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1.
Broad bean (Vicia faba), an annual plant bearing extrafloral nectaries (EFN) at the base of the upper leaves, is regularly infested by two aphid species, Aphis fabae and Acyrthosiphon pisum. EFN and A. fabae are commonly attended by the ant, Lasius niger, while Ac. pisum usually remains uninfested. Sugar concentration and sugar composition of extrafloral nectar did not change significantly after aphid infestation. The sugar concentration was significantly higher in EFN (c. 271 µg µl-1) than in the honeydew of A. fabae (37.5 µg µl-1). The presence of small A. fabae colonies had no significant effect on ant attendance of EFN, which remained at the same level as that on plants without A. fabae. Obviously, there was no significant competitive effect between the two sugar sources. We suggest that the high sugar concentration in the extrafloral nectar may outweigh the higher quality (due to the presence of melezitose) and quantity of aphid honeydew. Ants and the presence of EFN influenced aphid colony growth. While A. fabae colonies generally grew better in the presence of ants, Ac. pisum colonies declined on plants with EFN or A. fabae colonies. We conclude that EFN may provide some degree of protection for V. faba against those sucking herbivores that are not able to attract ants.  相似文献   

2.
Mandibles and stomachs of three Antarctic shrimp species (Nematocarcinus lanceopes, Notocrangon antarcticus, and Chorismus antarcticus) were investigated by means of scanning electron microscopy. Transmission electron microscopy of the midgut glands was applied to find out the nutritional status of the animals, which contained a broad variety of food items in their stomachs. In contrast to the Antarctic krill, the three carideans possess a dual filter system; primary filters in the cardia and secondary filters in the pyloric chamber. Morphologically, the mandibles and stomachs of the three species vary considerably; however, their food items are similar. We conclude that stomach contents do not really reflect the different modes of life or habitat preferences of the investigated species.  相似文献   

3.
In-situ estimates of fast-ice algal productivity at Cape Evans, McMurdo Sound, in 1999 were lower than at the same site in previous years. Under-ice irradiance was between 0 and 8 µmol photons m-2 s-1; the ice was between 1.9 and 2.0 m thick and the algal biomass averaged 150 mg chl a m-2, although values as high as 378 mg chl a m-2 were recorded. Production on 11 and 12 November was between 0.053 and 1.474 mg C m-2 h-1. When the data from 11 November were fitted to a hyperbolic tangent function, a multilinear regression gave estimates for Pmax of 0.571 nmol O2 cm-2 s-1, an ! of 0.167 nmol O2 cm-2 s-1 µmol-1 photons m-2 s-1 and an Ek of 3.419 µmol photons m-2 s-1. A Pmax of 2.674 nmol O2 cm-2 s-1, an ! of 0.275 nmol O2 cm-2 s-1 µmol-1 photons m-2 s-1, r of 0.305 nmol O2 cm-2 s-1 and an Ek of 9.724 µmol-1 photons m-2 s-1 were estimated from the 12 November data. The sea-ice algal community was principally comprised of Nitzschia stellata, Entomoneis kjellmanii and Berkeleya adeliensis. Other taxa present included N. lecointei, Fragilariopsis spp., Navicula glaciei, Pleurosigma spp. and Amphora spp. Variations in the method for estimating the thickness of the diffusive boundary layer were not found to significantly affect the measurements of oxygen flux. However, the inability to accurately measure fine-scale variations in biomass is thought to contribute to the scatter of the P versus E data.  相似文献   

4.
The icefish Cryodraco antarcticus is common in deeper waters of the southern Scotia Arc and the high-Antarctic zone. A number of biological features of this species are presented, with new information collected from recent scientific surveys of the South Shetland Islands and South Orkney Islands. The species is closely related to Chaenocephalus aceratus, but can be distinguished by a number of meristic characters. The two species are similar in size, colouration and body shape, and have a number of aspects of their life-cycle in common, such as reproduction, length-weight relationship and diet. These two species appear to occupy a very similar niche in the southern Scotia Arc ecosystem. However, Cryodraco antarcticus is less abundant and uses different spatial components of shelf areas, replacing Chaenocephalus aceratus in deeper water and high-Antarctic regions.  相似文献   

