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1.
Stable carbon- (δ13C), nitrogen- (δ15N) and hydrogen (δD) isotope profiles in feathers of migratory Great Reed Warblers Acrocephalus arundinaceus recaptured for 2 or more years in 6 successive years were examined to test whether the isotope profiles of individual warblers appeared to be consistent between years. Similar isotopic signatures in successive years suggested that individual birds tended to return and grow their feathers in Afro-tropical wintering habitats that generate similar δ13C, δ15N and δD signatures. Previous studies have shown that Great Reed Warblers exhibit strong natal and breeding philopatry, with most of the surviving birds returning to the breeding site. The present study of feather δ13C, δ15N and δD isotopic values demonstrate the year-to-year fidelity might also include the African moulting sites in this migratory species.  相似文献   

2.
Five microhabitat types with varying degrees of bird influence were examined. Soils were collected from open polygons, under mosses and bird nests on a nunatak with breeding snow petrels (Pagodroma nivea) and from open polygons and under mosses on a non-bird nunatak. Nutrient levels (total N and P, nitrate, nitrite and ammonia), moisture levels and δ 15N values were determined and the organic processes of nitrogen fixation (acetylene reduction) and soil respiration (CO2 flux) were examined. Nests represented the most favourable microhabitat type for soil respiration having the highest nutrient levels and most favourable temperature and moisture regimes. The soils under mosses were also favourable and appear to act as a nutrient sink for nutrients originating from the nests. The open polygons were the least favourable for biological activity. There was little nitrogen fixation in any of the soils except for the soils under mosses from the non-bird nunatak. Fixation is possibly limited in favourable microhabitat types on the bird nunatak by high nitrogen levels. These results were confirmed by the δ 15N results, which had high values typical of a seabird signal in the soils from the bird nunatak and values near zero, typical of soils containing fixed nitrogen, on the non-bird nunatak. Received: 3 March 1997 / Accepted: 30 March 1998  相似文献   

3.
To investigate migratory connectivity in the Reed Warbler Acrocephalus scirpaceus, we analysed (1) all available sub-Saharan ringing recoveries and (2) stable isotopes in feathers grown in Africa sampled at 17 European breeding sites across a migratory divide. A cluster analysis of ringing recoveries showed remarkable connectivity between breeding and non-breeding grounds. Two main clusters represented populations taking the two main migratory routes [southwesterly (SW) and southeasterly (SE)]. Stable isotope analysis confirmed the separation of wintering areas of SW- and SE-migrating populations. Higher δ15N values in feathers of SE-migrating birds indicated that they occupied more xeric biome types. Values of δ13C that did not differ significantly among populations were higher than those from feathers of known European origin and indicated a C4 biome. Three populations with an unknown migratory direction were assigned to the SE-migrating populations on the basis of δ15N values.  相似文献   

4.
It is widely accepted that stable isotope ratios in inert tissues such as feather keratin reflect the dietary isotopic signature at the time of the tissue synthesis. However, some elements such as stable nitrogen isotopes can be affected by individual physiological state and nutritional stress. Using malaria infection experiment protocols, we estimated the possible effect of malaria parasite infections on feather carbon (δ13C) and nitrogen (δ15N) isotope signatures in juvenile common crossbills Loxia curvirostra. The birds were experimentally infected with Plasmodium relictum (lineage SGS1) and P. ashfordi (GRW2), two widespread parasites of passerines. Experimental birds developed heavy parasitemia of both parasites and maintained high levels throughout the experiment (33 days). We found no significant difference between experimental and control birds in both δ13C and δ15N values of feathers re-grown. The study shows that even heavy primary infections of malaria parasites do not affect feather δ13C and δ15N isotopic signatures. The results of this experiment demonstrate that feather isotope values of wild-caught birds accurately reflect the dietary isotopic sources at the time of tissue synthesis even when the animal’s immune system might be challenged due to parasitic infection.  相似文献   

