Showing papers on "Ecosystem published in 2004"

Ecological linkages between aboveground and belowground biota.

Abstract: All terrestrial ecosystems consist of aboveground and belowground components that interact to influence community- and ecosystem-level processes and properties. Here we show how these components are closely interlinked at the community level, reinforced by a greater degree of specificity between plants and soil organisms than has been previously supposed. As such, aboveground and belowground communities can be powerful mutual drivers, with both positive and negative feedbacks. A combined aboveground-belowground approach to community and ecosystem ecology is enhancing our understanding of the regulation and functional significance of biodiversity and of the environmental impacts of human-induced global change phenomena.

Regime Shifts, Resilience, and Biodiversity in Ecosystem Management

Abstract: ▪ Abstract We review the evidence of regime shifts in terrestrial and aquatic environments in relation to resilience of complex adaptive ecosystems and the functional roles of biological diversity in this context. The evidence reveals that the likelihood of regime shifts may increase when humans reduce resilience by such actions as removing response diversity, removing whole functional groups of species, or removing whole trophic levels; impacting on ecosystems via emissions of waste and pollutants and climate change; and altering the magnitude, frequency, and duration of disturbance regimes. The combined and often synergistic effects of those pressures can make ecosystems more vulnerable to changes that previously could be absorbed. As a consequence, ecosystems may suddenly shift from desired to less desired states in their capacity to generate ecosystem services. Active adaptive management and governance of resilience will be required to sustain desired ecosystem states and transform degraded ecosystems...

Impact of climate change on marine pelagic phenology and trophic mismatch.

Abstract: Phenology, the study of annually recurring life cycle events such as the timing of migrations and flowering, can provide particularly sensitive indicators of climate change. Changes in phenology may be important to ecosystem function because the level of response to climate change may vary across functional groups and multiple trophic levels. The decoupling of phenological relationships will have important ramifications for trophic interactions, altering food-web structures and leading to eventual ecosystem-level changes. Temperate marine environments may be particularly vulnerable to these changes because the recruitment success of higher trophic levels is highly dependent on synchronization with pulsed planktonic production. Using long-term data of 66 plankton taxa during the period from 1958 to 2002, we investigated whether climate warming signals are emergent across all trophic levels and functional groups within an ecological community. Here we show that not only is the marine pelagic community responding to climate changes, but also that the level of response differs throughout the community and the seasonal cycle, leading to a mismatch between trophic levels and functional groups.

Plant functional markers capture ecosystem properties during secondary succession

Abstract: Although the structure and composition of plant communities is known to influence the functioning of ecosystems, there is as yet no agreement as to how these should be described from a functional perspective. We tested the biomass ratio hypothesis, which postulates that ecosystem properties should depend on species traits and on species contribution to the total biomass of the community, in a successional sere following vineyard abandonment in the Mediterranean region of France. Ecosystem-specific net primary productivity, litter decomposition rate, and total soil carbon and nitrogen varied significantly with field age, and correlated with community-aggregated (i.e., weighed according to the relative abundance of species) functional leaf traits. The three easily measurable traits tested, specific leaf area, leaf dry matter content, and nitrogen concentration, provide a simple means to scale up from organ to ecosystem functioning in complex plant communities. We propose that they be called ''functional markers,'' and be used to assess the impacts of community changes on ecosystem properties induced, in particular, by global change drivers.

The Evolution of Modern Eukaryotic Phytoplankton

Abstract: The community structure and ecological function of contemporary marine ecosystems are critically dependent on eukaryotic phytoplankton. Although numerically inferior to cyanobacteria, these organisms are responsible for the majority of the flux of organic matter to higher trophic levels and the ocean interior. Photosynthetic eukaryotes evolved more than 1.5 billion years ago in the Proterozoic oceans. However, it was not until the Mesozoic Era (251 to 65 million years ago) that the three principal phytoplankton clades that would come to dominate the modern seas rose to ecological prominence. In contrast to their pioneering predecessors, the dinoflagellates, coccolithophores, and diatoms all contain plastids derived from an ancestral red alga by secondary symbiosis. Here we examine the geological, geochemical, and biological processes that contributed to the rise of these three, distantly related, phytoplankton groups.

