Showing papers in "Ecological Monographs in 2020"

Total ecosystem carbon stocks of mangroves across broad global environmental and physical gradients

Abstract: Mangroves sequester large quantities of carbon (C) that become significant sources of greenhouse gases when disturbed through land-use change. Thus, they are of great value to incorporate into climate change adaptation and mitigation strategies. In response, a global network of mangrove plots was established to provide policy-relevant ecological data relating to interactions of mangrove C stocks with climatic, tidal, plant community, and geomorphic factors. Mangroves from 190 sites were sampled across five continents encompassing large biological, physical, and climatic gradients using consistent methodologies for the quantification of total ecosystem C stocks (TECS). Carbon stock data were collected along with vegetation, physical, and climatic data to explore potential predictive relationships. There was a 28-fold range in TECS (79-2,208 Mg C/ha) with a mean of 856 ± 32 Mg C/ha. Belowground C comprised an average 85% of the TECS. Mean soil depth was 216 cm, ranging from 22 to >300 cm, with 68 sites (35%) exceeding a depth of 300 cm. TECS were weakly correlated with metrics of forest structure, suggesting that aboveground forest structure alone cannot accurately predict TECS. Similarly, precipitation was not a strong predictor of TECS. Reasonable estimates of TECS were derived via multiple regression analysis using precipitation, soil depth, tree mass, and latitude (R2 = 0.54) as variables. Soil carbon to a 1 m depth averaged 44% of the TECS. Limiting analyses of soil C stocks to the top 1 m of soils result in large underestimates of TECS as well as in the greenhouse gas emissions that would arise from their conversion to other land uses. The current IPCC Tier 1 default TECS value for mangroves is 511 Mg C/ha, which is only 60% of our calculated global mean. This study improves current assessments of mangrove C stocks providing a foundation necessary for C valuation related to climate change mitigation. We estimate mangroves globally store about 11.7 Pg C: an aboveground carbon stock of 1.6 Pg C and a belowground carbon stock of 10.2 Pg C). The differences in the estimates of total ecosystem carbon stocks based on climate, salinity, forest structure, geomorphology, or geopolitical boundaries are not as much of an influence as the choice of soil depth included in the estimate. Choosing to limit soils to a 1 m depth resulted in estimates of 1 m depth resulted in global carbon stock estimates that exceeded 11.2 Pg C.

Carbon budget of the Harvard Forest Long‐Term Ecological Research site: pattern, process, and response to global change

Abstract: How, where, and why carbon (C) moves into and out of an ecosystem through time are long-standing questions in biogeochemistry. Here, we bring together hundreds of thousands of C-cycle observations at the Harvard Forest in central Massachusetts, USA, a mid-latitude landscape dominated by 80–120-yr-old closed-canopy forests. These data answered four questions: (1) where and how much C is presently stored in dominant forest types; (2) what are current rates of C accrual and loss; (3) what biotic and abiotic factors contribute to variability in these rates; and (4) how has climate change affected the forest’s C cycle? Harvard Forest is an active C sink resulting from forest regrowth following land abandonment. Soil and tree biomass comprise nearly equal portions of existing C stocks. Net primary production (NPP) averaged 680–750 g C m 2 yr ; belowground NPP contributed 38–47% of the total, but with large uncertainty. Mineral soil C measured in the same inventory plots in 1992 and 2013 was too heterogeneous to detect change in soil-C pools; however, radiocarbon data suggest a small but persistent sink of 10–30 g C m 2 yr . Net ecosystem production (NEP) in hardwood stands averaged ~300 g C m 2 yr . NEP in hemlock-dominated forests averaged ~450 g C m 2 yr 1 until infestation by the hemlock woolly adelgid turned these stands into a net C source. Since 2000, NPP has increased by 26%. For the period 1992–2015, NEP increased 93%. The increase in mean annual temperature and growing season length alone accounted for ~30% of the increase in productivity. Interannual variations in GPP and NEP were also correlated with increases in red oak biomass, forest leaf area, and canopy-scale lightuse efficiency. Compared to long-term global change experiments at the Harvard Forest, the C sink in regrowing biomass equaled or exceeded C cycle modifications imposed by soil warming, N saturation, and hemlock removal. Results of this synthesis and comparison to simulation models suggest that forests across the region are likely to accrue C for decades to come but may be disrupted if the frequency or severity of biotic and abiotic disturbances increases. Manuscript received 23 January 2020; accepted 22 May 2020. Corresponding Editor: Yude Pan. 16 Corresponding Author. E-mail: aabarker@fas.harvard.edu Article e01423; page 1 Ecological Monographs, 90(4), 2020, e01423 © 2020 by the Ecological Society of America

Effects of soil microbes on plant competition: a perspective from modern coexistence theory

