Showing papers on "Ecosystem published in 2001"
TL;DR: Larger numbers of species are probably needed to reduce temporal variability in ecosystem processes in changing environments and to determine how biodiversity dynamics, ecosystem processes, and abiotic factors interact.
Abstract: The ecological consequences of biodiversity loss have aroused considerable interest and controversy during the past decade. Major advances have been made in describing the relationship between species diversity and ecosystem processes, in identifying functionally important species, and in revealing underlying mechanisms. There is, however, uncertainty as to how results obtained in recent experiments scale up to landscape and regional levels and generalize across ecosystem types and processes. Larger numbers of species are probably needed to reduce temporal variability in ecosystem processes in changing environments. A major future challenge is to determine how biodiversity dynamics, ecosystem processes, and abiotic factors interact.
4,070 citations
TL;DR: For example, a recent review of the early phase of the coastal eutrophication problem can be found in this article, where the authors suggest that the early (phase I) con- ceptual model was strongly influenced by limnologists, who began intense study of lake eutrophicication by the 1960s.
Abstract: A primary focus of coastal science during the past 3 decades has been the question: How does anthropogenic nutrient enrichment cause change in the structure or function of nearshore coastal ecosystems? This theme of environmental science is recent, so our conceptual model of the coastal eutrophication problem continues to change rapidly In this review, I suggest that the early (Phase I) con- ceptual model was strongly influenced by limnologists, who began intense study of lake eutrophication by the 1960s The Phase I model emphasized changing nutrient input as a signal, and responses to that signal as increased phytoplankton biomass and primary production, decomposition of phytoplankton- derived organic matter, and enhanced depletion of oxygen from bottom waters Coastal research in recent decades has identified key differences in the responses of lakes and coastal-estuarine ecosystems to nutrient enrichment The contemporary (Phase II) conceptual model reflects those differences and includes explicit recognition of (1) system-specific attributes that act as a filter to modulate the responses to enrichment (leading to large differences among estuarine-coastal systems in their sensitivity to nu- trient enrichment); and (2) a complex suite of direct and indirect responses including linked changes in: water transparency, distribution of vascular plants and biomass of macroalgae, sediment biogeochem- istry and nutrient cycling, nutrient ratios and their regulation of phytoplankton community composition, frequency of toxic/harmful algal blooms, habitat quality for metazoans, reproduction/growth/survival of pelagic and benthic invertebrates, and subtle changes such as shifts in the seasonality of ecosystem functions Each aspect of the Phase II model is illustrated here with examples from coastal ecosystems around the world In the last section of this review I present one vision of the next (Phase III) stage in the evolution of our conceptual model, organized around 5 questions that will guide coastal science in the early 21st century: (1) How do system-specific attributes constrain or amplify the responses of coastal ecosystems to nutrient enrichment? (2) How does nutrient enrichment interact with other stressors (toxic contaminants, fishing harvest, aquaculture, nonindigenous species, habitat loss, climate change, hydro- logic manipulations) to change coastal ecosystems? (3) How are responses to multiple stressors linked? (4) How does human-induced change in the coastal zone impact the Earth system as habitat for humanity and other species? (5) How can a deeper scientific understanding of the coastal eutrophication problem be applied to develop tools for building strategies at ecosystem restoration or rehabilitation?
2,658 citations
TL;DR: These results help resolve debate over biodiversity and ecosystem functioning, show effects at higher than expected diversity levels, and demonstrate, for these ecosystems, that even the best-chosen monocultures cannot achieve greater productivity or carbon stores than higher-diversity sites.
Abstract: Plant diversity and niche complementarity had progressively stronger effects on ecosystem functioning during a 7-year experiment, with 16-species plots attaining 2.7 times greater biomass than monocultures. Diversity effects were neither transients nor explained solely by a few productive or unviable species. Rather, many higher-diversity plots outperformed the best monoculture. These results help resolve debate over biodiversity and ecosystem functioning, show effects at higher than expected diversity levels, and demonstrate, for these ecosystems, that even the best-chosen monocultures cannot achieve greater productivity or carbon stores than higher-diversity sites.
2,091 citations
TL;DR: Meta-analysis is used to synthesize data on the response of soil respiration, net N mineralization, and aboveground plant productivity to experimental ecosystem warming at 32 research sites representing four broadly defined biomes, including high (latitude or altitude) tundra, low tundara, grassland, and forest.
Abstract: Climate change due to greenhouse gas emissions is predicted to raise the mean global temperature by 1.0–3.5°C in the next 50–100 years. The direct and indirect effects of this potential increase in temperature on terrestrial ecosystems and ecosystem processes are likely to be complex and highly varied in time and space. The Global Change and Terrestrial Ecosystems core project of the International Geosphere-Biosphere Programme has recently launched a Network of Ecosystem Warming Studies, the goals of which are to integrate and foster research on ecosystem-level effects of rising temperature. In this paper, we use meta-analysis to synthesize data on the response of soil respiration, net N mineralization, and aboveground plant productivity to experimental ecosystem warming at 32 research sites representing four broadly defined biomes, including high (latitude or altitude) tundra, low tundra, grassland, and forest. Warming methods included electrical heat-resistance ground cables, greenhouses, vented and unvented field chambers, overhead infrared lamps, and passive night-time warming. Although results from individual sites showed considerable variation in response to warming, results from the meta-analysis showed that, across all sites and years, 2–9 years of experimental warming in the range 0.3–6.0°C significantly increased soil respiration rates by 20% (with a 95% confidence interval of 18–22%), net N mineralization rates by 46% (with a 95% confidence interval of 30–64%), and plant productivity by 19% (with a 95% confidence interval of 15–23%). The response of soil respiration to warming was generally larger in forested ecosystems compared to low tundra and grassland ecosystems, and the response of plant productivity was generally larger in low tundra ecosystems than in forest and grassland ecosystems. With the exception of aboveground plant productivity, which showed a greater positive response to warming in colder ecosystems, the magnitude of the response of these three processes to experimental warming was not generally significantly related to the geographic, climatic, or environmental variables evaluated in this analysis. This underscores the need to understand the relative importance of specific factors (such as temperature, moisture, site quality, vegetation type, successional status, land-use history, etc.) at different spatial and temporal scales, and suggests that we should be cautious in "scaling up" responses from the plot and site level to the landscape and biome level. Overall, ecosystem-warming experiments are shown to provide valuable insights on the response of terrestrial ecosystems to elevated temperature.
