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141.
Plant species composition and community structure were compared among four sites in an upland black spruce community in northwestern Ontario. One site had remained undisturbed since the 1930s and three had been disturbed by either logging, fire, or both logging and fire. Canonical correspondence ordination analyses indicated that herbaceous species composition and abundance differed among the disturbance types while differences in the shrub and tree strata were less pronounced. In the herb stratum Pleurozium schreberi, Ptilium crista-castrensis and Dicranum polysetum were in greatest abundance on the undisturbed forest site, while the wildfire and burned cutover sites were dominated by Epilobium angustifolium and Polytrichum juniperinum. The unburned harvested site was dominated by Epilobium angustifolium, Cornus canadensis and Pleurozium schreberi. Species richness was lower on the undisturbed site than on any of the disturbed sites while species diversity (H) and evenness (Hill's E5) were higher on the unburned harvested site than on the other sites. Results suggest that herb re-establishment is different among harvested and burned sites in upland black spruce communities and we hypothesize that differences in the characteristics of the disturbance were responsible, in particular, the impact of burning on nutrient availability. These differences need to be taken into account in determining the effects of these disturbances on biodiversity and long-term ecosystem management. 相似文献
142.
D. Rayfield J.W.S. Longhurst P.S. Ramsden J.A. Dinsdale R.M. Elliott D.E. Conlan 《The Environmentalist》1998,18(1):3-13
The transport-induced air quality problems of the Greater Manchester region are examined. The problems are defined in terms of economic and social factors leading to the development of a car-based economy. The impacts of transport emissions on the natural and built environment and human health are discussed. The administrative, legislative and pollution control responses to the problem are examined. The nature of future air quality problems will require an integration of traffic management, land use, public transport and pollution control policies if they are to be satisfactorily addressed. A series of policy recommendations towards this end are included. 相似文献
143.
Allozyme and mitochondrial DNA variation in yellowfin tuna (Thunnus albacares) from the Pacific Ocean 总被引:1,自引:0,他引:1
Samples of yellowfin tuna (Thunnus albacares) collected in 1991 and 1992 from the western, central and eastern regions of the Pacific Ocean were examined for genetic variability. Four polymorphic allozyme loci (ADA
*, FH
*, GPI-S
* and GPI-F
*) were examined in all samples and a fifth polymorphism (GDA
*) was examined in western and central samples only. Samples were also screened for mitochondrial DNA variation following restriction analysis by two enzymes (BcII and EcoRI) detecting polymorphic cut sites. Eighteen mtDNA haplotypes were revealed, with an overall nucleon diversity of 0.678. A subset of individuals screened for eight restriction enzymes had an overall nucleon diversity of 0.724 and a mean nucleotide diversity per sample of 0.359%. No significant spatial heterogeneity was detected for alleles at the ADA
*, FH
*, GPI-S
* and GDA
* loci nor for the mtDNA haplotypes. Significant heterogeneity was detected for GPI-F
*. At this locus, the two eastern samples (southern California and northern Mexico) were not significantly different from each other but were significantly different (P<0.001) from the five western/central samples (Philippines, Coral Sea, Kiribati, Hawaii-91 and Hawaii-92). GPI-F
*
100 was the most common allele in western and central regions, GPI-F
*
75 the most common in eastern samples. 相似文献
144.
Mytilus edulis planulatus (Lamarck) were collected from Howden, South-east Tasmania in autumn 1981. Interaction effects of cadmium, copper and zinc during accumulation by mussels exposed for ten days to all three metals simultaneously were examined in a series of experiments in which each metal was tested at three concentrations. In general, interaction effects were most evident at the highest concentrations tested (20 g l-1 Cd; 20 g l-1 Cu; 200 g l-1 Zn) and led to a reduction in the accumulation of cadmium and an increase in that of copper and zinc. More specifically, high levels of zinc caused a decrease in cadmium uptake and an increase in copper accumulation. The presence of copper resulted in depressed cadmium accumulation while zinc accumulation increased. Cadmium tended to enhance zinc accumulation, but copper accumulation was only affected to any great extent when zinc was also present. 相似文献
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149.
Michael R. Elliott Patricia F. Waller Trivellore E. Raghunathan Jean T. Shope Roderick J. A. Little 《Journal of Safety Research》2000,31(4):229-242
This analysis examines the ability of previous offenses to predict future high-risk offenses, and similarly, the ability of crashes to predict future high-risk crashes, using the complete driver history data (up to 9 years) for a set of young Michigan subjects. As expected, those with previous ticketed offenses or reported crashes are at greater risk for future offenses or crashes; with a previous-year serious offense doubling the odds of serious offenses during the subsequent year, and a previous-year at-fault crash increasing the odds of subsequent-year at-fault crashes by nearly 50%. There is modest evidence that serious offenses and at-fault crashes may better predict subsequent behavior in females and in more experienced drivers. This latter finding is also evidenced by the fact that records of these young drivers are less predictive of subsequent driving history than is true for records of all drivers in general found in other studies. This suggests that, in the early stages of driving, offenses and crashes are, at least in part, attributable to inexperience, and hence, characteristic of all beginning drivers. 相似文献
150.
How energy costs affect foraging decisions is poorly understood for marine animals. To provide data relevant to this topic,
we examined the relationship between activity levels and foraging behavior by attaching activity recorders to 29 chick-rearing
wing-propelled diving birds (thick-billed murres, Uria lomvia) in 1999–2000. We connected the activity during the final dive bout with the prey item we observed being fed to the chicks.
After accounting for changes in activity level with depth, activity was highest during the final dive of a dive bout, reflecting
maneuvring during prey capture. Pelagic prey items, especially invertebrates (amphipods), were associated with higher depth-corrected
activity, leading to shorter dives for a given depth (presumably due to higher oxygen consumption rates) and, thus, shorter
search times (lower bottom time for a given depth). Pelagic prey items were likely captured during active pursuit, with the
birds actively seeking and pursuing schooling mid-water prey. In contrast, benthic prey involved low activity and extended
search times, suggesting that the birds slowly glided along the bottom in search for prey hidden in the sediments or rocks.
We concluded that activity levels are important in determining the foraging tactics of marine predators.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献