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41.
42.
Abstract. The characteristics of microhabitats of established Pinus sylvestris and Betula seedlings were studied in a small windthrow gap in a mature P. sylvestris-dominated forest in the Petkeljärvi National Park in eastern Finland. Seedlings were strongly clustered in disturbed microhabitats, particularly uprooting pits and mounds, formed by tree falls. They covered 3% of the 0.3.ha study area consisting of the gap and some of the forest edge. Although Betula occurred only as scattered individuals in the dominant canopy layer of the forest, it accounted for 30% of the seedlings found in the study area. Betula regeneration was almost completely restricted to pits and mounds, where 91% of the seedlings were found. Uprooting spots were also the most important regeneration microhabitats for Pinus, where 60% of the seedlings grew, even though the seedlings were found in other substrates as well, particularly on sufficiently decomposed coarse wood. Undisturbed field- and bottom-layer vegetation had effectively hindered tree seedling establishment, which emphasises the role of soil disturbance for regeneration. While the establishment of seedlings was found to be clearly determined by the availability of favourable regeneration microhabitats, the early growth of seedlings was affected by a complex interaction of environmental variables, including the type of microhabitat, radiation environment and interferences caused by competing seedlings and adjacent trees. In the most important regeneration microhabitats, i.e. in uprooting pits and on mounds, the distributions of the local elevations of Pinus and Betula seedlings were different. Pinus seedlings occurred closer to ground level, i.e. on the fringes of pits and lower on mounds, while Betula seedlings grew deeper in pits and higher on mounds. The position of the Betula seedlings indicate that they may have a competitive advantage over Pinus seedlings in the dense seedling groups occurring in uprooting spots. We suggest that this initial difference in Pinus and Betula establishment may affect the subsequent within-gap tree species succession and can, in part, explain the general occurrence of Betula in conifer-dominated boreal forests. 相似文献
43.
Turnover of glucose and acetate coupled to reduction of nitrate, ferric iron and sulfate and to methanogenesis in anoxic rice field soil 总被引:8,自引:0,他引:8
Turnover of glucose and acetate in the presence of active reduction of nitrate, ferric iron and sulfate was investigated in anoxic rice field soil by using [U-(14)C]glucose and [2-(14)C]acetate. The turnover of glucose was not much affected by addition of ferrihydrite or sulfate, but was partially inhibited (60%) by addition of nitrate. Nitrate addition also strongly reduced acetate production from glucose while ferrihydrite and sulfate addition did not. These results demonstrate that ferric iron and sulfate reducers did not outcompete fermenting bacteria for glucose at endogenous concentrations. Nitrate reducers may have done so, but glucose fermentation may also have been inhibited by accumulation of toxic denitrification intermediates (nitrite, NO, N(2)O). Addition of nitrate resulted in complete inhibition of CH(4) production from [U-(14)C]glucose and [2-(14)C]acetate. However, addition of ferrihydrite or sulfate decreased the production of (14)CH(4) from [U-(14)C]glucose by only 70 and 65%, respectively. None of the electron acceptors significantly increased the production of (14)CO(2) from [U-(14)C]glucose, but all increased the production of (14)CO(2) from [2-(14)C]acetate. Uptake of acetate was faster in the presence of either nitrate, ferrihydrite or sulfate than in the unamended control. Addition of ferrihydrite and sulfate reduced (14)CH(4) production from [2-(14)C]acetate by 83 and 92%, respectively. Chloroform completely inhibited the methanogenic consumption of acetate. It also inhibited the oxidation of acetate, completely in the presence of sulfate, but not in the presence of nitrate or ferrihydrite. Our results show that, besides the possible toxic effect of products of nitrate reduction (NO, NO(2)(-) and N(2)O) on methanogens, nitrate reducers, ferric iron reducers and sulfate reducers were active enough to outcompete methanogens for acetate and channeling the flow of electrons away from CH(4) towards CO(2) production. 相似文献
44.
The coldwater crayfish Orconectes eupunctus is endemic to the Spring and Eleven Point Rivers of Arkansas and Missouri, and appears to have been displaced from a portion
of its range by the recently introduced ringed crayfish Orconectes neglectus. We examined competition among juveniles as a potential mechanism for this crayfish species displacement through laboratory
and field experiments. Orconectes eupunctus juveniles survived and grew in stream cages in their former range, implicating biotic interactions rather than habitat degradation
in the displacement. Laboratory experiments revealed O. neglectus juveniles were dominant in the presence of limited food, whereas size rather than species determined occupancy of limited
shelter. In a field competition experiment using stream cages, O. neglectus juveniles did not inhibit growth or reduce survival of O. eupunctus juveniles. Consequently, laboratory evidence of O. neglectus dominance did not correspond with competition under field conditions. Combined with previous studies examining the effects
of O. neglectus on O. eupunctus, these results suggest that competition may not be a factor in this crayfish species displacement. Alternate mechanisms for
the apparent displacement of O. eupunctus by O. neglectus, such as differential predation or reproductive interference, should be investigated. 相似文献
45.
Influence of plant resistance at the third trophic level: interactions between parasitoids and entomopathogenic fungi of cereal aphids 总被引:2,自引:0,他引:2
Host-plant resistance can affect herbivorous insects and their natural enemies such as parasitoids and entomopathogenic fungi.