1,988 citations
TL;DR: In this paper, the possible responses of ecosystem processes to rising atmospheric CO2 concentration and climate change are illustrated using six dynamic global vegetation models that explicitly represent the interactions of ecosystem carbon and water exchanges with vegetation dynamics.
Abstract: The possible responses of ecosystem processes to rising atmospheric CO2 concentration and climate change are illustrated using six dynamic global vegetation models that explicitly represent the interactions of ecosystem carbon and water exchanges with vegetation dynamics. The models are driven by the IPCC IS92a scenario of rising CO2 (Wigley et al. 1991), and by climate changes resulting from effective CO2 concentrations corresponding to IS92a, simulated by the coupled ocean atmosphere model HadCM2-SUL. Simulations with changing CO2 alone show a widely distributed terrestrial carbon sink of 1.4‐3.8 Pg C y ‐1 during the 1990s, rising to 3.7‐8.6 Pg C y ‐1 a century later. Simulations including climate change show a reduced sink both today (0.6‐ 3.0 Pg C y ‐1 ) and a century later (0.3‐6.6 Pg C y ‐1 ) as a result of the impacts of climate change on NEP of tropical and southern hemisphere ecosystems. In all models, the rate of increase of NEP begins to level off around 2030 as a consequence of the ‘diminishing return’ of physiological CO2 effects at high CO2 concentrations. Four out of the six models show a further, climate-induced decline in NEP resulting from increased heterotrophic respiration and declining tropical NPP after 2050. Changes in vegetation structure influence the magnitude and spatial pattern of the carbon sink and, in combination with changing climate, also freshwater availability (runoff). It is shown that these changes, once set in motion, would continue to evolve for at least a century even if atmospheric CO2 concentration and climate could be instantaneously stabilized. The results should be considered illustrative in the sense that the choice of CO2 concentration scenario was arbitrary and only one climate model scenario was used. However, the results serve to indicate a range of possible biospheric responses to CO2 and climate change. They reveal major uncertainties about the response of NEP to climate
1,982 citations
TL;DR: Seasonal contrasts between allochthonous prey supply and in situ prey biomass determine the importance of reciprocal subsidies in mutual trophic interactions between contiguous habitats.
Abstract: Mutual trophic interactions between contiguous habitats have remained poorly understood despite their potential significance for community maintenance in ecological landscapes. In a deciduous forest and stream ecotone, aquatic insect emergence peaked around spring, when terrestrial invertebrate biomass was low. In contrast, terrestrial invertebrate input to the stream occurred primarily during summer, when aquatic invertebrate biomass was nearly at its lowest. Such reciprocal, across-habitat prey flux alternately subsidized both forest birds and stream fishes, accounting for 25.6% and 44.0% of the annual total energy budget of the bird and fish assemblages, respectively. Seasonal contrasts between allochthonous prey supply and in situ prey biomass determine the importance of reciprocal subsidies.
1,134 citations
920 citations
TL;DR: The data reinforce the hypothesis that terrestrial species richness from the equator to the poles is ultimately governed by a synergism between climate and coarse-scale topographic heterogeneity.
Abstract: The search for a common cause of species richness gradients has spawned more than 100 explanatory hypotheses in just the past two decades. Despite recent conceptual advances, further refinement of the most plausible models has been stifled by the difficulty of compiling high-resolution databases at continental scales. We used a database of the geographic ranges of 2,869 species of birds breeding in South America (nearly a third of the world's living avian species) to explore the influence of climate, quadrat area, ecosystem diversity, and topography on species richness gradients at 10 spatial scales (quadrat area, ≈12,300 to ≈1,225,000 km2). Topography, precipitation, topography × latitude, ecosystem diversity, and cloud cover emerged as the most important predictors of regional variability of species richness in regression models incorporating 16 independent variables, although ranking of variables depended on spatial scale. Direct measures of ambient energy such as mean and maximum temperature were of ancillary importance. Species richness values for 1° × 1° latitude-longitude quadrats in the Andes (peaking at 845 species) were ≈30–250% greater than those recorded at equivalent latitudes in the central Amazon basin. These findings reflect the extraordinary abundance of species associated with humid montane regions at equatorial latitudes and the importance of orography in avian speciation. In a broader context, our data reinforce the hypothesis that terrestrial species richness from the equator to the poles is ultimately governed by a synergism between climate and coarse-scale topographic heterogeneity.
805 citations
TL;DR: This review focuses on tree and forest responses at boreal and temperate latitudes, ranging from the cellular to the ecosystem level, and management is critical for a positive response of forest growth to a warmer climate.
