Showing papers on "Ecosystem published in 1990"

Biological Feedbacks in Global Desertification

Abstract: Studies of ecosystem processes on the Jornada Experimental Range in southern New Mexico suggest that longterm grazing of semiarid grasslands leads to an increase in the spatial and temporal heterogeneity of water, nitrogen, and other soil resources. Heterogeneity of soil resources promotes invasion by desert shrubs, which leads to a further localization of soil resources under shrub canopies. In the barren area between shrubs, soil fertility is lost by erosion and gaseous emissions. This positive feedback leads to the desertification of formerly productive land in southern New Mexico and in other regions, such as the Sahel. Future desertification is likely to be exacerbated by global climate warming and to cause significant changes in global biogeochemical cycles.

Biological invasions and ecosystem processes : towards an integration of population biology and ecosystem studies

Abstract: Much of the recent progress in ecosystem ecology can be traced to studies which have examined the responses of ecosystems to disturbance (Odum 1969). For example, early studies of forest clear-felling (cf. Hesselman 1917, in Stalfelt (1960) demonstrated that soil nutrient availability is usually enhanced in harvested sites. More recently, studies at the Hubbard Brook Experimental Forest (Likens et al. 1970, Bormann and Likens 1979), in Sweden (Tamm et al. 1974, Wiklander 1981), and elsewhere documented that forest cutting alters watershed-level hydrology and nutrient losses; longer-term measurements have documented the reestablishment of biotic regulation of water and nutrient cycling during secondary succession (Bormann and Likens 1979).

Species effects on nitrogen cycling: a test with perennial grasses

Abstract: To test for differing effects of plant species on nitrogen dynamics, we planted monocultures of five perennial grasses (Agropyron repens, Agrostis scabra, Poa pratensis, Schizachyrium scoparium, and Andropogon gerardi) on a series of soils ranging from sand to black soil. In situ net N mineralization was measured in the monocultures for three years. By the third year, initially identical soils under different species had diverged up to 10-fold in annual net mineralization. This divergence corresponded to differences in the tissue N concentrations, belowground lignin concentrations, and belowground biomasses of the species. These results demonstrate the potential for strong feedbacks between the species composition of vegetation and N cycling. If individual plant species can affect N mineralization and N availability, then competition for N may lead to positive or negative feedbacks between the processes controlling species composition and ecosystem processes such as N and C cycling. These feedbacks create the potential for alternative stable states for the vegetation-soil system given the same initial abiotic conditions.

The Significance of Soil Microbial Biomass Estimations

Abstract: This chapter discusses the significance of utilizing microbial biomass values for investigating nutrient cycling in disturbed and nondisturbed ecosystems. Most biomass estimations are from temperate zone soils under agricultural cultivation, grassland, or forest. The estimates of pool sizes for microbial biomass C, N, P, and S suggest that these pools are large enough to have significant impacts on plant nutrient availability. Mineralization of plant nutrients from soil organic matter is a major function of the soil microbial biomass. Crop rotations affecting microbial biomass would make it possible to manage residues and rotations for the benefit of each crop. For soils, microbial biomass measurements may provide the information needed for ecosystem-level monitoring for initial disturbance and recovery. The role of microbial biomass in global nutrient cycles is well established, as significant N, P, and S transfers occur in terrestrial ecosystems.

