Showing papers by "Helmholtz Centre for Environmental Research - UFZ published in 2000"

Ecological buffering mechanisms in savannas : a unifying theory of long-term tree-grass coexistence

Abstract: Despite the large spatial extent and the obvious importance of the savanna biome, not to mention several decades of savanna research, the origin, age, nature, and dynamics of savannas are not well understood. Basically, the question surrounding the presence or existence of savannas focuses on the long-term coexistence of the dominating life forms – trees and grasses. How do these two very different components coexist, without one of them dominating the other, and what mechanisms determine the proportion of each? Earlier equilibrium concepts have recently been replaced by non-equilibrium concepts, and the current view is that tree-grass interactions in savannas cannot be predicted by a simple model. Instead, many interacting factors operating at various spatial and temporal scales contribute to creating and maintaining savanna physiognomy. In this paper we analyse a number of studies from savannas in different parts of the world and discuss whether a general pattern can be perceived behind the numerous factors influencing the presence of savannas systems. On the basis of this analysis we propose a new unifying concept of savanna existence, i.e., the concept of ecological buffering mechanisms. In contrast to previous approaches to explain tree-grass coexistence in savannas, the concept of buffering mechanisms does not focus on equilibria or non-equilibria, steady states of the system or domains of attraction. Instead, in the concept of ecological buffering mechanisms we suggest that it is much more useful to focus on the boundaries of savanna existence itself and to investigate the mechanisms that allow a savanna to persist in critical situations where this system is driven to its boundaries, e.g., pure grasslands or tropical forests. The concept of ecological buffering mechanisms integrates both earlier concepts of ecological theory and general ideas on savanna dynamics as well as specific studies of savannas in different parts of the world.

Exotic plant species invade diversity hot spots: the alien flora of northwestern Kenya

Abstract: We analysed the distribution of native and alien plant species across 20 ecogeographic zones of northwestern Kenya. The source pool for the majority of aliens was Europe and America. Thus, the source pool has a biogeographic bias which explains the low proportion of aliens in the tropics: most species in the European or American source pool are not well adapted to tropical conditions. As expected, native and alien plant species showed an area effect. Correcting for this area effect, species rich zones showed a higher proportion of alien plant species in their flora. At the analysed scale, species richness of native plant communities does not increase the resistance to invasions and alien plant species invade diversity hotspots. Compared to the other ecogeographic zone, the urban area around Nairobi showed an increased richness in alien and native plant species. This is very similar to findings in Europe, although the history of urbanisation is much shorter in Kenya. The species turnover between zones (β-diversity) shows a similar pattern in native and alien plant species. Within a very short time scale the alien plant species mapped the biogeographic patterns of natives, although the geography of human activities influences the propagule pressure.

From snapshot information to long-term population dynamics of acacias by a simulation model.

Abstract: The African Acacia species A. raddiana is believed to be endangered in the Negev desert of Israel. The ecology of this species is not well understood. The main idea of our study is to learn more about the long-term population dynamics of these trees using snapshot information in the form of size frequency distributions. These distributions are highly condensed indices of population dynamics acting over many years. In this paper, we analyse field data on recruitment, growth, and mortality and use an existing simulation model of the population dynamics of A. raddiana (SAM) to produce contrasting scenarios of these live history processes that are based on the analysed field evidence. The main properties of simulated as well as observed tree size frequency distributions are characterised with Simpson's index of dominance and a new permutation index. Finally, by running the SAM model under the different scenarios, we study the effect of these different processes on simulated size frequency distributions (pattern) and we compare them to size distributions observed in the field, in order to identify the processes acting in the field. Our study confirms rare recruitment events as a major factor shaping tree size frequency distributions and shows that the paucity of recruitment has been a normal feature of A. raddiana in the Negev over many years. Irregular growth, e.g., due to episodic rainfall, showed a moderate influence on size distributions. Finally, the size frequency distributions observed in the Negev reveal the information that, in this harsh environment, mortality of adult A. raddiana is independent of tree size (age).

Determination of arsenic species in water, soils and plants.