Water Pulses and Biogeochemical Cycles in Arid and Semiarid Ecosystems

Abstract: The episodic nature of water availability in arid and semiarid ecosystems has significant consequences on belowground carbon and nutrient cycling. Pulsed water events directly control belowground processes through soil wet-dry cycles. Rapid soil microbial response to incident moisture availability often results in almost instantaneous C and N mineralization, followed by shifts in C/N of microbially available substrate, and an offset in the balance between nutrient immobilization and mineralization. Nitrogen inputs from biological soil crusts are also highly sensitive to pulsed rain events, and nitrogen losses, particularly gaseous losses due to denitrification and nitrate leaching, are tightly linked to pulses of water availability. The magnitude of the effect of water pulses on carbon and nutrient pools, however, depends on the distribution of resource availability and soil organisms, both of which are strongly affected by the spatial and temporal heterogeneity of vegetation cover, topographic position and soil texture. The 'inverse texture hypothesis' for net primary production in water-limited ecosystems suggests that coarse-textured soils have higher NPP than fine-textured soils in very arid zones due to reduced evaporative losses, while NPP is greater in fine-textured soils in higher rainfall ecosystems due to increased water-holding capacity. With respect to belowground processes, fine-textured soils tend to have higher water-holding capacity and labile C and N pools than coarse-textured soils, and often show a much greater flush of N mineralization. The result of the interaction of texture and pulsed rainfall events suggests a corollary hypothesis for nutrient turnover in arid and semiarid ecosystems with a linear increase of N mineralization in coarse-textured soils, but a saturating response for fine-textured soils due to the importance of soil C and N pools. Seasonal distribution of water pulses can lead to the accumulation of mineral N in the dry season, decoupling resource supply and microbial and plant demand, and resulting in increased losses via other pathways and reduction in overall soil nutrient pools. The asynchrony of resource availability, particularly nitrogen versus water due to pulsed water events, may be central to understanding the consequences for ecosystem nutrient retention and long-term effects on carbon and nutrient pools. Finally, global change effects due to changes in the nature and size of pulsed water events and increased asynchrony of water availability and growing season will likely have impacts on biogeochemical cycling in water-limited ecosystems.

Progressive Nitrogen Limitation of Ecosystem Responses to Rising Atmospheric Carbon Dioxide

Abstract: A highly controversial issue in global biogeochemistry is the regulation of terrestrial carbon (C) sequestration by soil nitrogen (N) availability. This controversy translates into great uncertainty in predicting future global terrestrial C sequestration. We propose a new framework that centers on the concept of progressive N limitation (PNL) for studying the interactions between C and N in terrestrial ecosystems. In PNL, available soil N becomes increasingly limiting as C and N are sequestered in long-lived plant biomass and soil organic matter. Our analysis focuses on the role of PNL in regulating ecosystem responses to rising atmospheric carbon dioxide concentration, but the concept applies to any perturbation that initially causes C and N to accumulate in organic forms. This article examines conditions under which PNL may or may not constrain net primary production and C sequestration in terrestrial ecosystems. While the PNL-centered framework has the potential to explain diverse experimental...

Detritus, trophic dynamics and biodiversity

Abstract: Traditional approaches to the study of food webs emphasize the transfer of local primary productivity in the form of living plant organic matter across trophic levels. However, dead organic matter, or detritus, a common feature of most ecosystems plays a frequently overlooked role as a dynamic heterogeneous resource and habitat for many species. We develop an integrative framework for understanding the impact of detritus that emphasizes the ontogeny and heterogeneity of detritus and the various ways that explicit inclusion of detrital dynamics alters generalizations about the structure and functioning of food webs. Through its influences on food web composition and dynamics, detritus often increases system stability and persistence, having substantial effects on trophic structure and biodiversity. Inclusion of detrital heterogeneity in models of food web dynamics is an essential new direction for ecological research.

Endangered Ecosystems of the United States: A Preliminary Assessment of Loss and Degradation