Abstract: Although trees have responded to global warming in the past - to temperatures higher than they are now - the rate of change predicted in the 21st century is likely to be unprecedented. Greenhouse gas emissions could cause a 3-6°C increase in mean land surface temperature at high and temperate latitudes. Despite this, few experiments have isolated the effects of temperature for this scenario on trees and forests. This review focuses on tree and forest responses at boreal and temperate latitudes, ranging from the cellular to the ecosystem level. Adaptation to varying temperatures revolves around the trade-off between utilizing the full growing season and minimizing frost damage through proper timing of hardening in autumn and dehardening in spring. But the evolutionary change in these traits must be sufficiently rapid to compensate for the temperature changes. Many species have a positive response to increased temperature - but how close are we to the optima? Management is critical for a positive response of forest growth to a warmer climate, and selection of the best species for the new conditions will be of vital importance. Contents Summary 369 I. Introduction 370 II. Photosynthesis and respiration 370 III. Soil organic matter decomposition and mineralization 373 IV. Phenology and frost hardiness 376 V. Whole tree experimental responses to warming 380 VI. Changes in species distribution at warmer temperatures 381 VII. Adaptation and evolution 383 VIII. Ecosystem level responses to warming 387 Acknowledgements 390 References 390 Appendix I. Temperature response functions 399.
754 citations
TL;DR: In this paper, the concurrent effects of increasing atmospheric CO2 concentration, climate variability, and cropland establishment and abandonment on terrestrial carbon storage between 1920 and 1992 were assessed using a standard simulation protocol with four process-based terrestrial biosphere models.
Abstract: The concurrent effects of increasing atmospheric CO2 concentration, climate variability, and cropland establishment and abandonment on terrestrial carbon storage between 1920 and 1992 were assessed using a standard simulation protocol with four process-based terrestrial biosphere models. Over the long-term (1920-1992), the simulations yielded a time history of terrestrial uptake that is consistent (within the uncertainty) with a long-term analysis based on ice core and atmospheric CO2 data. Up to 1958, three of four analyses indicated a net release of carbon from terrestrial ecosystems to the atmosphere caused by cropland establishment. After 1958, all analyses indicate a net uptake of carbon by terrestrial ecosystems, primarily because of the physiological effects of rapidly rising atmospheric CO2. During the 1980s the simulations indicate that terrestrial ecosystems stored between 0.3 and 1.5 Pg C yr(-1), which is within the uncertainty of analysis based on CO2 and O-2 budgets. Three of the four models indicated tin accordance with O-2 evidence) that the tropics were approximately neutral while a net sink existed in ecosystems north of the tropics. Although all of the models agree that the long-term effect of climate on carbon storage has been small relative to the effects of increasing atmospheric CO2 and land use, the models disagree as to whether climate variability and change in the twentieth century has promoted carbon storage or release. Simulated interannual variability from 1958 generally reproduced the El Nino/Southern Oscillation (ENSO)-scale variability in the atmospheric CO2 increase, but there were substantial differences in the magnitude of interannual variability simulated by the models. The analysis of the ability of the models to simulate the changing amplitude of the seasonal cycle of atmospheric CO2 suggested that the observed trend may be a consequence of CO2 effects, climate variability, land use changes, or a combination of these effects. The next steps for improving the process-based simulation of historical terrestrial carbon include (1) the transfer of insight gained from stand-level process studies to improve the sensitivity of simulated carbon storage responses to changes in CO2 and climate, (2) improvements in the data sets used to drive the models so that they incorporate the timing, extent, and types of major disturbances, (3) the enhancement of the models so that they consider major crop types and management schemes, (4) development of data sets that identify the spatial extent of major crop types and management schemes through time, and (5) the consideration of the effects of anthropogenic nitrogen deposition. The evaluation of the performance of the models in the context of a more complete consideration of the factors influencing historical terrestrial carbon dynamics is important for reducing uncertainties in representing the role of terrestrial ecosystems in future projections of the Earth system.
743 citations
TL;DR: Research, modeling, and monitoring have shown that reducing nutrient sources by 40% would improve water quality, but less than originally thought, and the lack of reductions in nutrient concentrations in some streams and tidal waters and field research suggest that soil conservation-based management strategies are less effective than assumed.
Abstract: Chesapeake Bay has been the subject of intensive research on cultural eutrophication and extensive efforts to reduce nutrient inputs. In 1987 a commitment was made to reduce controllable sources of nitrogen (N) and phosphorous (P) by 40% by the year 2000, although the causes and effects of eutrophication were incompletely known. Subsequent research, modeling, and monitoring have shown that: (i) the estuarine ecosystem had been substantially altered by increased loadings of N and P of approximately 7- and 18-fold, respectively; (ii) hypoxia substantially increased since the 1950s; (iii) eutrophication was the major cause of reductions in submerged vegetation; and (iv) reducing nutrient sources by 40% would improve water quality, but less than originally thought. Strong public support and political commitment have allowed the Chesapeake Bay Program to reduce nutrient inputs, particularly from point sources, by 58% for P and 28% for N. However, reductions of nonpoint sources of P and N were projected by models to reach only 19% and 15%, respectively, of controllable loadings. The lack of reductions in nutrient concentrations in some streams and tidal waters and field research suggest that soil conservation-based management strategies are less effective than assumed. In 1997, isolated outbreaks of the toxic dinoflagellate Pfiesteria piscicida brought attention to the land application of poultry manure as a contributing factor to elevated soil P and ground water N concentrations. In addition to developing more effective agricultural practices, emerging issues include linking eutrophication and living resources, reducing atmospheric sources of N, enhancing nutrient sinks, controlling sprawling suburban development, and predicting and preventing harmful algal blooms.
TL;DR: This overview summarizes the current state of knowledge of microbial transformations of nutrients in mangrove ecosystems and illustrates the important contributions these microorganisms make to the productivity of the ecosystems.
