Ecological Monographs最新文献

Because of the first observations in the 1900s of the oligotrophic and eutrophic states of lakes, researchers have been interested in the process that makes lakes become turbid because of high phytoplankton biomass. Definitions of eutrophication have multiplied and diversified since the mid-20th century, more than for any other ecological process. Reasons for the high number of definitions might be that the former ones did not sufficiently describe their causes and/or consequences. Global change is bringing eutrophication more into the spotlight than ever, highlighting the need to find consensus on a common definition, or at least to explain and clarify why there are different meanings of the term eutrophication. To find common patterns, we analyzed 138 definitions that were classified by a multiple correspondence factor analysis (MCA) into three groups. The first group contains the most generic scientific definitions but many of these limit the causes to increased nutrient availability. A single definition takes into account all causes but would require additional work to clarify the process itself. Nutrient pollution, which is by far the primary cause of eutrophication in the Anthropocene, has generated a second group of environmental definitions that often specify the primary producers involved. Those definitions often mention the iconic consequences of nutrient pollution, such as increased algal biomass, anoxia/hypoxia and reduced biodiversity. The third group contains operational definitions, focusing on the consequences of nutrient pollution, for ecosystem services and therefore associated with ecosystem management issues. This group contains definitions related to regulations, mainly US laws and European directives. These numerous definitions, directly derived from the problem of nutrient pollution, have enlarged the landscape of definitions, and reflect the need to warn, legislate and implement a solution to remedy it. Satisfying this demand should not be confused with scientific research on eutrophication and must be based on communicating knowledge to as many people as possible using the simplest possible vocabulary. We propose that operational definitions (groups 2 and 3) should name the process “nutrient pollution,” making it possible to refine (scientific) definitions of eutrophication and to expand on other challenges such as climate warming, overfishing, and other nonnutrient-related chemical pollutions.

Climate-driven alterations to disturbance regimes are increasingly disrupting patterns of recovery in many biomes. Here, we examine the impact of disturbance and subsequent level of recovery in live hard coral cover on the Great Barrier Reef (GBR) across the last three decades. We demonstrate that a preexisting pattern of infrequent disturbances of limited spatial extent has changed to larger and more frequent disturbances, dominated by marine heatwaves and severe tropical cyclones. We detected an increase in the impact (measured as coral loss) across 265 individual disturbance impacts on 131 reefs in a 36-year dataset (1985–2022). Additionally, the number of survey reefs impacted by disturbance has increased each decade from 6% in the 1980s to 44% in the 2010s, as has the frequency of mass coral bleaching across the GBR, which has increased between 19% and 28% per year, and cyclones (3%–5% per year), resulting in less time for recovery. Of the 265 disturbance impacts we recorded, complete recovery to the highest levels of coral cover recorded earlier in this study (the “historical benchmark”) occurred only 62 (23%) times. Of the 23% of disturbance impacts that resulted in complete recovery to historical benchmarks, 34/62 recovered to their benchmark in 2021 or 2022. Complete recovery was more likely when the historical benchmark was <25% live hard coral cover. The lack of recovery was attributed to recovery time windows becoming shorter due to increases in the frequency of cyclones and of thermal stress events that result in mass coral bleaching episodes. These results confirm that climate change is contributing to ecosystem-wide changes in the ability of coral reefs to recover.

The increasing density of woody plants threatens the integrity of grassy ecosystems. It remains unclear if such encroachment can be explained mostly by direct effects of resources on woody plant growth or by indirect effects of disturbances imposing tree recruitment limitation. Here, we investigate whether woody plant functional traits provide a mechanistic understanding of the complex relationships between these resource and disturbance effects. We first assess the role of rainfall, soil fertility, texture, and geomorphology to explain variation in woody plant encroachment (WPE) following livestock grazing and consequent fire suppression across the Serengeti ecosystem. Second, we explore trait-environment relationships and how these mediate vegetation response to fire suppression. We find that WPE is strongest in areas with high soil fertility, high rainfall, and intermediate catena positions. These conditions also promote woody plant communities characterized by small stature and seed sizes smaller relative to a comparative baseline within the Serengeti ecosystem, alongside high recruit densities (linked to a recruitment-stature trade-off). The positioning of species along this “recruitment-stature axis” predicted woody stem density increase in livestock sites. Structural equation modeling suggested a causal pathway where environmental factors shape the community trait composition, subsequently influencing woody recruit numbers. These numbers, in turn, predicted an area's vulnerability to WPE. Our study underscores the importance of trait-environment relationships in predicting the impact of human alterations on local vegetation change. Understanding how environmental factors directly (resources) and indirectly (legacy effects and plant traits) determine WPE supports the development of process-based ecosystem structure and function models.