Spatial and Temporal Variation in Tropical Fish Trophic Networks

Abstract: Observed properties of natural food webs have both important theoretical and important management implications Four lowland aquatic food webs were investigated over the course of two years: a large swamp and a small stream in Costa Rica, and a similar swamp and stream in the Venezuelan llanos Each local ecosystem differed from the three others with respect to environmental changes associated with seasonal rainfall Phylogenetic composition and diversity of biotas also varied among systems Volumetric proportional utilization coefficients from fish gut contents were used as estimates of the intensity of predator—prey interactions An annual and two or more seasonal food webs were constructed for each local community Aquatic communities were defined operationally using common fish species as consumers, and using the sink subweb associated with the top predator of each system A computer calculated a variety of food—web statistics and plotted food—web diagrams containing either (a) all observed trophic links (predator—prey interactions), or (b) subsets with weak links eliminated at prescribed thresholds Individual community food webs contained from 58 (stream, Costa Rica) to 104 (swamp, Venezuela) interactive taxonomic units and from 208 to 1243 total trophic links Food—web parameters were very sensitive to changes in level of link threshold Web connectance and related parameters converged near link threshold 004 (utilization coefficients <004 eliminated) in a variety of inter—web comparisons Despite large differences in assemblage composition and attributes of the physical environment, distributions of trophic levels calculated according to a trophic continuum algorithm were very similar among study systems Herbivores, detritivores, and their direct predators formed the largest proportions of fishes in each assemblage, followed by omnivores and secondary carnivores Fishes that fed at more than one trophic interval were extremely common in all food webs Analysis of covariance was used to compare structural features of different webs across a range of link thresholds Extensive among—site variation in food—web parameters was associated with differences in species richness and environmental differences associated with rainfall patterns, physiography, and gross primary production Seasons generally influenced food—web parameters less than did site differences Relative importance of detritus, aquatic primary production, and terrestrial production in aquatic food webs varied seasonally in each system Detritus, derived primarily from aquatic macrophytes, was an important pathway in both tropical swamp ecosystems Aquatic primary productivity comprised the largest fraction of fish diets during the wet season in the Venezuelan swamp, but it formed the major component of fish diets during the dry season at all other sites Based on comparisons using 13 webs, two—thirds of the pairings among six food—web parameters used (number of nodes, compartmentation, connectance, average number of prey per node, average number of predators per node, ratio of consumer nodes to total nodes) were positively intercorrelated Several food—web relationships previously described as constant (eg, connectance x species richness constancy, species scaling law, link—species scaling law) were not confirmed by my data These earlier food—web trends are extremely sensitive to methodological biases, especially decisions regarding the degree of taxonomic lumping of species into trophic units Although food webs have unique emergent properties and spinoff a number of potentially informative macrodescriptors, empirical studies must achieve greater precision and uniformity before analyses can be performed across different systems Several problems and potential resolutions are discussed

Predicting long-term patterns of mass loss, nitrogen dynamics, and soil organic matter formation from initial fine litter chemistry in temperate forest ecosystems

Abstract: Long-term decomposition data are presented for several types of foliar and fine root litter in different stands in Wisconsin and Massachusetts, USA Changes in mass remaining as well as nitrogen

Ecosystem approach to a global nitrous oxide budget

Abstract: An approach using relationships among soil fertility, nitrogen cycling, and nitrous oxide production is presented in order to estimate nitrous oxide flux from humid tropical forests. The effects of human disturbance are also investigated. It is noted that recent estimates suggest that 6-10 million ha. of tropical forest are cleared permanently each year. The results of measurements of nitrous oxide flux from cleared and burned sites in Costa Rica and Brazil, in conjunction with satellite-based classifications of ecosystem types, are used to calculate average hourly fluxes for a given region. In the NASA intensive site near Manaus, Brazil, it was found that although pasture areas cover only 11 percent of the region they account for 40 percent of the nitrous oxide flux. This analysis of the fluxes of trace gases through consideration of the gradients of factors that control both fluxes and ecosystem properties and processes will prove useful for extrapolating fluxes and for calculating budgets of nitrous oxide, methane, nonmethane hydrocarbons, and carbon dioxide.

Microbial Ectoenzymes in Aquatic Environments

Abstract: During the past decade an increasing number of ecological studies have considered the complexity of aquatic environments. One major outcome of these studies has been an accelerated interest in the role of microheterotrophs and the mode by which organic matter is made available to them. The heterotrophic microorganisms are the key level at which the metabolism of the whole ecosystem is affected, i.e., nutrient cycling, organic matter transformation and mineralization, and energy flow. The measurement of microbial activity in natural waters is very important for understanding the dynamic aspects of the functioning of the whole ecosystem.