Abstract: Growing evidence shows that soil microbes affect plant coexistence in a variety of systems. However, since these systems vary in the impacts microbes have on plants and in the ways plants compete with each other, it is challenging to integrate results into a general predictive theory. To this end, we suggest that the concepts of niche and fitness difference from modern coexistence theory should be used to contextualize how soil microbes contribute to plant coexistence. Synthesizing a range of mechanisms under a general plant–soil microbe interaction model, we show that, depending on host specificity, both pathogens and mutualists can affect the niche difference between competing plants. However, we emphasize the need to also consider the effect of soil microbes on plant fitness differences, a role often overlooked when examining their role in plant coexistence. Additionally, since our framework predicts that soil microbes modify the importance of plant–plant competition relative to other factors for determining the outcome of competition, we suggest that experimental work should simultaneously quantify microbial effects and plant competition. Thus, we propose experimental designs that efficiently measure both processes and show how our framework can be applied to identify the underlying drivers of coexistence. Using an empirical case study, we demonstrate that the processes driving coexistence can be counterintuitive, and that our general predictive framework provides a better way to identify the true processes through which soil microbes affect coexistence.

Overfishing and the ecological impacts of extirpating large parrotfish from Caribbean coral reefs

Abstract: The unique traits of large animals often allow them to fulfill functional roles in ecosystems that small animals cannot. However, large animals are also at greater risk from human activities. Thus, it is critical to understand how losing large animals impacts ecosystem function. In the oceans, selective fishing for large animals alters the demographics and size structure of numerous species. While the community‐wide impacts of losing large animals are a major theme in terrestrial research, the ecological consequences of removing large animals from marine ecosystems remain understudied. Here, we combine survey data from 282 sites across the Caribbean with a field experiment to investigate how altering the size structure of parrotfish populations impacts coral reef communities. We show that Caribbean‐wide, parrotfish populations are skewed toward smaller individuals, with fishes <11 cm in length comprising nearly 70% of the population in the most heavily fished locations vs. ~25% at minimally fished sites. Despite these differences in size structure, sites had similar overall parrotfish biomass. As a result, algal cover was unrelated to parrotfish biomass and instead, was negatively correlated with the density of large parrotfishes. To mechanistically explore how large parrotfishes shape benthic communities, we manipulated fishes’ access to the benthos to create three distinct fish communities with different size structure. We found that excluding large or large and medium‐sized parrotfishes did not alter overall parrotfish grazing rates but caused respective 4‐ and 10‐fold increases in algal biomass. Unexpectedly, branching corals benefited from excluding large parrotfishes whereas the growth of mounding coral species was impaired. Similarly, removing large parrotfishes led to unexpected increases in coral recruitment that were absent when both large and medium bodied fishes were excluded. Our data highlight the unique roles of large parrotfishes in driving benthic dynamics on coral reefs and suggests that diversity of size is an important component of how herbivore diversity impacts ecosystem function on reefs. This study adds to a growing body of literature revealing the ecological ramifications of removing large animals from ecosystems and sheds new light on how fishing down the size structure of parrotfish populations alters functional diversity to reshape benthic reef communities.

Xylem vessel-diameter–shoot-length scaling: ecological significance of porosity types and other traits

Abstract: Flowering plants predominantly conduct water in tubes known as vessels, with vessel diameter playing a crucial role in plant adaptation to climate and reactions to climate change. The importance of vessels makes it essential to understand how and why vessel diameter, plant height, and other ecological factors are interrelated. Although shoot length is by far the main driver of variation in mean vessel diameter across angiosperms, much remains to be understood regarding the factors accounting for the abundant variation around the y-axis in plots of mean species vessel diameter against shoot length. Here, we explore the potential role of porosity types, wood density, leaf phenology, background imperforate tracheary element type, vasicentric tracheids, vascular tracheids, perforation plate type, and successive cambia in causing variation in the y-intercept or slope of the mean species vessel-diameter– and vesseldensity–shoot-length associations at the shoot tip and base. We detected numerous cases of ecologically significant variation. For example, latewood vessels of ring porous species scale with a lower slope than earlywood, i.e., latewood vessels are relatively narrow in taller plants. This pattern is likely the result of selection favoring freezing-induced embolism resistance via narrow vessels. Wood density was negatively associated with vessel diameter, with low wood density plants having wider vessels for a given height. Species with scalariform perforation plates scale with a lower shoot base vessel-diameter–shoot-length slope, likely reflecting selection against scalariform plates in wide vessels. In other cases, functional groups scaled similarly. For example, species with successive cambia did not differ from those with conventional single cambia in their mean vessel-diameter–shoot-length scaling, rejecting our prediction that species with successive cambia should have narrower vessels for a given shoot length. They did, however, have fewer vessels per unit shoot cross-sectional area than plants of similar heights, likely because vessels have longer functional lifespans (and therefore are fewer) in species with successive cambia. Our methods illustrate how vessel diameter can be studied taking shoot length into account to detect ecologically important variation and construct theory regarding plant adaptation via the hydraulic system that includes plant size as a vital element.

