Ecosystem Responses to Community Disassembly

Toxic nonpreferred species accelerate the natural restoration of plant productivity and diversity in degraded grasslands

Long-term overgrazing may lead to the degradation of grasslands which are often characterized by an increase in nonpreferred species, especially toxic plants. However, the impact of these toxic nonpreferred species on the restoration processes of degraded grasslands is not well understood, particularly their interactions with soil properties and other plant functional groups. To address this knowledge gap, we conducted an in situ grazing exclusion experiment in a temperate degraded grassland of Inner Mongolia, China. The objective of this study was to investigate how toxic nonpreferred plants influence the recovery of plant diversity and productivity in degraded grasslands and whether these effects can be explained by changes in soil properties. Our findings revealed that Stellera chamaejasme, a toxic nonpreferred species widely distributed in North China, directly altered plant community composition and improved species diversity in degraded grasslands dominated by Asteraceae plants. The presence of S. chamaejasme could inhibit Asteraceae abundance and increase soil copper content in this study area, because Asteraceae plants have a high copper accumulation capacity. Within the communities with S. chamaejasme, the alleviation of soil copper limitation to plants may subsequently enhance the abundance and aboveground productivity of Poaceae and Forbs. Our study demonstrated that the strong direct and indirect interactions of toxic nonpreferred species with other ecosystem components promoted competitive release in terms of biomass accumulation and species diversity. The change of soil limiting microelements content caused by toxic species exerts an important mediation function during the recovery process of degraded grasslands. Thus, these toxic nonpreferred species can act primarily as accelerators for the restoration of community structure and ecosystem function in degraded grasslands.

Past and recent anthropogenic pressures drive rapid changes in riverine fish communities

Understanding how and why local communities change is a pressing task for conservation, especially in freshwater systems. It remains challenging because of the complexity of biodiversity changes, driven by the spatio-temporal heterogeneity of human pressures. Using a compilation of riverine fish community time series (93% between 1993 and 2019) across the Palaearctic, Nearctic and Australasia realms, we assessed how past and recent anthropogenic pressures drive community changes across both space and time. We found evidence of rapid changes in community composition of 30% per decade characterized by important changes in the dominant species, together with a 13% increase in total abundance per decade and a 7% increase in species richness per decade. The spatial heterogeneity in these trends could be traced back to the strength and timing of anthropogenic pressures and was mainly mediated by non-native species introductions. Specifically, we demonstrate that the negative effects of anthropogenic pressures on species richness and total abundance were compensated over time by the establishment of non-native species, a pattern consistent with previously reported biotic homogenization at the global scale. Overall, our study suggests that accounting for the complexity of community changes and its drivers is a crucial step to reach global conservation goals.

Inverse priority effects: A role for historical contingency during species losses

Communities worldwide are losing multiple species at an unprecedented rate, but how communities reassemble after these losses is often an open question. It is well established that the order and timing of species arrival during community assembly shapes forthcoming community composition and function. Yet, whether the order and timing of species losses can lead to divergent community trajectories remains largely unexplored. Here, we propose a novel framework that sets testable hypotheses on the effects of the order and timing of species losses-inverse priority effects-and suggests its integration into the study of community assembly. We propose that the order and timing of species losses within a community can generate alternative reassembly trajectories, and suggest mechanisms that may underlie these inverse priority effects. To formalize these concepts quantitatively, we used a three-species Lotka-Volterra competition model, enabling to investigate conditions in which the order of species losses can lead to divergent reassembly trajectories. The inverse priority effects framework proposed here promotes the systematic study of the dynamics of species losses from ecological communities, ultimately aimed to better understand community reassembly and guide management decisions in light of rapid global change.

Humanity's diverse predatory niche and its ecological consequences

Although humans have long been predators with enduring nutritive and cultural relationships with their prey, seldom have conservation ecologists considered the divergent predatory behavior of contemporary, industrialized humans. Recognizing that the number, strength and diversity of predator-prey relationships can profoundly influence biodiversity, here we analyze humanity's modern day predatory interactions with vertebrates and estimate their ecological consequences. Analysing IUCN 'use and trade' data for ~47,000 species, we show that fishers, hunters and other animal collectors prey on more than a third (~15,000 species) of Earth's vertebrates. Assessed over equivalent ranges, humans exploit up to 300 times more species than comparable non-human predators. Exploitation for the pet trade, medicine, and other uses now affects almost as many species as those targeted for food consumption, and almost 40% of exploited species are threatened by human use. Trait space analyses show that birds and mammals threatened by exploitation occupy a disproportionally large and unique region of ecological trait space, now at risk of loss. These patterns suggest far more species are subject to human-imposed ecological (e.g., landscapes of fear) and evolutionary (e.g., harvest selection) processes than previously considered. Moreover, continued overexploitation will likely bear profound consequences for biodiversity and ecosystem function.

Climate-driven convergent evolution in riparian ecosystems on sky islands

Climate-induced evolution will determine population persistence in a changing world. However, finding natural systems in which to study these responses has been a barrier to estimating the impact of global change on a broad scale. We propose that isolated sky islands (SI) and adjacent mountain chains (MC) are natural laboratories for studying long-term and contemporary climatic pressures on natural populations. We used greenhouse common garden trees to test whether populations on SI exposed to hot and dry climates since the end of the Pleistocene have phenotypically diverged from populations on MC, and if SI populations have converged in these traits. We show: (1) populations of Populus angustifolia from SI have diverged from MC, and converged across SI, in reproductive and productivity traits, (2) these traits (cloning and aboveground biomass, respectively) are significantly correlated, suggesting a genetic linkage between them, and (3) the trait variation is driven by both natural selection and genetic drift. These shifts represent potentially beneficial phenotypes for population persistence in a changing world. These results suggest that the SI-MC comparison is a natural laboratory, as well as a predictive framework, for studying long-term responses to climate change across the globe.

