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101.
Dale E. Heydlauff 《Environmental management》1996,20(6):963-966
The quality and sustainability of the natural environment is a matter of inestimable value and is critical to public health and welfare. All species have a purpose, and they exist for the betterment of other species. It is, therefore, incumbent on all humans to do their part in the preservation of this vast, diverse ecosystem called Earth. All humans are the beneficiaries, the ultimate customers, of a sound environment—water that is safe to drink, air that can be breathed, and soil that will sustain crops. There must be a commitment to leaving a clean and healthy planet for generations to follow, an earth which is enhanced, not diminished, by humans' presence. 相似文献
102.
In northeastern America, thousands of kilometers of utility rights-of-way (ROWs) have to be managed to prevent the establishment of a tall vegetation cover that does not comply with safety and maintenance regulations. Recent decades have seen the emergence of ecologically based vegetation control strategies to reduce environmental impacts as well as maintenance costs. One such strategy is to take advantage of competitive herbaceous covers to limit tree invasion. This approach, however, as well as its fundamental underlying principles, has been little scrutinized. In this article, (1) we present the main ecological concepts supporting the use of a herbaceous cover to limit tree invasion, emphasizing naturally forested ecosystems of northeastern America. They include reported evidence of stable plant communities and an overview of potential underlying mechanisms of inhibition. (2) We then review field applications, specifically testing the ability of seeded herbaceous covers to control tree invasion in ROWs. (3) We discuss unresolved issues relevant to management and research. The available evidence suggests that seeding herbaceous covers in ROWs can help control tree invasion, but many issues still limit broad-scale applications. The various interactions that govern plant community dynamics are far from being fully understood, so selecting species still largely depends on an empirical approach. Patterns of resistance to tree invasion must be investigated over a wide range of spatial, historical, and environmental contexts to determine effective management and seeding practices that will lead to broad-scale applications. We suggest establishing communities rather than single dominant species and using as much as possible native species to limit risks of invasion.Published online 相似文献
103.
齐建国 《再生资源与循环经济》2011,4(7):4-6
中国在后国际金融危机时代要保持国民经济持续稳定快速增长,必须大力发展循环经济,全面提升国际竞争力。中国经济所处的发展阶段决定了不仅要在包括战略性新兴产业的高新技术领域抢占国际竞争的制高点,更要在国内市场需求快速增长的传统产业领域,通过自主创新抢占国际竞争的制高点。 相似文献
104.
IntroductionOneofthemajordrawbacksofalleycroppingsystemsinsemiaridlowfertilityconditionsiscompetitionforavailablewaterandnutrients.Competitionistobeexpectedifthetreeandthecrophaveoverlappingactiverootsystemsinspaceandtime.Aknowledgeofthespatialdistr… 相似文献
105.
Sperm competition and the level of polyandry in a bushcricket with large nuptial gifts 总被引:4,自引:0,他引:4
Leon?R.?HockhamEmail author Jefferson?A.?Graves Michael?G.?Ritchie 《Behavioral ecology and sociobiology》2004,57(2):149-154
Variation in paternity due to sperm competition or post-copulatory female choice has a major influence on animal mating system evolution and on the levels of genetic variability in natural populations. However, there are relatively few studies comparing the outcome of sperm-competition experiments in the laboratory with natural variation in polyandry among families from the field. In the bushcricket Ephippiger ephippiger, females mate multiply, and the males provide them with a large, nutritious, and probably expensive, donation at mating. We examined paternity in a series of laboratory matings, where females mated with two males, and amongst a series of families collected from a natural population. In the laboratory, paternity was highly bimodally distributed: 24% of families had offspring fathered by the first male to mate, 68% by the second male (in only 8% was paternity shared). In the field, paternity was more mixed: only 27% of families had a single father, 14% had more than two fathers, whilst 59% had two fathers. While unsuccessful matings may contribute to the highly biased paternity in the laboratory, they cannot fully explain the high incidence of complete P2 families. Nonrandom sperm utilisation is therefore likely. Greater sperm mixing in the field probably results from females mating with more males, but the distribution of paternity also reflects an active process of nonrandom sperm utilisation. Confidence of paternity due to last male advantage may be relatively high in this species, and therefore may have facilitated the evolution of the large spermatophore in E. ephippiger.Communicated by D. Gwynne 相似文献
106.
Despite widespread recognition that intersexual interactions shape reproductive strategies, studies of male competition do
not typically include effects imposed by females. In cannibalistic redback spiders, escalated fighting between rival suitors
is predicted, as males are unlikely to mate with more than one female, and strong first-male sperm precedence favours mating
with virgins. In staged competitions for matings between size mismatched rivals, smaller males adopted an alternative sneaking
strategy. However, despite initial agonistic interactions, larger males did not pursue or incapacitate smaller males. When
inter-male competition occurred, females struck at males frequently, although strikes were rarely seen when males courted
in the absence of a rival. After minimal fighting, larger males engaged in significant courtship (3 h) rather than killing
inferior rivals. Prolonged courtship was favoured by female behaviour, as males that attempted rapid copulation (smaller,
sneaking males) were cannibalised before mating was completed. This premature cannibalism significantly decreases paternity
in redback spiders. Thus, significant features of male competitive behaviour (i.e. prolonged courtship by larger males) may
be predicted with consideration of the female’s response to male reproductive strategies. Although the effect of females may
be more subtle in systems without the extreme reversed size-dimorphism of redbacks, these results suggest that female interests
should be explicitly considered when studying inter-male interactions. 相似文献
107.
