Abstract: We examined the roles of dispersal mechanism, a biological barrier; light availability, an environmental barrier; and level of disturbance, a physical barrier, in explaining the spatial patterns of exotic plant species along road and stream segments in a forest landscape in the western Cascade Range of Oregon (U.S.A). The presence or absence of 21 selected exotic plant species and light levels were observed along 0.3- to 1.0-km transects within four habitat types. Each habitat represented a different level of disturbance: high-use roads, low-use roads, abandoned roads, and streams in the H. J. Andrews Experimental Forest. Nearly 300 50 × 2–m sampling units were surveyed along five transects in each habitat type. We used ordination (nonmetric multidimensional scaling) and logistic regression to analyze data. All of the nearly 200 sampling units along roads with high and low levels of vehicle traffic contained at least one exotic plant species, and some contained as many as 14. Streams that were most recently disturbed by floods 20–30 years ago and abandoned spur roads with no traffic for 20–40 years also had numerous exotic species. Roads and streams apparently serve multiple functions that enhance exotic species invasion in this landscape: they act as corridors or agents for dispersal, provide suitable habitat, and contain reservoirs of propagules for future episodes of invasion. Species-specific dispersal mechanisms, habitat characteristics, and disturbance history each explain some, but not all, of the patterns of exotic species invasion observed in this study. 相似文献
Benthic organisms can significantly alter the physical properties of marine sediments, but it has hitherto been difficult to assess and quantify the effects of bioturbation. In situ geophysical techniques offer new methods for measuring these effects: measurement of acoustic shear-wave velocity and electrical resistivity allows nondestructive assessment of the properties of the grain framework and pore-fluid matrix, respectively, of the seabed sediment. The influence of burrowing invertebrates on the structural properties of sandy sediments at intertidal locations on the coast of Wales (UK) was investigated during the periol 1986–1987 using these techniques. Three species (Arenicola marina, Corophium arenarium and Lanice conchilega) were selected on the basis of their contrasting styles of burrow construction. All three species produced measurable and significant, although different, changes in bed properties. They modified shear-wave propagation through the bed by changing bed rigidity: while A. marina and C. arenarium decreased rigidity by creating open burrows, L. conchilega increased rigidity by building shell-lined tubes. All produced a decrease in electrical resistivity by altering porosity and/or tortuosity, which implies an increase in permeability; these changes were attributable not only to the presence of the burrows but also to modification of the between-burrow sediment texture and bed properties. 相似文献
In the 1970s, the Mauritius Kestrel ( Falco punctatus ) was the most endangered bird of prey in the world, at one time with only two known pairs surviving in the remnant native forest of the Black River Gorges (ca. 4,000 ha). At the end of the 1991–1992 breeding season, a minimum of 30 nesting pairs and more than 170 individuals were distributed in four separate forested areas, thanks mainly to manipulation of the reproductive potential of the wild pairs, to captive propagation, and to reintroduction (restocking). Since 1984, 139 young have been reared from 618 eggs laid by captive kestrels, and 147 from 265 wild eggs incubated and hatched in the laboratory; 235 young kestrels have been released on Mauritius by hacking and fostering. Adjustments in feeding and nesting habits of kestrels hacked and released outside the Black River Gorges in areas dominated by exotic vegetation and agriculture have allowed these kestrels to survive and reproduce in an array of previously unused habitats. Now that the kestrels have been released from dependence on the remnant and dying native forest, a viable population of more than 100 nesting pairs should be achievable in a few more years. 相似文献
Carbon monoxide (CO) in the surface sea waters is produced predominantly by photochemical processes, oxidized by micro-organisms and outgassed to the atmosphere. to assess carbon monoxide flux from the oceans to the atmosphere, the photochemical production and microbial oxidation of carbon monoxide in the oceanic mixed-layer was investigated during several oeanographic cruises and in the laboratory. the photoproduction rate of carbon monoxide was found to be well correlated to the concentration of dissolved organic carbon (DOC) in coastal and open ocean surface waters. Taking a global average carbon monoxide production rate of 10 ± 2 nmole litre-1 (mg DOC hr)-1 in the surface open ocean water, and 25 ± 7 nmole litre-1 (mg DOC hr)-1 in coastal sea water, at cloud-free summer solar noon, the photochemical production of carbon monoxide in the global oceans is estimated to be at a rate of 1200 ± 200 Tg CO y-1. the microbial carbon monoxide turnover time in the mixed-layer was observed to range from hours in a coastal estuary to 16 days in the Pacific along 1057deg; W in dark incubations. Natural sunlight can largely inhibit the microbial consumption of carbon monoxide in surface water. On a global scale, microbial consumption is responsible for the loss of less than 10% of photochemical produced carbon monoxide in the surface ocean. Field measurements have shown that the net transport of carbon monoxide from the euphotic zone to the underlying deeper ocean water is limited and that the overall life time in surface sea waters is less than 3-4 hours. When combined, these field measurements with the photoproduction and microbial consumption rates obtained, we estimate the oceanic flux to the atmosphere is about 1000 ± 200 Tg CO y-1, which represents the largest single source of atmospheric carbon monoxide. 相似文献
Understanding how cities can transform organic waste into a valuable resource is critical to urban sustainability. The capture and recycling of phosphorus (P), and other essential nutrients, from human excreta is particularly important as an alternative organic fertilizer source for agriculture. However, the complex set of socio-environmental factors influencing urban human excreta management is not yet sufficiently integrated into sustainable P research. Here, we synthesize information about the pathways P can take through urban sanitation systems along with barriers and facilitators to P recycling across cities. We examine five case study cities by using a sanitation chains approach: Accra, Ghana; Buenos Aires, Argentina; Beijing, China; Baltimore, USA; and London, England. Our cross-city comparison shows that London and Baltimore recycle a larger percentage of P from human excreta back to agricultural lands than other cities, and that there is a large diversity in socio-environmental factors that affect the patterns of recycling observed across cities. Our research highlights conditions that may be “necessary but not sufficient” for P recycling, including access to capital resources. Path dependencies of large sanitation infrastructure investments in the Global North contrast with rapidly urbanizing cities in the Global South, which present opportunities for alternative sanitation development pathways. Understanding such city-specific social and environmental barriers to P recycling options could help address multiple interacting societal objectives related to sanitation and provide options for satisfying global agricultural nutrient demand.
Polychlorinated biphenyl (PCB) input-output balance studies were performed on five male volunteers, aged between 24 and 30, for periods of 8-14 days in 1998. Dietary exposure was quantified by the duplicate meals method and varied between 220 and 460 ng of sigmaPCB (sum of 20 congeners) per day for each of the five individuals over the study period. Dietary intake was dominated by congeners 118, 138, 153 and 180. Average faecal outputs for the five volunteers were 50-290 ng of sigmaPCB (sum of 20 congeners) per day for each of the five individuals over the same period and was dominated by the same four congeners. Whilst the total PCB fluxes were therefore into the body (i.e., accumulation), important differences were noted for different individual congeners. PCBs 44, 47, 49, 52, 60, 66, 101, 105, 110, 118, 149, 151 and 183 all showed net absorption for all five volunteers. Some congeners showed a net absorption in some of the individuals but net excretion in others, as seen by other workers. These congeners (PCBs 138, 153, 180, 187 and 194) are all higher chlorinated congeners and lack meta-para-vicinal hydrogen atoms. There were differences in the net absorption/excretion between individuals, which appeared to be a function of body fat index (BFI). The volunteers with the lowest BFIs showed net excretion for the greatest number of congeners, whilst the individual with the highest BFI was a net absorber of all the congeners studied. The problems in determining and interpreting absorption efficiency values for use in quantitative exposure assessments are discussed. Various factors that influence net absorption of PCBs and other persistent organic pollutants are identified. These include compound properties (including susceptibility to metabolism), the individual's fat status and balance, exposure history and diet. 相似文献