Abstract: We report estimates of declines of natural ecosystems in the United States, provide a rationale for ecosystem-level conservation, discuss decline and threat as criteria for conservation, and relate ecosystem losses to endangerment at species and population levels. Ecosystems are defined generally and at various spatial scales and include vegetation types, plant associations, natural communities, and habitats defined by floristics, structure, age, geography, condition, and other ecologically relevant factors. The methodology for this report consisted of a literature review and a survey of conservation agencies and professionals. The results of this preliminary study indicated significant losses of biodiversity at the ecosystem level in the United States. The most substantial losses were summarized by listing ecosystems as critically endangered (>98% decline), endangered (85-98% decline), and threatened (70-84% decline). We identified more than 30 critically endangered, 58 endangered, and more than 38 threatened ecosystems. Losses of all kinds of ecosystems have been most pronounced in the South, Northeast, and Midwest, and in California. We suggest that integrated conservation plans for all ecosystems be developed in each ecoregion of the United States, starting with types and regions that sustained the greatest losses and are at greatest risk of further loss. Conservation plans could be based on detailed studies of ecosystem status and trends and include quantitative analyses of ecosystem decline, ecological consequences of loss and degradation, and current and potential threats to each ecosystem. Ecosystem conservation need not be restricted to pristine sites, which are now almost nonexistent. Rather, management and, where possible, restoration plans for native biodiversity in partially disturbed sites should be considered.

Ecosystem stability and compensatory effects in the Inner Mongolia grassland

Abstract: Numerous studies have suggested that biodiversity reduces variability in ecosystem productivity through compensatory effects; that is, a species increases in its abundance in response to the reduction of another in a fluctuating environment. But this view has been challenged on several grounds. Because most studies have been based on artificially constructed grasslands with short duration, long-term studies of natural ecosystems are needed. On the basis of a 24-year study of the Inner Mongolia grassland, here we present three key findings. First, that January-July precipitation is the primary climatic factor causing fluctuations in community biomass production; second, that ecosystem stability (conversely related to variability in community biomass production) increases progressively along the hierarchy of organizational levels (that is, from species to functional group to whole community); and finally, that the community-level stability seems to arise from compensatory interactions among major components at both species and functional group levels. From a hierarchical perspective, our results corroborate some previous findings of compensatory effects. Undisturbed mature steppe ecosystems seem to culminate with high biodiversity, productivity and ecosystem stability concurrently. Because these relationships are correlational, further studies are necessary to verify the causation among these factors. Our study provides new insights for better management and restoration of the rapidly degrading Inner Mongolia grassland.

Grazing systems, ecosystem responses, and global change

Abstract: ▪ Abstract Managed grazing covers more than 25% of the global land surface and has a larger geographic extent than any other form of land use. Grazing systems persist under marginal bioclimatic and edaphic conditions of different biomes, leading to the emergence of three regional syndromes inherent to global grazing: desertification, woody encroachment, and deforestation. These syndromes have widespread but differential effects on the structure, biogeochemistry, hydrology, and biosphere-atmosphere exchange of grazed ecosystems. In combination, these three syndromes represent a major component of global environmental change.

Ecosystem carbon storage in arctic tundra reduced by long-term nutrient fertilization

Abstract: Global warming is predicted to be most pronounced at high latitudes, and observational evidence over the past 25 years suggests that this warming is already under way. One-third of the global soil carbon pool is stored in northern latitudes, so there is considerable interest in understanding how the carbon balance of northern ecosystems will respond to climate warming. Observations of controls over plant productivity in tundra and boreal ecosystems have been used to build a conceptual model of response to warming, where warmer soils and increased decomposition of plant litter increase nutrient availability, which, in turn, stimulates plant production and increases ecosystem carbon storage. Here we present the results of a long-term fertilization experiment in Alaskan tundra, in which increased nutrient availability caused a net ecosystem loss of almost 2,000 grams of carbon per square meter over 20 years. We found that annual aboveground plant production doubled during the experiment. Losses of carbon and nitrogen from deep soil layers, however, were substantial and more than offset the increased carbon and nitrogen storage in plant biomass and litter. Our study suggests that projected release of soil nutrients associated with high-latitude warming may further amplify carbon release from soils, causing a net loss of ecosystem carbon and a positive feedback to climate warming.

Scaling of c:n:p stoichiometry in forests worldwide: implications of terrestrial redfield‐type ratios