Andreas Vermeulen Sierk Engels Klaus Peter Sauer 《Behavioral ecology and sociobiology》2008,63(1):77-83
Variation in traits closely related to fitness is expected to be low. This is because these traits are under directional selection
and the best genotype should prevail. However, there have been a number of studies demonstrating the existence of considerable
variance in sexually selected traits, which is generally known as the lek paradox. Accordingly, earlier studies found substantial
variation in sperm transfer rates in Panorpa vulgaris. Aiming at finding the mechanism that maintains this variation, we analyzed the condition dependence and the narrow sense
heritability of sperm transfer rates. Food deprivation in the larval/adult phase caused a decrease in the males’ capability
of saliva secretion resulting in shorter copulations and a reduced number of transferred sperm. There was a positive correlation
between mean sperm transfer rates and mean body mass. Additionally, intermale variation in sperm transfer rates decreased
with increasing food availability. Hence, we suggest that sperm transfer rates in P. vulgaris are influenced by adult feeding history. Heritability analyses of sperm transfer rates did not provide significant results,
which is consistent with the general hypothesis that additive genetic variance in traits closely related to fitness is small.
Since a trait’s potential to respond to selection is proportional to the amount of contained additive genetic variance, the
ascertained small heritability provides a satisfying explanation for the maintenance of substantial variation in sperm transfer
rates. 相似文献
108.
Attila?HettyeyEmail author J.?Dale?Roberts 《Behavioral ecology and sociobiology》2007,61(8):1303-1310
In species where males use alternative reproductive tactics and male phenotypes are confronted with different risks of sperm
competition, theory predicts that between-male-type differences in sperm expenditure may evolve. In the frog Crinia georgiana big males can monopolize females, whereas small males often engage in polyandrous matings. Consequently, big males may experience
a lower risk of sperm competition than do small males. We tested if the predictions from theoretical models can be applied
to the mating system of C. georgiana. Our results showed that small males do not have larger testes relative to their body size compared to their larger counterparts
and that the efficiency with which sperm number, size, motility, and longevity are produced by the testes does not differ
between small and large males in the predicted way. These results are not in alignment with predictions from a loaded raffle
model of sperm competition on sperm expenditure in males with alternative phenotypes. The plasticity in mating tactics used
by C. georgiana males and a high intraseasonal variation in male densities may have prevented the evolution of enhanced sperm performance
in smaller males. A fair raffle in the sperm competition game played by C. georgiana males could also explain the observed patterns in sperm traits. Future investigations determining the parameters responsible
for the deviation from theoretical predictions in this system will test the degree to which current theoretical models can
indeed be applied to species with plastic reproductive tactics. 相似文献
109.
BRADLEY CAIN ANTONY B. WANDERA SUSAN G. SHAWCROSS W. EDWIN HARRIS BARRY STEVENS‐WOOD STEPHEN J. KEMP BENSON OKITA‐OUMA PHILLIP C. WATTS 《Conservation biology》2014,28(2):594-603
A central premise of conservation biology is that small populations suffer reduced viability through loss of genetic diversity and inbreeding. However, there is little evidence that variation in inbreeding impacts individual reproductive success within remnant populations of threatened taxa, largely due to problems associated with obtaining comprehensive pedigree information to estimate inbreeding. In the critically endangered black rhinoceros, a species that experienced severe demographic reductions, we used model selection to identify factors associated with variation in reproductive success (number of offspring). Factors examined as predictors of reproductive success were age, home range size, number of nearby mates, reserve location, and multilocus heterozygosity (a proxy for inbreeding). Multilocus heterozygosity predicted male reproductive success (p< 0.001, explained deviance >58%) and correlated with male home range size (p < 0.01, r2 > 44%). Such effects were not apparent in females, where reproductive success was determined by age (p < 0.01, explained deviance 34%) as females raise calves alone and choose between, rather than compete for, mates. This first report of a 3‐way association between an individual male's heterozygosity, reproductive output, and territory size in a large vertebrate is consistent with an asymmetry in the level of intrasexual competition and highlights the relevance of sex‐biased inbreeding for the management of many conservation‐priority species. Our results contrast with the idea that wild populations of threatened taxa may possess some inherent difference from most nonthreatened populations that necessitates the use of detailed pedigrees to study inbreeding effects. Despite substantial variance in male reproductive success, the increased fitness of more heterozygous males limits the loss of heterozygosity. Understanding how individual differences in genetic diversity mediate the outcome of intrasexual competition will be essential for effective management, particularly in enclosed populations, where individuals have restricted choice about home range location and where the reproductive impact of translocated animals will depend upon the background distribution in individual heterozygosity. Efectos de la Endogamia Sesgada por el Sexo sobre el Éxito Reproductivo y el Rango del Tamaño de Hábitat del Rinoceronte Negro, Especie en Peligro Crítico 相似文献
110.
高速铁路对国内民航旅客运输的替代效应测度 总被引:1,自引:0,他引:1
中国高速铁路的快速建设,对国内民航客运的发展产生了较大的影响。基于事后分析视角,从直达联系视角建立了国内高铁—民航运输的空间叠合网络,定量分析了高铁建设前后民航旅客运输的变化特征,并基于航空客流增速变化识别出高铁影响下的四种航段类型及其空间分布。在此基础上,基于2007-2014年全国277对高铁—民航运输重叠城市对的8年面板数据,以航空客流为因变量,建立随机效应模型,研究了高速铁路介入后对国内民航客运系统、北上广枢纽机场以及其他机场航线的替代效应。结论表明:(1)高速铁路对重叠网络的民航旅客运输产生了较强的替代效应,且高铁列车速度越快、城市间距离越短,替代效应越明显。(2)由于航班网络联系的多元化、旅客出行偏好选择以及对时间敏感性不同,枢纽机场航线较非枢纽机场航线受高铁建设的影响相对较小。未来随着高铁网络的进一步完善,其对航空客运的影响仍值得持续关注。 相似文献