Abstract: Ion chromatographic separation coupled with ICP-MS was used to determine arsenic species in plant and soil extracts. A scheme for growth, harvesting, sample pre-treatment and analysis was developed for the arsenic species to enable determination. Preliminary results obtained with ten herb plants grown on arsenic-contaminated soil compared to non-contaminated soil show a heterogeneous pattern of accumulation rate, metabolization and detoxification mechanisms in monocots and dicots. Arsenite appears to be the major component in plants with good growth. Organic arsenic species were even detected at very low concentrations (< 150 μg kg–1 (dry mass)).

Fixed‐effect‐level toxicity equivalents—a suitable parameter for assessing ethoxyresorufin‐O‐deethylase induction potency in complex environmental samples

Abstract: Within the scope of bioassay-directed identification of dioxin-like toxicants in complex environmental samples, EC50-based and fixed-effect-level-based 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) toxicity equivalents (TEQs) were compared to assess 7-ethoxyresorufin-O-deethylase (EROD) inducing potency of sediment fractions using the rainbow trout liver cell line RTL-W1 as bioassay system. Toxicity equivalents on the basis of fixed effect levels are suggested in order to minimize interpretation problems due to the superposition of enzyme-inducing and enzyme-inhibiting effects. Bioassay-directed fractionation of a contaminated sediment extract in the industrial region of Bitterfeld (Germany) based on fixed-effect-level TEQs indicated high dioxin-like activity in the lipophilic sediment fractions containing the prototypic arylhydrocarbon receptor (AhR) agonists polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs). However, only a small part of the EROD induction could be attributed to the PCBs and PAHs that were analyzed. Significant EROD induction occurred also with some of the more polar fractions.

Do spatial effects play a role in the spatial distribution of desert-dwelling Acacia raddiana ?

Abstract: . We investigated the spatial pattern of A. raddiana in the Negev desert of Israel in order to gain insights into the factors and processes driving the dynamics of this species. Using a scale-dependent measure, the ring statistic, we analysed both patterns observed in the field and time series of spatial tree distributions produced by a simulation model. In the field, random spacing was the predominant pattern observed. However seedlings were clumped on small scales. We ran the model under two contrasting scenarios representing hypotheses that explain the clumping of seedlings and the random distribution of trees. One hypothesis is that there is spatial heterogeneity in seed distribution, germination and seedling mortality, but that these heterogeneities are not correlated with each other in space. The second hypothesis assumes a correlation between these heterogeneities leading to areas suitable for establishment. However, the suitability of the sites is temporally variable. Furthermore, the second hypothesis assumes density-dependent tree mortality due to competition. Both hypotheses lead to spatial distributions that are in qualitative agreement with the patterns observed in the field. Therefore, the classical view that a clumped seedling distribution and a random pattern of older trees is due to clumped regeneration and density-dependent mortality may not hold for Acacia trees in the Negev.

Hydroxylated metabolites of 2,4-dichlorophenol imply a fenton-type reaction in Gloeophyllum striatum.

Abstract: While degrading 2,4-dichlorophenol, two strains of Gloeophyllum striatum, a basidiomycetous fungus causing brown rot decay of wood, simultaneously produced 4-chlorocatechol and 3,5-dichlorocatechol. These metabolites were identified by comparing high-performance liquid chromatography retention times and mass spectral data with those of chemically synthesized standards. Under similar conditions, 3-hydroxyphthalic hydrazide was generated from phthalic hydrazide, a reaction assumed to indicate hydroxyl radical formation. Accordingly, during chemical degradation of 2,4-dichlorophenol by Fenton's reagent, identical metabolites were formed. Both activities, the conversion of 2,4-[U-(14)C]dichlorophenol into (14)CO(2) and the generation of 3-hydroxyphthalic hydrazide, were strongly inhibited by the hydroxyl radical scavenger mannitol and in the absence of iron. These results provide new evidence in favor of a Fenton-type degradation mechanism operative in Gloeophyllum.