Abstract: Mangrove communities are recognized as highly productive ecosystems that provide large quantities of organic matter to adjacent coastal waters in the form of detritus and live animals (fish, shellfish). The detritus serves as a nutrient source and is the base of an extensive food web in which organisms of commercial importance take part. In addition, mangrove ecosystems serve as shelter, feeding, and breeding zones for crustaceans, mollusks, fish of commercial importance, and resident and migratory birds. Although mangroves in the United States are protected, the systematic destruction of these ecosystems elsewhere is increasing. Deforestation of mangrove communities is thought to be one of the major reasons for the decrease in the coastal fisheries of many tropical and subtropical countries. There is evidence to propose a close microbe-nutrient-plant relationship that functions as a mechanism to recycle and conserve nutrients in the mangrove ecosystem. The highly productive and diverse microbial community living in tropical and subtropical mangrove ecosystems continuously transforms nutrients from dead mangrove vegetation into sources of nitrogen, phosphorus, and other nutrients that can be used by the plants. In turn, plant-root exudates serve as a food source for the microorganisms living in the ecosystem with other plant material serving similarly for larger organisms like crabs. This overview summarizes the current state of knowledge of microbial transformations of nutrients in mangrove ecosystems and illustrates the important contributions these microorganisms make to the productivity of the ecosystems. To conserve the mangrove ecosystems, which are essential for the sustainable maintenance of coastal fisheries, maintenance and restoration of the microbial communities should be undertaken. Inoculation of mangrove seedlings with plant-growth-promoting bacteria may help revegetate degraded areas and create reconstructed mangrove ecosystems.
TL;DR: It is found that the enhanced biomass accumulation in response to elevated levels of CO2 or nitrogen, or their combination, is less in species-poor than inspecies-rich assemblages.
Abstract: Human actions are causing declines in plant biodiversity, increases in atmospheric CO2 concentrations and increases in nitrogen deposition; however, the interactive effects of these factors on ecosystem processes are unknown. Reduced biodiversity has raised numerous concerns, including the possibility that ecosystem functioning may be affected negatively, which might be particularly important in the face of other global changes. Here we present results of a grassland field experiment in Minnesota, USA, that tests the hypothesis that plant diversity and composition influence the enhancement of biomass and carbon acquisition in ecosystems subjected to elevated atmospheric CO2 concentrations and nitrogen deposition. The study experimentally controlled plant diversity (1, 4, 9 or 16 species), soil nitrogen (unamended versus deposition of 4 g of nitrogen per m2 per yr) and atmospheric CO2 concentrations using free-air CO2 enrichment (ambient, 368 micromol mol-1, versus elevated, 560 micromol mol-1). We found that the enhanced biomass accumulation in response to elevated levels of CO2 or nitrogen, or their combination, is less in species-poor than in species-rich assemblages.
TL;DR: In this paper, the authors used the Hudson River catchment of North America to investigate the magnitude of net heterotrophy in aquatic ecosystems and found that respiration (R ) exceeds autochthonous gross primary production (GPP) in a majority of aquatic ecosystems.
Abstract: For a majority of aquatic ecosystems, respiration ( R ) exceeds autochthonous gross primary production (GPP). These systems have negative net ecosystem production ((NEP) = (GPP) - R ) and ratios of (GPP)/ R of <1. This net heterotrophy can be sustained only if aquatic respiration is subsidized by organic inputs from the catchment. Such subsidies imply that organic materials that escaped decomposition in the terrestrial environment must become susceptible to decomposition in the linked aquatic environment. Using a moderate-sized catchment in North America, the Hudson River (catchment area 33 500 km 2 ), evidence is presented for the magnitude of net heterotrophy. All approaches (CO 2 gas flux; O 2 gas flux; budget and gradient of dissolved organic C; and the summed components of primary production and respiration within the ecosystem) indicate that system respiration exceeds gross primary production by ~200 g C m -2 year -1 . Highly 14 C-depleted C of ancient terrestrial origin (1000-5000 years old) may be an important source of labile organic matter to this riverine system and support this excess respiration. The mechanisms by which organic matter is preserved for centuries to millennia in terrestrial soils and decomposed in a matter of weeks in a river connect modern riverine metabolism to historical terrestrial conditions.
TL;DR: In this article, a new marine ecosystem model designed for the global domain is presented, and model output is compared with field data from nine different locations, including high latitudes to the mid-ocean gyres.
Abstract: A new marine ecosystem model designed for the global domain is presented, and model output is compared with field data from nine different locations. Field data were collected as part of the international Joint Global Ocean Flux Study (JGOFS) program, and from historical time series stations. The field data include a wide variety of marine ecosystem types, including nitrogen- and iron-limited systems, and different physical environments from high latitudes to the mid-ocean gyres. Model output is generally in good agreement with field data from these diverse ecosystems. These results imply that the ecosystem model presented here can be reliably applied over the global domain. The model includes multiple potentially limiting nutrients that regulate phytoplankton growth rates. There are three phytoplankton classes, diatoms, diazotrophs, and a generic small phytoplankton class. Growth rates can be limited by available nitrogen, phosphorus, iron, and/or light levels. The diatoms can also be limited by silicon. The diazotrophs are capable of nitrogen fixation of N2 gas and cannot be nitrogen-limited. Calcification by phytoplankton is parameterized as a variable fraction of primary production by the small phytoplankton group. There is one zooplankton class that grazes the three phytoplankton groups and a large detrital pool. The large detrital pool sinks out of the mixed layer, while a smaller detrital pool, representing dissolved organic matter and very small particulates, does not sink. Remineralization of the detrital pools is parameterized with a temperature-dependent function. We explicitly model the dissolved iron cycle in marine surface waters including inputs of iron from subsurface sources and from atmospheric dust deposition. © 2001 Elsevier Science Ltd. All rights reserved.