Resilience of Avian Communities to Urbanization and Climate Change: an Integrative Review

The concept of ecological resilience is widely used to assess how species and ecosystems respond to external stressors but is applied infrequently at the level of the community or to chronic, ongoing disturbances. In this review, we first discuss the concept of ecological resilience and methods for quantifying resilience in ecological studies. We then synthesize existing evidence for the resilience of avian communities to climate change and urbanization, two chronic disturbances that are driving global biodiversity loss, and conclude with recommendations for future directions. We only briefly discuss the theoretical framework behind ecological resilience and species-specific responses to these two major disturbances, because numerous reviews already exist on these topics. Current research suggests strong heterogeneity in the responses and resilience of bird communities to urbanization and climate change, although community disassembly and reassembly is high following both disturbances. To advance our understanding of community resilience to these disturbances, we recommend five areas of future study (1) the development of a standardized, comprehensive community resilience index that incorporates both adaptive capacity and measures of functional diversity, (2) measurement/modeling of both community resistance and recovery in response to disturbance, (3) multi-scale and/or multi-taxa studies that include three-way interactions between plants, animals, and climate, (4) studies that incorporate interactions between disturbances, and (5) increased understanding of interactions between ecological resilience and socio-ecological dynamics. Advancement in these areas will enhance our ability to predict and respond to the rapidly accelerating effects of climate change and urbanization.

Anthropogenic disruptions to longstanding patterns of trophic-size structure in vertebrates

Diet and body mass are inextricably linked in vertebrates: while herbivores and carnivores have converged on much larger sizes, invertivores and omnivores are, on average, much smaller, leading to a roughly U-shaped relationship between body size and trophic guild. Although this U-shaped trophic-size structure is well documented in extant terrestrial mammals, whether this pattern manifests across diverse vertebrate clades and biomes is unknown. Moreover, emergence of the U-shape over geological time and future persistence are unknown. Here we compiled a comprehensive dataset of diet and body size spanning several vertebrate classes and show that the U-shaped pattern is taxonomically and biogeographically universal in modern vertebrate groups, except for marine mammals and seabirds. We further found that, for terrestrial mammals, this U-shape emerged by the Palaeocene and has thus persisted for at least 66 million years. Yet disruption of this fundamental trophic-size structure in mammals appears likely in the next century, based on projected extinctions. Actions to prevent declines in the largest animals will sustain the functioning of Earth's wild ecosystems and biomass energy distributions that have persisted through deep time.

Watershed Ecohydrological Processes in a Changing Environment: Opportunities and Challenges

Basin ecohydrological processes are essential for informing policymaking and social development in response to growing environmental problems. In this paper, we review watershed ecohydrology, focusing on the interaction between watershed ecological and hydrological processes. Climate change and human activities are the most important factors influencing water quantity and quality, and there is a need to integrate watershed socioeconomic activities into the paradigm of watershed ecohydrological process studies. Then, we propose a new framework for integrated watershed management. It includes (1) data collection: building an integrated observation network; (2) theoretical basis: attribution analysis; (3) integrated modeling: medium- and long-term prediction of ecohydrological processes by human–nature interactions; and (4) policy orientation. The paper was a potential solution to overcome challenges in the context of frequent climate extremes and rapid land-use change.

Trait-based approaches to global change ecology: moving from description to prediction

Trait-based approaches are increasingly recognized as a tool for understanding ecosystem re-assembly and function under intensifying global change. Here we synthesize trait-based research globally (n = 865 studies) to examine the contexts in which traits may be used for global change prediction. We find that exponential growth in the field over the last decade remains dominated by descriptive studies of terrestrial plant morphology, highlighting significant opportunities to expand trait-based thinking across systems and taxa. Very few studies (less than 3%) focus on predicting ecological effects of global change, mostly in the past 5 years and via singular traits that mediate abiotic limits on species distribution. Beyond organism size (the most examined trait), we identify over 2500 other morphological, physiological, behavioural and life-history traits known to mediate environmental filters of species' range and abundance as candidates for future predictive global change work. Though uncommon, spatially explicit process models-which mechanistically link traits to changes in organism distributions and abundance-are among the most promising frameworks for holistic global change prediction at scales relevant for conservation decision-making. Further progress towards trait-based forecasting requires addressing persistent barriers including (1) matching scales of multivariate trait and environment data to focal processes disrupted by global change, and (2) propagating variation in trait and environmental parameters throughout process model functions using simulation.

Antigonon leptopus invasion is associated with plant community disassembly in a Caribbean island ecosystem

Invasions by non-native plant species are widely recognized as a major driver of biodiversity loss. Globally, (sub-)tropical islands form important components of biodiversity hotspots, while being particularly susceptible to invasions by plants in general and vines in particular. We studied the impact of the invasive vine A. leptopus on the diversity and structure of recipient plant communities on the northern Caribbean island St. Eustatius. We used a paired-plot design to study differences in species richness, evenness and community structure under A. leptopus-invaded and uninvaded conditions. Community structure was studied through species co-occurrence patterns. We found that in plots invaded by A. leptopus, species richness was 40–50% lower, and these plots also exhibited lower evenness. The magnitude of these negative impacts increased with increasing cover of A. leptopus. Invaded plots also showed higher degrees of homogeneity in species composition. Species co-occurrence patterns indicated that plant communities in uninvaded plots were characterized by segregation, whereas recipient plant communities in invaded plots exhibited random co-occurrence patterns. These observations suggest that invasion of A. leptopus is not only associated with reduced species richness and evenness of recipient communities in invaded sites, but also with a community disassembly process that may reduce diversity between sites. Given that A. leptopus is a successful invader of (sub-)tropical islands around the globe, these impacts on plant community structure highlight that this invasive species could be a particular conservation concern for these systems.