Disturbance and recovery of large floodplain rivers

Abstract: Disturbance in a river-floodplain system is defined as an unpredictable event that disrupts structure or function at the ecosystem, community, or population level. Disturbance can result in species replacements or losses, or shifts of ecosystems from one persistent condition to another. A disturbance can be a discrete event or a graded change in a controlling factor that eventually exceeds a critical threshold. The annual flood is the major driving variable that facilitates lateral exchanges of nutrients, organic matter, and organisms. The annual flood is not normally considered a disturbance unless its timing or magnitude is “atypical.” The record flood of 1973 had little effect on the biota at a long-term study site on the Mississippi River, but the absence of a flood during the 1976–1977 Midwestern drought caused short- and long-term changes. Body burdens of contaminants increased temporarily in key species, because of increased concentration resulting from reduced dilution. Reduced runoff and sediment input improved light penetration and increased the depth at which aquatic macrophytes could grow. Developing plant beds exerted a high degree of biotic control and were able to persist, despite the resumption of normal floods and turbidity in subsequent years. In contrast to the discrete event that disturbed the Mississippi River, a major confluent, the Illinois River, has been degraded by a gradual increase in sediment input and sediment resuspension. From 1958 to 1961 formerly productive backwaters and lakes along a 320-km reach of the Illinois River changed from clear, vegetated areas to turbid, barren basins. The change to a system largely controlled by abiotic factors was rapid and the degraded condition persists. Traditional approaches to experimental design are poorly suited for detecting control mechanisms and for determining the critical thresholds in large river-floodplains. Large river-floodplain systems cannot be manipulated or sampled as easily as small streams, and greater use should be made of man-made or natural disturbances and environmental restoration as opportunistic experiments to measure thresholds and monitor the recovery process.

Disturbance regimes, resilience, and recovery of animal communities and habitats in lotic ecosystems

Abstract: Disturbance regime is a critical organizing feature of stream communities and ecosystems. The position of a given reach in the river basin and the sediment type within that reach are two key determinants of the frequency and intensity of flow-induced disturbances. We distinguish between predictable and unpredictable events and suggest that predictable discharge events are not disturbances. We relate the dynamics of recovery from disturbance (i.e., resilience) to disturbance regime (i.e., the disturbance history of the site). The most frequently and predictably disturbed sites can be expected to demonstrate the highest resilience. Spatial scale is an important dimension of community structure, dynamics, and recovery from disturbance. We compare the effects on small patches (⩽1 m2) to the effects of large reaches at the river basin level. At small scales, sediment movements and scour are major factors affecting the distribution of populations of aquatic insects or algae. At larger scales, we must deal with channel formation, bank erosion, and interactions with the riparian zone that will affect all taxa and processes. Our understanding of stream ecosystem recovery rests on our grasp of the historical, spatial, and temporal background of contemporary disturbance events.

Recovery of lotic macroinvertebrate communities from disturbance

Abstract: Ecosystem disturbances produce changes in macrobenthic community structure (abundances, biomass, and production) that persist for a few weeks to many decades. Examples of disturbances with extremely long-term effects on benthic communities include contamination by persistent toxic agents, physical changes in habitats, and altered energy inputs. Stream size, retention, and local geomorphology may ameliorate the influence of disturbances on invertebrates. Disturbances can alter food webs and may select for favorable genotypes (e.g., insecticidal resistance). Introductions of pesticides into lotic ecosystems, which do not result in major physical changes within habitats, illustrate several factors that influence invertebrate recovery time from disturbance. These include: (1) magnitude of original contamination, toxicity, and extent of continued use; (2) spatial scale of the disturbance; (3) persistence of the pesticide; (4) timing of the contamination in relation to the life history stages of the organisms; (5) vagility of populations influenced by pesticides; and (6) position within the drainage network. The ability of macroinvertebrates to recolonize denuded stream habitats may vary greatly depending on regional life histories, dispersal abilities, and position within the stream network (e.g., headwaters vs larger rivers). Although downstream drift is the most frequently cited mechanism of invertebrate recolonization following disturbance in middle- and larger-order streams, evidence is presented that shows aerial recolonization to be potentially important in headwater streams. There is an apparent stochastic element operating for aerial recolonization, depending on the timing of disturbance and flight periods of various taxa. Available evidence indicates that recolonization of invertebrate taxa without an aerial adult stage requires longer periods of time than for those that possess winged, terrestrial adult stages (i.e., most insects). Innovative, manipulative experiments are needed in order to address recolonization mechanisms of animals inhabiting streams that differ in size, latitude, disturbance frequency and magnitude, as well as the potential influence of early colonists on successional sequences of species following disturbance.