Abstract: Inspired by the importance of globally well-constrained carbon:nitrogen: phosphorus (C:N:P) ratios in planktonic biomass to the understanding of nutrient cycles and biotic feedbacks in marine ecosystems, we looked for analogous patterns in forest ecosystems worldwide. We used data from the literature to examine the stoichiometry of C, N, and P in forest foliage and litter on both global and biome levels. Additionally, we examined the scaling of nutrient investments with biomass and production both globally and within biomes to determine if and when these ratios respond to macroscale ecosystem properties (such as nutrient availability). We found that, while global forest C:N:P ratios in both foliage and litter were more variable than those of marine particulate matter, biome level (temperate broadleaf, temperate coniferous, and tropical) ratios were as constrained as marine ratios and statistically distinct from one another. While we were more interested in the relative constancy of the C:N:P ratios than their numerical value we did note, as have others, that the atomic ratios calculated for foliage (1212:28:1) and litter (3007:45:1) reflect the increased proportion of C-rich structural material characteristic of terrestrial vegetation. Carbon : nutrient ratios in litter were consistently higher than in comparable foliar data sets, suggesting that resorption of nutrients is a globally important mechanism, particularly for P. Litter C:N ratios were globally constant despite biome-level differences in foliar C:N; we speculate that this strong coupling may be caused by the significant contribution of immobile cell wall bound proteins to the total foliar N pool. Most ratios scaled isometrically across the range of biomass stocks and production in all biomes sug- gesting that ratios arise directly from physiological constraints and are insensitive to factors leading to shifts in biomass and production. There were, however, important exceptions to this pattern: nutrient investment in broadleaf forest litter and coniferous forest foliage increased disproportionately relative to C with increasing biomass and production sug- gesting a systematic influence of macroscopic factors on ratios.

Ecosystem Properties and Forest Decline in Contrasting Long-Term Chronosequences

Abstract: During succession, ecosystem development occurs; but in the long-term absence of catastrophic disturbance, a decline phase eventually follows. We studied six long-term chronosequences, in Australia, Sweden, Alaska, Hawaii, and New Zealand; for each, the decline phase was associated with a reduction in tree basal area and an increase in the substrate nitrogen-to-phosphorus ratio, indicating increasing phosphorus limitation over time. These changes were often associated with reductions in litter decomposition rates, phosphorus release from litter, and biomass and activity of decomposer microbes. Our findings suggest that the maximal biomass phase reached during succession cannot be maintained in the long-term absence of major disturbance, and that similar patterns of decline occur in forested ecosystems spanning the tropical, temperate, and boreal zones.

Land-use choices: balancing human needs and ecosystem function

Abstract: Conversion of land to grow crops, raise animals, obtain timber, and build cities is one of the foundations of human civilization. While land use provides these essential ecosystem goods, it alters a range of other ecosystem functions, such as the provisioning of freshwater, regulation of climate and biogeochemical cycles, and maintenance of soil fertility. It also alters habitat for biological diversity. Balancing the inherent trade-offs between satisfying immediate human needs and maintaining other ecosystem functions requires quantitative knowledge about ecosystem responses to land use. These responses vary according to the type of land-use change and the ecological setting, and have local, short-term as well as global, longterm effects. Land-use decisions ultimately weigh the need to satisfy human demands and the unintended ecosystem responses based on societal values, but ecological knowledge can provide a basis for assessing the trade-offs.

Climate change uncouples trophic interactions in an aquatic ecosystem

Abstract: The largest uncertainty in forecasting the effects of climate change on eco- systems is in understanding how it will affect the nature of interactions among species. Climate change may have unexpected consequences because different species show unique responses to changes in environmental temperatures. Here we show that increasingly warmer springs since 1962 have disrupted the trophic linkages between phytoplankton and zoo- plankton in a large temperate lake because of differing sensitivity to vernal warming. The timing of thermal stratification and the spring diatom bloom have advanced by more than 20 days during this time period. A long-term decline in Daphnia populations, the keystone herbivore, is associated with an expanding temporal mismatch with the spring diatom bloom and may have severe consequences for resource flow to upper trophic levels.

Hierarchy of responses to resource pulses in arid and semi-arid ecosystems.

Abstract: In arid/semi-arid ecosystems, biological resources, such as water, soil nutrients, and plant biomass, typically go through periods of high and low abundance. Short periods of high resource abundance are usually triggered by rainfall events, which, despite of the overall scarcity of rain, can saturate the resource demand of some biological processes for a time. This review develops the idea that there exists a hierarchy of soil moisture pulse events with a corresponding hierarchy of ecological responses, such that small pulses only trigger a small number of relatively minor ecological events, and larger pulses trigger a more inclusive set and some larger ecological events. This framework hinges on the observation that many biological state changes, where organisms transition from a state of lower to higher physiological activity, require a minimal triggering event size. Response thresholds are often determined by the ability of organisms to utilize soil moisture pulses of different infiltration depth or duration. For example, brief, shallow pulses can only affect surface dwelling organisms with fast response times and high tolerance for low resource levels, such as some species of the soil micro-fauna and -flora, while it takes more water and deeper infiltration to affect the physiology, growth or reproduction of higher plants. This review first discusses how precipitation, climate and site factors translate into soil moisture pulses of varying magnitude and duration. Next, the idea of the response hierarchy for ecosystem processes is developed, followed by an exploration of the possible evolutionary background for the existence of response thresholds to resource pulses. The review concludes with an outlook on global change: does the hierarchical view of precipitation effects in ecosystems provide new perspectives on the future of arid/semiarid lands?

