping competition from the resident, because invader clumping means that the resident is no longer the only source of competition.

The usual conception of character displacement is of resource competitors differentiating to specialize on different prey in order to reduce competition. However, traits that underlie many predator–prey interactions, such as chase-evade speeds, gape limitation, and toxin concentrations, do not permit such specialization, but instead result in unidirectional evolutionary arms races. Here, we develop and analyze an evolutionary model of predator–prey interactions to explore whether character displacement will still occur when such unidirectional traits define the species interactions, and if so, what environmental conditions foster or retard differentiation. Character displacement in predators and prey does occur, and this differentiation is driven by fitness component trade-offs. Instead of specialization or compartmentalization in which different sets of species have strong interactions, differentiation in this model causes a nested community structure in which species of predators and prey have the same rank interaction strengths with species at the other trophic level. Also, analyses of the model predict that character displacement is fostered in environments with higher productivity, weaker stressors, and lower structural complexity. Model comparisons suggest that character displacement should occur over a broader set of environmental conditions when traits permit prey specialization than when traits foster arms races. These results highlight how different types of phenotypic traits that underlie species interactions shape the species diversification and the structure of the resulting community.

Community interaction webs describe both direct and indirect interactions among species. Changes in direct interactions often become noticeable soon after a perturbation, but time lags in the responses of many species may delay the appearance of indirect effects and lead to temporal or spatial variation in interaction webs. Accurately identifying these shifts in the field requires time-specific, spatially differentiated interaction webs. We explore how variation in browsing affects interaction webs in a long-unburned chaparral shrubland near the central California coast. Most prior work in chaparral focused on rapid changes for <5 years after a wildfire that were assumed to determine community patterns until the next fire. Here, we report the results of the first 15 years of an ongoing experiment monitoring how interaction webs in long-unburned chaparral (at least 100 years postfire) respond to experimental variation in browsing by deer and rabbits on dominant shrubs (Arctostaphylos pumila, Ceanothus cuneatus var. rigidus, and Ericameria ericoides). We hypothesized that variation in browsing would directly affect foodplants, indirectly modify growth and survival of other shrubs, and impact habitat needed by herbaceous plants. We found a dynamic web of plant–herbivore and plant–plant interactions that responded rapidly to changes in deer browsing on Ceanothus followed by indirect interactions that continued developing over several years, affecting shrubs, open space, herbaceous plants, and small mammals. Experimental variation in the intensity of deer browsing led to temporal and spatial changes in interactions that produced three different community interaction webs. With deer, community webs were complex, having numerous direct and indirect interactions. Removing deer simplified the community web, changed outcomes of interactions, and reduced open space and herbaceous plant densities. Finally, changes in Ceanothus morphology without deer allowed woodrats to browse these shrubs, with negative impacts on Ceanothus growth and survival. General field observations also showed that all three alternative interaction webs occurred naturally at our fieldsite, varying across space and over time. Long-unburned chaparral communities browsed by deer maintain high biological diversity, but maintenance of this diversity involves many key direct and indirect biotic interactions.

During the past century, the fundamental niche, the complete set of environments that allow an individual, population, or species to persist, has shaped ecological thinking. It is a crucial concept connecting population dynamics, spatial ecology, and evolutionary theory, and a prerequisite for predictive ecological models at a time of rapid environmental change. Yet, its properties have eluded quantification, particularly for mobile, cognitively complex organisms. These difficulties are mainly a result of the separation between niche theory and field data, and the dichotomy between environmental and geographical spaces. Here, I combine recent mathematical and statistical results linking habitats to population growth, to achieve a quantitative and intuitive understanding of the fundamental niches of animals. I trace the development of niche ideas from the early steps of ecology to their use in modern statistical and conservation practice. I examine how animal mobility and behavior may blur the division between geographical and environmental space. I discuss how the central models of population and spatial ecology lead to a concise mathematical equation for the fundamental niche of animals and demonstrate how fitness parameters can be understood and directly estimated by fitting this model simultaneously to data on population growth and spatial distributions. I first illustrate these concepts theoretically for territorial species. I then fit the fundamental niche model to a data set of house sparrow colonies to quantify how a species of selective animals can increase their fitness in heterogeneous environments. This work confirms ideas that had been anticipated in the historical niche literature. Specifically, within traditionally defined environmental spaces, habitat heterogeneity and behavioral plasticity make the fundamental niche more complex and malleable than was historically envisaged. However, once examined in higher-dimensional environmental spaces, accounting for spatial heterogeneity, the niche is more predictable than recently suspected. This re-evaluation quantifies how organisms might buffer themselves from change by bending the boundaries of viable environmental space and offers a framework for designing optimal habitat interventions to protect biodiversity or obstruct invasive species. It therefore promotes the fundamental niche as a key concept for understanding animal responses to changing environments and a central tool for environmental management.