Grassland biogeochemistry: Links to atmospheric processes

Abstract: Regional modeling is an essential step in scaling plot measurements of biogeochemical cycling to global scales for use in coupled atmosphere-biosphere studies. We present a model of carbon and nitrogen biogeochemistry for the U.S. Central Grasslands region based on laboratory, field, and modeling studies. Model simulations of the geography of C and N biogeochemistry adequately fit observed data. Model results show geographic patterns of cycling rates and element storage to be a complex function of the interaction of climatic and soil properties. The model also includes regional trace gas simulation, providing a link between studies of atmospheric geochemistry and ecosystem function. The model simulates nitrogenous trace gas emission rates as a function of N turnover and indicates that they are variable across the grasslands. We studied effects of changing climate using information from a global climate model. Simulations showed that increases in temperature and associated changes in precipitation caused increases in decomposition and long-term emission of Co2 from grassland soils. Nutrient release associated with the loss of soil organic matter caused increases in net primary production, demonstrating that nutrient interactions are a major control over vegetation response to climate change.

Carbon balance in tussock tundra under ambient and elevated atmospheric CO2.

Abstract: Whole ecosystem CO2 flux under ambient (340 μl/l) and elevated (680 μl/l) CO2 was measured in situ in Eriophorum tussock tundra on the North Slope of Alaska. Elevated CO2 resulted in greater carbon acquisition than control treatments and there was a net loss of CO2 under ambient conditions at this upland tundra site. These measurements indicate a current loss of carbon from upland tundra, possibly the result of recent climatic changes. Elevated CO2 for the duration of one growing season appeared to delay the onset of dormancy and resulted in approximately 10 additional days of positive ecosystem flux. Homeostatic adjustment of ecosystem CO2 flux (sum of species' response) was apparent by the third week of exposure to elevated CO2. Ecosystem dark respiration rates were not significantly higher at elevated CO2 levels. Rapid homeostatic adjustment to elevated CO2 may limit carbon uptake in upland tundra. Abiotic factors were evaluated as predictors of ecosystem CO2 flux. For chambers exposed to ambient and elevated CO2 levels for the duration of the growing season, seasonality (Julian day) was the best predictor of ecosystem CO2 flux at both ambient and elevated CO2 levels. Light (PAR), soil temperature, and air temperature were also predictive of seasonal ecosystem flux, but only at elevated CO2 levels. At any combination of physical conditions, flux of the elevated CO2 treatment was greater than that at ambient. In short-term manipulations of CO2, tundra exposed to elevated CO2 had threefold greater carbon gain, and had one half the ecosystem level, light compensation point when compared to ambient CO2 treatments. Elevated CO2-acclimated tundra had twofold greater carbon gain compared to ambient treatments, but there was no difference in ecosystem level, light compensation point between elevated and ambient CO2 treatments. The predicted future increases in cloudiness could substantially decrease the effect of elevated atmospheric CO2 on net ecosystem carbon budget. These analyses suggest little if any long-term stimulation of ecosystem carbon acquisition by increases in atmospheric CO2.

Climatic change and its ecological implications at a subantarctic island.