Ecosystem consequences of bird declines

Abstract: We present a general framework for characterizing the ecological and societal consequences of biodiversity loss and applying it to the global avifauna. To investigate the potential ecological consequences of avian declines, we developed comprehensive databases of the status and functional roles of birds and a stochastic model for forecasting change. Overall, 21% of bird species are currently extinction-prone and 6.5% are functionally extinct, contributing negligibly to ecosystem processes. We show that a quarter or more of frugivorous and omnivorous species and one-third or more of herbivorous, piscivorous, and scavenger species are extinction-prone. Furthermore, our projections indicate that by 2100, 6–14% of all bird species will be extinct, and 7–25% (28–56% on oceanic islands) will be functionally extinct. Important ecosystem processes, particularly decomposition, pollination, and seed dispersal, will likely decline as a result.

Climate Impact on Plankton Ecosystems in the Northeast Atlantic

Abstract: It is now widely accepted that global warming is occurring, yet its effects on the world's largest ecosystem, the marine pelagic realm, are largely unknown. We show that sea surface warming in the Northeast Atlantic is accompanied by increasing phytoplankton abundance in cooler regions and decreasing phytoplankton abundance in warmer regions. This impact propagates up the food web (bottom-up control) through copepod herbivores to zooplankton carnivores because of tight trophic coupling. Future warming is therefore likely to alter the spatial distribution of primary and secondary pelagic production, affecting ecosystem services and placing additional stress on already-depleted fish and mammal populations.

Genetic diversity enhances the resistance of a seagrass ecosystem to disturbance

Abstract: Motivated by recent global reductions in biodiversity, empirical and theoretical research suggests that more species-rich systems exhibit enhanced productivity, nutrient cycling, or resistance to disturbance or invasion relative to systems with fewer species. In contrast, few data are available to assess the potential ecosystem-level importance of genetic diversity within species known to play a major functional role. Using a manipulative field experiment, we show that increasing genotypic diversity in a habitat-forming species (the seagrass Zostera marina) enhances community resistance to disturbance by grazing geese. The time required for recovery to near predisturbance densities also decreases with increasing eelgrass genotypic diversity. However, there is no effect of diversity on resilience, measured as the rate of shoot recovery after the disturbance, suggesting that more rapid recovery in diverse plots is due solely to differences in disturbance resistance. Genotypic diversity did not affect ecosystem processes in the absence of disturbance. Thus, our results suggest that genetic diversity, like species diversity, may be most important for enhancing the consistency and reliability of ecosystems by providing biological insurance against environmental change.

Soil water balance and ecosystem response to climate change.

Abstract: Some essential features of the terrestrial hydrologic cycle and ecosystem response are singled out by confronting empirical observations of the soil water balance of different ecosystems with the results of a stochastic model of soil moisture dynamics. The simplified framework analytically describes how hydroclimatic variability (especially the frequency and amount of rainfall events) concurs with soil and plant characteristics in producing the soil moisture dynamics that in turn impact vegetation conditions. The results of the model extend and help interpret the classical curve of Budyko, which relates evapotranspiration losses to a dryness index, describing the partitioning of precipitation into evapotranspiration, runoff, and deep infiltration. They also provide a general classification of soil water balance of the world ecosystems based on two governing dimensionless groups summarizing the climate, soil, and vegetation conditions. The subsequent analysis of the links among soil moisture dyna...

