ISSN:1051-0761    

DOI:10.2307/1942109

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Much sampling to detect and quantify human environmental disturbances is flawed by a lack of appropriate replication. BACI (Before—After—Control—Impact) designs have only a single control location, and any conclusions from them are illogical. Asymmetrical designs using one putatively impacted and several control locations can reliably detect a variety of environmental impacts, including those that do not affect long—run mean abundances, but do alter temporal variance. When abundances of populations in different locations show temporal interaction, the asymmetrical designs allow tests for impact that are not possible in BACI designs. Asymmetrical designs are also extendable to sample at hierarchical spatial and temporal scales. The power of tests using asymmetrical designs is great for non—interactive sets of abundances, but greatest for pulse (short—term) responses to disturbances, large alterations of temporal variance, or combinations of temporal variance, or combinations of sustained, press responses in mean abundance coupled with altered temporal heterogeneity. Power in temporally interactive sets of data is generally poor. Alternatives to pre—disturbance sampling, including generalized assessment of spatial and temporal variances and experimental impacts, may provide better guidance for detection of human disturbances.

ISSN:1051-0761    

DOI:10.2307/1942110

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

The power of any test of an environmental impact is simultaneously constrained by (1) the variability of the data, (2) the magnitude of the putative impact, and (3) the number of independent sampling events. In the context of the Before–After—Control–Impact design with Paired sampling (BACIP), the variability of interest is the temporal variation in the estimated differences in a parameter (e.g., population density) between two unperturbed sites. The challenges in designing a BACIP study are to choose appropriate parameters to measure and to determine the adequate number and timing of sampling events. Two types of studies that are commonly conducted can provide useful information in designing a BACIP study. These are (1) long—term studies that provide estimates of the natural temporal and spatial variability of environmental parameters and (2) spatial surveys around already—perturbed areas ("After—only" studies) that can suggest the magnitude of impacts. Here we use data from a long—term study and an After—only study to illustrate their potential contributions to the design of BACIP studies. The long—term study of parameters sampled at two undisturbed sites yielded estimates of natural temporal variability. Between site differences in chemical—physical parameters (e.g., elemental concentration) and in individual—based biological parameters (e.g., body size) were quite consistent through time, while differences in population—based parameters (e.g., density) were more variable. Serial correlation in the time series of differences was relatively small and did not appear to vary among the parameter groups. The After—only study yielded estimates of the magnitude of impacts through comparison of sites near and distant from a point—source discharge. The estimated magnitude of effects was greatest for population—based parameters and least for chemical—physical parameters, which tended to balance the statistical power associated with these two parameter groups. Individual—based parameters were intermediate in estimates of effect size. Thus, the ration of effect size to variability was greatest for individual—based parameters and least for population and chemical—physical parameters. The results suggest that relatively few of the population and chemical—physical parameters could provide adequate power given the time constraints of most studies. This indicates that greater emphasis on individual—based parameters is needed in field assessments of environmental impacts. It will be critical to develop and test predictive models that link these impacts with effects on populations.

ISSN:1051-0761    

DOI:10.2307/1942111

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Long—term studies provide useful insight into the functioning of populations and communities. Gaining a long—term perspective may be important in providing back—ground information upon which BACI (Before–After—Control–Impact)—type impact studies can be designed or in identifying many types of impact, particularly those of a large—scale and diffuse nature. Although marine soft—bottom communities commonly exhibit patchy distributions, in many long—term studies few replicates, from each site, are collected on each occasion. To illustrate potential problems encountered when spatial variation confounds temporal patterns, we use data collected from a monitoring program of the intertidal sandflat communities of Manukau Harbour, New Zealand. Randomization techniques were used to generate possible temporal sequences, based on a sample size similar to that used in many long—term studies. In the generated sequences the timing and size of annual density changes were often different from the observed sequence, and indicators of trends in density were often missed. In order to provide accurate descriptions of temporal variability in the soft—bottom communities of estuaries and coastal embayments, low bias and highly precise density estimates from each sampling occasion are required.