5.
The diet of the house mouse (Mus musculus domesticus), introduced to the Kerguelen archipelago in the 1800s, was studied at monthly intervals from August 1997 to July 1998 in the closed communities of Acaena magellanica, the main habitat of mice on Guillou Island. The analysis of 291 stomach contents showed that this opportunistic rodent included a variety of items in its diet: earthworms (Dendrodrilus rubidus tenuis, Microscolex kerguelensis), caterpillars of a flightless moth (Pringleophaga kerguelensis), weevil adults and larvae (Ectemnorrhinus spp.), seeds of Acaena magellanica, and floral parts of dandelion (Taraxacum officinale). The animal prey were dominant in its diet all year round, except in summer. Based on the presence of chaetae in the stomach contents, our results show that earthworms are an important prey for the house mouse at Kerguelen. The consequences of these food habits for the invertebrate communities of the subantarctic islands are discussed.  相似文献   

6.
Water availability and carbon isotope discrimination in conifers   总被引:22,自引:0,他引:22  
The stable C isotope composition ('13C) of leaf and wood tissue has been used as an index of water availability at both the species and landscape level. However, the generality of this relationship across species has received little attention. We compiled literature data for a range of conifers and examined relationships among landscape and environmental variables (altitude, precipitation, evaporation) and '13C. A significant component of the variation in '13C was related to altitude (discrimination decreased with altitude in stemwood, 2.53‰ km-1 altitude, r2=0.49, and in foliage, 1.91‰ km-1, r2=0.42), as has been noted previously. The decrease in discrimination with altitude was such that the gradient in CO2 partial pressure into the leaf (Pa-Pi) and altitude were generally unrelated. The ratio of precipitation to evaporation (P/E) explained significant variation in Pa-Pi of stemwood (r2=0.45) and foliage (r2=0.27), but only at low (<0.8) P/E. At greater P/E there was little or no relationship, and other influences on '13C probably dominated the effect of water availability. We also examined the relationship between plant drought stress (O) and '13C within annual rings of stemwood from Pinus radiata and Pinus pinaster in south-western Australia. Differential thinning and fertiliser application produced large differences in the availability of water, nutrients and light to individual trees. At a density of 750 stems ha-1, O and '13C were less (more negative) than at 250 stems ha-1 indicating greater drought stress and less efficient water use, contrary to what was expected in light of the general relationship between discrimination and P/E. The greater '13C of trees from heavily thinned plots may well be related to an increased interception of radiation by individual trees and greater concentrations of nutrients in foliage - attributes that increase rates of photosynthesis, reduce Pi and increase '13C. '13C was thus modified to a greater extent by interception of radiation and by nutrient concentrations than by water availability and the '13C-O relationship varied between thinning treatments. Within treatments, the relationship between '13C and O was strong (0.38<r2<0.58). We conclude that '13C may well be a useful indicator of water availability or drought stress, but only in seasonally dry climates (P/E<1) and where variation in other environmental factors can be accounted for.  相似文献   

7.
The lipid content, fatty acid composition and calorific value of seven species of mesopelagic deep-sea fish of the family Myctophidae and the mackerel icefish, Champsocephalus gunnari, important in the diet of Southern Ocean marine predators, are presented. Fish were sampled at the Kerguelen Plateau (KP) and Macquarie Ridge (MR) in the Indian and Pacific sectors of the Southern Ocean respectively, to examine geographic variation in lipid compositon. All species of myctophid from KP and Electrona antarctica from MR were high in lipid content (6-18% wet mass), particularly Gymnoscopelus nicholsi (18%) and E. antarctica (15%). The mackerel icefish, and G. fraseri and Protomyctophum tenisoni from MR were generally lower in lipid content (3-5%) and varied significantly in fatty acid composition from KP species. KP myctophids were high in calorific content (9.3 kJ g-1 wet mass) when compared with icefish (5.4 kJ g-1 wet mass) and other published values for prey items of marine predators such as squid (1.7-4.5 kJ g-1). KP myctophids were distinguished from each other and from C. gunnari and MR specimens by cluster and discriminant function analysis using six fatty acids (16:0, 18:1̩, 20:1̩, 22:1Ὃ, 20:5̣, 22:6̣). Findings presented here highlight trophic links between high-latitude fish and their prey and emphasise the importance of myctophids as a significant energy source for marine predators foraging in the Polar Frontal Zone.  相似文献   