Riparian deforestation, stream narrowing, and loss of stream ecosystem services

Abstract: A study of 16 streams in eastern North America shows that riparian deforestation causes channel narrowing, which reduces the total amount of stream habitat and ecosystem per unit channel length and compromises in-stream processing of pollutants. Wide forest reaches had more macroinvertebrates, total ecosystem processing of organic matter, and nitrogen uptake per unit channel length than contiguous narrow deforested reaches. Stream narrowing nullified any potential advantages of deforestation regarding abundance of fish, quality of dissolved organic matter, and pesticide degradation. These findings show that forested stream channels have a wider and more natural configuration, which significantly affects the total in-stream amount and activity of the ecosystem, including the processing of pollutants. The results reinforce both current policy of the United States that endorses riparian forest buffers as best management practice and federal and state programs that subsidize riparian reforestation for stream restoration and water quality. Not only do forest buffers prevent nonpoint source pollutants from entering small streams, they also enhance the in-stream processing of both nonpoint and point source pollutants, thereby reducing their impact on downstream rivers and estuaries.

Whole-lake carbon-13 additions reveal terrestrial support of aquatic food webs

Abstract: Ecosystems are supported by organic carbon from two distinct sources. Endogenous carbon is produced by photosynthesis within an ecosystem by autotrophic organisms. Exogenous carbon is produced elsewhere and transported into ecosystems. Consumers may use exogenous carbon with consequent influences on population dynamics, predator-prey relationships and ecosystem processes(1). For example, exogenous inputs provide resources that may enhance consumer abundance beyond levels supported by within-system primary production(2). Exogenous fluxes of organic carbon to ecosystems are often large, but this material is recalcitrant and difficult to assimilate, in contrast to endogenously produced organic matter, which is used more easily(3,4). Here we show, by the experimental manipulation of dissolved inorganic C-13 in two lakes, that internal primary production is insufficient to support the food webs of these ecosystems. Additions of NaH (CO3)-C-13 enriched the C-13 content of dissolved inorganic carbon, particulate organic carbon, zooplankton and fish. Dynamics of C-13 indicate that 40-55% of particulate organic carbon and 22-50% of zooplankton carbon are derived from terrestrial sources, showing that there is significant subsidy of these ecosystems by organic carbon produced outside their boundaries.

How soil moisture, rain pulses, and growth alter the response of ecosystem respiration to temperature

Abstract: [1] In this paper, we analyzed 3 years of carbon flux data from continuous eddy covariance measurements to investigate how soil moisture, rain pulses, and growth alter the response of ecosystem respiration to temperature. The data were acquired over an annual grassland and from the grass understory of an oak/grass savanna ecosystem in California. We observed that ecosystem respiration was an exponential function of soil temperature during the winter wet season and a jump in ecosystem respiration occurred, at comparable temperatures, during the spring growth period. The depletion of the moisture from the soil reservoir, during spring, limited ecosystem respiration after its volumetric water content dropped below a threshold of 0.15 m 3 m � 3 . The senescence of grass during the summer switched the source of ecosystem respiration to heterotrophic bacteria and fungi. During the summer, respiration proceeded at a low basal rate (about 0.10 to 0.3 g C m � 2 d � 1 ), except when summer rain events stimulated large dynamic pulses in heterotrophic respiration. Peak respiratory pulses were on the order of 60–80 times baseline and could not be explained by functions that depend on mean soil moisture and temperature. We found that the magnitude of the respiratory pulses was inversely related to its prerain value and that the time constant, describing the exponential decay of the respiratory pulses after the rain event, was a function of the amount of rainfall. The amount of carbon lost, in association with a few summer rain events, was greater at the site with higher primary productivity and soil carbon content. INDEX TERMS: 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 1615 Global Change: Biogeochemical processes (4805); 1866 Hydrology: Soil moisture; 3322 Meteorology and Atmospheric Dynamics: Land/atmosphere interactions; KEYWORDS: respiration, carbon balance, savanna, biosphere-atmosphere interactions, eddy covariance Citation: Xu, L., D. D. Baldocchi, and J. Tang (2004), How soil moisture, rain pulses, and growth alter the response of ecosystem respiration to temperature, Global Biogeochem. Cycles, 18, GB4002, doi:10.1029/2004GB002281.

