Ecological Monographs最新文献

Relationships between biodiversity and ecosystem functioning (BEF) are typically investigated separately in different ecosystem types, often neglecting connections across ecosystem boundaries. Here, we examined the cross-boundary relationships between terrestrial and aquatic biodiversity and terrestrial and aquatic ecosystem function (here productivity in terms of biomass). We collected a dataset from 100 Finnish boreal lakes for phytoplankton and zooplankton, and for trees and understory plants in the surrounding forest ecosystems. We explored the connections among climatic, catchment, and local environmental factors, and terrestrial and aquatic biodiversity and productivity using structural equation modeling (SEM). The results indicated cross-boundary connections between the two realms. Terrestrial biodiversity was associated with terrestrial productivity and connected to lake water chemistry directly and indirectly through terrestrial productivity. Water chemistry in turn was linked to aquatic biodiversity and productivity. Within both realms, biodiversity was positively associated with ecosystem productivity. The effects of biodiversity per se were weaker in the aquatic realm, in which nutrient availability was the strongest determinant of productivity. Our findings underscore the importance of exploring cross-ecosystem coupling, as the impacts of several global change drivers, such as climate and land-use change or eutrophication, extend beyond individual realms to transcend ecosystem boundaries. In particular, the combined effects of warming, eutrophication, and increasing terrestrial productivity are likely to increase the import of allochthonous nutrients to boreal lake ecosystems, resulting in enhanced primary productivity therein. As freshwater ecosystems integrate the effects of direct and indirect changes in their catchments, they serve as ideal settings for investigating cross-ecosystem coupling and act as valuable sentinels of climate and other global changes.

Ecological communities frequently exhibit remarkable taxonomic and trait diversity, and this diversity is consistently shown to regulate ecosystem function and resilience. However, ecologists lack a synthetic theory for how this diversity is maintained when species compete for limited resources, hampering our ability to project the future of biodiversity under climate change. Water-limited plant communities are an ideal system in which to study these questions given (1) the diversity of hydraulic traits they exhibit, (2) the importance of this diversity for ecosystem productivity and drought resilience, and (3) forecast changes to precipitation and evapotranspiration under climate change. We developed an analytically tractable model of water and light competition in age-structured perennial plant communities and demonstrated that high diversity is maintained through phenological division of the time between storms. We modeled a system where water arrives in the form of intermittent storms, between which plants consume the limited pool of soil water until it becomes dry enough that they must physiologically shut down to avoid embolism. Competition occurs because individuals, by consuming the shared water pool, cause their competitors to shut down earlier, harming their long-term growth and reproduction. When total precipitation is low, plants in the model compete only for water. However, increases in precipitation can cause the canopy to close and individuals to begin competing for light. Variation among species in the minimum soil water content at which they can sustain growth without embolizing leads to emergent phenological variation, as species will shut down at varying points between storm events. When this variation is paired with a trade-off such that species that shut down early are compensated by faster biomass accumulation, higher fecundity, or lower mortality, there is no limit to the number that can coexist. These results are robust to variation in both total precipitation and the time between storms. The model therefore offers a plausible explanation for how hydraulic trait diversity is maintained in a wide array of natural systems. More broadly, this work illustrates how the phenological division of an apparently singular resource can emerge because of common trade-offs and ultimately foster high taxonomic and trait diversity.

Whether, and which, individuals migrate or not is rapidly changing in many populations. Exactly how and why environmental change alters migration propensity is not well understood. We constructed density-dependent structured population models to explore conditions for the coexistence of migrants and residents. Our theoretical models were motivated by empirical data identified via a systematic literature review. We find that the equilibrium density in the season with the strongest density dependence of a strategy predicts whether the strategy will become dominant within the population. This equilibrium density represents strategy fitness in a seasonal environment and can be used to examine selection on migratory behavior. Whether partial migration can be maintained within a population depends on where in the annual cycle density dependence operates. Diversified bet-hedging, where parents produce a mix of migrants and residents, also maintains partial migration. Our study disentangles density-dependent and density-independent rates in a population with seasonal structure, potentially providing routes to explain the rapid change in migration strategies observed in many populations.