TL;DR: In this paper, the authors focus on how coral reef ecosystems relate to disturbance in an increasingly human-dominated environment and identify the spatial sources of resilience in dynamic seascapes and exemplify and discuss the relation between ecological memory (biological legacies, mobile link species, and support areas) and functional diversity for seascape resilience.
Abstract: There have been several earlier studies that addressed the influence of natural disturbance regimes on coral reefs. Humans alter natural disturbance regimes, introduce new stressors, and modify background conditions of reefs. We focus on how coral reef ecosystems relate to disturbance in an increasingly human-dominated environment. The concept of ecosystem resilience—that is, the capacity of complex systems with multiple stable states to absorb disturbance, reorganize, and adapt to change—is central in this context. Instead of focusing on the recovery of certain species and populations within disturbed sites of individual reefs, we address spatial resilience—that is, the dynamic capacity of a reef matrix to reorganize and maintain ecosystem function following disturbance. The interplay between disturbance and ecosystem resilience is highlighted. We begin the identification of spatial sources of resilience in dynamic seascapes and exemplify and discuss the relation between “ecological memory” (biological legacies, mobile link species, and support areas) and functional diversity for seascape resilience. Managing for resilience in dynamic seascapes not only enhances the likelihood of conserving coral reefs, it also provides insurance to society by sustaining essential ecosystem services.
TL;DR: It is demonstrated, in two long-term experiments in a desertified Mediterranean ecosystem, that inoculation with indigenous arbuscular mycorrhizal fungi and with rhizobial nitrogen-fixing bacteria not only enhanced the establishment of key plant species but also increased soil fertility and quality.
Abstract: Disturbance of natural plant communities is the first visible indication of a desertification process, but damage to physical, chemical, and biological soil properties is known to occur simultaneously. Such soil degradation limits reestablishment of the natural plant cover. In particular, desertification causes disturbance of plant-microbe symbioses which are a critical ecological factor in helping further plant growth in degraded ecosystems. Here we demonstrate, in two long-term experiments in a desertified Mediterranean ecosystem, that inoculation with indigenous arbuscular mycorrhizal fungi and with rhizobial nitrogen-fixing bacteria not only enhanced the establishment of key plant species but also increased soil fertility and quality. The dual symbiosis increased the soil nitrogen (N) content, organic matter, and hydrostable soil aggregates and enhanced N transfer from N-fixing to nonfixing species associated within the natural succession. We conclude that the introduction of target indigenous species of plants associated with a managed community of microbial symbionts is a successful biotechnological tool to aid the recovery of desertified ecosystems.
Scripps Institution of Oceanography1, University of Maryland, College Park2, Colorado State University3, University of New Mexico4, Swedish Museum of Natural History5, United States Forest Service6, University of Georgia7, Oregon State University8, Memorial University of Newfoundland9, Polish Academy of Sciences10
TL;DR: In this paper, the authors hypothesized that diversity is indeed important to ecosystem function in marine CTZs because high diversity maintains positive interactions among species (facilitation and mutualism), promoting stability and resistance to invasion or other forms of disturbance.
Abstract: Estuaries and coastal wetlands are critical transition zones (CTZs) that link land, freshwater habitats, and the sea. CTZs provide essential ecological functions, including decomposition, nutrient cycling, and nutrient production, as well as regulation of fluxes of nutrients, water, particles, and organisms to and from land, rivers, and the ocean. Sediment-associated biota are integral to these functions. Functional groups considered essential to CTZ processes include heterotrophic bacteria and fungi, as well as many benthic invertebrates. Key invertebrate functions include shredding, which breaks down and recycles organic matter; suspension feeding, which collects and transports sediments across the sediment–water interface; and bioturbating, which moves sediment into or out of the seabed. In addition, macrophytes regulate many aspects of nutrient, particle, and organism dynamics above- and belowground. Animals moving within or through CTZs are vectors that transport nutrients and organic matter across terrestrial, freshwater, and marine interfaces. Significant threats to biodiversity within CTZs are posed by anthropogenic influences; eutrophication, nonnutrient pollutants, species invasions, overfishing, habitat alteration, and climate change affect species richness or composition in many coastal environments. Because biotic diversity in marine CTZ sediments is inherently low whereas their functional significance is great, shifts in diversity are likely to be particularly important. Species introductions (from invasion) or loss (from overfishing or habitat alteration) provide evidence that single-species changes can have overt, sweeping effects on CTZ structure and function. Certain species may be critically important to the maintenance of ecosystem functions in CTZs even though at present there is limited empirical evidence that the number of species in CTZ sediments is critical. We hypothesized that diversity is indeed important to ecosystem function in marine CTZs because high diversity maintains positive interactions among species (facilitation and mutualism), promoting stability and resistance to invasion or other forms of disturbance. The complexity of interactions among species and feedbacks with ecosystem functions suggests that comparative (mensurative) and manipulative approaches will be required to elucidate the role of diversity in sustaining CTZ functions.
TL;DR: In this article, the authors used LANDSAT MSS to quantify changes in land-use and ecosystem services due to urban sprawl in Bexar County, TX, in which San Antonio is centered.
TL;DR: This review synthesizes the recent work on availability and plant uptake of amino acids with classic work on ON in soils to predict the ecological implications of ON use by plants given the influence of climatic change and N pollution.