In tree canopies, incoming solar radiation interacts with leaves and branches to generate temperature differences within and among leaves, presenting thermal opportunities and risks for leaf-dwelling ectotherms. Although leaf biophysics and insect thermal ecology are well understood, few studies have examined them together in single systems. We examined temperature variability in aspen canopies, Populus tremuloides, and its consequences for a common herbivore, the leaf-mining caterpillar Phyllocnistis populiella. We shaded leaves in the field and measured effects on leaf temperature and larval growth and survival. We also estimated larval thermal performance curves for feeding and growth and measured upper lethal temperatures. Sunlit leaves directly facing the incoming rays reached the highest temperatures, typically 3–8°C above ambient air temperature. Irradiance-driven increases in temperature, however, were transient enough that they did not alter observed growth rates of leaf miners. Incubator and ramping experiments suggested that larval performance peaks between 25 and 32°C and declines to zero between 35 and 40°C, depending on the duration of temperature exposure. Upper lethal temperatures during 1-h heat shocks were 42–43°C. When larvae were active in early spring, temperatures generally were low enough to depress rates of feeding and growth below their maxima, and only rarely did estimated mine temperatures rise beyond optimal temperatures. Observed leaf or mine temperatures never approached larval upper lethal temperatures. At this site during our experiments, larvae thus appeared to have a significant thermal safety margin; the more pressing problem was inadequate heat. Detailed information on mine temperatures and larval performance curves, however, allowed us to leverage long-term data sets on air temperature to estimate potential future shifts in performance and longer-term risks to larvae from lethally high temperatures. This analysis suggests that, in the past 20 years, larval performance has often been limited by cold and that the risk of heat stress has been low. Future warming will raise mean rates of feeding and growth but also the risk of exposure to injuriously or lethally high temperatures.

Climate warming and altered disturbance regimes are changing forest composition and structure worldwide. Given that species often exhibit individualistic responses to change, making predictions about the cumulative effects of multiple stressors across environmental gradients is challenging, especially in diverse communities. For example, warming temperatures are predicted to drive species upslope, whereas fire exclusion promotes the expansion of species at lower elevations where fire was historically frequent. We resampled 148 vegetation plots to assess 46 years (1969–2015) of species and community-level response to warming and fire exclusion in a topographically complex landscape in the Klamath Mountains, California, USA, a diverse region that served as a climate refugia throughout the Holocene. We compared cover and assessed change in the elevational distributions of 12 conifer species at different life stages (i.e., seedlings, saplings, canopy). We observed consistent but non-significant shifts upward in elevation for eight species, and a significant shift upward for one species, all of which were far less than expectations based on recent warming. Six species declined in total cover and another five declined in at least one life stage, whereas the drought- and fire-intolerant Abies concolor increased by 30.7%. The largest declines were at lower elevations in drought-tolerant, early-seral species (Pinus lambertiana and Pinus ponderosa) and at higher elevations for the shade-tolerant Abies magnifica var. shastensis and the regionally rare Abies lasiocarpa. Regionally rare (Picea engelmannii) and endemic (Picea breweriana) species had reductions in early life stages, portending future declines. Multivariate analyses revealed a high degree of inertia with a minor, but significant, shift in composition and a slight decrease in species turnover along the elevation gradient driven by the expansion of A. concolor. Our results indicate that most species are declining, especially at lower and mid-elevations where fire exclusion has increased the cover of shade-tolerant species and reduced the recruitment for fire-adapted species. Collectively, declines in most species, insufficient upward movement to track warming, reductions in drought- and fire-tolerant early-seral species, and an increase in a single, shade-tolerant species will leave these communities maladapted to projected climate scenarios and questions the potential for future climate refugia in this region.