5.
To understand the year-round ecology of seabirds it is necessary not only to study the birds in their breeding grounds, but also to gain information about their movements during the inter-breeding period. Especially for the smaller procellariiform species, such studies are still scarce, mainly due to methodological problems. The recovery rates of banded birds are low and satellite tracking devices still far too heavy to equip these small birds. Here, we present data on foraging areas of Wilson’s storm-petrel Oceanites oceanicus inferred from stable isotope analysis. We compared ratios of δ13C and δ15N between different life-history stages and between the breeding and inter-breeding period. Samples of adult and chick feathers, chick down and egg-white were taken between 1996 and 2005 on King-George-Island, South Shetland Islands. δ13C values can be clearly distinguished between the breeding and inter-breeding period. During the inter-breeding period, most pre-breeders foraged in the same area as breeders, but four pre-breeders were found to forage in latitudes north of the Subtropical Front. In the 2002 inter-breeding period adult birds wintered further north than in 2003, which is in line with the different locations of food rich frontal systems in these years. We show that isotope ratios of both δ13C and δ15N increase from egg white, over chick down to chick feathers. We suggest that this isotopic change, due to a change in both foraging location and diet between egg production and chick feeding, may be used to trace the shift from the use of maternal resources from the egg to the uptake of nutrients from the diet.  相似文献   

6.
Stable isotope analysis is an increasingly valuable tool in ecological studies and shows promise as a measure of nutritional stress in wild animals. Thus far, however, the only studies on endotherms that have conclusively shown changes in δ15N and δ13C values in response to nutritional stress were conducted on fasting animals and animals growing under extreme levels of food restriction. We conducted a laboratory experiment to test whether δ15N and δ13C values provide a general index of nutritional stress. We compared the isotopic composition of whole blood, liver, muscle and feathers between two groups of juvenile song sparrows (Melospiza melodia) hand-reared in captivity under identical conditions except for feeding regime. To verify that our experimental treatment induced a biologically meaningful level of nutritional stress, we simultaneously measured the effects on physiology, growth and development at multiple scales. While food-restricted birds were physiologically stressed, physically smaller, and showed poorer growth and brain development compared to ad libitum-fed birds, there was no effect of feeding regime on either δ15N or δ13C values in any tissue. Instead of a continuum where the level of change in 15N or 13C contents corresponds to the level of nutritional stress, we suggest there may be a threshold level of nutritional stress below which such isotopic changes are likely to be negligible.  相似文献   

7.
The little auk (Alle alle), a small and abundant planktivorous seabird that breeds in the high Arctic, has the potential to be used as a monitor of the composition and abundance of lower trophic-level zooplankton. We investigated age- and sex-related sources of variation in diet and stable isotope (δ13C and δ15N) values of little auks breeding in Spitsbergen during the summer of 2002 to evaluate this possibility. Stable isotope profiles of both adult and chick blood changed over the breeding season, with blood δ15N values increasing and δ13C values decreasing. This could represent a switch to higher trophic-level prey derived from more pelagic sources. However, while chick blood δ13C values followed those values in their meals, this was not the case for blood δ15N values, suggesting additional physiological mechanisms influencing blood δ15N values in growing chicks. Chicks had consistently lower δ15N values than their parents, which may indicate they were being fed on lower trophic-level prey items or may alternatively reflect complexities in chick blood δ15N values through the growth period. These results have several important implications for use of stable isotope analysis as a tool to detect changes in seabird diet and availability of lower trophic-level prey in high-Arctic marine environments. Until physiological aspects of stable isotope discrimination are well understood, we caution against using chicks of this seabird as any form of isotopic monitor.  相似文献   

8.
Recent studies have shown that the complementary analysis of mercury (Hg) concentrations and stable isotopic ratios of nitrogen (δ15N) and carbon (δ13C) can be useful for investigating the trophic influence on the Hg exposure and accumulation in marine top predators. In this study, we propose to evaluate the interspecies variability of Hg concentrations in phocids from polar areas and to compare Hg bioaccumulation between both hemispheres. Mercury concentrations, δ15N and δ13C were measured in fur from 85 individuals representing 7 phocidae species, a Ross seal (Ommatophoca rossii), Weddell seals (Leptonychotes weddellii), crabeater seals (Lobodon carcinophagus), harbour seals (Phoca vitulina), grey seals (Halichoerus grypus), ringed seals (Pusa hispida) and a bearded seal (Erignathus barbatus), from Greenland, Denmark and Antarctica. Our results showed a positive correlation between Hg concentrations and δ15N values among all individuals. Seals from the Northern ecosystems displayed greater Hg concentrations, δ15N and δ13C values than those from the Southern waters. Those geographical differences in Hg and stable isotopes values were likely due to higher environmental Hg concentrations and somewhat greater number of steps in Arctic food webs. Moreover, dissimilarities in feeding habits among species were shown through δ15N and δ13C analysis, resulting in an important interspecific variation in fur Hg concentrations. A trophic segregation was observed between crabeater seals and the other species, resulting from the very specific diet of krill of this species and leading to the lowest observed Hg concentrations.  相似文献   

