Impacts of Urbanization on Stream Habitat and Fish Across Multiple Spatial Scales

We analyzed the relation of the amount and spatial pattern of land cover with stream fish communities, in-stream habitat, and baseflow in 47 small southeastern Wisconsin, USA, watersheds encompassing a gradient of predominantly agricultural to predominantly urban land uses. The amount of connected impervious surface in the watershed was the best measure of urbanization for predicting fish density, species richness, diversity, and index of biotic integrity (IBI) score; bank erosion; and base flow. However, connected imperviousness was not significantly correlated with overall habitat quality for fish. Nonlinear models were developed using quantile regression to predict the maximum possible number of fish species, IBI score, and base flow for a given level of imperviousness. At watershed connected imperviousness levels less than about 8%, all three variables could have high values, whereas at connected imperviousness levels greater than 12% their values were inevitably low. Connected imperviousness levels between 8 and 12% represented a threshold region where minor changes in urbanization could result in major changes in stream condition. In a spatial analysis, connected imperviousness within a 50-m buffer along the stream or within a 1.6-km radius upstream of the sampling site had more influence on stream fish and base flow than did comparable amounts of imperviousness further away. Our results suggest that urban development that minimizes amount of connected impervious surface and establishes undeveloped buffer areas along streams should have less impact than conventional types of development.     

Denitrification Distributions in Four Valley and Ridge Riparian Ecosystems

/ Denitrification in riparian ecosystems can reduce the amount ofnitrogen transported from farm fields to streams. In this study, we examinedenitrification in four riparian ecosystems common to the Valley and Ridgephysiographic province in Pennsylvania, USA. The sites exhibit differentvegetation, are underlain by different rock types, and are downgradient offarm fields. Mean site denitrification rates ranging from 0.6 to 1.9 &mgr;gN/kg soil/day were measured using intact core incubation techniques. Thethree riparian sites covered with grass each exhibited greaterdenitrification rates than the wooded site. Denitrification rate wascorrelated with moisture content but not with nitrate-N or organic carboncontents. Denitrification rates were greatest near the soil surface and atpositions nearest the stream. Rates decreased uniformly with distance awayfrom the stream and also with depth in the soil for each site. While patternsof nitrate-N, moisture, and organic carbon content differ among the sites,their combined effects on denitrification support the observed, consistentdenitrification rate pattern.KEY WORDS: Denitrification; Riparian ecosystems     

Maximizing Offspring Production While Maintaining Genetic Diversity in Supplemental Breeding Programs of Highly Fecund Managed Species

Abstract:  Supplemental breeding is an intensive population management strategy wherein adults are captured from nature and spawned in controlled settings, and the resulting offspring are later released into the wild. To be effective, supplemental breeding programs require crossing strategies that maximize offspring production while maintaining genetic diversity within each supplemental year class. We used computer simulations to assess the efficacy of different mating designs to jointly maximize offspring production and maintain high levels of genetic diversity (as measured by the effective population size) under a variety of biological conditions particularly relevant to species with high fecundity and external fertilization, such as many fishes. We investigated four basic supplemental breeding designs involving either monogamous pairings or complete factorial designs (in which every female is mated to every male and vice versa), each with or without the added stipulation that all breeders contribute equally to the total reproductive output. In general, complete factorial designs that did not equalize parental contributions came closest to the goal of maximizing offspring production while still maintaining relatively large effective population sizes. Next, we estimated the effective population size of 10 different supplemental year classes within the breeding program of the robust redhorse (Moxostoma robustum). Two year classes failed to produce progeny, whereas successful year classes used partial factorial designs to realize effective sizes ranging from 2 to 26 individuals. On average, a complete factorial design could increase the effective size of each robust redhorse supplemental year class by 19%.     

Interspecific Variation in Susceptibility of Frog Tadpoles to the Pathogenic Fungus Batrachochytrium dendrobatidis