Biodiversity underpins the functional integrity of ecosystems. At present, our understanding of the relationship between biodiversity and ecosystem functioning (BEF) is essentially based on manipulative experiments. Compelling data at large spatial scales are scarce, especially for river networks. BEF patterns across landscapes are complex because they unfold in the context of environmental gradients and compositional turnover of natural communities. Leaf litter decomposition, a pivotal ecosystem process in streams, is no exception to this context dependency. The dendritic structure of river networks plus the unidirectional water flow shape both environmental conditions and the distribution of leaf resources and consumers. However, it is difficult to predict how spatial gradients of resource and consumer composition can overlap across a river network, and thus govern spatial patterns of decomposition. Here, we investigated the capacity of macroinvertebrate biodiversity to control decomposition rates of heterogeneous leaf resources at the river-network scale. We deployed five litterbags containing either one of four single leaf species or a mixture of all species at 51 sites across the Thur River network (Switzerland). We measured litter decomposition rates, variation of decomposition among leaf resources, and the effect of leaf litter diversity on decomposition. We found that decomposition rates decreased from headwaters to downstream reaches mainly due to the parallel decrease in the abundance of key shredder taxa (namely, Amphinemura, Nemoura, Leuctra, Habroleptoides, and Stenophylacini). Macroinvertebrate diversity had a minor, negative effect on decomposition rates. However, high functional macroinvertebrate diversity at the reach scale reduced the variation of decomposition among leaf resources, thus alleviating nutritional constraints exerted by nutrient-poor leaf resources. Furthermore, litter mixtures were preferably decomposed by communities with low evenness and dominated by a few taxa. These findings point to a critical role of macroinvertebrates in controlling litter decomposition at the network scale beyond environmental effects. While shredder abundance and community composition are key to determining decomposition rates across the river network, functional diversity is important in decreasing the variation of decomposition rates among leaf resources. Our results stress the importance of biodiversity controlling ecosystem functioning not only at the local but also at the river network scale.

Beta diversity—the variation among community compositions in a region—is a fundamental measure of biodiversity. Most classic measures have posited that beta diversity is maximized when each community has a distinct, nonoverlapping set of species. However, this assumption overlooks the ecological significance of species interactions and non-additivity in ecological systems, where the function and behavior of species depend on other species in a community. Here, we introduce a geometric approach to measure beta diversity as the hypervolume of the geometric embedding of a metacommunity. Besides considering compositional distinctiveness as in classic metrics, this geometric measure explicitly incorporates species associations and captures the idea that adding a unique, species-rich community to a metacommunity increases beta diversity. We show that our geometric measure is closely linked to and naturally extends previous information- and variation-based measures. Additionally, we provide a unifying geometric framework for widely adopted extensions of beta diversity. Applying our geometric measures to empirical data, we address two long-standing questions in beta diversity research—the latitudinal pattern of beta diversity and the effect of sampling effort—and present novel ecological insights that were previously obscured by the limitations of classic approaches. In sum, our geometric approach offers a new and complementary perspective on beta diversity, is immediately applicable to existing data, and holds promise for advancing our understanding of the complex relationships between species composition, ecosystem functioning, and stability.