Repeated fire shifts carbon and nitrogen cycling by changing plant inputs and soil decomposition across ecosystems

Abstract: Fires shape the biogeochemistry and functioning of many ecosystems, and fire frequencies are changing across much of the globe. Frequent fires can change soil carbon (C) and nitrogen (N) storage by altering the quantity and chemistry of plant inputs through changes in plant biomass and composition as well as the decomposition of soil organic matter. How decomposition rates change with shifting inputs remains uncertain because most studies focus on the effects of single fires, where transient responses may not reflect responses to decadal changes in burning frequencies. Here, we sampled seven sites exposed to different fire frequencies. In four of the sites, we intensively sampled both soils and plant communities across four ecosystems in North America and Africa spanning tropical savanna, temperate coniferous savanna, temperate broadleaf savanna, and temperate coniferous forest ecosystems. Each site contained multiple plots burned frequently for 33–61 years and nearby plots that had remained unburned over the same period replicated at the landscape scale. Across all sites, repeatedly burned plots had 25–185% lower bulk soil C and N concentrations but also 2–10fold lower potential decomposition of organic matter compared to unburned sites. Soil C and N concentrations and extracellular enzyme activities declined with frequent fire because fire reduced both plant biomass inputs into soils and dampened the localized enrichment effect of tree canopies. Examination of soil extracellular enzyme activities revealed that fire decreased the potential turnover of organic matter in the forms of cellulose, starch, and chitin (P < 0.0001) but not polyphenol and lignin (P = 0.09), suggesting a shift in soil C and N cycling. Inclusion of dC data from three additional savanna sites (19–60 years of altered fire frequencies) showed that soil C losses were largest in sites where estimated tree inputs into soils declined the most (r = 0.91, P < 0.01). In conclusion, repeated burning reduced C and N storage, consistent with previous studies, but fire also reduced potential decomposition, likely contributing to slower C and N cycling. Trees were important in shaping soil C and N responses across sites, but the magnitude of tree effects differed and depended on how tree biomass inputs into soil responded to fire.

The impact of yeast presence in nectar on bumble bee behavior and fitness

Abstract: The presence of yeasts in pollen and floral nectar is rather the norm than the exception. Due to the metabolic activities of yeasts, sugar and amino acid composition of nectar often drastically change and may negatively impact the nutritional value of nectar for pollinators and hence insect fitness. On the other hand, the presence of yeasts in floral nectar may also increase its nutritional value due to yeast's probiotic effect and the release of yeast's metabolites. In this study, we investigated whether the presence of defined flower‐ and insect‐associated yeasts affected individual and colony fitness of the bumble bee pollinator Bombus terrestris. Specifically, we tested whether the presence of yeasts in nectar affected bumble bee foraging behavior and nectar consumption, individual growth and colony development, larval and queen mortality, and mating success. Quantitative analyses of sugar and amino acid profiles showed that nectar yeasts significantly affected the chemical composition of nectar. However, dual‐choice experiments indicated that yeast inoculation did not significantly affect foraging behavior or consumption rates. Nest development, on the other hand, was significantly affected by the presence of yeasts, but effects largely depended on species identity, with Candida bombiphila, Metschnikowia gruessii, and Rhodotorula mucilaginosa having the largest positive impact on colony growth. Interestingly, the effects at the colony level were more pronounced than at the individual level. In vitro growth tests further showed that yeasts have the potential to suppress the growth of the bumble bee gut pathogen Crithidia bombi. Overall, these results demonstrate that nectar‐inhabiting yeasts can have diverse effects on bumble bee fitness and therefore may mediate plant–pollinator mutualisms.

Quantifying water requirements of African ungulates through a combination of functional traits

Abstract: Climate and land use change modify surface water availability in African savannas. Surface water is a key resource for both wildlife and livestock and its spatial and temporal distribution is important for understanding the composition of large herbivore assemblages in savannas. Yet, the extent to which ungulate species differ in their water requirements remains poorly quantified. Here, we infer the water requirements of 48 African ungulates by combining six different functional traits related to physiological adaptations to reduce water loss, namely minimum dung moisture, relative dung pellet size, relative surface area of the distal colon, urine osmolality, relative medullary thickness, and evaporation rate. In addition, we investigated how these differences in water requirements relate to differences in dietary water intake. We observed strong correlations between traits related to water loss through dung, urine and evaporation, suggesting that ungulates minimize water loss through multiple pathways simultaneously, which suggests that each trait can thus be used independently to predict water requirements. Furthermore, we found that browsers and grazers had similar water requirements, but browsers are expected to be less dependent on surface water because they acquire more water through their diet. We conclude that these key functional traits are a useful way to determine differences in water requirements and an important tool for predicting changes in herbivore community assembly resulting from changes in surface water availability.