9.
Trophic polymorphism was recently reported in introduced bluegill (Lepomis macrochirus) in Lake Biwa, Japan, where three morphs are specialized in benthic invertebrates (benthivorous type), submerged aquatic plants (herbivorous type), and zooplankton (planktivorous type). We evaluated the long-term effects of food resource utilization by these trophic morphs using stable isotope ratios, δ15N and δ13C. A significant difference in δ15N was found between the benthivorous and planktivorous types. The planktivorous type had the higher δ15N value, which corresponded with the value expected from its prey, zooplankton. The lower δ15N value of the benthivorous type would be derived from the lower δ15N values of benthic prey organisms compared to zooplankton. These results support previous findings that the benthivorous and planktivorous types have different food resource utilization. In contrast, the δ15N and δ13C values of the herbivorous type were distinctly different from the expected values, indicating that this type was unlikely to utilize aquatic plants substantially, contradicting the results of the dietary analysis.  相似文献   

10.
Some procellariiform seabirds use a dual strategy for provisioning their chicks by alternating short (ST) and long (LT) foraging trips. Parent birds gain mass during LT but they lose mass while increasing the chick feeding frequency during ST. Self-feeding during LT is crucial for the success of ST because firstly most of the energy used during ST is likely to be derived from the energy stored during LT and secondly self-feeding during ST is presumed to be negligible. Self-feeding by adult procellariiforms is thus a key issue to understand allocation processes but it is still poorly known. We tested these predictions by using the stable isotope (δ15N and δ13C) technique on birds’ plasma and prey with the short-tailed shearwater Puffinus tenuirostris breeding at Tasmania as a model. Parent shearwaters returning to the colony after a LT have an Antarctic/subantarctic δ13C signature in their plasma (−23.8‰), thus indicating that they fed in cold waters, far away from their breeding colony, for their own maintenance. Parent birds returning to the colony after a ST also have a distant Antarctic/subantarctic δ13C signature in their plasma (−24.3‰), thus verifying that self-feeding is negligible during ST and that birds fast at that time, using energy stores built up in cold waters. Plasma δ15N values of adults (8.8‰) indicates they mainly prey upon zooplankton-eating organisms, probably mesopelagic myctophid fishes. A simple isotopic mixing model estimates that they consume by mass 87% myctophids and 13% subantarctic krill when self-feeding. Finally and as expected, the carbon isotopic signature of chick plasma (−22.2‰) was intermediate between those of high- and low-latitude marine organisms and is thus in agreement with chicks being fed with a large diversity of prey species caught by adult birds from Antarctic to Tasmanian waters. One main consequence of this system is that reproduction of a Tasmanian species is controlled by resources available at great distances from the breeding colony that drive allocation decisions of parent birds.  相似文献   

11.
Addressing spatial variability in nitrogen (N) availability in the Central Brazilian Amazon, we hypothesized that N availability varies among white-sand vegetation types (campina and campinarana) and lowland tropical forests (dense terra-firme forests) in the Central Brazilian Amazon, under the same climate conditions. Accordingly, we measured soil and foliar N concentration and N isotope ratios (δ15N) throughout the campina-campinarana transect and compared to published dense terra-firme forest results. There were no differences between white-sand vegetation types in regard to soil N concentration, C:N ratio and δ15N across the transect. Both white-sand vegetation types showed very low foliar N concentrations and elevated foliar C:N ratios, and no significant difference between site types was observed. Foliar δ15N was depleted, varying from −9.6 to 1.6‰ in the white-sand vegetations. The legume Aldina heterophylla had the highest average δ15N values (−1.5‰) as well as the highest foliar N concentration (2.1%) while the non-legume species had more depleted δ15N values and the average foliar N concentrations varied from 0.9 to 1.5% among them. Despite the high variation in foliar δ15N among plants, a significant and gradual 15N-enrichment in foliar isotopic signatures throughout the campina–campinarana transect was observed. Individual plants growing in the campinarana were significantly enriched in 15N compared to those in campina. In the white-sand N-limited ecosystems, the differentiation of N use seems to be a major cause of variations observed in foliar δ15N values throughout the campina–campinarana transect.  相似文献   