Warming Rates Alter Sequence of Disassembly in Experimental Communities

AbstractThe frequency, intensity, and duration of periods of extreme environmental warming are expected to rise over the next hundred years and play an increasing role in species loss resulting from climate change, and yet we know little about their potential future effects on variability in the composition of communities. This study analyzed patterns of species loss in a community of four rotifers and six ciliates exposed to different rates of extreme warming. Temperature of loss was positively correlated with warming rates for all species, consistent with theoretical frameworks suggesting that lower rates of warming increase exposure time and cumulative thermal stress at each temperature. The sequence of species loss during extreme warming depended on the environmental warming rate (i.e., warming rates had the capacity to drive reversals in the relative thermal tolerances of species), and changes in the sequence of species loss driven by the warming rate resulted in substantial variability in community composition. The results suggest that differences in warming rates across space and time may increase variability in community composition in ecosystems increasingly disturbed by extreme temperature, potentially altering interspecific interactions, the abiotic environment, and ecosystem function. Several ecological mechanisms may be responsible, singly or together, for changes in the sequence of species loss at different rates of warming, including (a) differences among species in their sensitivity to the intensity and duration of heat exposure, (b) the effects of warming rates on temperature-dependent interspecific interactions, and (c) differences in opportunities for evolution among species and across warming rates.

Species-specific responses of a marsh-forest ecotone plant community responding to climate change

Ecotones are responsive to environmental change and pave a path for succession as they move across the landscape. We investigated the biotic and abiotic filters to species establishment on opposite ends of a tidal marsh-forest ecotone that is moving inland in response to sea level rise. We transplanted four plant species common to the ecotone to the leading or trailing edge of the migrating ecotone, with and without caging to protect them from ungulate herbivores. We found that species exhibited an individualistic response to abiotic and biotic pressures in this ecotone; three species performed better at the leading edge of the ecotone in the coastal forest, whereas one performed better at the trailing edge in the marsh. Specifically, grass species Phragmites australis and Panicum virgatum grew more in the low light and low salinity conditions of the leading edge of the ecotone (forest), whereas the shrub Iva frutescens grew better in the high light, high salinity conditions of the trailing edge of the ecotone (marsh). Furthermore, of the four species, only P. australis was affected by the biotic pressure of herbivory by an introduced ungulate, Cervus nippon, which greatly reduced its biomass and survival at the leading edge (forest). P. australis is an aggressive invasive species and has been observed to dominate in the wake of migrating marsh-forest ecotones. Our findings detail the role of lower salinity stress to promote and herbivory pressure to inhibit the establishment of P. australis during shifts of this ecotone, and also highlight an interaction between two nonnative species, P. australis and C. nippon. Understanding migration of the marsh-forest ecotone and the factors controlling P. australis establishment are critical for marsh conservation in the face of sea level rise. More generally, our findings support the conclusion that the abiotic and biotic filters of a migrating ecotone shape the resulting community.

Overview of global status of plastic presence in marine vertebrates

The presence of plastic in the environment is generating impacts on all habitats and has become a major global problem in marine megafauna. Macroplastics can cause entanglement, ingestion and loss of suitable habitats. In addition to entanglement problems, there is evidence that plastics are entering the food web through ingestion by marine organisms, which could ultimately be affecting humans. Much of the available information on the impact of plastic in biota is scattered and disconnected due to the use of different methodologies. Here, we review the variety of approaches and protocols followed to assess macro- and microplastic ingestion in marine vertebrates such as sea turtles, cetaceans and fishes in order to offer a global overview of their current status. The analysis of 112 studies indicates the highest plastic ingestion in organisms collected in the Mediterranean and Northeast Indian Ocean with significant differences among plastic types ingested by different groups of animals, including differences in colour and the type of prevalent polymers. In sea turtles, the most prevalent types of plastics are white plastics (66.60%), fibres (54.54%) and LDPE polymer (39.09%); in cetaceans, white macro- and microplastics (38.31%), fibres (79.95%) and PA polymer (49.60%); and in fishes, transparent plastics (45.97%), fibres (66.71%) and polyester polymer (36.20%). Overall, clear fibre microplastics are likely the most predominant types ingested by marine megafauna around the globe.

Threshold responses of riverine fish communities to land use conversion across regions of the world

The growing human enterprise has sparked greater interest in identifying ecological thresholds in land use conversion beyond which populations or communities demonstrate abrupt nonlinear or substantive change in species composition. Such knowledge remains fundamental to understanding ecosystem resilience to environmental degradation and informing land use planning into the future. Confronting this challenge has been largely limited to inferring thresholds in univariate metrics of species richness and indices of biotic integrity and has largely ignored how land use legacies of the past may shape community responses of today. By leveraging data for 13,069 riverine sites from temperate, subtropical, and boreal climate zones on four continents, we characterize patterns of community change along diverse gradients of urbanization and agricultural land use, and identity threshold values beyond which significant alterations in species composition exists. Our results demonstrate the apparent universality by which freshwater fish communities are sensitive to even low levels of watershed urbanization (range of threshold values: 1%-12%), but consistently higher (and more variable) levels of agricultural development (2%-37%). We demonstrated that fish community compositional thresholds occurred, in general, at lower levels of watershed urbanization and agriculture when compared to threshold responses in species richness. This supports the notion that aggregated taxon-specific responses may better reflect the complexity of assemblage responses to land use development. We further revealed that the ghost of land use past plays an important role in moderating how current-day fish communities respond to land use intensification. Subbasins of the United States experiencing greater rates of past land use change demonstrated higher current-day thresholds. Threshold responses of community composition, such as those identified in our study, illustrate the need for globally coordinated efforts to prioritize country-specific management and policy initiatives that ensure that freshwater fish diversity is not inevitably lost in the future.