Biomes are large-scale ecosystems occupying large spaces. The biome concept should theoretically facilitate scientific synthesis of global-scale studies of the past, present, and future biosphere. However, there is neither a consensus biome map nor universally accepted definition of terrestrial biomes, making joint interpretation and comparison of biome-related studies difficult. “Desert,” “rainforest,” “tundra,” “grassland,” or “savanna,” while widely used terms in common language, have multiple definitions and no universally accepted spatial distribution. Fit-for-purpose classification schemes are necessary, so multiple biome-mapping methods should for now co-exist. In this review, we compare biome-mapping methods, first conceptually, then quantitatively. To facilitate the description of the diversity of approaches, we group the extant diversity of past, present, and future global-scale biome-mapping methods into three main families that differ by the feature captured, the mapping technique, and the nature of observation used: (1) compilation biome maps from expert elicitation, (2) functional biome maps from vegetation physiognomy, and (3) simulated biome maps from vegetation modeling. We design a protocol to measure and quantify spatially the pairwise agreement between biome maps. We then illustrate the use of such a protocol with a real-world application by investigating the potential ecological drivers of disagreement between four broadly used, modern global biome maps. In this example, we quantify that the strongest disagreement among biome maps generally occurs in landscapes altered by human activities and moderately covered by vegetation. Such disagreements are sources of bias when combining several biome classifications. When aiming to produce realistic biome maps, biases could be minimized by promoting schemes using observations rather than predictions, while simultaneously considering the effect of humans and other ecosystem engineers in the definition. Throughout this review, we provide comparison and decision tools to navigate the diversity of approaches to encourage a more effective use of the biome concept.

Imaging spectroscopy is emerging as a leading remote sensing method for quantifying plant biodiversity. The spectral variation hypothesis predicts that variation in plant hyperspectral reflectance is related to variation in taxonomic and functional identity. While most studies report some correlation between spectral and field-based (i.e., taxonomic and functional) expressions of biodiversity, the observed strength of association is highly variable, and the utility in applying spectral community properties to examine environmental drivers of communities remains unknown. We linked hyperspectral data acquired by airborne imaging spectrometers with precisely geolocated field plots to examine the spectral variation hypothesis along a temperate-to-boreal forest gradient in southern Québec, Canada. First, we examine the degree of association between spectral and field-based dimensions of canopy tree composition and diversity. Second, we ask whether the relationships between field-based community properties and the environment are reproduced when using spectral community properties. We found support for the spectral variation hypothesis with the strength of association generally greater for the functional than taxonomic dimension, but the strength of relationships was highly variable and dependent on the choice of method or metric used to quantify spectral and field-based community properties. Using a multivariate approach (comparisons of separate ordinations), spectral composition was moderately well correlated with field-based composition; however, the degree of association increased when univariately relating the main axes of compositional variation. Spectral diversity was most tightly associated with functional diversity metrics that quantify functional richness and divergence. For predicting canopy tree composition and diversity using environmental variables, the same qualitative conclusions emerge when hyperspectral or field-based data are used. Spatial patterns of canopy tree community properties were strongly related to the turnover from temperate-to-boreal communities, with most variation explained by elevation. Spectral composition and diversity provide a straightforward way to quantify plant biodiversity across large spatial extents without the need for a priori field observations. While commonly framed as a potential tool for biodiversity monitoring, we show that spectral community properties can be applied more widely to assess the environmental drivers of biodiversity, thereby helping to advance our understanding of the drivers of biogeographical patterns of plant communities.

Despite the importance of population structures throughout ecology, relatively little theoretical attention has been paid to understanding the implications of social groups for population dynamics. The dynamics of socially structured populations differ substantially from those of unstructured or metapopulation-structured populations, because social groups themselves may split, fuse, and compete. These “between-group processes” remain understudied as drivers of the dynamics of socially structured populations. Here, we explore the role of various between-group processes in the dynamics of socially structured populations. To do so, we analyze a model that includes births, deaths, migration, fissions, fusions, and between-group competition and flexibly allows for density dependence in each process. Both logistic growth and an Allee effect are considered for within-group density dependence. We show that the effect of various between-group processes is mediated by their influence on the stable distribution of group sizes, with the ultimate impact on the population determined by the interaction between within-group density dependence and the process's effect on the group size distribution. Between-group interactions that change the number of groups can lead to both negative and positive density dependence at the global population level (even if birth and death rates depend only on group size and not population size). We conclude with a series of case studies that illustrates different ways that age, sex, and class structure impact the dynamics of social populations. These case studies demonstrate the importance of group-formation mechanisms, the cost of having excess males in a group, and the potential drawbacks of generating too many reproductive individuals. In sum, our results make clear the importance of within-group density dependence, between-group dynamics, and the interactions between them for the population dynamics of social species and provide a flexible framework for modeling social populations.