Flower traits, habitat, and phylogeny as predictors of pollinator service: a plant community perspective

Abstract: Pollinator service is essential for successful sexual reproduction and long‐term population persistence of animal‐pollinated plants, and innumerable studies have shown that insufficient service by pollinators results in impaired sexual reproduction (“pollen limitation”). Studies directly addressing the predictors of variation in pollinator service across species or habitats remain comparatively scarce, which limits our understanding of the primary causes of natural variation in pollen limitation. This paper evaluates the importance of pollination‐related features, evolutionary history, and environment as predictors of pollinator service in a large sample of plant species from undisturbed montane habitats in southeastern Spain. Quantitative data on pollinator visitation were obtained for 191 insect‐pollinated species belonging to 142 genera in 43 families, and the predictive values of simple floral traits (perianth type, class of pollinator visitation unit, and visitation unit dry mass), phylogeny, and habitat type were assessed. A total of 24,866 pollinator censuses accounting for 5,414,856 flower‐minutes of observation were conducted on 510 different dates. Flowering patch and single flower visitation probabilities by all pollinators combined were significantly predicted by the combined effects of perianth type (open vs. restricted), class of visitation unit (single flower vs. flower packet), mass of visitation unit, phylogenetic relationships, and habitat type. Pollinator composition at insect order level varied extensively among plant species, largely reflecting the contrasting visitation responses of Coleoptera, Diptera, Hymenoptera, and Lepidoptera to variation in floral traits. Pollinator composition had a strong phylogenetic component, and the distribution of phylogenetic autocorrelation hotspots of visitation rates across the plant phylogeny differed widely among insect orders. Habitat type was a key predictor of pollinator composition, as major insect orders exhibited decoupled variation across habitat types in visitation rates. Comprehensive pollinator sampling of a regional plant community has shown that pollinator visitation and composition can be parsimoniously predicted by a combination of simple floral features, habitat type, and evolutionary history. Ambitious community‐level studies can help to formulate novel hypotheses and questions, shed fresh light on long‐standing controversies in pollination research (e.g., “pollination syndromes”), and identify methodological cautions that should be considered in pollination community studies dealing with small, phylogenetically biased plant species samples.

Links between soil microbial communities, functioning, and plant nutrition under altered rainfall in Australian grassland

Abstract: The size, frequency, and timing of precipitation events are predicted to become more variable worldwide. Despite these predictions, the importance of changes in precipitation in driving multiple above- and belowground ecosystem attributes simultaneously remains largely underexplored. Here, we carried out 3 yr of rainfall manipulations at the DRI-Grass facility, located in a mesic grassland in eastern Australia. Treatments were implemented through automated water reapplication and included +50% and −50% amount, reduced frequency of events, and an extreme summer drought. We evaluated the spatiotemporal responses of multiple ecosystem attributes including microbial biomass, community composition and activity, soil nutrient content and availability, and plant nutritional status to altered rainfall regimes. We found that changing precipitation patterns resulted in multiple direct and indirect changes in microbial communities and soil and plant nutrient content. Main results included greater availability of soil macronutrients and reduced availability of micronutrients under drought, and taxon-specific changes in the composition of soil microbial communities in response to altered rainfall. Moreover, using structural equation modeling, we showed that, in summer 2015, plant macronutrient contents, a widely used ecological indicator of pasture quality, were simultaneously explained by greater soil nutrient availability and the structure of soil microbial communities, and significantly reduced by lower rainfall. Plant micronutrients were also reduced by lower rainfall and explained by changes in microbial attributes. Despite treatment effects on many of the soil, microbial, and plant variables analyzed across the 3 yr of study, many of these ecosystem attributes varied greatly across sampling events. This resulted in many significant interactions between the rainfall treatments and experimental duration, suggesting complex system-level responses to changing rainfall in our grassland, and a high natural buffering capacity of the ecosystem to varying rainfall conditions. Some interactions manifested as changes in the coefficient of variation of ecosystem attributes, particularly in response to changes in the timing of precipitation events and the extreme summer drought. Finally, we posit that a detailed understanding of plant–soil–microbial interactions, and the role of climate in modifying these linkages, will be key for adapting the sustainability of grasslands to a future that will be shaped by climate change.