Community disassembly and disease: realistic-but not randomized-biodiversity losses enhance parasite transmission

Debates over the relationship between biodiversity and disease dynamics underscore the need for a more mechanistic understanding of how changes in host community composition influence parasite transmission. Focusing on interactions between larval amphibians and trematode parasites, we experimentally contrasted the effects of host richness and species composition to identify the individual and joint contributions of both parameters on the infection levels of three trematode species. By combining experimental approaches with field surveys from 147 ponds, we further evaluated how richness effects differed between randomized and realistic patterns of species loss (i.e. community disassembly). Our results indicated that community-level changes in infection levels were owing to host species composition, rather than richness. However, when composition patterns mirrored empirical observations along a natural assembly gradient, each added host species reduced infection success by 12-55%. No such effects occurred when assemblages were randomized. Mechanistically, these patterns were due to non-random host species assembly/disassembly: while highly competent species predominated in low diversity systems, less susceptible hosts became progressively more common as richness increased. These findings highlight the potential for combining information on host traits and assembly patterns to forecast diversity-mediated changes in multi-host disease systems.

Chemosensory deficiency may render island-dwelling lizards more vulnerable to invasive predators

Newly introduced predators constitute a major threat to prey populations worldwide. Insular prey animals in particular often do not succeed in overcoming their naivety towards alien predators, making them specifically vulnerable. Why this is the case remains incompletely understood. Here, we investigate how the ability to detect and respond to predator chemical cues varies among populations of the Dalmatian wall lizard, Podarcis melisellensis. Lizards were sampled from five locations in south-eastern Croatia (one mainland location and four islands) that varied in the composition of their predator community. We observed the lizards’ behaviour in response to chemical cues of native saurophagous snakes (the Balkan whip snake, Hierophis gemonensis, and eastern Montpellier snake, Malpolon insignitus) and an introduced mammalian predator (the small Indian mongoose, Herpestes auropunctatus – a species held responsible for the loss of numerous insular reptile populations worldwide). Mainland lizards showed elevated tongue-flick rates (indicative of scent detection) as well as behaviours associated with distress in response to scents of both native and introduced predators. In sharp contrast, island lizards did not alter their behaviour when confronted with any of the predator cues. Alarmingly, even lizards from islands with native predators (both snakes and mammals) and from an island on which mongooses were introduced during the 1920s were non-responsive. This suggests that insular populations are chemosensorily deprived. As failure at the predator-detection level is often seen as the most damaging form of naivety, these results provide further insight into the mechanisms that render insular-living animals vulnerable to invasive species.

Adaptation to agricultural pesticides may allow mosquitoes to avoid predators and colonize novel ecosystems

Human activities such as the application of agrochemicals may detrimentally disturb natural ecosystems, generating novel selection pressures. Here we examine how pesticides may influence community composition using the aquatic communities within bromeliad phytotelmata, and how adaptive responses to pesticides may influence community-level patterns. We first quantified the composition of macroinvertebrate communities from pesticide-free and pesticide-exposed locations. Complementary manipulative experiments where bromeliads were transplanted between pesticide-free and pesticide-exposed sites were then performed. Finally, pesticide bioassays on the most common predators (Mecistogaster modesta damselflies) and prey (Wyeomyia abebela mosquitoes) assessed a potential evolutionary mechanism that may influence community compositional differences. Our field survey revealed differences in W. abebela and M. modesta abundances between pesticide-free and pesticide-exposed areas. Our transplant experiment suggested compositional differences were not due to physical differences between bromeliads from different locations. Pesticide bioassays revealed that M. modesta from pesticide-free locations had higher innate pesticide tolerances than W. abebela from pesticide-free areas, but M. modesta larvae showed no evidence of adapted resistance as none were found where pesticides were used. Conversely, W. abebela larvae from pesticide-exposed locations had higher pesticide tolerances than individuals from pesticide-free sites, suggesting an adaptive response. This evolved resistance to pesticides may, therefore, allow W. abebela to colonize habitats free of the dominant predator in the system, explaining the higher W. abebela abundances in pesticide-exposed areas than in pesticide-free locations. We suggest that the total effect of novel stressors is driven by interactions between ecological and evolutionary processes.

Threshold Responses in the Taxonomic and Functional Structure of Fish Assemblages to Land Use and Water Quality: A Case Study from the Taizi River

Biological functional traits help to understand specific stressors that are ignored intaxonomic data analysis. A combination of biological functional traits and taxonomic data ishelpful in determining specific stressors which are of significance for fish conservation and riverbasin management. In the current study, the Taizi River was used as a case study to understand therelationships between the taxonomic and functional structure of fish and land use and waterquality, in addition to determining the thresholds of these stressors. The results showed thattaxonomic structure was significantly affected by the proportion of urban land and specificconductivity levels, while functional metrics were influenced by the proportions of farmland andforest. Threshold indicator taxa analysis found that Phoxinus lagowskii, Barbatula barbatula nuda,Odontobutis obscura, and Cobitis granoei had negative threshold responses along the gradients ofurban developments and specific conductivity. There was a significant change in fish taxonomiccomposition when the proportion of urban land exceeded a threshold of 2.6–3.1%, or specificconductivity exceeded a threshold of 369.5–484.5 μS/cm. Three functional features—habitatpreference, tolerance to disturbances, and spawning traits—showed threshold responses to theproportion of farmland and forest. The abundance of sensitive species should be monitored as partof watershed management, as sensitive species exhibit an earlier and stronger response to stressorsthan other functional metrics. Sensitive species had a positive threshold response to the proportionof forest at 80.1%. These species exhibited a negative threshold response to the proportion offarmland at 13.3%. The results of the current study suggest that the taxonomic and functionalstructure of fish assemblages are affected by land use and water quality. These parameters shouldbe integrated into routine monitoring for fish conservation and river basin management in the TaiziRiver. In addition, corresponding measures for improving river habitat and water quality shouldbe implemented according to the thresholds of these parameters.