ISSN:1051-0761    

DOI:10.2307/1942112

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

We use a model based on the size—structured von Foerster equation to describe how size—dependent growth and mortality rates, pulsed recruitment, and variability in growth affect the shape of a size distribution. The deterministic, equilibrium size distribution with constant recruitment increases with size when the difference between mortality rate and the rate at which growth rate decreases with size is positive (growth dominated), and decreases when it is negative (mortality dominated). Pulsed recruitment causes modes whose relative amplitudes are indicated by the corresponding constant recruitment case. For typical animal growth patterns, the distance between pulses decreases with age. Pulses merge and can be selectively obscured by variability in growth so that their relative amplitudes no longer correspond to the constant recruitment case. We use this information to evaluate why bimodality occurs in size distributions of the red sea urchin, Strongylocentrotus franciscanus, in some habitats, but not others. The mode at larger sizes, which occurs in all habitats, arises because the distribution is mortality dominated and the final sizes of individuals vary. The upper half of a second mode at smaller sizes is caused by higher mortality rates at sizes greater than the peak of that mode. The lower half may be due either to a refuge from predation under the spine canopy of adults or to sampling selectivity.

ISSN:1051-0761    

DOI:10.2307/1942113

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

In population modelling the functional forms of models are at least as important as are parameter values in expressing the underlying biology and in determining the outcome. Here the influence of differing biological assumptions as to predator interference on the forms of predator—prey models is discussed, and it is shown how this aspect of the underlying biology can influence the conclusions to be drawn from multispecies population models as to how "competition from natural predators" affects human harvesting of natural populations. The importance of these issues for the current controversy over the effects of marine mammals on fisheries is stressed.

ISSN:1051-0761    

DOI:10.2307/1942114

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Previous studies have shown that exposure to chemical contaminants is associated with a variety of disease conditions in feral fish. However, relatively little is known about the effects of toxicopathic disease or other types of impairment associated with chronic exposure to environmental contaminants on fish mortality rates. In this study, annual mortality rates were estimated in English sole (Pleuronectes vetulus) from heavily and minimally contaminated areas in Puget Sound (Washington state), and in sole with and without selected hepatic lesions, including neoplasms. Mortality rates were determined using catch curve analysis and linear regression techniques. For English sole 3 yr of age and above from urban and non—urban areas of Puget Sound combined, the annual mortality rate was 0.38. This estimate is similar to published mortality rates for English sole from northern Puget Sound and other flatfish species from the North Sea, Grand Banks, and western Canada. Mortality rates in sole from heavily contaminated sites or in sole with toxicopathic liver lesions were not significantly higher than those for English sole from Puget Sound as a whole. These findings suggest that although toxicant—related death due to disease or other impairment may contribute to mortality rates in English sole, at least in older animals, its impact appears to be overridden by other factors that could deplete English sole populations, such as fishing pressure, predation, or fluctuations in food supply. The major impact of chronic exposure to environmental contaminants may be on reproductive capacity, egg and larval viability, and survival of juveniles rather than on survival of adults.

ISSN:1051-0761    

DOI:10.2307/1942115

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

We test whether data from laboratory bioassays can be used to predict zooplankton responses during a whole—lake experiment using two rotifers, Keratella cochlearis and Keratella taurocephala. The acidification experiment was conducted in Little Rock Lake, Wisconsin, USA, which was divided into a reference basin maintained at a natural pH near 6.1 and a treatment basin which was acidified in 2—yr stages to pH values of 5.6, 5.2, and 4.7. Laboratory assays examined the effect of pH on reproduction under varied food conditions and survivorship without food. In the lake, the two rotifers showed strong and opposite responses to acidification: K. cochlearis decreased in abundance while K. taurocephala increased. In the laboratory bioassays, neither species was sensitive to pH when food conditions yielded high reproductive rates. When food was limited, K. cochlearis exhibited lower survivorship and a trend towards lower reproductive rates at lower pH. With limited food, K. taurocephala survivorship was either unaffected by pH or higher at high pH and its reproduction was slightly higher at intermediate pH. In situ experiments revealed that food conditions in the treatment basin lowered reproduction by K. cochlearis, indicating that a combined effect of food and pH caused its population decline. Neither food nor pH could explain the increase in K. taurocephala, which appeared to be linked to a reduction in its predators at lower pH. Overall, our analyses revealed substantial discrepancies between laboratory bioassays and in—lake responses. This was particularly the case for K. taurocephala, for which assays predicted no changes or a decline in abundance rather than the marked increase that actually occurred. Our results suggest that caution should be used in extending results from laboratory assays to natural ecosystems.