8.
Conifers decrease the amount of biomass apportioned to leaves relative to sapwood in response to increasing atmospheric evaporative demand. We determined how these climate-driven shifts in allocation affect the aboveground water relations of ponderosa pine growing in contrasting arid (desert) and humid (montane) climates. To support higher transpiration rates, a low leaf:sapwood area ratio (AL/AS) in desert versus montane trees could increase leaf-specific hydraulic conductance (KL). Alternatively, a high sapwood volume:leaf area ratio in the desert environment may increase the contribution of stored water to transpiration. Transpiration and hydraulic conductance were determined by measuring sap flow (JS) and shoot water potential during the summer (June-July) and fall (August-September). The daily contribution of stored water to transpiration was determined using the lag between the beginning of transpiration from the crown at sunrise and JS. In the summer, mean maximum JS was 31.80LJ.74 and 24.34Dž.05 g m-2 s-1 for desert and montane trees (a 30.6% difference), respectively. In the fall, JS was 25.33NJ.52 and 16.36dž.64 g m-2 s-1 in desert and montane trees (a 54.8% difference), respectively. JS was significantly higher in desert relative to montane trees during summer and fall (P<0.05). Predawn and midday shoot water potential and sapwood relative water content did not differ between environments. Desert trees had a 129% higher KL than montane trees in the summer (2.41᎒-5 versus 1.05᎒-5 kg m-2 s-1 MPa-1, P<0.001) and a 162% higher KL in the fall (1.97᎒-5 versus 0.75᎒-5 kg m-2 s-1 MPa-1, P<0.001). Canopy conductance decreased with D in all trees at all measurement periods (P<0.05). Maximum gC was 3.91 times higher in desert relative to montane trees averaged over the summer and fall. Water storage capacity accounted for 11 kg (11%) and 10.6 kg (17%) of daily transpiration in the summer and fall, respectively, and did not differ between desert and montane trees. By preventing xylem tensions from reaching levels that cause xylem cavitation, high KL in desert ponderosa pine may facilitate its avoidance. Thus, the primary benefit of low leaf:sapwood allocation in progressively arid environments is to increase KL and not to increase the contribution of stored water to transpiration.  相似文献   

9.
Adult fish may affect the growth and survival of conspecific larvae through a variety of pathways, including negative interactions via competition for shared limiting resources or via predation (i.e., cannibalism), and positive interactions due to the consumption of larval predators and via resource enhancement (i.e., presence of adults increases availability of larval prey). To examine the overall effect of adult bluegill sunfish (Lepomis macrochirus) on larval bluegill, we conducted a field experiment in which we manipulated adult densities and quantified larval growth and survival, prey abundance, invertebrate predator abundance, and cannibalism. The presence of adult bluegill had a negative effect on final larval mass. This response was consistent with competition for zooplankton prey. Adult bluegill reduced the abundance of large zooplankton (e.g., Chaoborus and Daphnia), which were the dominant prey of bluegill larvae in the absence of adults. Larvae in the no-adult treatment also had significantly more prey in their stomachs compared to larvae in the presence of adults. Larval survival was maximized at intermediate adult densities and the overall production of larvae peaked at intermediate adult densities. The higher larval survival at intermediate adult densities is attributed to a reduction in invertebrate predators in treatments with adult bluegill; invertebrate predators experienced an 80% reduction in the presence of adult fish. Decreased larval survival at the highest adult density was not due to resource limitation and may be due to cannibalism, which was not directly observed in our study, but has been observed in other studies.  相似文献   

10.
Replicated ecological studies in marine reserves and associated unprotected areas are valuable in examining top-down impacts on communities and the ecosystem-level effects of fishing. We carried out experimental studies in two temperate marine reserves to examine these top-down influences on shallow subtidal reef communities in northeastern New Zealand. Both reserves examined are known to support high densities of predators and tethering experiments showed that the chance of predation on the dominant sea urchin, Evechinus chloroticus, within both reserves was approximately 7 times higher relative to outside. Predation was most intense on the smallest size class (30-40 mm) of tethered urchins, the size at which urchins cease to exhibit cryptic behaviour. A high proportion of predation on large urchins could be attributed to the spiny lobster, Jasus edwardsii. Predation on the smaller classes was probably by both lobsters and predatory fish, predominantly the sparid Pagrus auratus. The density of adult Evechinus actively grazing the substratum in the urchin barrens habitat was found to be significantly lower at marine reserve sites (2.2ǂ.3 m-2) relative to non-reserve sites (5.5ǂ.4 m-2). There was no difference in the density of cryptic juveniles between reserve and non-reserve sites. Reserve populations were more bimodal, with urchins between 40 and 55 mm occurring at very low numbers. Experimental removal of Evechinus from the urchin barrens habitat over 12 months lead to a change from a crustose coralline algal habitat to a macroalgal dominated habitat. Such macroalgal habitats were found to be more extensive in both reserves, where urchin densities were lower, relative to the adjacent unprotected areas that were dominated by urchin barrens. The patterns observed provide evidence for a top-down role of predators in structuring shallow reef communities in northeastern New Zealand and demonstrate how marine reserves can reverse the indirect effects of fishing and re-establish community-level trophic cascades.  相似文献   