Predicting ecosystem vulnerability to biodiversity loss from community composition

Ecosystems vary widely in their responses to biodiversity change, with some losing function dramatically while others are highly resilient. However, generalizations about how species- and community-level properties determine these divergent ecosystem responses have been elusive because potential sources of variation (e.g., trophic structure, compensation, functional trait diversity) are rarely evaluated in conjunction. Ecosystem vulnerability, or the likely change in ecosystem function following biodiversity change, is influenced by two types of species traits: response traits that determine species' individual sensitivities to environmental change, and effect traits that determine a species' contribution to ecosystem function. Here we extend the response-effect trait framework to quantify ecosystem vulnerability and show how trophic structure, within-trait variance, and among-trait covariance affect ecosystem vulnerability by linking extinction order and functional compensation. Using in silico trait-based simulations we found that ecosystem vulnerability increased when response and effect traits positively covaried, but this increase was attenuated by decreasing trait variance. Contrary to expectations, in these communities, both functional diversity and trophic structure increased ecosystem vulnerability. In contrast, ecosystem functions were resilient when response and effect traits covaried negatively, and variance had a positive effect on resiliency. Our results suggest that although biodiversity loss is often associated with decreases in ecosystem functions, such effects are conditional on trophic structure, and the variation within and covariation among response and effect traits. Taken together, these three factors can predict when ecosystems are poised to lose or gain function with ongoing biodiversity change.

Patterns and drivers of fish extirpations in rivers of the American Southwest and Southeast

Effective conservation of freshwater biodiversity requires spatially explicit investigations of how dams and hydroclimatic alterations among climate regions may interact to drive species to extinction. We investigated how dams and hydroclimatic alterations interact with species ecological and life history traits to influence past extirpation probabilities of native freshwater fishes in the Upper and Lower Colorado River (CR), Alabama-Coosa-Tallapoosa (ACT), and Apalachicola-Chattahoochee-Flint (ACF) basins. Using long-term discharge data for continuously gaged streams and rivers, we quantified streamflow anomalies (i.e., departure "expected" streamflow) at the sub-basin scale over the past half-century. Next, we related extirpation probabilities of native fishes in both regions to streamflow anomalies, river basin characteristics, species traits, and non-native species richness using binomial logistic regression. Sub-basin extirpations in the Southwest (n = 95 Upper CR, n = 130 Lower CR) were highest in lowland mainstem rivers impacted by large dams and in desert springs. Dampened flow seasonality, increased longevity (i.e., delayed reproduction), and decreased fish egg sizes (i.e., lower parental care) were related to elevated fish extirpation probability in the Southwest. Sub-basin extirpations in the Southeast (ACT n = 46, ACF n = 22) were most prevalent in upland rivers, with flow dependency, greater age and length at maturity, isolation by dams, and greater distance upstream. Our results confirm that dams are an overriding driver of native fish species losses, irrespective of basin-wide differences in native or non-native species richness. Dams and hydrologic alterations interact with species traits to influence community disassembly, and very high extirpation risks in the Southeast are due to interactions between high dam density and species restricted ranges. Given global surges in dam building and retrofitting, increased extirpation risks should be expected unless management strategies that balance flow regulation with ecological outcomes are widely implemented.

Random species loss underestimates dilution effects of host diversity on foliar fungal diseases under fertilization

With increasing attention being paid to the consequences of global biodiversity losses, several recent studies have demonstrated that realistic species losses can have larger impacts than random species losses on community productivity and resilience. However, little is known about the effects of the order in which species are lost on biodiversity-disease relationships. Using a multiyear nitrogen addition and artificial warming experiment in natural assemblages of alpine meadow vegetation on the Qinghai-Tibetan Plateau, we inferred the sequence of plant species losses under fertilization/warming. Then the sequence of species losses under fertilization/warming was used to simulate the species loss orders (both realistic and random) in an adjacently novel removal experiment manipulating plot-level plant diversity. We explicitly compared the effect sizes of random versus realistic species losses simulated from fertilization/warming on plant foliar fungal diseases. We found that realistic species losses simulated from fertilization had greater effects than random losses on fungal diseases, and that species identity drove the diversity-disease relationship. Moreover, the plant species most prone to foliar fungal diseases were also the least vulnerable to extinction under fertilization, demonstrating the importance of protecting low competence species (the ability to maintain and transmit fungal infections was low) to impede the spread of infectious disease. In contrast, there was no difference between random and realistic species loss scenarios simulated from experimental warming (or the combination of warming and fertilization) on the diversity-disease relationship, indicating that the functional consequences of species losses may vary under different drivers.

What North America's skeleton crew of megafauna tells us about community disassembly

Functional trait diversity is increasingly used to model future changes in community structure despite a poor understanding of community disassembly's effects on functional diversity. By tracking the functional diversity of the North American large mammal fauna through the End-Pleistocene megafaunal extinction and up to the present, I show that contrary to expectations, functionally unique species are no more likely to go extinct than functionally redundant species. This makes total functional richness loss no worse than expected given similar taxonomic richness declines. However, where current species sit in functional space relative to pre-anthropogenic baselines is not random and likely explains ecosystem functional changes better than total functional richness declines. Prehistoric extinctions have left many extant species functionally isolated and future extinctions will cause even more rapid drops in functional richness.

Functional redundancy and sensitivity of fish assemblages in European rivers, lakes and estuarine ecosystems

The impact of species loss on ecosystems functioning depends on the amount of trait similarity between species, i.e. functional redundancy, but it is also influenced by the order in which species are lost. Here we investigated redundancy and sensitivity patterns across fish assemblages in lakes, rivers and estuaries. Several scenarios of species extinction were simulated to determine whether the loss of vulnerable species (with high propensity of extinction when facing threats) causes a greater functional alteration than random extinction. Our results indicate that the functional redundancy tended to increase with species richness in lakes and rivers, but not in estuaries. We demonstrated that i) in the three systems, some combinations of functional traits are supported by non-redundant species, ii) rare species in rivers and estuaries support singular functions not shared by dominant species, iii) the loss of vulnerable species can induce greater functional alteration in rivers than in lakes and estuaries. Overall, the functional structure of fish assemblages in rivers is weakly buffered against species extinction because vulnerable species support singular functions. More specifically, a hotspot of functional sensitivity was highlighted in the Iberian Peninsula, which emphasizes the usefulness of quantitative criteria to determine conservation priorities.

Response diversity, nonnative species, and disassembly rules buffer freshwater ecosystem processes from anthropogenic change

Integrating knowledge of environmental degradation, biodiversity change, and ecosystem processes across large spatial scales remains a key challenge to illuminating the resilience of earth's systems. There is now a growing realization that the manner in which communities will respond to anthropogenic impacts will ultimately control the ecosystem consequences. Here, we examine the response of freshwater fishes and their nutrient excretion - a key ecosystem process that can control aquatic productivity - to human land development across the contiguous United States. By linking a continental-scale dataset of 533 fish species from 8100 stream locations with species functional traits, nutrient excretion, and land remote sensing, we present four key findings. First, we provide the first geographic footprint of nutrient excretion by freshwater fishes across the United States and reveal distinct local- and continental-scale heterogeneity in community excretion rates. Second, fish species exhibited substantial response diversity in their sensitivity to land development; for native species, the more tolerant species were also the species contributing greater ecosystem function in terms of nutrient excretion. Third, by modeling increased land-use change and resultant shifts in fish community composition, land development is estimated to decrease fish nutrient excretion in the majority (63%) of ecoregions. Fourth, the loss of nutrient excretion would be 28% greater if biodiversity loss was random or 84% greater if there were no nonnative species. Thus, ecosystem processes are sensitive to increased anthropogenic degradation but biotic communities provide multiple pathways for resistance and this resistance varies across space.

Differential responses and mechanisms of productivity following experimental species loss scenarios

Experiments that simulate nonrandom species loss from natural communities can offer a fundamentally different understanding of the impacts of species loss on ecosystem function and their underlying mechanisms compared to seeding experiments where species are randomly assembled from a local species pool. We examined the mechanisms underlying changes in primary productivity following experimental species loss scenarios in Mongolian grassland. The range of species loss scenarios was based on natural patterns of species abundance that reflect the species' contributions to ecosystem processes. We found a clear reduction in productivity due to species loss only when species were lost randomly. Grassland productivity was relatively robust following nonrandom species loss scenarios. Even in the context of density compensation, the decrease in dominant trait values for leaf height would explain the reduction in productivity with random species loss. In contrast, the maintenance of dominant trait values of key productivity traits such as leaf dry matter content and leaf height might contribute to the maintenance of productivity in response to nonrandom species loss. Our experiment demonstrated that the responses and mechanisms of primary productivity to species loss differ according to the scenarios of species loss in natural grassland communities. The effects of diversity on productivity might be weak in mature natural systems when species loss is nonrandom. Understanding the consequences of realistic species loss on ecosystem functioning based on field-based removal experiments will give insights into real conservation strategies in the face of global biodiversity change.

Disassembly of a tadpole community by a multi-host fungal pathogen with limited evidence of recovery

Emerging infectious diseases can cause host community disassembly, but the mechanisms driving the order of species declines and extirpations following a disease outbreak are unclear. We documented the community disassembly of a Neotropical tadpole community during a chytridiomycosis outbreak, triggered by the generalist fungal pathogen, Batrachochytrium dendrobatidis (Bd). Within the first 11 months of Bd arrival, tadpole density and occupancy rapidly declined. Species rarity, in terms of tadpole occupancy and adult relative abundance, did not predict the odds of tadpole occupancy declines. But species losses were taxonomically selective, with glassfrogs (Family: Centrolenidae) disappearing the fastest and tree frogs (Family: Hylidae) and dart-poison frogs (Family: Dendrobatidae) remaining the longest. We detected biotic homogenization of tadpole communities, with post-decline communities resembling one another more strongly than pre-decline communities. The entire tadpole community was extirpated within 22 months following Bd arrival, and we found limited signs of recovery within 10 years post-outbreak. Because of imperfect species detection inherent to sampling species-rich tropical communities and the difficulty of devising a single study design protocol to sample physically complex tropical habitats, we used simulations to provide recommendations for future surveys to adequately sample diverse Neotropical communities. Our unique data set on tadpole community composition before and after Bd arrival is a valuable baseline for assessing amphibian recovery. Our results are of direct relevance to conservation managers and community ecologists interested in understanding the timing, magnitude, and consequences of disease outbreaks as emerging infectious diseases spread globally.

Grazing maintains native plant diversity and promotes community stability in an annual grassland

Maintaining native biodiversity in grasslands requires management and mitigation of anthropogenic changes that have altered resource availability, grazing regimes, and community composition. In California (USA), high levels of atmospheric nitrogen (N) deposition have facilitated the invasion of exotic grasses, posing a threat to the diverse plant and insect communities endemic to serpentine grasslands. Cattle grazing has been employed to mitigate the consequences of exotic grass invasion, but the ecological effects of grazing in this system are not fully understood. To characterize the effects of realistic N deposition on serpentine plant communities and to evaluate the efficacy of grazing as a management tool, we performed a factorial experiment adding N and excluding large herbivores in California's largest serpentine grassland. Although we observed significant interannual variation in community composition related to climate in our six-year study, exotic cover was consistently and negatively correlated with native plant richness. Sustained low-level N addition did not influence plant community composition, but grazing reduced grass abundance while maintaining greater native forb cover, native plant diversity, and species richness in comparison to plots excluding large herbivores. Furthermore, grazing increased the temporal stability of plant communities by decreasing year-to-year variation in native forb cover, native plant diversity, and native species richness. Taken together, our findings demonstrate that moderate-intensity cattle grazing can be used to restrict the invasive potential of exotic grasses and maintain native plant communities in serpentine grasslands. We hypothesize that the reduced temporal variability in serpentine plant communities managed by grazing may directly benefit populations of the threatened Edith's Bay checkerspot butterfly (Euphydryas editha bayensis).

Convergence in mycorrhizal fungal communities due to drought, plant competition, parasitism, and susceptibility to herbivory: consequences for fungi and host plants

Plants and mycorrhizal fungi influence each other's abundance, diversity, and distribution. How other biotic interactions affect the mycorrhizal symbiosis is less well understood. Likewise, we know little about the effects of climate change on the fungal component of the symbiosis or its function. We synthesized our long-term studies on the influence of plant parasites, insect herbivores, competing trees, and drought on the ectomycorrhizal fungal communities associated with a foundation tree species of the southwestern United States, pinyon pine (Pinus edulis), and described how these changes feed back to affect host plant performance. We found that drought and all three of the biotic interactions studied resulted in similar shifts in ectomycorrhizal fungal community composition, demonstrating a convergence of the community towards dominance by a few closely related fungal taxa. Ectomycorrhizal fungi responded similarly to each of these stressors resulting in a predictable trajectory of community disassembly, consistent with ecological theory. Although we predicted that the fungal communities associated with trees stressed by drought, herbivory, competition, and parasitism would be poor mutualists, we found the opposite pattern in field studies. Our results suggest that climate change and the increased importance of herbivores, competitors, and parasites that can be associated with it, may ultimately lead to reductions in ectomycorrhizal fungal diversity, but that the remaining fungal community may be beneficial to host trees under the current climate and the warmer, drier climate predicted for the future.

Climate relicts and their associated communities as natural ecology and evolution laboratories

Climate relicts, marginal populations that have become isolated via climate-driven range shifts, preserve ecological and evolutionary histories that can span millennia. Studies point to climate relicts as 'natural laboratories' for investigating how long-term environmental change impacts species and populations. However, we propose that such research should be expanded to reveal how climate change affects 'interacting' species in ways that reshape community composition and evolution. Biotic interactions and their community and ecosystem effects are often genetically based and driven by associations with foundation species. We discuss evolution in climate relicts within the context of the emerging fields of community and ecosystem genetics, exploring the idea that foundation relicts are also natural community and ecosystem laboratories and windows to future landscapes.

Realistic diversity loss and variation in soil depth independently affect community-level plant nitrogen use

Numerous experiments have demonstrated that diverse plant communities use nitrogen (N) more completely and efficiently, with implications for how species conservation efforts might influence N cycling and retention in terrestrial ecosystems. However, most such experiments have randomly manipulated species richness and minimized environmental heterogeneity, two design aspects that may reduce applicability to real ecosystems. Here we present results from an experiment directly comparing how realistic and randomized plant species losses affect plant N use across a gradient of soil depth in a native-dominated serpentine grassland in California. We found that the strength of the species richness effect on plant N use did not increase with soil depth in either the realistic or randomized species loss scenarios, indicating that the increased vertical heterogeneity conferred by deeper soils did not lead to greater complementarity among species in this ecosystem. Realistic species losses significantly reduced plant N uptake and altered N-use efficiency, while randomized species losses had no effect on plant N use. Increasing soil depth positively affected plant N uptake in both loss order scenarios but had a weaker effect on plant N use than did realistic species losses. Our results illustrate that realistic species losses can have functional consequences that differ distinctly from randomized losses, and that species diversity effects can be independent of and outweigh those of environmental heterogeneity on ecosystem functioning. Our findings also support the value of conservation efforts aimed at maintaining biodiversity to help buffer ecosystems against increasing anthropogenic N loading.

Non-random biodiversity loss underlies predictable increases in viral disease prevalence

Disease dilution (reduced disease prevalence with increasing biodiversity) has been described for many different pathogens. Although the mechanisms causing this phenomenon remain unclear, the disassembly of communities to predictable subsets of species, which can be caused by changing climate, land use or invasive species, underlies one important hypothesis. In this case, infection prevalence could reflect the competence of the remaining hosts. To test this hypothesis, we measured local host species abundance and prevalence of four generalist aphid-vectored pathogens (barley and cereal yellow dwarf viruses) in a ubiquitous annual grass host at 10 sites spanning 2000 km along the North American West Coast. In laboratory and field trials, we measured viral infection as well as aphid fecundity and feeding preference on several host species. Virus prevalence increased as local host richness declined. Community disassembly was non-random: ubiquitous hosts dominating species-poor assemblages were among the most competent for vector production and virus transmission. This suggests that non-random biodiversity loss led to increased virus prevalence. Because diversity loss is occurring globally in response to anthropogenic changes, such work can inform medical, agricultural and veterinary disease research by providing insights into the dynamics of pathogens nested within a complex web of environmental forces.

Dietary guild composition and disaggregation of avian assemblages under climate change

Climate change is expected to cause geographic redistributions of species. To the extent that species within assemblages have different niche requirements, assemblages may no longer remain intact and dis- and reassemble at current or new geographic locations. We explored how climate change projected by 2100 may transform the world's avian assemblages (characterized at a 110 km spatial grain) by modeling environmental niche-based changes to their dietary guild structure under 0, 500, and 2000 km-dispersal distances. We examined guild structure changes at coarse (primary, high-level, and mixed consumers) and fine (frugivores, nectarivores, insectivores, herbivores, granivores, scavengers, omnivores, and carnivores) ecological resolutions to determine whether or not geographic co-occurrence patterns among guilds were associated with the magnitude to which guilds are functionally resolved. Dietary guilds vary considerably in their global geographic prevalence, and under broad-scale niche-based redistribution of species, these are projected to change very heterogeneously. A nondispersal assumption results in the smallest projected changes to guild assemblages, but with significant losses for some regions and guilds, such as South American insectivores. Longer dispersal distances are projected to cause greater degrees of disassembly, and lead to greater homogenization of guild composition, especially in northern Asia and Africa. This arises because projected range gains and losses result in geographically heterogeneous patterns of guild compensation. Projected decreases especially of primary and mixed consumers most often are compensated by increases in high-level consumers, with increasing uncertainty about these outcomes as dispersal distance and degree of guild functional resolution increase. Further exploration into the consequences of these significant broad-scale ecological functional changes at the community or ecosystem level should be increasingly on the agenda for conservation science.