Abstract: Marion Island (47°S, 38°E) has one of the most oceanic climates on earth, with consistently low air temperatures, high precipitation, constantly high humidity, and low incident radiation. Since 1968 mean surface air temperature has increased significantly, by 0.025° C year-1. This was strongly associated with corresponding changes in sea surface temperature but only weakly, or not at all, with variations in radiation and precipitation. We suggest that changing sealevel (atmospheric and oceanic) circulation patterns in the region underlie all of these changes. Sub-Antarctic terrestrial ecosystems are characterized by being species-poor and having a simple trophic structure. Marion Island is no exception and a scenario is presented of the implications of climatic change for the structure and functioning of its ecosystem. Primary production on the island is high and consequently the vegetation has a large annual requirement for nutrients. There are no macroherbivores and even the insects play only a small role as herbivores, so most of the energy and nutrients incorporated in primary production go through a detritus, rather than grazing, cycle. Ameliorating temperatures and increasing CO2 levels are expected to increase productivity and nutrient demand even further. However, most of the plant communities occur on soils which have especially low available levels of nutrients and nutrient mineralization from organic reserves is the main bottleneck in nutrient cycling and primary production. Increasing temperatures will not significantly enhance microbially-mediated mineralization rates since soil microbiological processes on the island are strongly limited by waterlogging, rather than by temperature. The island supports large numbers of soil macro-arthropods, which are responsible for most of the nutrient release from peat and litter. The activities of these animals are strongly temperature dependent and increasing temperature will result in enhanced nutrient availability, allowing the potential for increased primary production due to elevated temperature and CO2 levels to be realized. However, housemice occur on the island and have an important influence on the ecosystem, mainly by feeding on soil invertebrates. The mouse population is strongly temperature-limited and appears to be increasing, possibly as a result of ameliorating temperatures. We suggest that an increasing mouse population, through enhanced predation pressure on soil invertebrates, will decrease overall rates of nutrient cycling and cause imbalances between primary production and decomposition. This, along with more direct effects of mice (e.g. granivory) has important implications for vegetation succession and ecosystem structure and functioning on the island. Some of these are already apparent from comparisons with nearby Prince Edward Island where mice do not occur. Other implications of climatic change for the island are presented which emphasize the very marked influences that invasive organisms have on ecosystem structure and functioning. We suggest that changing sealevel circulation patterns, by allowing opportunities for colonization by new biota, may have an even more important influence on terrestrial sub-Antarctic ecosystems than is suggested merely on the basis of associated changes in temperature or precipitation.

Denitrification In Aquatic Sediments

Abstract: Nutrients (N and P) are important in controlling primary production in aquatic ecosystems. The availability of N or P within an ecosystem is partly a function of the rates of external inputs to the system and permanent removal within the system by biological, chemical and/or physical processes. N is a limiting nutrient in many estuaries, coastal and continental shelf marine systems (Ryther and Dunstan, 1971), some lakes (Keeney, 1973; Gerhart and Likens, 1975), and some streams and rivers (Grimm and Fisher, 1986). Denitrification in the sediments or anoxic water removes N from aquatic systems as the gaseous nitrogen produced diffuses into the atmosphere. Denitrification in aquatic systems, therefore, decreases the amount of nitrogen available for primary production, as well as decreases the transport of N to downstream or offshore waters. In aquatic systems that receive N from pollution sources, the removal of a portion of that N via denitrification may help control the degree of eutrophication of the system.

Signy Island as a Paradigm of Biological and Environmental Change in Antarctic Terrestrial Ecosystems

Abstract: Terrestrial ecosystems throughout much of Antarctica are exhibiting local changes in their environment which are, either directly or indirectly, influencing biological processes. Central to these changes is the present climatic warming being experienced, especially in the more northerly regions, which is causing substantial recession of glaciers and ice fields and creating new surfaces on which biota may become established. Other temporal changes occur as a function of the natural process of colonisation, growth and ecosystem development but which may be strongly influenced by minor variations in climate or other components of the environment. Examples of such environmental, and consequently of biological, changes are presented for one climatically and ecologically sensitive locality, Signy Island in the South Orkney Islands, to illustrate the diversity of changes which may affect the structure and dynamics of Antarctic ecosystems in general. These are discussed in terms of ecological change resulting from long-term climatic trends, short-term climatic (especially summer temperature) fluctuations, plant colonisation and growth, community development, and environmental perturbation. A plea is made for implementing long-term monitoring studies to determine the direction and rate of environmental and ecological changes, with particular regard to assessing the resiliance of ecosystems to and their recovery from these phenomena. The Antarctic environment offers probably the most significant baseline to which global atmospheric changes may be related. The predicted trend in global warming implies disturbing consequences for the future integrity of Antarctica’s, or indeed the world’s, environment and biota. However, it offers ideal opportunities to study the cause-and-effect relationship of ecological change and, from this, to develop a strong management policy for the active use and conservation of the Antarctic biome.