Abstract:  As part of an overall biodiversity crisis many amphibian populations are in decline throughout the world. Numerous causes have been invoked to explain these declines. These include habitat destruction, climate change, increasing levels of ultraviolet radiation, environmental contamination, and the introduction of non-native species and diseases. Several types of pathogens have been implicated in contributing to amphibian population declines: viruses, bacteria, oomycetes, and fungi. One particular fungus, the chytridiomycete Batrachochytrium dendrobatidis may have caused amphibian population declines in several regions. This pathogen causes chytridiomycosis, which is fatal to newly metamorphic and adult amphibians of certain species. We present experimental evidence that larval stages may also be susceptible to exposure to Batrachochytrium . There was, however, differential sensitivity to B. dendrobatidis in larvae we examined. In laboratory experiments, larval western toads (  Bufo boreas ) exposed to B. dendrobatidis experienced increased mortality and behaviors that suggested they were affected by exposure compared with unexposed control tadpoles. Larvae of Cascades frogs (  Rana cascadae ), bullfrogs ( R. catesbeiana ), and Pacific treefrogs ( Hyla regilla ) did not die after exposure to Batrachochytrium and appeared to behave normally. R. cascadae larvae exposed to B. dendrobatidis , however, showed an increase incidence in mouthpart abnormalities, a characteristic effect of chytridiomycosis, compared with unexposed controls. These results show that Batrachochytrium can negatively affect some species of amphibians at the larval stage and not others. The implications of interspecific variation in susceptibility to fungal infection are broad.     

The Novel and Endemic Pathogen Hypotheses: Competing Explanations for the Origin of Emerging Infectious Diseases of Wildlife

Abstract:  Chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis , is an emerging infectious disease implicated in declines of amphibian populations around the globe. An emerging infectious disease is one that has recently been discovered; has recently increased in incidence, geography, or host range; or is newly evolved. For any given outbreak of an emerging disease, it is therefore possible to state two hypotheses regarding its origin. The novel pathogen hypothesis states that the disease has recently spread into new geographic areas, whereas the endemic pathogen hypothesis suggests that it has been present in the environment but recently has increased in host range or pathogenicity. Distinguishing between these hypotheses is important, because the conservation measures needed to slow or stop the spread of a novel pathogen are likely to differ from those needed to prevent outbreaks of an endemic pathogen. Population genetics may help discriminate among the possible origins of an emerging disease. Current evidence suggests chytridiomycosis may be a novel pathogen being spread worldwide by carriers; until we know how much genetic variation to expect in an endemic strain, however, we cannot yet conclude that B. dendrobatidis is a novel pathogen.     

The measurement of job control

Subjective or perceived control over job-related activities or events is a frequently measured construct in organizational stress research. Karasek (1985) assessed perceived control as both decision authority and skill discretion at work (job decision latitude). Ganster (1989b; Dwyer and Ganster, 1991) developed a multidimensional or general measure of worker control, as well as a specific measure of work predictability. Because little published psychometric data exist for these scales, we investigated the item-level measurement properties of Karasek's and Ganster's measures. We hypothesized two separate, two-factor solutions, decision authority and skill discretion, for the job decision latitude scale, and general control and predictability, for the work control scale. The dimensionality of both measures was assessed in multiple, independent samples using confirmatory factor analyses (LISREL) with maximum likelihood estimation. Simultaneous solutions across samples were used to determine the fit of the factor models to the data. The hypothesized two-factor solutions were confirmed for both Karasek's and Ganster's scales, although item refinement is indicated. We also investigated the relative independence between Karasek's and Ganster's scales and found a lack of independence between the general control and decision authority items in one sample. © 1997 John Wiley & Sons, Ltd.     

Use of Substitute Species in Conservation Biology

Abstract:  In conservation biology, researchers often want to study the reasons why an endangered population is faring poorly but are unable to study it directly for logistical or political reasons. Instead they study a species that substitutes for the one of concern in the hope that it will cast light on the conservation problem. Here we outline the assumptions underlying this approach. Substitutes can be different populations or species and may be chosen because they are similar biologically to the target or representatives of a constellation of species of which the target is one. They also may be used to develop a predictive model to which the conservation target can be related. For substitutes to be appropriate, they should share the same key ecological or behavioral traits that make the target sensitive to environmental disturbance and the relationship between population vital rates and level of disturbance should match that of the target. These conditions are unlikely to pertain in most circumstances and the use of substitute species to predict endangered populations' responses to disturbance is questionable.     

Refugee Density and Dependence: Practical Implications of Camp Size

Different refugee camps may have widely differing morbidity and mortality rates. Some of these differences are ascribed to environmental factors. This paper reviews the key issues relating to one environmental factor: the size of the refugee camp, and provides a tentative theoretical framework for examining the effect of camp size on refugees. This effect may not be considered because aid workers chronically underestimate the value of the refugees' contribution to their own survival. Large camps settle great numbers of refugees to the hinterland of the camp and limit their access to resources available there. This may increase refugee dependency and vulnerability. There is some slight evidence from the analysis of data provided by Mercer (1992) that child mortality rates (aged 0–4 years) are positively correlated with camp size (as inferred from child populations). If other factors allow, it might be wise for camp planners to try to limit camps to a size which allows refugees reasonable access to local resources.