Flower exposure to high temperature reduces the production, viability, and performance of pollen, ovules, and seeds, which in turn impairs individual fecundity and risks the survival of populations. Autonomous floral cooling could alleviate the effects of flower exposure to harmful temperatures, yet investigations on thermal ecology of flowers in hot environments are needed to evaluate the reality, magnitude, and ecological significance of thermoregulatory cooling. This paper reports a study on the thermal ecology of the flower heads (=capitula) of 15 species of summer-blooming Asteraceae, tribe Cardueae, from hot-dry habitats in the southern Iberian Peninsula. Temperature inside (T_in) and outside (T_out) capitula were assessed under natural field conditions using two complementary sampling and measurement procedures, which provided information on the relationships between the two temperatures at the levels of individual capitula (“continuous recording”) and local plant populations (“instantaneous measurements”). Baselines for the T_in–T_out relationship in the absence of physiological activity were obtained by exposing dehydrated capitula to variable ambient temperatures in the field. To assess whether the co-flowering capitula of summer-blooming Asteraceae defined collectively a distinct thermal layer, the vertical distribution of capitula relative to the ground was quantified. Bees visiting capitula were watched and temperature of the air beside the visited capitulum was measured. Results were remarkably similar for all plant species. The capitula experienced high ambient temperatures during long periods, yet their interior was cooler than the air most of the time, with temperature differentials (ΔT = T_in − T_out) often approaching, and sometimes exceeding −10°C. The relationship between T_in and T_out was best described by a composite of one steep and one shallow linear relationship separated by a breakpoint (Ψ, interspecific range = 25–35°C). Capitula were only weakly thermoregulated when T_out < Ψ, but switched to closely thermoregulated cooling when T_out > Ψ. Narrow vertical distributions of capitula above the ground and similar cooling responses by all species resulted in a “refrigerated floral layer” where most bees foraged at T_out > Ψ and presumably visited cooled capitula. Thermoregulatory refrigeration of capitula (“thermal engineering”) can benefit not only plant reproduction by reducing pollen and ovule exposure to high temperatures during the summer but also the populations of bee pollinators and other floricolous insects.

The rising introduction of invasive species through trade networks threatens biodiversity and ecosystem services. Yet, we have a limited understanding of how transportation networks determine spatiotemporal patterns of range expansion. This knowledge gap may stem from two reasons. First, current analytical models fail to integrate the invader's life-history dynamics with heterogeneity in human-mediated dispersal patterns. Second, classical statistical methods often fail to provide reliable estimates of model parameters, such as the time and place of species introduction and life-history characteristics, due to spatial biases in the presence-only records and lack of informative demographic data. To address these gaps, we first formulate an age-structured metapopulation model that uses a probability matrix to emulate human-mediated dispersal patterns. The model reveals that an invader spreads radially along the shortest network path, such that the inter-patch network distances decrease with increasing traffic volume and reproductive value of hitchhikers. Next, we propose a hierarchical Bayesian statistical method to estimate model parameters using presence-only data and prior demographic knowledge. To show the utility of the statistical approach, we analyze zebra mussel (Dreissena polymorpha) expansion in North America through the inland commercial shipping network. Our analysis suggests that zebra mussels might have been introduced before 1981, indicating a lag of 5 years between the time of introduction and first detection in late 1986. Furthermore, using our statistical model, we estimated a one in three chance that they were introduced near Kingsville (Ontario, Canada), where they were first reported. We also find that survival, fecundity, and dispersal during early life (1–2 years) play a critical role in determining the expansion success of these mollusks. These results underscore the importance of fusing prior scientific knowledge with observation and demographic processes in a Bayesian framework for conceptual and practical understanding of how invasive species spread by human agency.

Batrachochytrium dendrobatidis (Bd) is a globally distributed fungal pathogen of amphibians that has contributed to one of the largest disease-related biodiversity losses in wildlife. Bd is regularly viewed through the lens of a global wildlife epizootic because the spread of highly virulent genetic lineages has resulted in well-documented declines and extinctions of multiple amphibian species. However, the current state of Bd occurrence, host range, host impacts, and ecological drivers remains poorly understood outside of the most negatively affected amphibian species and regions. Our objective was to describe the macroecology of Bd occurrence and infection intensity on caudates (salamanders) across the United States and to compare these patterns with better-studied anurans (frogs and toads). We collected swabs from 11,183 amphibians at 609 sites from 54 species across the United States from 2015 to 2017. We analyzed the prevalence and intensity of Bd infection jointly using a Bayesian hurdle model with covariates of site-level temperature and precipitation, as well as individual characteristics and species identification. Bd was distributed widely across sites and species sampled across the spatial extent of the conterminous United States. We found that Bd prevalence and intensity were most strongly influenced by temperature in the month preceding sampling and by differences among taxon groups. We estimated that temperature had a strong and nonlinear influence on both Bd prevalence and intensity with peak infection at intermediate temperatures and lower infection at low and high temperatures. We found Caudate hosts tended to have higher prevalence than Anuran hosts and Anuran hosts tended to have higher intensity at optimal temperatures for Bd infection. Our findings suggest that Bd has an amphibian-wide host range, temperature gradients exert a strong influence on Bd, and enzootic transmission likely encompasses a much larger spatial and species distribution than previously recognized across North America.