Simulated long-term vegetation response to alternative stocking strategies in savanna rangelands

Abstract: Increasing cover by woody vegetation, prevalent in semiarid savanna rangelands throughout the world, is a degrading process attributed to the grazing impact as a major causal factor. We studied grazing effects on savanna vegetation dynamics under alternative stocking strategies with a spatially explicit grid-based simulation model grounded in Kalahari (southern Africa) ecology. Plant life histories were modeled for the three major life forms: perennial grasses, shrubs, annuals. We conducted simulation experiments over a range of livestock utilization intensities for three alternative scenarios of small scale grazing heterogeneity, and two alternative strategies: fixed stocking versus adaptive stocking tracking herbage production. Additionally, the impact of the duration of the management planning horizon was studied, by comparing community response and mean stocking rates after 20 and 50 years. Results confirmed a threshold behavior of shrub cover increase: at low, subcritical utilization intensity little change occurred; when utilization intensity exceeded a threshold, shrub cover increased drastically. For both stocking strategies, thresholds were highly sensitive to grazing heterogeneity. At a given critical utilization intensity, the long term effect of grazing depended on the level of grazing heterogeneity: whereas under low heterogeneity, shrub cover remained unchanged, a large increase occurred under highly heterogeneous grazing. Hence, information on spatial grazing heterogeneity is crucial for correct assessment of the impact of livestock grazing on vegetation dynamics, and thus for the assessment of management strategies. Except for the least heterogeneous grazing scenario, adaptive stocking allowed a more intensive utilization of the range without inflating the risk of shrub cover increase. A destabilizing feedback between rainfall and herbage utilization was identified as the major cause for the worse performance of fixed compared to adaptive stocking, which lacks this feedback. Given the usually high grazing heterogeneity in semiarid rangelands, adaptive stocking provides a management option for increasing herbage utilization and thus returns of livestock produce without increasing degradation risks.

Mammals in South American drylands: faunal similarity and trophic structure

Abstract: We compared the fauna of small mammals (less than 500 g body weight) among five major South American drylands (Atacama, Altiplano, Monte, Patagonia and Caatinga) and found considerable heterogeneity and distinctiveness in species richness and composition between these biomes. From a total of 89 recorded species, 76 of them are restricted to only one of these drylands. The highland desert, or Altiplano, is the biome with the highest number of species. Despite the marked differences in the composition of the mammalian fauna, the trophic structure shows a rather consistent pattern: herbivores are the most important trophic group in all drylands. This consistency seems to be more the result of phylogenetic inertia than of similar ecological processes. Our results are compared with recent studies on desert small mammals across continents.

Long-term dynamics in arid and semiarid ecosystems - synthesis of a workshop

Abstract: According to the United Nations (UNEP, Agenda 21), approximately one sixth of the world’s population, 70% of all drylands with a total area of 3.6 billion hectares, and a quarter of the total land surface of the earth is endangered by desertification. The most obvious consequences are increasing poverty and the growing damage to 3.3 billion hectares of rangeland. These impressive figures illustrate the importance of developing mechanisms for combating desertification. However, there is an increasing number of scientists who doubt the accuracy of these figures and who speak of ‘the myth of desertification’ as a publicity tool (e.g., Thomas & Middleton 1994). Their criticism is based on the idea that dryland ecosystems might after all be well adapted to disturbances and may exhibit good recovery characteristics. Nevertheless, it has been increasingly recognized that arid and semiarid ecosystems often exhibit complex non-equilibrium dynamics involving complicated nonlinear processes and stochastic event-driven behavior (Westoby et al. 1989; Walker 1993). Vegetation changes generally occur unpredictably in the short term (years) in response to rainfall, and episodically in the long term (several decades) in response to rare events, or due to grazing pressure, climatic change, altered disturbance regimes, or a combination of these factors. The complex dynamics of arid and semiarid ecosystems and especially the mismatch between observation times (years) and time scales of vegetation change (centuries) make it difficult to fully understand their long-term dynamics (Wiegand et al. 1995; Jeltsch et al. 1996). Hence, it is extremely difficult to establish whether an area is undergoing progressive, long-term decline in biodiversity and productivity (desertification), or whether it is merely suffering shortterm drought, from which the land may recover if human impact is reduced or eliminated. But for developing management strategies for sustained animal production and species conservation, it is necessary to know how resilient arid and semiarid ecosystems really are, and to assess their potential to recover from serious natural or human disturbances or stress. Long-term ecological monitoring and interdisciplinary approaches to interaction between rainfall, geology and ecology are required in order to gain a better understanding of long-term dynamics in arid and semiarid systems. These approaches include both field studies and modelling studies. The latter can help to extrapolate local information and knowledge gathered on shorter time scales to larger spatial and temporal scales. This knowledge originating from field and modeling studies forms an indispensable basis for identifying key factors and processes that shape the potential for both desertification and recovery therefrom. In the following we briefly summarize and link various papers (all this issue) on field and modeling studies that are based on contributions to the INTECOL symposium ‘Long-term dynamics in arid and semi-arid ecosystems’ in Florence, 1998, or that were inspired through discussions during and after the symposium