Factors controlling sediment community respiration in woodland stream ecosystems

Abstract: In shaded woodland streams, sediment community respiration is an important measure of decomposition of organic matter. In situ rates of community respiration were measured in sediments from shaded woodland streams at the Hubbard Brook Experimental Forest (HBEF), USA, by estimating rates of CO2 production in low-disturbance benthic chambers. Rates of sediment community respiration (range: 26-340 mg C * m-2 * d) were closely correlated with amounts of sediment organic matter, but not with water column DOC or sediment size structure. Analysis of literature data indicated similar couplings between community respiration and sediment organic matter in other streams and in lakes and marine systems. However, respiration per unit organic matter was 23-fold higher in lakes and marine systems than in woodland streams, apparently due to differences in quality of organic matter (algae and macrophytes vs terrestrial detritus). In HBEF streams, community respiration was elevated in sediments of organic debris dams, suggesting that organic debris dams are focal sites of metabolism and nutrient regeneration in the stream channel.

Recovery of lotic periphyton communities after disturbance

Abstract: Periphyton communities represent potentially excellent candidates for assessing the recovery of lotic ecosystems after disturbance. These communities are ubiquitous, relatively easy to sample and measure (in terms of total community biomass), have short generation times, and may influence the recovery rates of higher trophic levels. The first section of this article analyzes how site availability, species availability, and differential species performance influence periphyton successional dynamics. This background information provides a foundation for understanding how periphytic organisms respond after a disturbance. The second section of this article analyzes how periphyton communities respond to four different types of disturbance (flood events, desiccation, organic nutrient enrichment, and toxic metal exposure). Although data are limited, it is concluded that the fast growth rates and short generation times of periphytic organisms, coupled with their flexible life history strategies and good dispersal ability, allow lotic periphyton communities to recover relatively quickly after a disturbance. In addition, disturbance type and severity, local environmental conditions, and site-specific factors also will influence recovery rates.

Recovery processes in lotic ecosystems: Limits of successional theory

Abstract: The concept of succession has a distinguished history in general ecology and has been applied to stream ecosystems with some success. Succession in streams is largely secondary, follows initial floristics models, and occurs through a variety of mechanisms. The process is moderately predictable but is highly influenced by “climatic” factors, particularly nutrient chemistry. In desert streams, succession does not result in a climax state. While evidence is slim, succession may not be a significant process in streams of certain types or in certain regions.

Adélie and Chinstrap Penguins: Their Potential as Monitors of the Southern Ocean Marine Ecosystem

Abstract: Analysis of annual variability in the parameters recommended by CCAMLR for ecosystem monitoring revealed considerable variation among the parameters for which we have sufficient data to make an assessment. Interpretation of the observed variability was greatly enhanced when several parameters were considered simultaneously. In concert, the parameters should be useful for assessing perturbations in the Antarctic ecosystem. The choice of Adelie and chinstrap penguins as key indicator species seems an excellent one, given their very different responses to the highly variable conditions in the Antarctic Peninsula region, their species-specific breeding patterns, and the asynchrony between their respective breeding seasons. If a time series of data becomes available, it may be possible to differentiate between fishery-induced and natural perturbations in the ecosystem.

Changes in microbial nutrient status during secondary succession and its modification by earthworms.