The relationship between trophic level and body size in fishes depends on functional traits

Abstract: Predators typically are larger than their prey, and consequently, trophic level should increase with body size. Whereas this relationship has helped in developing predictions about food web structure and dynamics in mesocosms and simple communities, a trophiclevel–body-size relationship may not exist for all kinds of communities or taxa, especially those with many non-carnivorous species. Moreover, functional traits associated with trophic level generally have not been considered. Herein, we examine the correlation between trophic level and body size in fishes and how this relationship may vary in relation to functional traits (body dimensions, mouth size and orientation, tooth shape, gill rakers, and gut length) and trophic guilds (carnivorous vs. non-carnivorous). We analyzed data from morphological measurements and dietary analyses performed on thousands of specimens from freshwater and estuarine habitats across three zoogeographic regions (Neartic, Neotropical, and Afrotropical). A positive relationship between trophic level and body size was only found for carnivorous fishes. No relationship was found when all species were analyzed together, rejecting the idea that trophic level is positively related with body size in fishes generally. This result was consistent even when using either body mass or standard length as the measure of body size, and trophic level for either species (average values) or individual specimens as the response variable. At the intraspecific level, trophic level varied consistently with size for one third of the species, among which only 40% had positive relationships. Body depth, tooth shape, and mouth width were all associated with the trophic-level–body-size relationship. Overall, predators with conical or triangular serrated teeth, large mouths, and elongated/and/or fusiform bodies tend to have positive trophic-level–body-size relationships, whereas primarily non-carnivorous species with unicuspid or multicuspid teeth, deep bodies and small to medium sized mouth gapes tended to have negative relationships. Given the diverse ecological strategies encompassed by fishes, trophic level and food web patterns and processes should not be inferred based solely on body size. Research that integrates multiple functional traits with trophic ecology will improve understanding and predictions about food web structure and dynamics.

Urban soil carbon and nitrogen converge at a continental scale

Abstract: U.S. National Science FoundationNational Science Foundation (NSF) [EF-1065548, 1065737, 1065740, 1065741, 1065772, 1065785, 1065831, 121238320]

An assessment of population size and demographic drivers of the Bearded Vulture using integrated population models

Abstract: Conventional approaches for the assessment of population abundance or trends are usually based on a single source of information, such as counts or changes in demographic parameters. However, these approaches usually neglect some of the information needed to properly understand the population as a whole, such as assessments of the non-breeding proportion of the population and the drivers of population change. The Bearded Vulture Gypaetus barbatus is a threatened species and its Pyrenean population (the largest in Europe) inhabits parts of Spain, Andorra, and France. We developed an Integrated Population Model (IPM) using data from a long-term study (1987–2016) in the three countries, including capture–mark–recapture of 150 marked individuals, to assess population size and age structure at the whole population scale, and obtain estimates of survival and breeding parameters of this population. The breeding population experienced a geometric mean population increase of 3.3% annually, falling to 2.3% during the last 10 yr. The adult proportion of the population increased with time, from 61% to 73%. There were 365 (95% Bayesian credible interval [BCI]: 354–373) adult breeding birds in 2016, representing 49% of the adult population and 36% of the total population (estimated at 1,026 individuals, 95% BCI: 937–1,119). The large number of non-breeding adults probably led to higher mean age of first reproduction than previously estimated, and to an estimated 30–35% of territories occupied by polyandrous trios. Population growth rate was positively and strongly correlated with adult survival, which had a much greater effect on population growth than productivity. The effects of subadult and juvenile survival on population growth were weaker. We found strong evidence for a density-dependent decrease in juvenile survival, productivity and adult survival, leading to reduced population growth with increased population size. Our approach allowed us to identify important conservation issues related to the management of supplementary feeding sites and geographic expansion of this population. Our study supports the use of IPMs as a tool to understand long-lived species, allowing simultaneous estimates of the non-breeding size of the population (which is critical for understanding population functioning), better estimates of population parameters, and assessment of demographic drivers.

Reviewing the role of plant litter inputs to forested wetland ecosystems: leafing through the literature

Abstract: The input of senescent terrestrial leaf litter into soil and aquatic ecosystems is one of the most massive cyclic subsidies on Earth, particularly within forested ecosystems. For freshwater systems embedded within forests, litter inputs provide a vital source of energy and nutrients that allows greater production than in situ resources can provide. In return, freshwater food webs can provide an enormous amount of material to the terrestrial landscape through biotic respiration, photosynthesis, and organism emergence. Most research concerning this important aquatic‐terrestrial link has focused on lotic ecosystems (i.e., streams and rivers); far less attention has been given to its role in lentic systems (i.e., wetlands and lakes). A focus on small forested wetlands is particularly important, as these systems account for a disproportionate amount of global carbon flux relative to their spatial coverage, and the decomposition of leaf litter is a major contributor. Here, we review six themes: (1) the evidence for the role of leaf litter inputs as an ecologically important subsidy in forested wetlands; (2) the bottom‐up effects of quantitative and qualitative variation in litter inputs; (3) how diversity in litter mixtures can alter ecological functioning; (4) evidence for top‐down consequences of litter inputs through toxic effects on predators and parasites, and the alteration of predator–prey interactions; (5) the relevance of our review to other research fields by considering the role of litter inputs relative to other types of subsidies and environmental gradients (e.g., temperature, canopy cover, and hydrology); and (6) the interaction of litter subsidies with anthropogenic disturbances. We conclude by highlighting several high‐priority research questions and providing suggestions for future research on the role of litter subsidies in freshwater ecosystems.