Novel lysimeter techniques : a basis for the improved investigation of water, gas, and solute transport in soils

Abstract: For many years lysimeters have been proven to be effective tools in assessing and predicting the effects of current land use and future land use changes in catchment areas on both water and solute balances. Although due to the diverse aspects of mass transport modelling, many different types of lysimeters exist, water and solute balances are by no means fully understood, especially in post-mining areas. To tackle this problem, a new piece of equipment has been developed which enables the actual weight of a lysimeter to be precisely measured. The newly designed device, which has been used for one of the experiments described in this paper, permits the weighing of for example a 2 m3 lysimeter vessel with an accuracy down to 30 g. The second newly developed appliance presented here is the GAMS (Gas-Migration-Simulator). Basically comparable to a lysimeter, the difference is that the GAMS allows the detailed investigation of soil-gas migration processes and their dependence on parameters like the diffusion coefficient and the gas permeability of the soil, alterations of the groundwater level and on various external influences such as changes of the actual meteorological conditions. These two newly developed techniques are described in this paper, and their respective suitability is demonstrated on the basis of data sets recorded during initial experiments. Neue Lysimetertechniken — eine Grundlage zur verbesserten Untersuchung des Wasser-, Gas- und Stofftransportes in Boden Lysimeter haben sich als effektives Hilfsmittel zur Bewertung und Voraussage der von gegenwartigen Landnutzungen und kunftigen Landnutzungsanderungen in Einzugsgebieten ausgehenden Wirkungen auf den Wasser- und Stoffhaushalt bewahrt. Obwohl fur die modellhafte Beschreibung von Stoffbilanzen verschiedene Lysimeteranlagen verfugbar sind, konnte der Wasser- und Stoffhaushalt in Bergbaufolgelandschaften bisher nicht vollstandig aufgeklart werden. Zur Losung dieses Problems wurde eine neue Generation von Lysimetern entwickelt, mit denen beispielsweise die Wagung von 2 m3 grosen Bodensaulen mit einer Genauigkeit von 30 g moglich ist. Bei der zweiten hier vorgestellten Neuentwicklung handelt es sich um einen Gas-Migrations-Simulator (GAMS). Der grundsatz-lich mit dem Bauprinzip eines Lysimeters vergleichbare GAMS ermoglicht im Gegensatz zu herkommlichen Lysimetern die detaillierte Untersuchung der Bodengasmigration bzw. der Abhangigkeit dieser Migrationsprozesse von verschiedenen Einflussgrosen. Relevante einflussnehmende Parameter sind beispielsweise der Diffusionskoeffizient und die Gaspermeabilitat des Bodens, Ande-rungen des Grundwasserstandes oder externe Einflusse wie tageszyklische oder langerfristige Schwankungen der meteorologischen Verhaltnisse. Die beiden Neuentwicklungen werden anhand erster Untersuchungsergebnisse beschrieben, und es werden Schlussfolgerungen fur die weitere Arbeit abgeleitet.