Programs that incentivize private landowners to create habitats that offset losses due to conversion and climate change are increasingly being used to bolster sensitive wildlife populations. In the Central Valley of California, shorebird habitat incentive programs pay landowners to create additional habitat during the non-breeding season by flooding their fields. However, it remains unclear how successful these programs have been in supporting baseline shorebird population needs or meeting established population goals, particularly in the face of changing environmental conditions. To address these questions, we used bioenergetics modeling to estimate shorebird food energy needs over four consecutive years that had the highest annual mean air temperatures ever recorded in California, and included years of extreme drought, as well as the second wettest winter on record. Our objectives were to (1) characterize annual variability in the timing and magnitude of shorebird food energy shortfalls, (2) estimate the contributions that incentive programs made to meeting these needs, and (3) develop recommendations for implementation of future habitat programs to advance shorebird conservation in the region. Overall, we found a high level of consistency in the timing and magnitude of habitat shortfalls, especially in fall, despite large differences in annual rainfall, a result that was unexpected, but that emphasizes how highly managed the hydrological system is in the Central Valley. We also found that the magnitude of both fall and spring energy shortfalls increased, relative to recent (2007–2014) estimates, perhaps due to aberrantly warm conditions. Incentive programs implemented to provide supplemental habitat were somewhat effective in reducing shortfalls for the assumed baseline population, but there were consistent unmet habitat needs when there were not enough shallow open water foraging areas available. Strategies to offset these remaining food energy deficits include scaling up habitat investments, adjusting the timing of habitat programs to better match the migration patterns of the birds, and adapting programs to new geographies. To the extent that there is variability in annual habitat need we recommend implementing a dynamic conservation approach. This involves scaling the amount of additional habitat created to match the shifting needs of the birds to maximize return on investment.

Ecological and evolutionary processes can occur at similar time scales and, hence, influence one another. There has been much progress in developing metrics that quantify contributions of ecological and evolutionary components to trait change over time. However, many empirical evolutionary ecology studies document trait differentiation among populations structured in space. In both time and space, the observed differentiation in trait values among populations and communities can be the result of interactions between nonevolutionary (phenotypic plasticity, changes in the relative abundance of species) and evolutionary (genetic differentiation among populations) processes. However, the tools developed so far to quantify ecological and evolutionary contributions to trait changes are implicitly addressing temporal dynamics because they require directionality of change from an ancestral to a derived state. Identifying directionality from one site to another in spatial studies of eco-evolutionary dynamics is not always possible and often not meaningful. We suggest three modifications to existing partitioning metrics so they allow quantifying ecological and evolutionary contributions to changes in population and community trait values across spatial locations in landscapes. Applying these spatially modified metrics to published empirical examples shows how these metrics can be used to generate new empirical insights and to facilitate future comparative analyses. The possibility of applying eco-evolutionary partitioning metrics to populations and communities in natural landscapes is critical as it will broaden our capacity to quantify eco-evolutionary interactions as they occur in nature.

Understanding long-term forest fire histories of boreal landscapes is instrumental for parameterizing climate–fire interactions and the role of humans affecting natural fire regimes. The eastern sections of the European boreal zone currently lack a network of annually resolved and centuries-long forest fire histories. To fill in this knowledge gap, we dendrochronologically reconstructed the 600-year fire history of a middle boreal pine-dominated landscape of the southern part of the Republic of Komi, Russia. We combined the reconstruction of fire cycle (FC) and fire occurrence with the data on the village establishment and climate proxies and discussed the relative contribution of climate versus human land use in shaping historic fire regimes. Over the 1340–1610 ce period, the territory had a FC of 66 years (with the 90% confidence envelope of 56.8 and 78.6 years). Fire activity increased during the 1620–1730 ce period, with the FC reaching 32 years (31.0–34.7 years). Between 1740–1950, the FC increased to 47 years (41.9–52.0). The most recent period, 1960–2010, marks FC's historic maximum, with the mean of 153 years (102.5–270.3). Establishment of the villages, often as small harbors on the Pechora River, was associated with a non-significant increase in fire occurrence in the sites nearest the villages (p = 0.07–0.20). We, however, observed a temporal association between village establishment and fire occurrence at the scale of the whole studied landscape. There was no positive association between the former and the FC. In fact, we documented a decline in the area burned, following the wave of village establishment during the second half of the 1600s and the first half of the 1700s. The lack of association between the dynamics of FC and the dates of village establishments, and the significant association between large fire years and the early and latewood pine chronologies, used as historic drought proxy, indirectly suggests that the climate was the primary control of the landscape-level FCs in the studied forests. Pine-dominated forests of the Komi Republic may hold a unique position as the ecosystem with the shortest history of human-related shifts in fire cycles across the European boreal region.