Arbuscular mycorrhizae and terrestrial ecosystem processes

Abstract: Arbuscular mycorrhizal fungi (AMF; phylum Glomeromycota) are ubiquitous in terrestrial ecosystems. Despite their acknowledged importance in ecology, most research on AMF has focused on effects on individual plant hosts, with more recent efforts aimed at the level of the plant community. Research at the ecosystem level is less prominent, but potentially very promising. Numerous human-induced disturbances (including global change and agro-ecosystem management) impinge on AMF functioning; hence study of this symbiosis from the ecosystem perspective seems timely and crucial. In this paper, I discuss four (interacting) routes via which AMF can influence ecosystem processes. These include indirect pathways (through changes in plant and soil microbial community composition), and direct pathways (effects on host physiology and resource capture, and direct mycelium effects). I use the case study of carbon cycling to illustrate the potentially pervasive influence of AMF on ecosystem processes. A limited amount of published research on AMF ecology is suited for direct integration into ecosystem studies (because of scale mismatch or ill-adaptation to the ‘pools and flux’ paradigm of ecosystem ecology); I finish with an assessment of the tools (experimental designs, response variables) available for studying mycorrhizae at the ecosystem scale.

Bioturbators enhance ecosystem function through complex biogeochemical interactions

Abstract: Predicting the consequences of species loss is critically important, given present threats to biological diversity such as habitat destruction, overharvesting and climate change. Several empirical studies have reported decreased ecosystem performance (for example, primary productivity) coincident with decreased biodiversity, although the relative influence of biotic effects and confounding abiotic factors has been vigorously debated. Whereas several investigations focused on single trophic levels (for example, grassland plants), studies of whole systems have revealed multiple layers of feedbacks, hidden drivers and emergent properties, making the consequences of species loss more difficult to predict. Here we report functionally important organisms and considerable biocomplexity in a sedimentary seafloor habitat, one of Earth's most widespread ecosystems. Experimental field measurements demonstrate how the abundance of spatangoid urchins--infaunal (in seafloor sediment) grazers/deposit feeders--is positively related to primary production, as their activities change nutrient fluxes and improve conditions for production by microphytobenthos (sedimentatry microbes and unicellular algae). Declines of spatangoid urchins after trawling are well documented, and our research linking these bioturbators to important benthic-pelagic fluxes highlights potential ramifications for productivity in coastal oceans.

Fish invasion restructures stream and forest food webs by interrupting reciprocal prey subsidies

Abstract: Habitat alteration and biotic invasions are the two leading causes of global environmental change and biodiversity loss. Recent innovative experiments have shown that habitat disturbance can have drastic effects that cascade to adjacent ecosystems by altering the flow of resource subsidies from donor systems. Likewise, exotic species in- vasions could alter subsidies and affect distant food webs, but very few studies have tested this experimentally. Here we report evidence from a large-scale field experiment in northern Japan that invasion of nonnative rainbow trout (Oncorhynchus mykiss) interrupted reciprocal flows of invertebrate prey that drove stream and adjacent riparian forest food webs. Rainbow trout usurped terrestrial prey that fell into the stream, causing native Dolly Varden charr (Salvelinus malma) to shift their foraging to insects that graze algae from the stream bottom. This indirectly increased algal biomass, but also decreased biomass of adult aquatic insects emerging from the stream to the forest. In turn, this led to a 65% reduction in the density of riparian-specialist spiders in the forest. Thus, species invasions can interrupt flows of resources between interconnected ecosystems and have effects that propagate across their boundaries, effects that may be difficult to anticipate without in-depth understanding of food web relationships.

Carbon sequestration in temperate grassland ecosystems and the influence of management, climate and elevated CO2

Abstract: Contents I. Introduction 2 II. Carbon in temperate grasslands 2 III. The process of carbon sequestration in soils 4 IV. Tracking carbon movement 9 V. Models of soil carbon dynamics 10 VI. Management effects on carbon sequestration 11 VII. Climate-change effects on carbon sequestration 12 VIII. Response to elevated CO2 13 IX. Conclusions 14 References 14 Summary The substantial stocks of carbon sequestered in temperate grassland ecosystems are located largely below ground in roots and soil. Organic C in the soil is located in discrete pools, but the characteristics of these pools are still uncertain. Carbon sequestration can be determined directly by measuring changes in C pools, indirectly by using 13C as a tracer, or by simulation modelling. All these methods have their limitations, but long-term estimates rely almost exclusively on modelling. Measured and modelled rates of C sequestration range from 0 to > 8 Mg C ha−1 yr−1. Management practices, climate and elevated CO2 strongly influence C sequestration rates and their influence on future C stocks in grassland soils is considered. Currently there is significant potential to increase C sequestration in temperate grassland systems by changes in management, but climate change and increasing CO2 concentrations in future will also have significant impacts. Global warming may negate any storage stimulated by changed management and elevated CO2, although there is increasing evidence that the reverse could be the case.