Abstract: Microbial biomass, nutrient (N and P) status, and carbon and nutrient limitation of the microflora were investigated in soils from five different sites (field, 5-, 12-, and about 50-year-old fallow, beechwood), which represent different stages of a secondary succession from a wheat field to the climax ecosystem of a beechwood on limestone. In addition, the effect of faeces production by the substrate feeding earthworm species Octolasion lacteum (Orley) on the nutrient status of the soil microflora of these sites was studied. Humus had accumulated in the soil of the third fallow site, with an enhanced biomass of microflora. However, in the beechwood soil, which had the highest humus content, microbial biomass was lower than in the soil of the third fallow site and similar to that of the field and the two younger fallow sites. In general, soil microbial biomass was little affected by the passage of soil through the gut of O. lacteum. The soil microflora of the field, the 5-, 12-, and about 50-year-old fallow was limited by carbon, whereas in the beechwood soil phosphorus limited microbial growth. NItrogen availability to the soil microflora was low in the two younger fallow sites and high in the field and the third fallow. In the beechwood soil nitrogen supply did not affect microbial carbon utilization. Application of phosphorus stimulated glucose mineralization in the soil of the field, the third fallow, and the beechwood, but not in the two younger fallow sites. Therefor, the nutrient status of the soil microflora seems to have changed during secondary succession: presumably, during the first phase the availability of nitrogen decreased, whereas during the second phase microbial phosphorus supply became more important, which resulted in phosphorus limitation of the soil microflora in the climax ecosystem. The passage of soil through the gut of O. lacteum caused an alteration in the microbial nutrient status. Generally, microbial growth in earthworm casts was limited by carbon. The relative effect of the gut passage of the soils on microbial carbon utilization seems to increase during succession. Therefore, the effect of decomposer invertebrates on microbial nutrient supply seems to increase during secondary succession. In general, nitrogen did not limit microbial carbon utilization in earthworm casts. Phosphorus requirements of the soil microflora were lowered by the gut passage of the soil of the third fallow site and the beechwood, which indicates an increased phosphorus supply in earthworm casts. Howerver, this additional supply was not sufficient to enable optimal carbon utilization by the soil microflora. The results indicate that the effect of decomposer invertebrates on the soil microflora depends on the nutrient status of the ecosystem.

Influence of mangrove detritus in an estuarine ecosystem

Abstract: This study has determined the relationship and extent of mangrove reduced carbon flow and its contribution to the diet of higher consumers in a nearby seagrass bed using stable isotope techniques. In a seagrass bed approximately 80 m from a riverine mangrove community, detritus from seagrass or other marine sources of carbon such as epiphytes (Stilophora rhizodes) or phytoplankton the major contributor of reduced carbon. Mangroves make only a localized contribution to the food chain, thus contributing a relatively low proportion of reduced carbon to this seagrass community. The major primary producers in southern Florida fringe coastal communities are seagrasses and mangroves. Thalassia testudinum dominates the seagrass beds offshore, with Halodule wrightii as a secondary species. Rhizophora mangle dominates and Avicennia germinans and Laguncularia racemosa contribute significantly to the riverine and fringe forests. The detrital material formed by the partial decomposition of the leaves of these vascular plants may be the basis of trophic food chains in coastal ecosystems of southern Florida. Mangrove forests are net exporters of particulate organic carbon (Golley et a1., 1962; Heald, 1969; Lugo et al., 1976). Further, mangrove carbon supports food chains within riverine and basin communities along the southwest coast of Florida (Odum and Heald, 1975; Zieman et a1., 1984). In areas where seagrass and mangrove communities are in close association, however, the relative contribution of the seagrass and mangrove detritus to the food chain is difficult to determine. Mangrove estuarine environments are important nursery areas for fish populations, yet large numbers of organisms are supported by seagrass beds, the magnitude being related to the seagrass cover density (Zieman et a1., 1984). This paper addresses the question of whether the influence of mangrove detrital flow is far reaching or localized. To answer this question we used stable carbon isotope abundance analysis of organisms from a mangrove riverine community and two seagrass communities, one of which was geographically close to a mangrove stand and the other was without any apparent mangrove detritus input. Stable isotope analysis provides a powerful tool to determine carbon flow in this particular ecosystem because there is a significant difference in the carbon-I 3 content of the primary producers in question (mangroves and seagrasses, Zieman et al., 1984). Further, consumers of mangroves and seagrass can be identified by isotopic analysis, because diet switching and turnover experiments for carbon clearly show that diet is the primary determinant of the animal isotopic composition (see Fry and Arnold, 1982). Unfortunately, nitrogen isotope analysis cannot be used in this situation since in some cases there are no differences in natural 15N abundances between seagrass and mangrove detritus (Zieman et a1., 1984).