11.
W. Block  P. Convey 《Polar Biology》2001,24(10):764-770
Body-water content of field-fresh samples of the springtail, Cryptopygus antarcticus Willem (Collembola, Isotomidae) was measured at monthly intervals over 11 years (1984-1995) at Signy Island, in the maritime Antarctic. A clear annual cycle of variation in water content was observed, with maxima in the austral spring and autumn, and minima in midwinter and midsummer. There was no overall trend during the 11-year study, in contrast to an earlier analysis of the initial 1984-1987 period, which demonstrated a significant increase in body-water content (from 56.6 to 66.0% fresh weight). It is suggested that, between 1984 and 1987, water stress on C. antarcticus in its environment declined, and thereafter stabilised between 1988 and 1995. Springtail body-water content between 1984 and 1995 showed significant increases in several months, particularly in autumn and early winter, with decreases in midsummer. This was consistent with the predicted consequences of the pattern of regional climatic warming in the maritime Antarctic, where small increments in temperature have effectively increased the length of the potential biologically active period. C. antarcticus responds rapidly to local and short-term variations in environmental conditions and will be able to take advantage of increases in the thermal energy budget and growing season length. It is predicted that climate warming could lead to a reduction in life-cycle duration, an increase in population density and extension of geographical range.  相似文献   

12.
Terrestrial arthropods that reside in Antarctica are exposed to considerable periods of environmental stress, thus factors that promote identification of favorable microhabitats are extremely important. In this study, we report the presence of chemical cues that induce oviposition and aggregation in two species of Antarctic collembolans, Cryptopygus antarcticus and Friesea grisea. Responses of the Collembola were enhanced by low temperatures but were not altered by humidity. One of the major physiological benefits derived from an aggregation was a substantial reduction in water loss rates. Although F. grisea and C. antarcticus were commonly found in cross-species aggregations, we found no evidence to suggest cross-species attraction. When individuals were exposed to areas previously occupied by groups of Collembola, more eggs were laid. Thus, chemicals released by the collembolans appear to induce both aggregation and oviposition in these Antarctic species.  相似文献   

13.
Elite white maize lines W506 and M37W were transformed with a selectable marker gene (bar) and a reporter gene (uidA) or the polygalacturonase-inhibiting protein (pgip) gene after bombardment of cultured immature zygotic embryos using the particle inflow gun. Successful transformation with this device did not require a narrow range of parameters, since transformants were obtained from a wide range of treatments, namely pre-culture of the embryos for 4-6 days, bombardment at helium pressures of 700-900 kPa, selection-free culture for 2-4 days after bombardment and selection on medium containing bialaphos at 0.5-2 mg l-1. However, bombardments with helium pressures below 700 kPa yielded no transformants. The culture of immature zygotic embryos of selected elite white maize lines on medium containing 2 mg l-1 2,4-dichlorophenoxyacetic acid and 20 mM L-proline proved to be most successful for the production of regenerable embryogenic calli and for the selection of putative transgenic calli on bialaphos-containing medium after transformation. Transgenic plants were obtained from four independent transformation events as confirmed by Southern blot analysis. Transmission of the bar and uidA genes to the T4 progeny of one of these transformation events was demonstrated by Southern blot analysis and by transgene expression. In this event, the transgenes bar and uidA were inserted in tandem.  相似文献   

14.
Invasive species, where successful, can devastate native communities. We studied the dynamics of the invasive Argentine ant, Linepithema humile, for 7 years in Jasper Ridge, a biological preserve in northern California. We monitored the distributions at the hectare scale of native ant taxa and L. humile in the spring and fall from 1993 to 1999. We also studied the invasion dynamics at a finer resolution by searching for ants in 1-m2 plots. Our results are similar at both scales. The distributions of several native species are not random with regard to L. humile; the distributions of several epigeic species with similar habitat affinities overlap much less frequently than expected with the distribution of L. humile. We found that season had a significant influence on the distributions of L. humile and several native taxa. Over the 7-year period, L. humile has increased its range size in Jasper Ridge largely at the expense of native taxa, but there is seasonal and yearly variation in this rate of increase. Studies of invasions in progress which sample across seasons and years may help to predict the spread and effects of invasive species.  相似文献   