12.
To determine whether stable isotope measurements of bird feathers can be used to identify moulting (interbreeding) foraging areas of adult seabirds, we examined the stable-carbon (δ13C) and nitrogen (δ15N) isotopic composition of feathers of chicks and adults of black-browed albatrosses (Diomedea melanophrys) from Kerguelen Islands, southern Indian Ocean. Albatross chicks are fed primarily fish (75% by mass), the diet being dominated by various species of the family Nototheniidae and Channichthyidae which commonly occur in the shelf waters in the vicinity of the colony. δ13C and δ15N values in chick feathers, which are grown in summer in the breeding area, were lower than values in adult feathers, which are grown in winter (δ13C: –19.6‰ versus –17.6‰ and δ15N: 12.4‰ versus 15.7‰, respectively). No differences in δ13C and δ15N values were found in adult wing feathers moulted in 1993 and 1994 and in adult feathers formed at the beginning, middle and end of the 1994 moulting period. These data are consistent with adults moulting in the same area and feeding at the same trophic level from one year to the next and with no major changes in foraging ecology within a given moulting season; they suggest that foraging grounds were different in summer and winter and that these differed in their stable-isotope signature. Changes in both feather δ13C and δ15N values indicated feeding south of the Subtropical Front (STF) during chick rearing, which is in agreement with the known foraging ecology at this time and feeding north of the STF during moult. This, together with band recoveries from adult birds, indicates that black-browed albatrosses from Kerguelen Islands wintered in subtropical waters off southern Australia. The stable-isotope markers in feathers, therefore, have the potential for locating moulting areas of migratory seabird species moving between isotopically distinct regions and for investigating seabirds’ foraging ecology during the poorly known interbreeding period. Such information is needed for studies of year-round ecology of seabirds as well as for their conservation and the long-term monitoring of the pelagic environment. Received: 28 June 1999 / Accepted: 14 September 1999  相似文献   

13.
Carbon and nitrogen stable isotopes are frequently used to study energy sources and food web structure in ecosystems, and more recently, to study the effects of anthropogenic stress on aquatic ecosystems. We investigated the effect of nutrient enrichment on δ13C and δ15N in fine (FPOM), coarse (CPOM) particulate organic matter, periphyton, invertebrates and fish in nine boreal streams in south-central Sweden. In addition, we analysed the diet of benthic consumers using stable isotope data. Increases in δ15N of periphyton (R 2 = 0.88), CPOM (0.78), invertebrates (0.92) and fish (0.89) were related to nutrient enrichment. In contrast, δ13C signatures did not change along the nutrient gradient. Our results show that δ15N has potential as a sensitive indicator of nutrient enrichment in boreal streams. Carbon and nitrogen isotopes failed to elucidate putative diets of selected aquatic consumers. Indeed, comparison of low- and high-impact sites showed that δ13C of many consumers were found outside the ranges of basal resource δ13C. Moreover, ranges of basal resource δ13C and δ15N overlapped at both low and high sites, making discrimination between the importance of allochthonous and autochthonous production difficult. Our findings show that a fractionation rate of 3.4‰ is not always be appropriate to assess trophic interactions, suggesting that more studies are needed on fractionation rates along gradients of impairment. Handling editor: M. Power  相似文献   

14.
Human sewage and septic waste are significant sources of nutrient loading to many aquatic ecosystems. Ecologically relevant nitrogen sources can be traced by analyzing nitrogen stable isotope ratios (δ15N signatures) in aquatic plants. Elevated δ15N signatures can suggest increased uptake of nitrogen derived from human and/or animal waste. In the current study, Vallisneria americana, a freshwater angiosperm, was collected from several locations in Upper Saranac Lake, NY, USA. Samples were also collected from Lake George, NY and the Sassafras River, MD, USA. Plant material was analyzed for δ15N and % N; some samples were also analyzed for δ13C, % C, and % P. Results suggest that there is variation in septic inputs to Upper Saranac Lake, with some areas of the lake receiving more input than others. Results also show that increased watershed population density is correlated with elevated δ15N signatures of Vallisneria americana. Taken together, these results suggest that nitrogen stable isotope analysis of aquatic plant tissue is an effective method for assessing and monitoring septic inputs to freshwater ecosystems.  相似文献   