Global change: Translating plant ecophysiological responses to ecosystems.

Abstract: The physiological responses of plants to elevated CO 2 have not been incorporated into most models of ecosystem function under changed climate. These responses are now well documented, and recent work demonstrates that they can be readily included in ecosystem models. Simulations show that the effects of elevated CO 2 levels on transpiration and gas exchange will increase the sensitivity of community structure (particularly of forests) to climate change.

Succession in Abandoned Fields. Studies in Central Bohemia, Czechoslavakia.

Abstract: 1. Introduction.- 2. General characteristics of the region.- 3. Abandoned fields in the region.- 3.1 General characteristics.- 3.2 Intensively studied fields.- 3.3 Principal plant species of the studied fields.- 4. Dynamics of populations and communities.- 4.1 Changes in vegetation during succession.- 4.1.1 Plant populations.- 4.1.2 Life histories of principal plant populations, including their allelopathic interferences.- 4.1.3 Horizontal structure.- 4.1.4 Successional and seasonal changes in biomass and production.- 4.2 Heterotrophic organisms.- 4.2.1 Soil micromycetes.- 4.2.2 Collembola and other microarthropods.- 4.2.3 Small mammals.- 5. Selected ecosystem processes and functions.- 5.1 Microclimate.- 5.2 Water balance.- 5.2.1 Soil moisture, transpiration, water saturation deficit and sublethal deficit.- 5.2.2 Root water potential of dominants.- 5.3 Mineral nutrients.- 5.3.1 General soil chemical characteristics.- 5.3.2 Seasonal dynamics of phosphorus.- 5.3.3 Dynamics of potassium, calcium and sodium.- 5.4 Decomposition and release of nutrients.- 6. Reaction to perturbations.- 6.1 Mowing.- 6.2 Vehicle perturbation.- 6.3 Nutrient additives.- 6.3.1 Effects of phosphorus addition on interactions between the plant dominants.- 6.3.2 Phosphorus leaching experiments.- 6.3.3 Potassium additives.- 6.4 Fly ash application.- 6.5 Effect of herbicides.- 7. Synthesis.- 7.1 Vegetational dynamics.- 7.2 Water and nutrient economy.- 7.3 Stability.- 7.4 Abandoned fields in the landscape.- 8. Summary.- References.

Australia's unique biota: implications for ecological processes.

Abstract: The infertility of the soil and the climatic varia- ectothermic vertebrates. It has also resulted in high preva- bility in Australia are the most extreme of all the continents lence of fire and the evolution of a fire-adapted biota. and, after a long period of relative isolation, have resulted Within the wet-dry tropics of Australia, the greatest in a highly characteristic biota. Like those of the tropics of diversity of plants and animals is located in the extensive, Asia, Africa and South America, the biota of tropical but structurally simple and somewhat unproductive, euca- Australia has its major biogeographic affinities with the lypt forests and woodlands (savannas). The wetlands and biota of its own continent. The eucalypts, the phyllodinous wet forests, being more productive, have more vertebrate acacias and many of the vertebrate groups are distinctively herbivores and larger vertebrates predators but shorter food Australian. The radiation of some biotic groups are clearly chains. Social insects dominate in the savannas, having sub- consequences of the evolution of particular resources in stantial impacts on all aspects of the functioning of the abundance (e.g. parrots and honeyeaters), but others are savannas. Frequent fires of low intensity have resulted in a probably largely historical (e.g. marsupials). range of patterns of animal utilization of fire with conse- The low productivity of native Australian ecosystems has quences for community structure and function. The Austra- favoured the occurrence of mutualistic relationships. It has lian savanna biota is clearly a distinctive eucalypt also meant that spatial heterogeneity has great functional woodland biota rather than an attenuated rainforest biota as importance for spatial heterogeneity as small areas of high- in South America or a grassland biota as in Africa. er productivity are crucial refuges and are sites of intense