15.
Photosynthetic gas exchange, chlorophyll fluorescence, nitrogen use efficiency, and related leaf traits of native Hawaiian tree ferns in the genus Cibotium were compared with those of the invasive Australian tree fern Sphaeropteris cooperi in an attempt to explain the higher growth rates of S. cooperi in Hawaii. Comparisons were made between mature sporophytes growing in the sun (gap or forest edge) and in shady understories at four sites at three different elevations. The invasive tree fern had 12-13 cm greater height increase per year and approximately 5 times larger total leaf surface area per plant compared to the native tree ferns. The maximum rates of photosynthesis of S. cooperi in the sun and shade were significantly higher than those of the native Cibotium spp (for example, 11.2 and 7.1 µmol m-2 s-1, and 5.8 and 3.6 µmol m-2 s-1 respectively for the invasive and natives at low elevation). The instantaneous photosynthetic nitrogen use efficiency of the invasive tree fern was significantly higher than that of the native tree ferns, but when integrated over the life span of the frond the differences were not significant. The fronds of the invasive tree fern species had a significantly shorter life span than the native tree ferns (approximately 6 months and 12 months, respectively), and significantly higher nitrogen content per unit leaf mass. The native tree ferns growing in both sun and shade exhibited greater photoinhibition than the invasive tree fern after being experimentally subjected to high light levels. The native tree ferns recovered only 78% of their dark-acclimated quantum yield (Fv/Fm), while the invasive tree fern recovered 90% and 86% of its dark-acclimated Fv/Fm when growing in sun and shade, respectively. Overall, the invasive tree fern appears to be more efficient at capturing and utilizing light than the native Cibotium species, particularly in high-light environments such as those associated with high levels of disturbance.  相似文献   

16.
Mesozooplankton community structure and grazing impact were investigated at 13 stations in the Polar Frontal Zone during the second Marion Offshore Variability Ecosystem Study (MOVES II), conducted during April 2002. Total integrated chl- a biomass ranged between 11.17 and 28.34 mg chl- a m-2 and was always dominated by nano- and picophytoplankton (<20 µm). Throughout the study, small copepods, mainly Oithona similis and Ctenocalanus vanus, numerically dominated the mesozooplankton community, composing up to 85% (range 30-85%) of the total abundance. Grazing activity of the four most abundant copepods ( O. similis, C. vanus, Calanus simillimus and Clausocalanus spp.), constituting up to 93% of total mesozooplankton abundance, was investigated using the gut fluorescence technique. Results of gut fluorescence analyses indicated that Calanus simillimus and Clausocalanus spp. exhibited diel variability in gut pigments with maximum values recorded at night. In contrast, O. similis and Ctenocalanus vanus did not demonstrate diel variation in gut pigment contents. Ingestion rates of the four copepods ranged from 23.23 to 1462,02 ng(pigm.)ind-1day-1. The combined grazing impact of the four copepods ranged between 1 and 36% of the phytoplankton standing stock per day, with the highest daily impact occurring at stations occupied in the vicinity of the Antarctic Polar Front (~35.86% at station 23). Among the copepods, O. similis and Ctenocalanus vanus represented the most important consumers of phytoplankton biomass, collectively responsible for up to 89% (range 15-89%) of the total daily grazing impact. Carbon specific ingestion rates of the copepods varied between 42 and 320% body carbon per day.  相似文献   

17.
Na+-K+-Cl- cotransport and Na+/K+ATPase were studied by immunohistochemistry in the kidney and urinary bladder of Trematomus bernacchii and Chionodraco hamatus. The activity was correlated to the density of mitochondria. The first segment of the renal proximal tubule was more active than the second one. In T. bernacchii and the temperate marine teleost Pagellus bogaraveo, the immunoreactivity for the antibody to cotransporters and to the !-subunit of the sodium pump was stronger than in the icefish. This difference indicates in the kidney of the icefish, a weaker secretory activity, a consequent lower osmolarity in the lumen and lower water loss, which correlates well with the need for a greater blood volume in the icefish. The epithelium of the urinary bladder in T. bernacchii, where intense immunostaining was observed, was composed of columnar cells. In C. hamatus the columnar cells, where the immunostaining was weaker, lined only a portion of the urinary bladder, the other region being composed of cuboidal cells.  相似文献   

