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Facilitation by habitat modifiers is common in ecological communities, but the potential for temporal and spatial variations in environmental conditions to modify the outcome of these interactions and influence the strength of feedbacks is poorly understood. Suspension-feeding bivalves are important habitat modifiers that can facilitate surrounding communities by providing refuge from predation and changing boundary flows and through the production of organically enriched biodeposits. However, numerous studies have highlighted the problem of finding generalizable patterns. We tested the strength and generality of the relationship between the large suspension-feeding bivalve Atrina zelandica and surrounding macrofauna and hypothesized that facilitation by Atrina is conditional and modulated by site-specific suspended sediment concentration (SSC), which influences the quantity and quality of biodeposit production. We found temporally consistent patterns of higher rates of biodeposition and increased abundance and species richness in close proximity to Atrina under low SSC conditions. Facilitation strength decreased with increasing SSC, suggesting that the facilitation effect of Atrina is reduced and reversed along this environmental stress gradient. 相似文献
96.
Feedbacks between bivalve density, flow, and suspended sediment concentration on patch stable states
We explore the role of biophysical feedbacks occurring at the patch scale (spatial scale of tens of meters) that influence bivalve physiological condition and affect patch stability by developing a numerical model for the pinnid bivalve, Atrina zelandica, in cohesive sediments. Simulated feedbacks involve bivalve density, flow conditions (assumed to be primarily influenced by local water depth and peak current speed), suspended sediment concentration (evaluated through a balance between background concentration, deposition, and erosion), and changes in the physiology of Atrina derived from empirical study. The model demonstrates that high bivalve density can lead to skimming flow and to a concomitant decrease in resuspension that will affect suspended sediment concentration over the patch directly feeding back on bivalve physiology. Consequently, for a given flow and background suspended sediment load, the stability of a patch directly depends on the size and density of bivalves in the patch. Although under a range of conditions patch stability is ensured independently of bivalve density, simulations clearly indicate that sudden changes in bivalve density or suspended sediment concentration can substantially affect patch structure and lead to different stable states. The model highlights the role of interactions between organisms, flow, and broader scale environmental conditions in providing a mechanistic explanation for the patchy occurrence of benthic suspension feeders. 相似文献
97.
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
98.
Lam JC Tanabe S Chan SK Lam MH Martin M Lam PK 《Environmental pollution (Barking, Essex : 1987)》2006,144(3):790-801
Concentrations of 22 trace elements were determined in green turtle (Chelonia mydas) eggs collected from Hong Kong. Concentrations of selenium, lead and nickel in these eggs were generally higher than those reported in other studies. The predicted no effect concentrations (PNEC; ng/g wet weight) of Pb (1000), Se (340 and 6000 for the worst-case and best-case scenarios, respectively) and Ni (17) in the green turtle eggs were estimated. Hazard quotients (HQs) estimate that Se (HQs: 0.2-24.5) and Ni (HQs: 4.0-26.4) may pose some risks to the turtles. Our study also found that concentrations of Ag, Se, Zn, Hg and Pb in the shell of the turtle eggs were significantly correlated with levels in the whole egg contents (yolk+albumen). Once the precise relationships of specific elements are established, egg-shell concentrations may be used as a non-lethal, non-invasive, surrogate for predicting whole egg burden of certain contaminants in marine turtles. 相似文献
99.
Background, Aim and Scope
Due to their large potential for manifold applications, the use of nanoparticles is of increasing importance. As large amounts
of nanoparticles may reach the environment voluntarily or by accident, attention should be paid on the potential impacts on
the environment. First studies on potential environmental effects of photocatalytic TiO2 nanoparticles have been performed
on the basis of widely accepted, standardized test systems which originally had been developed for the characterization of
chemicals. The methods were adapted to the special requirements of testing photocatalytic nanoparticles.
Materials and Methods:
Suspensions of two different nanoparticles were illuminated to induce their photocatalytic activity. For testing, the growth
inhibition test with the green alga Desmodesmus subspicatus and the immobilization test with the daphnid Daphnia magna were
selected and performed following the relevant guidelines (algae: ISO 8692, OECD 201, DIN 38412-33; daphnids: ISO 6341, OECD
202, DIN 38412-30). The guidelines were adapted to meet the special requirements for testing photocatalytic nanoparticles.
Results:
The results indicate that it is principally possible to determine the ecotoxicity of nanoparticles. It was shown that nanoparticles
may have ecotoxicological effects which depend on the nature of the particles. Both products tested differ in their toxicity.
Product 1 shows a clear concentration-effect curve in the test with algae (EC50: 44 mg/L). It could be proven that the observed
toxicity was not caused by accompanying contaminants, since the toxic effect was comparable for the cleaned and the commercially
available product. For product 2, no toxic effects were determined (maximum concentration: 50 mg/L). In the tests with daphnids,
toxicity was observed for both products, although the concentration effect-curves were less pronounced. The two products differed
in their toxicity; moreover, there was a difference in the toxicity of illuminated and non-illuminated products.
Discussion:
Both products differ in size and crystalline form, so that these parameters are assumed to contribute to the different toxicities.
The concentration-effect curves for daphnids, which are less-pronounced than the curves obtained for algae, may be due to
the different test organisms and/or the differing test designs. The increased toxicity of pre-illuminated particles in the
tests with daphnids demonstrates that the photocatalytic activity of nanoparticles lasts for a period of time.
Conclusions:
The following conclusions can be drawn from the test results: (I) It is principally possible to determine the ecotoxicity
of (photocatalytic) nanoparticles. Therefore, they can be assessed using methods comparable to the procedures applied for
assessing soluble chemicals. - (II) Nanoparticles may exert ecotoxicological effects, which depend on the specific nanoparticle.
- (III) Comparable to traditional chemicals, the ecotoxicity depends on the test organisms and their physiology. - (IV) The
photocatalytic activity of nanoparticles lasts for a relevant period of time. Therefore, pre-illumination may be sufficient
to detect a photocatalytic activity even by using test organisms which are not suitable for application in the pre-illumination-phase.
Recommendations and Perspectives:
First results are presented which indicate that the topic 'ecotoxicity and environmental effects of nanoparticles' should
not be neglected. In testing photocatalytic nanoparticles, there are still many topics that need clarification or improvement,
such as the cause for an observed toxicity, the improvement of the test design, the elaboration of a test battery and an assessment
strategy.
On the basis of optimized test systems, it will be possible to test nanoparticles systematically. If a potential risk by specific
photocatalytic particles is known, a risk-benefit analysis can be performed and, if required, risk reducing measures can be
taken. 相似文献
100.
The required coefficient of friction (RCOF) is one of the critical elements in determining whether a slip incident might occur. Most researchers, when measuring RCOF, do not differentiate between the two feet of the same participant under the same walking condition. Results from a recent study (Chang et al., 2010) indicated that the stochastic distributions of the RCOF of both feet were different in 76% of the cases. Using the previous data (Chang et al., 2010), this paper presents a comparison of the RCOF for both feet of 50 participants under four level walking conditions resulting from two footwear types and two walking speeds using a t-test, commonly used by safety professionals to compare two pools of data. The current results indicated that 78% of the RCOF data showed a statistically significant difference between the RCOF from two feet for the same participant under each walking condition. The results of the logistic regression analysis used to identify factors that contributed to the outcome of the t-test indicated that the walking speed was the only factor with a statistical significance (p = 0.044). 相似文献