Pulse of dissolved organic matter alters reciprocal carbon subsidies between autotrophs and bacteria in stream food webs

Abstract: Summary Soils are currently leaching out their organic matter at an increasing pace and darkening aquatic ecosystems due to climate and land use change, or recovery from acidification. The implications for stream biogeochemistry and food webs remain largely unknown, notably the metabolic balance (biotic CO2 emissions), reciprocal subsidies between autotrophs and bacteria, and trophic transfer efficiencies. We use a flow food web approach to test how a small addition of labile dissolved organic matter affects the strength and dynamics of the autotrophs-bacteria interaction in streams. Our paired streams whole-ecosystem experimental approach combined with continuous whole-stream metabolism and stable isotope probing allowed to unravel carbon fluxes in the control and treatment streams. We increased the natural supply of dissolved organic matter for three weeks by only 12% by continuously adding 0.5 mg L−1 of sucrose with a δ13C signature different from the natural organic matter. Both photosynthesis and heterotrophic respiration increased rapidly following C addition, but this was short lived due to N and P stoichiometric constraints. The resulting peak in heterotrophic respiration was of similar magnitude to natural peaks in the control observed when soils were hydrologically connected to the streams and received soil derived carbon. Carbon reciprocal subsidies between autotrophs and bacteria in the control stream accounted for about 50% of net primary production and 75% of bacterial production, under low flow conditions when stream water was hydrologically disconnected from soil water. The reciprocal subsidies were weaker by 33% (autotrophs to bacteria) and 55% (bacteria to autotrophs) in the treatment relative to the control. Net primary production relied partly (11% in the control) on natural allochthonous dissolved organic carbon via the CO2 produced by bacterial respiration. Many large changes in ecosystem processes were observed in response to the sucrose addition. The light use efficiency of the autotrophs increased by 37%. Ecosystem respiration intensified by 70%, and the metabolic balance became relatively more negative, i.e. biotic CO2 emissions increased by 125%. Heterotrophic respiration and production increased by 89%, and this was reflected by a shorter (−40%) uptake length (SwOC) and faster (+92%) mineralisation velocity of organic carbon. The proportion of DOC flux respired and organic carbon use efficiency by bacteria increased by 112%. Macroinvertebrate consumer density increased by 72% due to sucrose addition and consumer production was 1.8 times higher in the treatment than in the control at the end of the experiment. The trophic transfer efficiencies from resources to consumers were similar between the control and the treatment (2-5%). Synthesis. Part of the carbon derived from natural allochthonous organic matter can feed the autotrophs via the CO2 produced by stream bacterial respiration, intermingling the green and brown webs. The interaction between autotrophs and bacteria shifted from mutualism to competition with carbon addition under nutrient limitation (N, P) increasing biotic CO2 emissions. Without nutrient limitation, mutualism could be reinforced by a positive feedback loop, maintaining the same biotic CO2 emissions. A small increase in dissolved organic carbon supply from climate and land use change could have large effects on stream food web and biogeochemistry with implications for the global C cycle under stoichiometric constraints.

Food quantity–quality interactions and their impact on consumer behavior and trophic transfer

Abstract: Food quantity–quality interactions determine growth rates and reproductive success of consumers and thereby regulate community dynamics and food web structure. Predator–prey models that shape our conceptual understanding of foraging ecology typically rely on the parametrization of fixed consumer responses to either food quantity or food quality. In nature, however, consumers optimize their fitness by responding simultaneously to changes in food quantity and quality. Therefore, we assessed consumer responses to changing food environments using a new fitness optimization model that accounted for food quality–quantity interactions to better capture the regulatory flexibility of consumers. Our simulations demonstrated that the impact of food quality on important consumer traits can be altered or even reversed by changes in food quality. Low food quality, for example, affected feeding rates negatively at low food concentrations but triggered surplus feeding at high food concentrations. The scope of surplus feeding was thereby mainly dependent on dynamics of nutrient digestion and in contrast to previous assumptions, energy costs of feeding played a minor role. Further, the regulation of digestive enzyme production, a crucial factor determining assimilation efficiencies, was strongly dependent on whether nonessential or essential nutrients were limiting growth. Consequently, not only the degree but also the type of nutrient limitation mediated the impact of the food environment on consumers’ fitness. At the community level, food quality was key in shaping predator–prey biomass ratios. High food qualities resulted in top‐heavy systems with larger consumer than prey biomass. Decreases of prey digestibility or the availability of essential nutrients, however, triggered a switch from inverted to classical pyramid shapes of bi‐trophic systems. The impact of food quantity on trophic transfer and emerging structural ecosystem properties thus critically hinges on behavioral and physiological responses of consumers. The inclusion of the regulatory flexibility of consumers is therefore an essential next step to improve predator–prey models and our conceptual understanding of trophic interactions.