15.
Despite the recognition of the functional role of Hymenoptera (ants, bees and wasps) and Isoptera (termites) in tropical ecosystems, their detailed feeding habits are not well known. To examine the feeding habits of these groups, we measured nitrogen (N) and carbon (C) stable isotope ratios (δ15N and δ13C) of hymenopterans (12 families, ≥16 genera and ≥32 species) and isopterans (one family and 10 species) collected in a tropical rain forest, Sarawak, Malaysia. We compared the isotopic signatures of these insects to those previously reported for other consumers collected in the same forest. The δ15N and δ13C values of these insects overlapped with those of the other consumers, indicating that they have access to diverse C and N sources in the forest. The δ15N values of ants and termites indicated that their feeding habits range along a continuum from herbivory (i.e. dependent on honeydew and nectar) to predation and from wood-feeders to soil-feeders, respectively. In addition, the δ15N values of wasps varied greatly from −0.1‰ (Braconidae sp.) to 8.6‰ (Bembix sp.), suggesting that their feeding habits also range from omnivory to predation. The ant species Camponotus gigas had δ13C values similar to those of invertebrate detritivores and omnivores rather than to those of invertebrate herbivores, although the diet of this species consists mostly of honeydew. This discrepancy suggests that the ant uses carbohydrates as an energy source, the isotopic signatures of which are not well retained in the body tissues. Values of both δ15N and δ13C of the predatory army ant Leptogenys diminuta and the soil-feeding termite Dicuspiditermes nemorosus did not differ significantly, indicating that both trophic level and the humification of feeding substrates can increase the isotopic signatures of terrestrial consumers.  相似文献   

16.
The purpose of this study was to assess if there was trophic niche overlap of silver carp (Hypophthalmichthys molitrix) and bighead carp (H. nobilis) in four large freshwater ecosystems from southern China using stable carbon and nitrogen isotopes (δ13C and δ15N). Multivariate analysis of variance (MANOVA) on the δ13C and δ15N values measured from muscle tissue indicates trophic niche overlap in one unproductive and one highly productive large system and trophic niche segregation in two systems with moderate watershed size and productivity. For these two coexisting planktivorous fish, which were hitherto believed to occupy different trophic niches, this study demonstrated that the degree of their trophic niche overlap varied according to ecosystem properties.  相似文献   

17.
Contamination in marine foodwebs is nowadays of great environmental concern owing to the increasing levels of pollution in marine ecosystems from different anthropogenic sources. Seabirds can be used as indicators of regional contaminant patterns across large temporal and spatial scales. We analysed Hg, Se and Pb levels as well as stable isotope ratios of C (13C/12C, δ13C) and N (15N/14N, δ15N) in breeding- and winter-season feathers on males and females of two related shearwater species, providing information on spatiotemporal patterns of contaminants as well as the influence of the trophic ecology of these seabirds on contaminant levels. During the breeding season, Se and Pb concentrations were highest at the Cape Verde archipelago, showing no differences among the other colonies or between the sexes. However, Hg levels varied among colonies, being highest in the Mediterranean, probably resulting from the larger emissions and fallout of this pollutant in Europe. Feathers grown during breeding also showed sexual differences in Hg concentrations and δ13C. Differences in Hg concentration between sexes are mainly due to egg-laying decontamination in females. In contrast, differences in Hg among colonies are probably related to differences in trophic ecology, as indicated by δ13C and δ15N measurements. Contaminant concentrations in winter-grown feathers did not show any relationship with stable isotope values but were affected by contaminant loads associated with the breeding season. These findings suggest that the interpretation of contaminant levels of migratory species from feathers moulted out of the breeding season should be made with caution because those values could reflect exposures to contaminants acquired during the breeding season. We conclude that factors other than feeding ecology may play an important role in the interpretation of contaminant levels and their annual dynamics at several spatial scales. Consideration of the relevant temporal context provided by isotopic signatures and contaminant concentrations is important in deciphering contaminant information based on various tissues. Xavier Ruiz: deceased 27 April 2008.  相似文献   