Global climate change is predicted to cause range shifts in the mosquito species that transmit pathogens to humans and wildlife. Recent modeling studies have sought to improve our understanding of the relationship between temperature and the transmission potential of mosquito-borne pathogens. However, the role of the vertebrate host population, including the importance of host behavioral defenses on mosquito feeding success, remains poorly understood despite ample empirical evidence of its significance to pathogen transmission. Here, we derived thermal performance curves for mosquito and parasite traits and integrated them into two models of vector–host contact to investigate how vertebrate host traits and behaviors affect two key thermal properties of mosquito-borne parasite transmission: the thermal optimum for transmission and the thermal niche of the parasite population. We parameterized these models for five mosquito-borne parasite transmission systems, leading to two main conclusions. First, vertebrate host availability may induce a shift in the thermal optimum of transmission. When the tolerance of the vertebrate host to biting from mosquitoes is limited, the thermal optimum of transmission may be altered by as much as 5°C, a magnitude of applied significance. Second, thresholds for sustained transmission depend nonlinearly on both vertebrate host availability and temperature. At any temperature, sustained transmission is impossible when vertebrate hosts are extremely abundant because the probability of encountering an infected individual is negligible. But when host biting tolerance is limited, sustained transmission will also not occur at low host population densities. Furthermore, our model indicates that biting tolerance should interact with vertebrate host population density to adjust the parasite population thermal niche. Together, these results suggest that vertebrate host traits and behaviors play essential roles in the thermal properties of mosquito-borne parasite transmission. Increasing our understanding of this relationship should lead us to improved predictions about shifting global patterns of mosquito-borne disease.

Ecological networks describe the interactions between different species, informing us how they rely on one another for food, pollination, and survival. If a species in an ecosystem is under threat of extinction, it can affect other species in the system and possibly result in their secondary extinction as well. Understanding how (primary) extinctions cause secondary extinctions on ecological networks has been considered previously using computational methods. However, these methods do not provide an explanation for the properties that make ecological networks robust, and they can be computationally expensive. We develop a new analytic model for predicting secondary extinctions that requires no stochastic simulation. Our model can predict secondary extinctions when primary extinctions occur at random or due to some targeting based on the number of links per species or risk of extinction, and can be applied to an ecological network of any number of layers. Using our model, we consider how false negatives and positives in network data affect predictions for network robustness. We have also extended the model to predict scenarios in which secondary extinctions occur once species lose a certain percentage of interaction strength, and to model the loss of interactions as opposed to just species extinction. From our model, it is possible to derive new analytic results such as how ecological networks are most robust when secondary species are of equal degree. Additionally, we show that both specialization and generalization in the distribution of interaction strength can be advantageous for network robustness, depending upon the extinction scenario being considered.

Animals that feed socially can sometimes better locate prey, often by transferring information about food that is patchy, dense, and temporally and spatially unpredictable. Information transfer is a potential benefit of living in breeding colonies where unsuccessful foragers can more readily locate successful ones and thereby improve feeding efficiency. Most studies on social foraging have been short term, and how long-term environmental change affects both foraging strategies and the associated benefits of coloniality is generally unknown. In the colonial Cliff Swallow (Petrochelidon pyrrhonota), we examined how social foraging, information transfer, and feeding ecology changed over a 40-year period in western Nebraska. Relative to the 1980s, Cliff Swallows in 2016–2022 were more likely to forage solitarily or in smaller groups, spent less time foraging, were more successful as solitaries, fed in more variable locations, and engaged less in information transfer at the colony site. The total mass of insects brought back to nestlings per parental visit declined over the study. The diversity of insect families captured increased over time, and some insect taxa dropped out of the diet, although the three most common insect families remained the same over the decades. Nestling Cliff Swallow body mass at 10 days of age and the number of nestlings surviving per nest declined more sharply with colony size in 2015–2022 than in 1984–1991 at sites where the confounding effects of ectoparasites were removed. Adult body mass during the provisioning of nestlings was lower in more recent years, but the change did not vary with colony size. The reason(s) for the reduction in social foraging and information transfer over time is unclear, but the consequence is that colonial nesting may no longer offer the same fitness advantages for Cliff Swallows as in the 1980s. The results illustrate the flexibility of foraging behavior and dynamic shifts in the potential selective pressures for group living.

The discipline of ecology seeks to understand how ecosystems, communities, and populations are regulated. A ubiquitous mechanism of population regulation of consumers is that capturing energy and nutrients in sufficient quantities for survival and reproduction becomes more difficult as population density increases. Extensive evidence has revealed that populations of large herbivores are often regulated by density dependence, defined as the reduction in the per-capita population growth rate that occurs as populations grow large. Diminished body mass of individuals has been repeatedly observed in high-density populations, implicating compromised nutrition as the primary cause of density dependence. However, there is no general explanation for why these nutritional deficiencies occur. Recent work demonstrated that reduced food intake rates resulting from the functional response of herbivores to depleted plant biomass does not provide a sensible explanation for density dependence because rates of food intake of herbivores are often insensitive to changes in plant biomass. A new model of feedbacks from plant biomass to herbivores shows how reduced nutrition of herbivores can result from increased dilution of nutrients in the plant tissue they consume as populations grow, even when their rate of consumption of plants remains constant. The model contains parameters that can be scaled to body mass, allowing unusually general predictions. The model shows that convex, concave, and linear relationships between the per-capita growth rate and population density can arise from the effects of depletion of plant biomass by herbivore foraging. The model is the first to explicitly include spatial variance in the nutritional quality of plants as a general driver of herbivore population dynamics. I show how regulation of herbivore abundance by plant nutrients can occur, even when a large fraction of the consumable plant biomass remains uneaten, providing a simple, mechanistic explanation for bottom-up control of population dynamics of primary consumers in a “green world.”