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The Science of Nature - 相似文献
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Ulrich Struck Maren Voss Bodo von Bodungen Nicolai Mumm 《Die Naturwissenschaften》1998,85(12):597-603
15 N of bulk sediment, organic carbon concentrations, and abundances of exoskeletons of Bosmina
longispina
maritima in the sediment, the data are used to evaluate significant sources of nitrogen in the food web over the past century. Nitrogen
isotopic composition of bulk sediments ranges from 2.5 to 4.5?, that of exokeletons varies between 0.4 and 6.2?. The two are
positively correlated. A marked increase in the abundance of Bosmina since 1965 (from less than 500 specimen to more than 5000 specimen cm3 of sediment) is correlated with a significant increase in sedimentary organic carbon concentrations (from 4% to more than
10%). The isotopic data do not identify increased land-derived nitrate as the dominant nitrogen source fuelling the increase.
Instead, we postulate that nitrogen fixation by diazotrophic bacteria has been one of the larger sources of nitrogen in the
Baltic Sea, as it is today.
Received: 29 April 1998 / Accepted in revised form: 13 August 1998 相似文献
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Insects are considered the most important predators of seed cones, the female reproductive structures of conifers, prior to seed dispersal. Slightly more than 100 genera of insects are known to parasitize conifer seed cones. The most diverse (i.e., number of species) of these genera is Megastigmus (Hymenoptera: Torymidae), which comprises many important seed pests of native and exotic conifers. Seed chalcids, Megastigmus spp., lay eggs inside the developing ovules of host conifers and, until recently, oviposition was believed to occur only in fertilized ovules. Ovule development begins just after pollination, but stops if cells are not fertilized. The morphological stage of cone development at the time of oviposition by seed chalcids has been established for many species; however, knowledge of ovule development at that time has been documented for only one species, M. spermotrophus. Megastigmus spermotrophus oviposits in Douglas-fir ovules after pollination but before fertilization. Unlike the unfertilized ovules, those containing a M. spermotrophus larva continue to develop, whether fertilized or not, stressing the need to broaden our understanding of the insect–plant interactions for this entire genus. To achieve this task, we reviewed the scientific literature and assembled information pertaining to the timing of oviposition and to the pollination and fertilization periods of their respective host(s). More specifically, we were searching for circumstantial evidence that other species of Megastigmus associated with conifers could behave (i.e., oviposit before ovule fertilization) and impact on female gametophyte (i.e., prevent abortion) like M. spermotrophus. The evidence from our compilation suggests that seed chalcids infesting Pinaceae may also oviposit before ovule fertilization, just like M. spermotrophus, whereas those infesting Cupressaceae seemingly oviposit after ovule fertilization. Based on this evidence, we hypothesize that all species of Megastigmus associated with Pinaceae can oviposit in unfertilized ovules, whereas those exploiting Cupressaceae cannot, and thus oviposit only in already fully developed fertilized seeds. Furthermore, we predict that the presence of a larva in unfertilized ovules of all Pinaceae will influence the development of the female gametophyte by preventing its abortion. This influence on the Pinaceae can be interpreted as an ability to parasitize any of the potential seeds present in a seed cone, and as such represents a much more efficient oviposition strategy than searching and locating only fertilized seeds. Concomitantly, this ability has likely led to an overestimation of the impact of the species of seed chalcid infesting Pinaceae on seed production. 相似文献