Anthropogenic disturbance drives dispersal syndromes, demography, and gene flow in amphibian populations

Abstract: There is growing evidence that anthropogenic landscapes can strongly influence the evolution of dispersal, particularly through fragmentation, and may drive organisms into an evolutionary trap by suppressing dispersal. However, the influence on dispersal evolution of anthropogenic variation in habitat patch turnover has so far been largely overlooked. In this study, we examined how human‐driven variation in patch persistence affects dispersal rates and distances, determines dispersal‐related phenotypic specialization, and drives neutral genetic structure in spatially structured populations. We addressed this issue in an amphibian, Bombina variegata, using an integrative approach combining capture–recapture modeling, demographic simulation, common garden experiments, and population genetics. B. variegata reproduces in small ponds that occur either in habitat patches that are persistent (i.e., several decades or more), located in riverine environments with negligible human activity, or in patches that are highly temporary (i.e., a few years), created by logging operations in intensively harvested woodland. Our capture–recapture models revealed that natal and breeding dispersal rates and distances were drastically higher in spatially structured populations (SSPs) in logging environments than in riverine SSPs. Population simulations additionally showed that dispersal costs and benefits drive the fate of logging SSPs, which cannot persist without dispersal. The common garden experiments revealed that toadlets reared in laboratory conditions have morphological and behavioral specialization that depends on their habitat of origin. Toadlets from logging SSPs were found to have higher boldness and exploration propensity than those from riverine SSPs, indicating transgenerationally transmitted dispersal syndromes. We also found contrasting patterns of neutral genetic diversity and gene flow in riverine and logging SSPs, with genetic diversity and effective population size considerably higher in logging than in riverine SSPs. In parallel, intrapatch inbreeding and relatedness levels were lower in logging SSPs. Controlling for the effect of genetic drift and landscape connectivity, gene flow was found to be higher in logging than in riverine SSPs. Taken together, these results indicate that anthropogenic variation in habitat patch turnover may have an effect at least as important as landscape fragmentation on dispersal evolution and the long‐term viability and genetic structure of wild populations.

The ecology and evolution of seed predation by Darwin's finches on Tribulus cistoides on the Galápagos Islands

Abstract: Predator–prey interactions play a key role in the evolution of species traits through antagonistic coevolutionary arms races. The evolution of beak morphology in the Darwin's finches in response to competition for seed resources is a classic example of evolution by natural selection. The seeds of Tribulus cistoides are an important food source for the largest ground finch species (Geospiza fortis, G. magnirostris, and G. conirostris) in dry months, and the hard spiny morphology of the fruits is a potent agent of selection that drives contemporary evolutionary change in finch beak morphology. Although the effects of these interactions on finches are well known, how seed predation affects the ecology and evolution of the plants is poorly understood. Here we examine whether seed predation by Darwin's finches affects the ecology and evolution of T. cistoides. We ask whether the intensity of seed predation and the strength of natural selection by finches on fruit defense traits vary among populations, islands, years, or with varying finch community composition (i.e., the presence/absence of the largest beaked species, which feed on T. cistoides most easily). We then further test whether T. cistoides fruit defenses have diverged among islands in response to spatial variation in finch communities. We addressed these questions by examining seed predation by finches in 30 populations of T. cistoides over 3 yr. Our study reveals three key results. First, Darwin's finches strongly influence T. cistoides seed survival, whereby seed predation varies with differences in finch community composition among islands and in response to interannual fluctuations in precipitation. Second, finches impose phenotypic selection on T. cistoides fruit morphology, whereby smaller and harder fruits with longer or more spines exhibited higher seed survival. Variation in finch community composition and precipitation also explains variation in phenotypic selection on fruit defense traits. Third, variation in the number of spines on fruits among islands is consistent with divergent phenotypic selection imposed by variation in finch community composition among islands. These results suggest that Darwin's finches and T. cistoides are experiencing an ongoing coevolutionary arms race, and that the strength of this coevolution varies in space and time.

Removal of grazers alters the response of tundra soil carbon to warming and enhanced nitrogen availability

Abstract: The circumpolar Arctic is currently facing multiple global changes that have the potential to alter the capacity of tundra soils to store carbon. Yet, predicting changes in soil carbon is hindered ...