ISSN:1051-0761    

DOI:10.2307/1942116

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Malaria, transmitted by anopheline mosquitoes, remains a serious health problem in the tropics. Most malaria eradication efforts focus on control of anopheline vectors. These efforts include the NASA Di—Mod project, whose current goal is to integrate remote sensing, geographic information systems (GIS), and field research to predict anopheline mosquito population dynamics in the Pacific coastal plain of Chiapas, Mexico. Field studies demonstrate that high larval production of Anopheles albimanus, the principal malaria vector in the plain, can be linked to a small number of larval habitat—types, determined by larval sampling and cluster analysis of wetlands in the coastal plain. Analysis of wet and dry season Landsat Thematic Mapper (TM) satellite imagery identified 16 land cover units within an 185—km2study area in the coastal zone. A hierarchical approach was used to link the larval habitat—types with the larger land cover units and make predictions of potential and actual low, medium, and high anopheline production. The TM—based map and GIS techniques were then used to predict differences in anopheline production at two villages, La Victoria and Efrain Gutierrez. La Victoria was predicted to have much higher Anopheles albimanus production, based upon a 2—10 times greater extent of medium— and high—producing land cover units in its vicinity. This difference between villages was independently supported by sampling (with light traps) of adults, which were 5—10 times more abundant in La Victoria.

ISSN:1051-0761    

DOI:10.2307/1942117

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

It is widely acknowledged (1) that meningeal helminths (Parelaphostrongylus tenuis) carried by white—tailed deer (Odocoileus virginianus) but fatal to moose (Alces alces) and other cervids, have caused widespread population declines where deer have invaded the ranges of other cervids, (2) that meningeal worms can alter the outcome of interspecific competition between cervid hosts, and (3) that moose populations persist on enzootic range in habitat refuges not frequented by deer. Further, some attempts to reintroduce other cervids to range now occupied by deer have failed. However, moose have been observed to persist on range sympatric with deer, there is evidence that at least one moose population introduced to range with deer is growing, and other events (such as habitat change) are confounded with the presumed effects of meningeal worms. Thus, it is still not clear whether the mortality of individual cervids attributed to meningeal worms necessarily has the effects on population dynamics ascribed to it. Using conventional host—macroparasite models, we describe the population dynamics of deer, moose, intermediate gastropod hosts, and the parasite to determine conditions under which the meningeal worm might be implicated in widespread declines of moose populations. The analysis resulted in three general scenarios depending on (1) the magnitude of parameters that affect transmission rates between different phases of the parasite life cycle, (2) the parasite—induced mortality rate of moose, and (3) the difference in comptitive ability between deer and moose. First, when larval transmission to intermediate hosts, consumption of intermediate hosts by definitive hosts, and parasite—induced mortality of moose were all small, and the competitive difference between definitive hosts was large, moose excluded deer. Second, moose and deer could coexist at either a stable or unstable equilibrium. Stable coexistence occurred when transmission rates, mortality rates, and competitive differences were all large; then deer outnumbered moose. Unstable coexistence at equilibrium occurred under the opposite conditions; moose then were more abundant than deer. Finally, deer could competitively exclude moose. To determine which of these cases was most plausible, we substituted available estimates for some parameters from published literature, solved the equations numerically, and conducted a sensitivity analysis. Overall, model solutions were most sensitive to the magnitude of the competitive effect of moose on deer, the parasite—induced mortality rate of moose, and the intrinsic rate of increase of the intermediate host–all parameters for which there is little empirical information. We conclude that, with the available empirical information, meningeal worm cannot be ruled out as a factor contributing to declines of moose populations. However, our analyses suggest that moose populations sympatric with deer, in many cases, should not become extinct. This implies that reintroductions of moose (and perhaps other cervids) to range now occupied by deer need not be presumed to fail necessarily. Moose may be able to coexist with deer, albeit at lower densities, even in the absence of habitat refuges from the disease.

ISSN:1051-0761    

DOI:10.2307/1942118

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY