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We used evolutionary programming to model innate migratory pathways of wildebeest in the Serengeti Mara Ecosystem, Tanzania and Kenya. Wildebeest annually move from the southern short-grass plains of the Serengeti to the northern woodlands of the Mara. We used satellite images to create 12 average monthly and 180 10-day surfaces from 1998 to 2003 of percentage rainfall and new vegetation. The surfaces were combined in five additive and three multiplicative models, with the weightings on rainfall and new vegetation from 0% to 100%. Modeled wildebeest were first assigned random migration pathways. In simulated generations, animals best able to access rainfall and vegetation were retained, and they produced offspring with similar migratory pathways. Modeling proceeded until the best pathway was stable. In a learning phase, modeling continued with the ten-day images in the objective function. The additive model, influenced 25% by rainfall and 75% by vegetation growth, yielded the best agreement, with a multi-resolution comparison to observed densities yielding 76.8% of blocks in agreement (kappa = 0.32). Agreement was best for dry season and early wet season (kappa = 0.22-0.57), and poorest for the late wet season (0.04). The model suggests that new forage growth is a dominant correlate of wildebeest migration. 相似文献
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Unraveling the impact of workforce age diversity on labor productivity: The moderating role of firm size and job security
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Previous literature has suggested both positive and negative effects of age diversity on labor productivity: positive because of the potential knowledge complementarities between employees of different ages and negative because of the age‐related value differences that might reduce cohesion and cooperation, hampering firm performance. Using a Belgian sample of 5892 organizational observations (2008–2011), we unraveled these countervailing effects in two ways. First, we built on prior studies to suggest that the effect of age diversity depends on the particular shape of the age distribution: positive when it is heterogeneous (i.e., variety) and negative when it is polarized (i.e., polarization). This was supported by our findings. Second, we explored the moderating impact of two contextual contingencies, firm size and job security. As expected, the positive effect of age variety is reinforced in large firms and in firms where job security is high. Although firm size also emphasizes the negative effect of age polarization on productivity, job security, unexpectedly, does not moderate this relationship. Our study offers a valuable contribution to the literature as it reveals the boundary conditions of the competing implications of age diversity and, thus, allows one to account for the inconclusive findings reported in previous literature. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
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Elizabeth M. Boone Matthew E. Rossheim Jenna R. Krall Robert M. Weiler 《Traffic injury prevention》2018,19(3):270-273
Objective: Research has found that mandatory motorcycle helmet laws increase helmet use and reduce motorcycle-related fatalities. However, the association between state moped helmet laws and helmet use in the United States has not been examined. This study investigated this association among a census of fatally injured moped riders in the United States.
Methods: A logistic regression model was constructed to analyze data extracted from the Fatality Analysis Reporting System (FARS) to examine risk factors for helmet nonuse among 572 moped riders fatally injured between 2011 and 2015.
Results: Fatally injured moped riders in states with universal helmet laws had 69 times the odds of wearing a helmet (P < .001).
Conclusions: Findings suggest that universal moped helmet laws increase helmet use. However, additional research is needed to examine helmet laws and use among nonfatally injured moped riders. 相似文献
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An assessment of tree health and trace element accumulation near a coal-fired generating station, Manitoba, Canada 总被引:2,自引:0,他引:2
A forest health assessment was performed in stands dominated by bur oak and trembling aspen to study the potential effects of airborne emissions from a 132 MW coal-fired station. Forty-two stands were sampled within a 16-km radius of the station for both foliar stress symptoms and trace element toxicology. The concentrations of tracer elements (As, Ba, Sr, and V) in the leaf litter were not spatially congruent with airborne emission deposition models (except Ba, which showed elevated levels immediately SE of the station), nor were they at phytotoxic levels. Elemental concentrations were significantly related to soil parameters including organic matter and texture. No patterns were found in forest health along directional or distance gradients from the generating station. Trembling aspen stands demonstrated little decline in general, but three of the 19 bur oak plots, all located on thin sandy soils developed on calcareous till, demonstrated branch dieback. In addition to poor soil conditions, two of these sites also had high water tables, and exhibited tree mortality. The bur oak decline did not appear to be related to emissions from the station, but is suspected to be a result of poor site quality, with urban development as a confounding factor. 相似文献
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Multiple stressors in amphibian communities: effects of chemical contamination, bullfrogs, and fish. 总被引:6,自引:0,他引:6
Michelle D Boone Raymond D Semlitsch Edward E Little Meaghan C Doyle 《Ecological applications》2007,17(1):291-301
A leading hypothesis of amphibian population declines is that combinations of multiple stressors contribute to declines. We examined the role that chemical contamination, competition, and predation play singly and in combination in aquatic amphibian communities. We exposed larvae of American toads (Bufo americanus), southern leopard frogs (Rana sphenocephala), and spotted salamanders (Ambystoma maculatum) to overwintered bullfrog tadpoles (R. catesbeiana), bluegill sunfish (Lepomis macrochirus), the insecticide carbaryl, and ammonium nitrate fertilizer in 1000-L mesocosms. Most significantly, our study demonstrated that the presence of multiple factors reduced survival of B. americanus and A. maculatum and lengthened larval periods of R. sphenocephala. The presence of bluegill had the largest impact on the community; it eliminated B. americanus and A. maculatum and reduced the abundance of R. sphenocephala. Chemical contaminants had the second strongest effect on the community with the insecticide, reducing A. maculatum abundance by 50% and increasing the mass of anurans (frogs and toads) at metamorphosis; the fertilizer positively influenced time and mass at metamorphosis for both anurans and A. maculatum. Presence of overwintered bullfrogs reduced mass and increased time to metamorphosis of anurans. While both bluegill and overwintered bullfrog tadpoles had negative effects on the amphibian community, they performed better in the presence of one another and in contaminated habitats. Our results indicate that predicting deleterious combinations from single-factor effects may not be straightforward. Our research supports the hypothesis that combinations of factors can negatively impact some amphibian species and could contribute to population declines. 相似文献
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刘聿太 《应用与环境生物学报》1995,(2)
从厌氧消化器中富集和分离出含有一株新的厌氢氧化丙酸盐细菌LYP的共培养物,LYP的唯一伴生菌是能利用氢和甲酸的亨氏甲烷螺菌(Methanospirillumhungateii)。该共培养物利用丙酸和少量的丁酸生成乙酸、CO2和甲烷。共培养物的生氏和产甲烷能力受乙酸或丁酸降解菌的促进。 相似文献
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Zachary O. Toledo J. Boone. Kauffman 《Journal of the American Water Resources Association》2001,37(6):1653-1663
ABSTRACT: Intact riparian zones are the product of an incredibly complex multitude of linkages between the geomorphic, hydrologic, and biotiè features of the ecosystem. Land‐use activities that sever or alter these linkages result in ecosystem degradation. We examined the relationship between riparian vegetation and channel morphology by sampling species composition and herbaceous root biomass in incised (down‐cut and widened) versus unincised (intact) sections of unconstrained reaches in three headwater streams in northeastern Oregon. Incision resulted in a compositional shift from wetland‐obligate plant species to those adapted to drier environments. Root biomass was approximately two times greater in unincised sections than incised sections and decreased with depth more rapidly in incised sections than in unincised sections. Total root biomass ranged from 2,153 g m‐2 to 4,759 g m‐2 in unincised sections and from 1,107 g m‐2 to 2,215 g m‐2 in incised sections. In unincised sections less than 50 percent of the total root biomass was found in the top 10 cm, with approximately 20 percent in successive 10‐cm depth increments. In contrast, incised sections had greater than 60 percent of the total root biomass in the top 10 cm, approximately 15 percent in the 10 to 20 cm depth, less than 15 percent in the 20 to 30 cm depth, and less than 10 percent in the 30 to 40 cm depth. This distribution of root biomass suggests a positive feedback between vegetation and channel incision: as incision progresses, there is a loss of hydrologic connectivity, which causes a shift to a drier vegetation assemblage and decreased root structure, resulting in a reduced erosive resistance capacity in the lower zone of the streambank, thereby allowing further incision and widening. 相似文献
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Pickett ST Cadenasso ML Grove JM Boone CG Groffman PM Irwin E Kaushal SS Marshall V McGrath BP Nilon CH Pouyat RV Szlavecz K Troy A Warren P 《Journal of environmental management》2011,92(3):331-362
Urban ecological studies, including focus on cities, suburbs, and exurbs, while having deep roots in the early to mid 20th century, have burgeoned in the last several decades. We use the state factor approach to highlight the role of important aspects of climate, substrate, organisms, relief, and time in differentiating urban from non-urban areas, and for determining heterogeneity within spatially extensive metropolitan areas. In addition to reviewing key findings relevant to each state factor, we note the emergence of tentative "urban syndromes" concerning soils, streams, wildlife and plants, and homogenization of certain ecosystem functions, such as soil organic carbon dynamics. We note the utility of the ecosystem approach, the human ecosystem framework, and watersheds as integrative tools to tie information about multiple state factors together. The organismal component of urban complexes includes the social organization of the human population, and we review key modes by which human populations within urban areas are differentiated, and how such differentiation affects environmentally relevant actions. Emerging syntheses in land change science and ecological urban design are also summarized. The multifaceted frameworks and the growing urban knowledge base do however identify some pressing research needs. 相似文献