Spatiotemporal patterns of microbial composition and diversity in precipitation

Abstract: Microbes in the atmosphere have broad ecological impacts, including the potential to trigger precipitation through species and strains that act as ice nucleation particles. To characterize spatiotemporal trends of microbial assemblages in precipitation we sequenced 16S (bacterial) and 18S (fungal) rRNA gene amplicon libraries collected from 72 precipitation events in three U.S. states (Idaho, Louisiana, and Virginia) over four seasons. We considered these data from the perspective of a novel metacommunity framework. In agreement with our heuristic, we found evidence for distinct mechanisms underlying the composition and diversity of bacterial and fungal assemblages in precipitation. Specifically, we determined that (1) bacterial operational taxonomic unit (OTU) composition of precipitation was strongly associated with macroscale drivers including season and high‐altitude characteristics of storms; (2) fungal OTU composition was strongly correlated with mesoscale drivers including particular spatial locations; (3) β‐diversity (heterogeneity of taxa among samples) for both bacteria and fungi was largely maintained by turnover of taxa; however, (4) bacterial assemblages had higher contributions to total β‐diversity from nestedness (i.e., lower richness assemblages were largely taxonomic subsets of richer assemblages), due to losses of taxa during dispersal, particularly among potential ice nucleation active bacteria; and (5) fungal assemblages had higher contributions to total β‐diversity from turnover due to OTU replacement. Spatiotemporal trends in precipitation‐borne metacommunities allowed delineation of a large number of statistically significant indicator taxa for particular sites and seasons, including trends for bacteria that are potentially ice nucleation active. Our findings advance understanding regarding the dispersion of aerosolized microbes via wet deposition, and the development of theory concerning potential assembly rules for bioaerosol assemblages.

Testing Darwin’s naturalization conundrum based on taxonomic, phylogenetic, and functional dimensions of vascular plants

Abstract: Charles Darwin posited two alternative hypotheses to explain the success of nonnative species based on their relatedness to incumbent natives: coexistence between them should be (i) more likely with greater relatedness (due to trait similarity that correlates with better matching to the environment), or (ii) less likely (due to biotic interference, such as competition). The paradox raised by the opposing predictions of these two hypotheses has been termed 9Darwin9s naturalization conundrum9 (DNC). Using plant communities measured repeatedly over a 31-year time span across an experimental fire gradient in an oak savanna (Minnesota, USA) we evaluated the DNC by explicitly incorporating taxonomic, functional and phylogenetic information. Our approach was based on 9focal-species9 such that the taxonomic, functional and phylogenetic structure of species co-occurring with a given nonnative species in local communities was quantified. We found three main results: first, nonnatives colonizers tended to co-occur most with closely related incumbent natives in recipient communities, except in the extreme ends of the fire gradient (i.e., communities with no fire and those subjected to high fire frequencies); second, with increasing fire frequency, nonnative species were functionally more similar to native species in recipient communities; third, functional similarity of co-occurring nonnatives and natives in recipient communities showed a consistent pattern over time, but the phylogenetic similarity shifted over time, suggesting that external forces (e.g., climate variability) are also relevant in driving the phylogenetic relatedness of nonnatives to natives in invaded communities. Our results provide insights for understanding the invasion dynamics across environmental gradients and highlight the importance of evaluating different dimensions of biodiversity in order to produce more powerful evaluations of species co-occurrence at different spatial and temporal scales.

Multiple metrics of latitudinal patterns in insect pollination and herbivory for a tropical-temperate congener pair

Abstract: The biotic interactions hypothesis posits that biotic interactions are more important drivers of adaptation closer to the equator, evidenced by “stronger” contemporary interactions (e.g. greater interaction rates) and/or patterns of trait evolution consistent with a history of stronger interactions. Support for the hypothesis is mixed, but few studies span tropical and temperate regions while experimentally controlling for evolutionary history. Here, we integrate field observations and common garden experiments to quantify the relative importance of pollination and herbivory in a pair of tropical‐temperate congeneric perennial herbs. Phytolacca rivinoides and P. americana are pioneer species native to the Neotropics and the eastern USA, respectively. We compared plant‐pollinator and plant‐herbivore interactions between three tropical populations of P. rivinoides from Costa Rica and three temperate populations of P. americana from its northern range edge in Michigan and Ohio. For some metrics of interaction importance, we also included three subtropical populations of P. americana from its southern range edge in Florida. This approach confounds species and region but allows us, uniquely, to measure complementary proxies of interaction importance across a tropical‐temperate range in one system. To test the prediction that lower‐latitude plants are more reliant on insect pollinators, we quantified floral display and reward, insect visitation rates, and self‐pollination ability (autogamy). To test the prediction that lower‐latitude plants experience more herbivore pressure, we quantified herbivory rates, herbivore abundance, and leaf palatability. We found evidence supporting the biotic interactions hypothesis for most comparisons between P. rivinoides and north‐temperate P. americana (floral display, insect visitation, autogamy, herbivory, herbivore abundance, and young‐leaf palatability). Results for subtropical P. americana populations, however, were typically not intermediate between P. rivinoides and north‐temperate P. americana, as would be predicted by a linear latitudinal gradient in interaction importance. Subtropical young‐leaf palatability was intermediate, but subtropical mature leaves were the least palatable, and pollination‐related traits did not differ between temperate and subtropical regions. These nonlinear patterns of interaction importance suggest future work to relate interaction importance to climatic or biotic thresholds. In sum, we found that the biotic interactions hypothesis was more consistently supported at the larger spatial scale of our study.