Abstract: The recently recognized importance of organic nitrogen (ON), particularly amino acids, to plant nutrition in many types of agricultural and natural ecosystems has raised questions about plant-microbe interactions, N availability in soils, and the ecological implications of ON use by plants in the light of climate change and N pollution. In this review we synthesize the recent work on availability and plant uptake of amino acids with classic work on ON in soils. We also discuss recent work on the use of natural abundance levels of 15N to infer N sources for plants. Reliance on ON is widespread among plants from many ecosystems. Authors have reached this conclusion based on laboratory studies of amino acid uptake by plants in pure culture, amino acid concentrations in soils, plant uptake of isotopically labeled amino acids in the field and in plant-soil microcosms, and from plant natural abundance values of 15N. The supply of amino acids to plants is determined mainly by the action of soil proteolytic enzymes, interactions between amino acids and the soil matrix, and competition between plants and microbes. Plants generally compete for a minor fraction of the total amino acid flux, but in some cases this forms a significant N resource, especially in ecosystems where microbial biomass undergoes large seasonal fluctuations and contributes labile ON to the soil. A quantitative understanding of ON use by plants is confounded by incomplete data on partitioning of ON between plants, mycorrhizal fungi, and competing soil microbes. Further research is needed to predict the ecological implications of ON use by plants given the influence of climatic change and N pollution.
TL;DR: Higher algal and total plant biomass and lower losses of total phosphorus resulted from the greater chance in species mixtures of a high algal biomass and the presence of sago pondweed, which physically filter particulate phosphorus from the water.
Abstract: Wetlands provide many important ecosystem services to human society1,2,3,4,5, which may depend on how plant diversity influences biomass production and nutrient retention4,6,7,8. Vascular aquatic plant diversity may not necessarily enhance wetland ecosystem functioning, however, because competition among these plant species can be strong, often resulting in the local dominance of a single species4,9. Here we have manipulated the species richness of rooted, submerged aquatic plant (macrophyte) communities in experimental wetland mesocosms. We found higher algal and total plant (algal plus macrophyte) biomass, as well as lower loss of total phosphorus, in mesocosms with a greater richness of macrophyte species. Greater plant biomass resulted from a sampling effect; that is, the increased chance in species mixtures that algal production would be facilitated by the presence of a less competitive species—in this case, crisped pondweed. Lower losses of total phosphorus resulted from the greater chance in species mixtures of a high algal biomass and the presence of sago pondweed, which physically filter particulate phosphorus from the water2,10,11. These indirect and direct effects of macrophyte species richness on algal production, total plant biomass and phosphorus loss suggest that management practices that maintain macrophyte diversity may enhance the functioning and associated services of wetland ecosystems.
TL;DR: It is suggested that approaches developed for quantitative genetics can be extended to ecosystem functioning by modeling the means and variances of phenotypes within a group of species, and that phenotypic variance within functional groups is linearly related to their ability to respond to environmental changes.
Abstract: Biodiversity plays a vital role for ecosystem functioning in a changing environment. Yet theoretical approaches that incorporate diversity into classical ecosystem theory do not provide a general dynamic theory based on mechanistic principles. In this paper, we suggest that approaches developed for quantitative genetics can be extended to ecosystem functioning by modeling the means and variances of phenotypes within a group of species. We present a framework that suggests that phenotypic variance within functional groups is linearly related to their ability to respond to environmental changes. As a result, the long-term productivity for a group of species with high phenotypic variance may be higher than for the best single species, even though high phenotypic variance decreases productivity in the short term, because suboptimal species are present. In addition, we find that in the case of accelerating environmental change, species succession in a changing environment may become discontinuous. Our work suggests that this phenomenon is related to diversity as well as to the environmental disturbance regime, both of which are affected by anthropogenic activities. By introducing new techniques for modeling the aggregate behavior of groups of species, the present approach may provide a new avenue for ecosystem analysis.
01 Jan 2001
TL;DR: The La Copita case study as mentioned in this paper showed that woody plant root biomass in upper soil horizons exceeded foliar litter inputs by one to two orders of magnitude, suggesting that belowground inputs of organic matter drive changes in soil physical and chemical properties subsequent to woody establishment in grasslands.
Abstract: Publisher Summary Woody plant encroachment has been widespread in grassland and savanna ecosystems over the past century. This phenomenon jeopardizes grassland biodiversity and threatens the sustainability of pastoral, subsistence, and commercial livestock grazing. When woody species increase in abundance and transform grasslands and savannas into shrublands and woodlands, the potential to alter land surface–atmosphere interactions and carbon and nitrogen sequestration and cycling at regional and global scales may be significant. The La Copita case study documents the rate and magnitude of change in ecosystem biogeochemistry that can occur when a subtropical dryland landscape is transformed from savanna grassland to woodland. Fluctuations in monthly woody plant root biomass in upper soil horizons exceeded foliar litter inputs by one to two orders of magnitude, suggesting that belowground inputs of organic matter drive changes in soil physical and chemical properties subsequent to woody plant establishment in grasslands. These results are of potential global significance, given that large areas of Africa, South America, North America, and Australia have been undergoing similar land cover changes over the past century. The demonstrated capacity for carbon sequestration in this semiarid system suggests a need to reevaluate traditional perspectives on woody plants in rangelands as governments and industries seek ways to mitigate greenhouse gas emissions.
TL;DR: The results show that the introduction of these mammals to New Zealand has caused far-ranging effects at both the community and ecosystem levels of resolution, with particularly adverse effects for indigenous plant com- munities and populations of most groups of litter-dwelling mesofauna and macrofauna.