Enhancing species distribution modeling by characterizing predator-prey interactions

Niche theory is a well-established concept integrating a diverse array of environmental variables and multispecies interactions used to describe species geographic distribution. It is now customary to employ species distribution models (SDMs) that use environmental variables in conjunction with species location information to characterize species' niches and map their geographic ranges. The challenge remains, however, to account for the biotic interactions of species with other community members on which they depend. We show here how to connect species spatial distribution and their dependence with other species by modeling spatially explicit predator-prey interactions, which we call a trophic interaction distribution model (TIDM). To develop the principles, we capitalized on data from Canada lynx (Lynx canadensis) reintroduced into Colorado. Spatial location information for lynx obtained from telemetry was used in conjunction with environmental data to construct an SDM. The spatial locations of lynx-snowshoe hare encounters obtained from snow-tracking in conjunction with environmental data were used to construct a TIDM. The environmental conditions associated with lynx locations and lynx-hare encounters identified through both SDM and TIDM revealed an initial transient phase in habitat use that settled into a steady state. Nevertheless, despite the potential for the SDM to broadly encompass all lynx hunting and nonhunting spatial locations, the spatial extents of the SDM and TIDM differed; about 40% of important lynx-snowshoe hare locations identified in the TIDM were not identified in the lynx-only SDM. Our results encourage greater effort to quantify spatial locations of trophic interactions among species in a community and the associated environmental conditions when attempting to construct models aimed at projecting current and future species geographic distributions.

Global climate change and the evolutionary ecology of ecosystem functioning

Environmental warming due to global climate change is an important stressor that stands to alter organismal physiology and, ultimately, carbon cycling in ecosystems. Yet the theoretical framework for predicting warming effects on whole-ecosystem carbon balance by way of changes in organismal physiology remains rudimentary. This is because ecosystem science has yet to embrace principles of evolutionary ecology that offer the means to explain how environmental stress on organisms mediates ecosystem carbon dynamics. Here, using selected case studies and a theoretical model, I sketch out one framework that shows how increases in animal metabolic rates in response to thermal stress lead to phenotypically plastic shifts in animal elemental demand, from nitrogen-rich proteins that support production to carbon-rich soluble carbohydrates that support elevated energy demands. I further show how such a switch in resource selection alters the fate of carbon between atmospheric versus animal, plant, and soil pools. The framework shows that animals, despite having relatively low biomass representation in ecosystems, can nonetheless have disproportionately larger effects on carbon cycling in ecosystems whose effects are exacerbated by environmental stressors like climate warming.

Life histories predict coral community disassembly under multiple stressors

Climate change is reshaping biological communities against a background of existing human pressure. Evaluating the impacts of multiple stressors on community dynamics can be particularly challenging in species-rich ecosystems, such as coral reefs. Here, we investigate whether life-history strategies and cotolerance to different stressors can predict community responses to fishing and temperature-driven bleaching using a 20-year time series of coral assemblages in Kenya. We found that the initial life-history composition of coral taxa largely determined the impacts of bleaching and coral loss. Prior to the 1998 bleaching event, coral assemblages within no-take marine reserves were composed of three distinct life histories - competitive, stress-tolerant and weedy- and exhibited strong declines following bleaching with limited subsequent recovery. In contrast, fished reefs had lower coral cover, fewer genera and were composed of stress-tolerant and weedy corals that were less affected by bleaching over the long term. Despite these general patterns, we found limited evidence for cotolerance as coral genera and life histories were variable in their sensitivities to fishing and bleaching. Overall, fishing and bleaching have reduced coral diversity and led to altered coral communities of 'survivor' species with stress-tolerant and weedy life histories. Our findings are consistent with expectations that climate change interacting with existing human pressure will result in the loss of coral diversity and critical reef habitat.

Rare species support vulnerable functions in high-diversity ecosystems

Around the world, the human-induced collapses of populations and species have triggered a sixth mass extinction crisis, with rare species often being the first to disappear. Although the role of species diversity in the maintenance of ecosystem processes has been widely investigated, the role of rare species remains controversial. A critical issue is whether common species insure against the loss of functions supported by rare species. This issue is even more critical in species-rich ecosystems where high functional redundancy among species is likely and where it is thus often assumed that ecosystem functioning is buffered against species loss. Here, using extensive datasets of species occurrences and functional traits from three highly diverse ecosystems (846 coral reef fishes, 2,979 alpine plants, and 662 tropical trees), we demonstrate that the most distinct combinations of traits are supported predominantly by rare species both in terms of local abundance and regional occupancy. Moreover, species that have low functional redundancy and are likely to support the most vulnerable functions, with no other species carrying similar combinations of traits, are rarer than expected by chance in all three ecosystems. For instance, 63% and 98% of fish species that are likely to support highly vulnerable functions in coral reef ecosystems are locally and regionally rare, respectively. For alpine plants, 32% and 89% of such species are locally and regionally rare, respectively. Remarkably, 47% of fish species and 55% of tropical tree species that are likely to support highly vulnerable functions have only one individual per sample on average. Our results emphasize the importance of rare species conservation, even in highly diverse ecosystems, which are thought to exhibit high functional redundancy. Rare species offer more than aesthetic, cultural, or taxonomic diversity value; they disproportionately increase the potential breadth of functions provided by ecosystems across spatial scales. As such, they are likely to insure against future uncertainty arising from climate change and the ever-increasing anthropogenic pressures on ecosystems. Our results call for a more detailed understanding of the role of rarity and functional vulnerability in ecosystem functioning.