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121.
122.
1. A sediment core from the shallow, hypertrophic Lake Søbygaard (mean depth ∼1 m; [TP] 310 μg P L−1) was analysed for subfossil remains to reconstruct chironomid community changes in relation to the succession and disappearance of aquatic macrophytes. 2. Species composition in the 1.10 m core indicates a succession from a 'naturally' eutrophic state to a hypertrophic state during recent centuries. Radiometric dating (210Pb) of the uppermost 20 cm of the sediment core (∼1932–93) indicates that sediment accumulation rate had doubled in recent decades. 3. Changes in chironomid assemblages were in close agreement with changes in both diatoms and macrophyte remains in the same core. Distinct changes in chironomid communities reflect the eutrophication process and macrophyte succession through Chara , Ceratophyllum and Potamogeton dominance to the present state, with complete loss of submerged vegetation and dominance by phytoplankton. 4. The co-occurrence and relationship between aquatic macrophyte diversity and recent subfossil chironomid assemblages were assessed from an additional 25 Danish lakes. There was good agreement between the macrophyte and chironomid-based lake groupings. Overall, a significant difference ( P <0.001) was found in chironomid assemblages among lakes in different macrophyte classes. In a pair-wise comparison, the poorly buffered mesotrophic lakes and the alkaline eutrophic lakes had significantly different chironomid assemblages. 5. Chironomid taxa commonly reported to be associated with macrophytes ( Cricotopus , Endochironomus and Glyptotendipes ) were shown also to be indicators of highly productive lakes lacking abundant submerged vegetation.  相似文献   
123.
The virulence of different pulsed-field gel electrophoresis (PFGE) types of Listeria monocytogenes was examined by monitoring their ability to invade Caco-2 cells. Strains belonging to seven different PFGE types originating from both foods and humans were included. No significant differences in invasiveness were detected between strains isolated from humans and those isolated from food. Strains belonging to PFGE type 1 expressed a significantly lower ability to invade cells compared to strains belonging to other PFGE types. Although strains of PFGE type 2 also seemed to invade at a low level, this was not significant in the present study. PFGE types 1 and 2 as well as type 14 are more frequently found in food than the four other PFGE types examined and moreover have a relatively low prevalence in humans compared to their prevalence in food. Thus, the hypothesis that some PFGE types are less virulent than others is supported by this study showing that certain PFGE types of L. monocytogenes commonly found in food are less invasive than others to Caco-2 cells. In contrast to the differences in invasion, identical intracellular growth rates between the different PFGE types were observed. In vivo studies of the actual ability of the strains to invade the liver and spleen of cimetidine-treated rats following an oral dose of 109 L. monocytogenes cells were performed for isolates of PFGE types 1, 2, 5, and 15. After 2 days, equal amounts of bacteria were observed in the liver and spleen of the rats for any of the PFGE types tested.  相似文献   
124.
The stoat and the least weasel are specialist predators of small rodents, and therefore their numbers are likely to depend on the availability of voles. These small predators are ecologically alike, but they differ somewhat in their diet. The stoat is larger in size than the least weasel and therefore capable of using a wider variety of prey species, while the least weasel is more restricted to small mammals. Voles in northern Fennoscandia exhibit cyclic dynamics of 3–5 years with large-scale spatial synchrony and geographical trends in cycle length and amplitude. We predicted that the cyclic dynamics of voles are reflected in the dynamics of their predators with slight differences between the stoat and the least weasel. In this study we use snow-tracking data to characterize the dynamics of small mustelids. The data were collected from different parts of Finland using permanent triangle-shaped census routes of 12 km in 1989 to 2003. Population fluctuations of small mustelids were generally multiannually periodic and in synchrony over large areas, but we did not find any clear geographical gradient in the attribute of small mustelid dynamics comparable to those observed in vole population fluctuations. Instead, we found a similar decreasing temporal trend in the abundances of both species as has been recently reported for voles.  相似文献   
125.
We studied experimentally how heterospecific attraction may affect habitat selection of migrant passerine birds in Finnish Lapland. We manipulated the densities of resident tit species (Parus spp.). In four study plots residents were removed before the arrival of the migrants in the first study year, and in four other plots their densities were increased by releasing caught individuals. In the second year the treatments of the areas were reversed, allowing paired comparisons within each plot. We also investigated the relative abundance of arthropods in the study plots by the sweep-net method. This allowed us to estimate the effect of food resources on the abundance of birds. The heterospecific attraction hypothesis predicts that densities of migrant species (especially habitat generalists) would be higher during increased resident density. Results supported this prediction. Densities and number of the most abundant migrant species were significantly higher when resident density was increased than when they were removed. On the species level the redwing (Turdus iliacus) showed the strongest positive response to the increased abundance of tits. Migrant bird abundances seemed not to vary in parallel with relative arthropod abundance, with the exception of the pied flycatcher (Ficedula hypoleuca) which showed a strongly positive correlation with many arthropod groups. The results of the experiment indicate that migrants can use resident tit species as a cue to a profitable breeding patch. The relationship between the abundance of the birds and arthropods suggests that annual changes in food resources during the breeding season probably do not have a very important effect on bird populations in these areas. The results stress the importance of positive interspecific interactions in structuring northern breeding bird communities. Received: 1 September 1997 / Accepted: 22 January 1998  相似文献   
126.
Several aspects of the life history of reindeer (Rangifer tarandus) are related to the nutritional condition of the animals. Moreover, compensatory growth and fattening in summer decreases with age. The interaction of tooth wear and the standing crop of lichens on age-related variation in body size and tissue reserves was examined to evaluate the proximate causes of density-dependent food limitation on life history parameters in female reindeer. Studies in nine semi-domesticated free-ranging reindeer herds showed that molar height depended on the mean standing crop of terrestrial lichens in winter habitats. The extent of tooth wear had the strongest effect on body reserves among the oldest females (11–14 years). This indicates that severe tooth wear limits the animals′ ability to process food efficiently and, hence, to maintain their body reserves. Both tooth wear and the biomass of lichens influenced body mass in old females, probably because on heavily exploited winter ranges reindeer had to use higher proportions of lower-ranking coarser foods, especially dwarf shrubs. Received: 1 December 1997 / Accepted: 13 July 1998  相似文献   
127.
128.
A study on sex-determination and karyotypic evolution inTetranychidae   总被引:1,自引:1,他引:0  
The chromosome complements of 45 species of spider mites (Tetranychidae) were studied, making the total number of species now examined in this family 57, approximately 10% of all species known. The chromosome numbers range fromn=2 ton=7. The modal number of the family is 3 (found in 44% of the species). It is argued that the ancestral number isn=2 (21% of the species).In the more primitive subfamily of theBryobiinae both thelytokous and arrhenotokous species occur, whereas the subfamily of theTetranychinae exclusively exhibits arrhenotoky. The karyotype evolution is discussed in connection with arrhenotoky. It is stated that karyotype information is a useful tool for the spider mite taxonomist.This study was financially supported by the Netherlands Foundation for the Advancement of Tropical Research (W.O.T.R.O.) and the Uyttenboogaart-Eliasen Foundation.  相似文献   
129.
Matrix-like particles were formed in sonicated suspensions of soluble chromogranins and phospholipids extracted from the isolated membrane phase of chromaffin granules. The artificial particles which sedimented into 0.4–0.6 M sucrose layers during isopycnic centrifugation, consisted of liposomal vesicles and granular aggregates indicative of lamellar and possibly hexagonal organization patterns of phospholipids and the aqueous phase containing the soluble chromogranins. The ultrastructure of the granular aggregates was strikingly similar to the matrix phase of the intact chromaffin granule.The dopamine-β-hydroxylase activity (EC 1.14.2.1) of the soluble chromogranin preparation was potentiated by the addition of the phospholipids and further enhanced by the sonication procedure. The enzymic activity was highest in the 0.4–0.6 M sucrose layers, rich in the artificial matrix-like particles.The artificially formed particles incorporated ATP by further sonication and the presence of ATP in the particle fractions did not inhibit the specific dopamine-β-hydroxylase activity, even at ratios of 4 and 2 μmoles ATP/mg protein in the 0.4 and 0.6 M sucrose layers, respectively. Noradrenaline was incorporated into the particle fractions in presence of ATP. However, the presence of noradrenaline above 3·10?6 M resulted in a 50–75% inhibition of the dopamine-β-hydroxylase activity of the artificial matrices.  相似文献   
130.
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