Body size and host range in European Heteroptera

Abstract: We used data on body size and host range of phytophagous Heteroptera in central Europe, an inverse measure of specialisation, to analyse the relationship of body size vs specialisation: 1) we found a clear positive relationship between body size and host range using species as independent data points. 2) However, a nested analysis of variance shows that most of the variance in body size occurred at higher taxonomic levels whereas most of the variance in host specialisation occurred between species. This suggests different phylogenetic inertia of body size and specialisation. Nevertheless, using means of different higher taxonomic levels there is still a significant positive correlation between body size and host range. 3) With more sophisticated methods of correcting for the phylogenetic relatedness between species, the positive correlation between body size and host range still holds, despite the different assumptions of each method. Thus, the relationship between body size and host range is a very robust pattern in true bugs.

Coupling of SPME and GC-AED for the Determination of Organometallic Compounds

Abstract: The solid-phase microextraction (SPME) technique coupled with gas chromatography and atomic emission detection was successfully applied for the determination of selected organometallic species of Pb, As and Hg in aqueous samples. To obtain a high extraction yield, the SPME conditions were optimised for each element by fibre selection and varying the exposure time, stirring rate, pH range and desorption time. All the organometallic compounds tested were extracted from the aqueous phase using SPME. The preconcentration factors attained ranged between 40 and 150, depending on the compound. Detection limits in the pg/L and ng/L ranges were achieved.

A Theoretical Study on the Metabolic Requirements Resulting from α-Ketoglutarate-Dependent Cleavage of Phenoxyalkanoates

Abstract: The etherolytic cleavage of phenoxyalkanoic acids in various bacteria is catalyzed by an alpha-ketoglutarate-dependent dioxygenase. In this reaction, the electron acceptor is oxidatively decarboxylated to succinate, whereas the proper substrate is cleaved by forming the oxidized alkanoic acid and the phenolic intermediate. The necessity of regenerating alpha-ketoglutarate and the consequences for the overall metabolism were investigated in a theoretical study. It was found that the dioxygenase mechanism is accompanied by a significant loss of carbon amounting to up to 62.5% in the assimilatory branch, thus defining the upper limit of carbon conversion efficiency. This loss in carbon is almost compensated for in comparison to a monooxygenase-catalyzed initial step when the dissimilatory efforts of the entire metabolism are included: the yield coefficients become similar. The alpha-ketoglutarate-dependent dioxygenase mechanism has more drastic consequences for microorganisms which are restricted in their metabolism to the first step of phenoxyalkanoate degradation by excreting the phenolic intermediate as a dead-end product. In the case of phenoxyacetate derivatives, the cleavage reaction would quickly cease due to the exhaustion of alpha-ketoglutarate and no growth would be possible. With the cleavage products of phenoxypropionate and phenoxybutyrate herbicides, i.e., pyruvate and succinate(semialdehyde), respectively, as the possible products, the regeneration of alpha-ketoglutarate will be guaranteed for stoichiometric reasons. However, the maintenance of the cleavage reaction ought to be restricted due to physiological factors owing to the involvement of other metabolic reactions in the pool of metabolites. These effects are discussed in terms of a putative recalcitrance of these compounds.

Diagnosis of Helicobacter pylori infection in children: is the 15N urine test more reliable than the 13C breath test?

Abstract: Background: The [13C]urea breath test is a convenient method to estimate Helicobacter pylori colonization non-invasively. As an alternative method, the [15N]urea urine test has been established. It is hypothesized that the urine test could be advantageous under some circumstances. The diagnostic value of the breath test might, for example, be jeopardized owing to fluctuating 13C isotope dilution by endogenous carbon dioxide. Methods: To compare the reliability and practicability of the two tests, 13C breath tests and 15N urine tests were performed simultaneously in three groups of patients: A) 36 adults under standardized conditions, B) 67 children who were allowed to move around during the test, and C) 18 children once without and once during standardized physical activity. Results: In the 36 adults there was a close correlation of the test results of the two methods (r = 0.88). In contrast, the correlation in the group of 67 children and infants was poor (r = 0.10). In the children with controlled activ...