18.
Analysis of the natural variations in stable isotope ratios in animal tissue may be a powerful tool to reveal the trophic position and feeding preferences of generalist predators such as lycosid spiders. In the present study, changes in 15N/14N and 13C/12C ratios in the lycosid spider Pardosa lugubris (Araneae: Lycosidae), fed with prey of different quality, were investigated. Experimental food chains included three trophic levels: prey media, prey organism and predator. In order to analyse the time course of stable isotope enrichment, different life stages of P. lugubris feeding on Drosophila melanogaster were studied. The 15N content of hatchlings of P. lugubris was significantly lower than that of their mothers, indicating the existence of nitrogen pools with different 15N signatures in female P. lugubris. With duration of feeding and progressing development, age and body weight, the 15N content in spiderlings increased. Starvation resulted in 15N and 13C enrichment in P. lugubris. Being fed with prey of different quality resulted in varying patterns of stable isotope enrichment. 15N but not 13C content consistently increased when fed on high quality prey (Heteromurus nitidus, D. melanogaster and a mixed diet consisting of H. nitidus and D. melanogaster). In contrast, low quality prey (Rhopalosiphum padi, Folsomia candida and a mixed diet consisting of H. nitidus, D. melanogaster and F. candida) resulted in deviations from postulated patterns of stable isotope enrichment with 15N content being similar to that of starving spiderlings. When fed on high quality prey, 15N enrichment of P. lugubris was close to ca. 3‰. It is concluded that the analysis of variations in the natural abundance of stable isotopes is particularly helpful in revealing the trophic structure of terrestrial food webs in which polyphagous feeders predominate, as is the case in litter and soil.  相似文献   

19.
The photosynthetic induction response under constant and fluctuating light was examined in naturally occurring saplings (about 0.5-2 m in height) of three shade-tolerant tree species, Pourouma bicolor spp digitata, Dicorynia guianensis, and Vouacapoua americana, growing in bright gaps and in the shaded understorey in a Neotropical rain forest. Light availability to saplings was estimated by hemispherical photography. Photosynthetic induction was measured in the morning on leaves that had not yet experienced direct sunlight. In Dicorynia, the maximum net photosynthesis rate (Amax) was similar between forest environments (ca 4 µmol m-2 s-1), whereas for the two other species, it was twice as high in gaps (ca 7.5) as in the understorey (ca 4.5). However, the time required to reach 90% of Amax did not differ among species, and was short, 7-11 min. Biochemical induction was fast in leaves of Pourouma, as about 3 min were needed to reach 75% of maximum carboxylation capacity (Vcmax); the two other species needed 4-5 min. When induction continued after reaching 75% of Vcmax, stomatal conductance increased in Pourouma only (ca 80%), causing a further increase in its net photosynthesis rate. When fully induced leaves were shaded for 20 min, loss of induction was moderate in all species. However, gap saplings of Dicorynia had a rapid induction loss (ca 80%), which was mainly due to biochemical limitation as stomatal conductance decreased only slowly. When leaves were exposed to a series of lightflecks separated by short periods of low light, photosynthetic induction increased substantially and to a similar extent in all species. Although Amax was much lower in old than in young leaves as measured in Dicorynia and Vouacapoua, variables of the dynamic response of photosynthesis to a change in light tended to be similar between young and old leaves. Old leaves, therefore, might remain important for whole-plant carbon gain, especially in understorey environments. The three shade-tolerant species show that, particularly in low light, they are capable of efficient sunfleck utilization.  相似文献   

20.
The diet of the South Georgia shag Phalacrocorax georgianus at Laurie Island, South Orkney Islands, was studied by the analysis of 420 pellets (regurgitated casts) collected from the 1995 to 1999 breeding seasons. Demersal-benthic fish were by far the main prey, followed by molluscs (mainly octopods and bivalves) and polychaetes. Harpagifer antarcticus (in 1995, 1998, 1999) and Lepidonotothen nudifrons (1996, 1997) were the most frequent prey, followed by Gobionotothen gibberifrons or Trematomus newnesi. Harpagifer antarcticus, followed by L. nudifrons or T. newnesi, was the most numerous prey in all of the seasons and predominated in mass in 1995 and 1996, followed by Notothenia coriiceps and L. nudifrons. In 1997 and 1999, G. gibberifrons and N. coriiceps were the species that most contributed to the diet whereas N. coriiceps and H. antarcticus contributed most in 1998. Our results differ from those reported for the South Orkney Islands in previous studies. These differences could be due to the use of different diet-analysis methods and to shags-related and/or fisheries-related changes in the food availability around the colonies. These alternative explanations are analysed and discussed.  相似文献   

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