Abstract: Forest dwelling browsing mammals, notably feral goats and deer, have been introduced to New Zealand over the past 220 years; prior to this such mammals were absent from New Zealand. The New Zealand forested landscape, therefore, presents an almost unique opportunity to determine the impacts of introduction of an entire functional group of alien animals to a habitat from which that group was previously absent. We sampled 30 long-term fenced exclosure plots in indigenous forests throughout New Zealand to evaluate community- and ecosystem-level impacts of introduced browsing mammals, emphasizing the decomposer subsystem. Browsing mammals often significantly altered plant community composition, reducing palatable broad-leaved species and promoting other less palatable types. Vegetation density in the browse layer was also usually reduced. Although there were some small but statis- tically significant effects of browsing on some measures of soil quality across the 30 locations, there were no consistent effects on components of the soil microfood web (com- prising microflora and nematodes, and spanning three consumer trophic levels); while there were clear multitrophic effects of browsing on this food web for several locations, com- parable numbers of locations showed stimulation and inhibition of biomasses or populations of food web components. In contrast, all microarthropod and macrofaunal groups were consistently adversely affected by browsing, irrespective of trophic position. Across the 30 locations, the magnitude of response of the dominant soil biotic groups to browsing mammals (and hence their resistance to browsers) was not correlated with the magnitude of vegetation response to browsing but was often strongly related to a range of other variables, including macroclimatic, soil nutrient, and tree stand properties. There were often strong significant effects of browsing mammals on species composition of the plant community, species composition of leaf litter in the litter layer, and composition of various litter-dwelling faunal groups. Across the 30 locations, the magnitude of browsing mammal effects on faunal community composition was often correlated with browser effects on litter layer leaf species composition but never with browser effects on plant community composition. Browsing mammals usually reduced browse layer plant diversity and often also altered habitat diversity in the litter layer and diversity of various soil faunal groups. Across the 30 locations, the magnitude of browser effects on diversity of only one faunal group, humus-dwelling nematodes, was correlated with browser effects on plant diversity. However, browser effects on diversity of diplopods and gastropods were correlated with browser effects on habitat diversity of the litter layer. Reasons for the lack of unidirectional relationships across locations between effects of browsers on vegetation community attri- butes and on soil invertebrate community attributes are discussed. Browsing mammals generally did not have strong effects on C mineralization but did significantly influence soil C and N storage on an areal basis for several locations. However the direction of these effects was idiosyncratic and presumably reflects different mechanisms by which browsers affect soil processes. While our study did not support hypotheses predicting consistent negative effects of browsing mammals on the decomposer subsystem through promotion of plant species with poorer litter quality, our results still show that the introduction of these mammals to New Zealand has caused far-ranging effects at both the community and ecosystem levels of resolution, with particularly adverse effects for indigenous plant com- munities and populations of most groups of litter-dwelling mesofauna and macrofauna.
TL;DR: In this paper, a reductive pathway for nitrogen cycling in upland tropical forest soils that decreases the amount of nitrate susceptible to leaching and denitrification, thus conserving nitrogen in the ecosystem was reported.
Abstract: The internal transformations of nitrogen in terrestrial ecosystems exert strong controls over nitrogen availability to net primary productivity, nitrate leaching into ground- water, and emissions of nitrogen-based greenhouse gas. Here we report a reductive pathway for nitrogen cycling in upland tropical forest soils that decreases the amount of nitrate susceptible to leaching and denitrification, thus conserving nitrogen in the ecosystem. Using '5N tracers we measured rates of dissimilatory nitrate reduction to ammonium (DNRA) in upland humid tropical forest soils averaging -0.6 p1g-g-'d-'. Rates of DNRA were three times greater than the combined N20 and N2 fluxes from nitrification and denitrification and accounted for 75% of the turnover of the nitrate pool. To determine the relative im- portance of ambient C, 02, and NO3 concentrations on rates of DNRA, we estimated rates of DNRA in laboratory assays using soils from three tropical forests (cloud forest, palm forest, and wet tropical forest) that differed in ambient C and 02 concentrations. Rates of DNRA measured in laboratory assays ranged from 0.5 to 9 (Lgg-'gd-' in soils from the three different forests and appeared to be primarily limited by the availability of NO3, as opposed to C Or 02. Tests of sterile soils indicated that the dominant reductive pathway for both NO2 and NO3 was biotic and not abiotic. Because NH4 is the form of N generally favored for assimilation by plants and microbes, and NO3 is easily lost from the ecosystem, the rapid and direct transformation of NO3 to NH4 via DNRA has the potential to play an important role in ecosystem N conservation.
TL;DR: Research programs in U.S. desert shrubland and pinyon-juniper woodland have shown that some environmental perturbations can cause wholesale reorganization of ecosystems and other changes may be buffered because of the compensatory dynamics of complementary species.
Abstract: Studies that combine experimental manipulations with long-term data collection reveal elaborate interactions among species that affect the structure and dynamics of ecosystems. Research programs in U.S. desert shrubland and pinyon-juniper woodland have shown that (i) complex dynamics of species populations reflect interactions with other organisms and fluctuating climate; (ii) genotype x environment interactions affect responses of species to environmental change; (iii) herbivore-resistance traits of dominant plant species and impacts of "keystone" animal species cascade through the system to affect many organisms and ecosystem processes; and (iv) some environmental perturbations can cause wholesale reorganization of ecosystems because they exceed the ecological tolerances of dominant or keystone species, whereas other changes may be buffered because of the compensatory dynamics of complementary species.
TL;DR: In this article, the authors systematically monitored the structure of ground arthropod communities for 12 months at 16 sites representing the four most abundant forms of urban land use (residential, industrial, agricultural, and desert remnant) in a rapidly growing metropolitan area (Phoenix, AZ).
TL;DR: The results indicate that floristically and edaphiinput of dead fine roots to mineral soil, the majority of cally distinct forest ecosystems also differ in microbial community composition and substrate use, consistent across aboveground litter is deposited in late autumn.
Abstract: northern Wisconsin, Christensen (1969) found evidence for such a relationship, wherein communities of soil The composition and diversity of biotic communities are controlled microfungi were correlated with the occurrence of forest by the availability of growth-limiting resources. Resource availability for microbial populations in soil is controlled by the amount and types vegetation (also see Christensen et al., 1962; Tresner et of organic compounds entering soil from plant litter. Because plant al., 1954). Notwithstanding these observations, we have communities differ in the amount and type of substrates entering a limited understanding of the manner in which the soil, we reasoned that the composition and function of soil microbial broader soil microbial community varies with plant comcommunities should differ with the dominant vegetation. We tested munity composition at a landscape-level scale. this idea by studying two sugar maple (Acer saccharum Marsh.)- Temporal patterns of root and leaf litter production, dominated and one oak (Quercus spp.)-dominated forest ecosystems combined with variation in the chemical composition in northern Lower Michigan that differ in rates of soil N cycling. We of these tissues, could influence the composition and used phospholipid fatty acid (PLFA) analysis to gain insight into function of soil microbial communities. Root litter typimicrobial community composition, and we used a subset of Biolog cally contains more N and lignin than leaf litter (Aber GN substrates found in root exudate to assess the metabolic capabilities soil microbial communities. Although microbial biomass did not et al., 1990; Vogt et al., 1986). The fact that roots and differ among ecosystems, principal components analysis of bacterial, leaves differ chemically could lead to changes in microactinomycetal, and fungal PLFAs clearly separated the microbial com- bial community composition and function, depending munities of the three ecosystems. Similarly, principal components on temporal variation in the proportion of leaf vs. root analysis separated microbial communities by differences in growth on litter entering soil. In sugar maple‐dominated forests, carbohydrates, organic acids, and amino acids. Discrimination among the greatest addition of root litter to soil occurs when microbial communities in the three ecosystems by PLFAs and sub- fine-root mortality peaks in early autumn (Hendrick and strate use occurred in spring, summer, and fall, but the individual Pregitzer, 1992), but substantial inputs also can occur PLFAs and substrates contributing to discrimination changed during throughout the growing season. Unlike the continuous the growing season. Our results indicate that floristically and edaphiinput of dead fine roots to mineral soil, the majority of cally distinct forest ecosystems also differ in microbial community composition and substrate use. This pattern was consistent across aboveground litter is deposited in late autumn. Seasonal the growing season and repeatedly occurred across relatively large variation in above- and belowground litter production land areas. could influence microbial community composition and function, depending on the types of substrates available for microbial metabolism.
TL;DR: In this article, the average concentrations of chlorophyll a and the estimated rates of primary production in the surface waters of the North Pacific Subtropical Gyre (NPSG) off Hawaii have more than doubled while the concentrations of dissolved silicate and phosphate have decreased during the past three decades.
Abstract: Oceanic productivity, fishery yields and the net marine sequestration of atmospheric greenhouse gases are all controlled by the structure and function of planktonic communities. Detailed paleoceanographic studies have documented abrupt changes in these processes over timescales ranging from centuries to millennia. Most of these major shifts in oceanic productivity and biodiversity are attributable to changes in Earth's climate, manifested through large-scale ocean–atmosphere interactions. By comparison, contemporary biodiversity and plankton community dynamics are generally considered to be “static”, in part due to the lack of a suitable time frame of reference, and the absence of oceanic data to document ecosystem change over relatively short timescales (decades to centuries). Here we show that the average concentrations of chlorophyll a (chl a) and the estimated rates of primary production in the surface waters of the North Pacific Subtropical Gyre (NPSG) off Hawaii have more than doubled while the concentrations of dissolved silicate and phosphate have decreased during the past three decades. These changes are accompanied by an increase in the concentration of chl b, suggesting a shift in phytoplankton community structure. We hypothesize that these observed ecosystem trends and other related biogeochemical processes in the upper portion of the NPSG are manifestations of plankton community succession in response to climate variations. The hypothesized photosynthetic population “domain shift” toward an ecosystem dominated by prokaryotes has altered nutrient flux pathways and affected food web structure, new and export production processes, and fishery yields. Further stratification of the surface ocean resulting from global warming could lead to even more enhanced selection pressures and additional changes in biogeochemical dynamics.
TL;DR: Variable diversity effects for species representative of marine coastal systems at both global and regional scales are demonstrated and evidence for an increase in complementary resource use as diversity increases and strong evidence for diversity effects in naturally assembled communities at a regional scale is shown.
Abstract: Revealing the consequences of species extinctions for ecosystem function has been a chief research goal and has been accompanied by enthusiastic debate. Studies carried out predominantly in terrestrial grassland and soil ecosystems have demonstrated that as the number of species in assembled communities increases, so too do certain ecosystem processes, such as productivity, whereas others such as decomposition can remain unaffected. Diversity can influence aspects of ecosystem function, but questions remain as to how generic the patterns observed are, and whether they are the product of diversity, as such, or of the functional roles and traits that characterize species in ecological systems. Here we demonstrate variable diversity effects for species representative of marine coastal systems at both global and regional scales. We provide evidence for an increase in complementary resource use as diversity increases and show strong evidence for diversity effects in naturally assembled communities at a regional scale. The variability among individual species responses is consistent with a positive but idiosyncratic pattern of ecosystem function with increased diversity.