18.
When using stable isotopes as dietary tracers it is essential to consider effects of nutritional state on isotopic fractionation. While starvation is known to induce enrichment of 15N in body tissues, effects of moderate food restriction on isotope signatures have rarely been tested. We conducted two experiments to investigate effects of a 50–55% reduction in food intake on δ15N and δ13C values in blood cells and whole blood of tufted puffin chicks, a species that exhibits a variety of adaptive responses to nutritional deficits. We found that blood from puffin chicks fed ad libitum became enriched in 15N and 13C compared to food-restricted chicks. Our results show that 15N enrichment is not always associated with food deprivation and argue effects of growth on diet–tissue fractionation of nitrogen stable isotopes (Δ15N) need to be considered in stable isotope studies. The decrease in δ13C of whole blood and blood cells in restricted birds is likely due to incorporation of carbon from 13C-depleted lipids into proteins. Effects of nutritional restriction on δ15N and δ13C values were relatively small in both experiments (δ15N: 0.77 and 0.41‰, δ13C: 0.20 and 0.25‰) compared to effects of ecological processes, indicating physiological effects do not preclude the use of carbon and nitrogen stable isotopes in studies of seabird ecology. Nevertheless, our results demonstrate that physiological processes affect nitrogen and carbon stable isotopes in growing birds and we caution isotope ecologists to consider these effects to avoid drawing spurious conclusions.  相似文献   

19.
We evaluated the potential utility of stable isotope analysis of tissues commonly archived by aquatic biologists. Previous studies with chemically preserved samples have shown contradictory results, which present an obstacle for the use of archived sample materials. We tested the effects of ethanol and formalin preservation on zooplankton and of ethanol on benthic macroinvertebrate δ13C and δ15N values. We found that neither formalin nor ethanol had a significant effect on δ13C and δ15N values of preserved zooplankton. Nor did ethanol significantly affect δ13C or δ15N values of macroinvertebrates. However, ethanol preservation slightly, but significantly decreased C:N ratios of both zooplankton and macroinvertebrates, probably reflecting some extraction of lipids. Overall, the effects of preservatives on δ13C and δ15N values that we observed were minor. We also compared δ13C and δ15N values analysed from roach scales and perch operculum bones with those analysed from muscle tissue. Decalcification of scales and operculum bones only slightly improved our comparison to muscle tissue δ13C and δ15N values. Decalcified scales had slightly higher δ13C and lower δ15N values. Similarly, decalcified operculum bones showed slightly increased δ13C and decreased δ15N values to those for fish muscle. Our results confirm that scales and operculum bones can provide a suitable proxy for fish muscle in isotope studies with minor correction. We conclude that various archived sample materials can indeed be used with confidence for historical reconstructions of freshwater food webs by stable isotope analysis. Handling editor: K. Martens  相似文献   

20.
Eshetu  Zewdu  Högberg  Peter 《Plant and Soil》2000,222(1-2):109-117
We used the natural abundance of 15N in soils in forests, pastures and cultivated lands in the Menagesha and Wendo-Genet areas of Ethiopia to make inferences about the N cycles in these ecosystems. Since we have described the history of these sites based on variations in 13C natural abundance, patterns of δ15N and δ13C values were compared to determine if shifts of 15N correlate with shifts of vegetation. At Menagesha, a > 500-yr-old planted forest, we found δ15N values from −8.8 to +3.5‰ in litter, from −3.5 to +4.5‰ in 0–10 cm soil layer, and from −1.5 to +6.8‰ at >20 cm soil depth. The low δ15N in litter and surface mineral soils suggests that a closed N cycle has operated for a long time. At this site, the low δ13C of the surface horizon and the high δ13C of the lower soil horizons is clear evidence of a long phase of C4 grass dominance or cultivation of C4 crops before the establishment of the forest >500 years ago. In contrast, at Wendo-Genet, high δ13C of soils reveals that most of the land has been uncovered by forests until recently. Soil δ15N was high throughout (3.4–9.8‰), and there were no major differences between forested, cultivated and pasture soils in δ15N values of surface mineral soils. The high δ15N values suggest that open N cycles operate in the Wendo-Genet area. From the points of view of soil fertility management, it is interesting that tall forest ecosystems with relatively closed N cycling could be established on the fairly steep slopes at Menagesha after a long period of grass vegetation cover or cultivation. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   

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