The strength of biodiversity–ecosystem functioning (BEF) relationships varies within and across studies, depending on the investigated ecosystem function and diversity facet (e.g., species richness or functional composition), limiting our ability to translate BEF results into recommendations for management and conservation. The variability in BEF relationships is particularly high when considering complex multitrophic communities and can be explained by food web contexts. Here we examine how different plant diversity facets affect biomass stocks and energy flows of each trophic group depending on their position in the trophic network. We used coupled aboveground–belowground multitrophic networks of energy dynamics, assembled across the experimental gradients of grassland plant species richness, functional diversity, and presence of plant functional groups. We compared the strengths of these diversity effects between trophic groups, trophic levels, aboveground versus belowground subnetworks, and types of ecosystem functions. Plant species richness, functional trait diversity, and the presence of legumes and grasses were influential drivers of ecosystem energetics. The effects of plant species richness across the food web often operated through mechanisms of plant functional-trait diversity. The effects of plant species richness attenuated across trophic levels. Legume presence strengthened the top-down control (predation) of primary consumers. We found an overall mismatch in the strength of diversity effects on flows versus stocks. Some trophic groups showed even contrasting direction in responses of their stocks and flows to plant diversity. This indicates that plant diversity constrains consumer functioning by means other than only altered consumer biomass. Responses of flows and stocks to plant diversity differed between trophic groups, and aboveground versus belowground parts. Individual stocks and energy flows were responsive to different biodiversity facets, highlighting the importance of the explicit consideration of individual functions and diversity facets for a comprehensive multitrophic understanding. For example, legume presence increased aboveground processes but reduced plant carbon uptake and belowground plant production. Plant communities containing legumes lost more biomass to herbivores, had faster decomposition, and channeled less energy to soil detritus. An important implication of these results is that targeted grassland management would profit from focusing on specific plant diversity facets depending on the ecosystem function or service of interest.

Deadwood stores about 8% of global carbon stock, and its decomposition is a key factor in forest ecosystems. Deadwood-associated (saproxylic) organisms constitute a food web that sustains a substantial part of biodiversity globally. After fungi, saproxylic beetles are the most prominent agents of structural deadwood decomposition in forests. They are often classified according to their presumed link to the deadwood decomposition gradient, generally as feeding on fresh wood, decayed wood, fungi, or predators. These classifications are, however, based on ecomorphological characters (e.g., trophic morphology, habitat use) while information on their diet is globally limited. Carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope ratios represent potential useful tracers to improve knowledge on the trophic ecology of this model group and the whole decomposition food web. We performed stable isotope analysis on 121 beetle species (530 samples) from a mixed-deciduous forest in Central Europe in order to (1) characterize drivers of saproxylic beetles' isotopic variability with respect to potential food sources along the wood decomposition gradient and in relation to the potentially key intrinsic factors such as phylogeny and body size and (2) to assess how isotope information matches with two trophic guild classifications based on ecomorphological characters which are commonly used in ecological studies. The analysis revealed a clear pattern of δ¹³C increase and simultaneous C:N ratio decrease across potential food sources along the gradient from fresh to decayed deadwood and fungi. Beetle phylogeny and body size explained a significant part of their isotope variability, with values of δ¹³C being lower in smaller species. After filtering out these effects, the δ¹³C values reflected the position of beetle species on the decomposition gradient only loosely. Fungi-feeding guilds had higher δ¹³C values than the guilds dependent on fresher deadwood, but otherwise the guilds were indistinguishable. Deadwood consumers did not differ from predators. The isotopic niches of different feeding guilds largely overlapped, and the large observed variation suggests that not only fungi feeders but species from most guilds may depend considerably on fungi and that mixed trophic strategies may be more common in the decomposition food web than currently acknowledged.