This tritrophic effect acts on interspecific interactions between the two groups of natural enemies distantly related in phylogenetic
terms. The intra- and extra-host aspects of the interaction between the cereal aphid parasitoid Aphidius rhopalosiphi and the entomopathogenic fungus Erynia neoaphidis developing on the grain aphid, Sitobion avenae, on resistant and susceptible wheat (Triticum aestivum) cultivars, were studied. The competitive outcome of the intra-host interaction depended on the timing of parasitoid oviposition
and fungal infection and was affected by wheat resistance. In particular, survival of the parasitoid was lower on the resistant
wheat cultivar than the susceptible wheat cultivar, when the competitive outcome of the interaction was favourable for either
parasitoid or fungal development. Before and after this period the influence of plant resistance was not significant. Furthermore,
the extra-host interaction was not affected by the wheat cultivar, although an increase in fungal infection of S. avenae was observed when parasitoids foraged in the experimental arena with sporulating aphid cadavers compared with foraging in
the absence of sporulating cadavers. Our results showed that the host plant may affect interspecific interactions between
parasitoids and fungi and that these interactions depended on the timing of parasitoid oviposition and fungal infection.
Received: 16 March 1998 / Accepted: 24 August 1998 相似文献
46.
J. P. Hanby 《American journal of physical anthropology》1980,52(4):549-564
Monkey groups are characterized in terms of their networks of relationships. Six groups, each consisting of one adult male, several adult females and immatures, were studied over the same time period. This provided data on interaction patterns within groups and the ways in which individuals' kinship, backgrounds, idiosyncrasies, age and sex affected their interactions. Consistencies and changes in group networks in the course of time and in the face of events such as births, deaths, separations, and introductions are given special attention. Principles of networks are discussed with reference to problems of social structure such as cohesion, permeability, communication, stability and competition. 相似文献
47.
Facilitation is an important driver of community assembly, and often overwhelms the effect of competition in stressed habitats. Thus, net effect of biotic interactions is often positive in stressed grasslands, where dominant species and litter can protect the subordinate species. Besides facilitation, niche partitioning can also support species coexistence leading to limiting similarity between subordinate species. Our aim was to provide a detailed analysis of fine-scale biotic interactions in stressed alkali grasslands. We supposed, that there are positive relationships between the main biomass fractions and species richness. We expected the expansion of trait ranges and the increase of trait dissimilarity with increasing biomass scores (total litter, green biomass of dominant species) and species richness. We studied the relationships between main biomass fractions, species richness, functional diversity and functional trait indices (ranges, weighted means and Rao indices). We used fine-scale biomass sampling in nine stands of dry alkali grasslands dominated by Festuca pseudovina. The detected relationships were always positive between the main biomass fractions (green biomass of dominant species, total litter and green biomass of subordinate species) and species richness. We found that the green biomass of dominant species and total litter increased ranges and dissimilarity of functional traits. Our results suggest that in dry alkali grasslands facilitation is crucial in shaping vegetation composition. The green biomass of dominant species and total litter increased the biomass production of subordinate species leading to overyielding. We found that mechanisms of facilitation and limiting similarity were jointly shaping the species coexistence in stressed grasslands, such as alkali grasslands. 相似文献
48.
49.
B. Leroi 《Population Ecology》1973,15(1):163-182
The leaf-mining larvae of the celery fly, P. heraclei, have the ability to leave their primary mine to bore a secondary mine in another leaflet or leaf. This phenomenon is always numerically significant under natural conditions (depending on the plots and the years, 38 to 97% of the larvae observed made such a change), although it is not an obligatory behavior. The distribution of the secondary mines on the leaflets and on the leaves varies with the importance of the migrations, with the total unoccupied and healthy leaf area and with the relative position of different leaves on the plant. These migrations are due to the insufficiency of healthy food available for the larvae: either because the quantity of parenchyma offered by a leaflet is too small for a larva, or because there is intraspecific competition following multiple egg-layings on the same leaflet, or because there is a deterioration of the quality of the parenchyma, particularly following the development of celery leaf-spot. The possibility for the larvae of P. heraclei to bore secondary mines appears to be an extremely important factor in the population dynamics of this species. This permits the reduction of the negative effects of intraspecific competition and celery leaf-spot, and permits the colonization by the larvae of young leaves not used by the females at the time of egg-laying. 相似文献
50.
Objectives:Plateauing of world records in sports has been suggested to reflect the limits of human physiology. Possible explanations include reduced doping or declining popularity that may even lead to a decrease in human performance. Such a decrease, however, has not yet been observed. We hypothesized that rather than a performance plateau, performance has recently declined.Methods:Fifteen athletic disciplines of the Austrian annual rankings were analyzed by regression statistics and the average best performance of the last 20 years compared to earlier periods.Results:The best performances occurred between 1980–1999 and were on average 2.56% (men) and 1.67% (women) better than between 2000–2019. This attenuation was significant in men in 200 m, 800 m, 1500 m, 10 km, long jump, javelin throw (p<0.05), high jump, pole vault, discus throw, shot put and hammer throw (p<0.001); and in women in 400 m, long jump, discus throw (p<0.05) and high jump (p<0.001). The greatest performance declines were observed in the men’s shot put (9.11%) and hammer throw (11.44%).Conclusions:The Austrian track and field annual best results show a performance decline following a peak, instead of a plateau. Future studies should address the causes and whether this also applies to other sports and countries. 相似文献