Feeding behavior in sexual and clonal strains of Poeciliopsis

Summary Sexual and clonal fish of the genus Poeciliopsis occur together in desert streams of Sonora, Mexico. Their coexistence has been explained in terms of niche partitioning for food and space. We examined predatory behavior that might influence niche relationships, and found significant differences among two coexisting sperm-dependent clonal strains and their two sexual progenitors. Handling time and prey manipulation of free-swimming (Artemia) and benthic (chironomid larvae) prey differed significantly among sexual and clonal strains. Analyses of gut contents from field-collected fish revealed that the laboratory estimates of predatory efficiency were related to their feeding behavior in nature. Behavior differences, such as those described herein, contribute to our understanding of the mechanisms of unisexual/bisexual coexistence in Poeciliopsis. Offprint requests to: R.C. Vrijenhoek     

“Applicability of the t-Test for Detecting Trends in Water Quality Variables,” by Robert H. Montgomery and Jim C. Loftis2

Montgomery and Loftis (1987) have listed several situations for which the t-test does not accurately reproduce Type I errors, and should therefore be avoided. Characteristics common to water quality data (skewness or other non-normality, presence of outliers and less-thans) also reduce the power of the t-test, in relation to nonparametric alternatives. Thus if one is interested in reaching correct decisions when trends or differences exist, and not just when they do not, the t-test should not be considered “robust” (in the sense of being generally applicable) when its assumptions are violated. Further, t-tests assume that differences in means are relevant (the mean is a good measure of central tendency), and that data groups differ by some additive amount. When all of these assumptions are recognized, and in light of the availability of truly robust and comparatively powerful non-parametric alternatives, we believe there is little applicability of the t-test for detecting trends or differences in water quality variables.     

A conceptual model and empirical analysis of children's acquisition of spatial knowledge

How adults and children come to understand, represent and behave within their spatial environment are topics of great interest to geographers, psychologists, environmental planners and laypeople. Considerable research and theory has been published on these and related topics. In this paper, we will review some of what is known and theorized about spatial cognition and then consider elements of our research program on the acquisition of spatial knowledge. We focus on two intimately related topics. The first is the development of a conceptual model of the knowledge structures and processes associated with acquiring, representing and accessing knowledge of a given environment. The conceptual model forms the basis for a formal computational process model intended as a simulation of actual knowledge and performance in way finding tasks. The second emphasis is an in-depth case study of the acquisition of spatial knowledge. The case study focuses on a single child acquiring knowledge of a lengthy route through an unfamiliar suburban neighborhood. It is presented as an empirical test of certain assumptions embodied within the conceptual model.Before introducing the conceptual model and the case study, we first review the state of current theory and data on spatial cognition and identify four central issues confronting researchers in this field. This review provides a necessary context for describing and evaluating our program of research. The second section of this paper discusses elements of the conceptual model and its relationship to other formal computational models. The third section considers specific hypotheses about the acquisition and representation of spatial knowledge and tests of these hypotheses from the single in-depth case study. The final discussion section of this paper is a reconsideration of the four issues raised in the first section and necessary and proposed extensions of the current research.     

Towards a national reference center for environmental information in The Netherlands: A review

For improvement of the accessibility and use of environmental information a prototype national reference center (CIMI) was constructed in the Netherlands in 1986. Target groups were identified on the basis of an analysis of the demand side. They were questioned about their needs in the field of environmental information. On the basis of these experiences an automated integrated system has been built that is composed of subsystems for reference to expertise in organizations (specialisms), literature, research, and databases of site-specific data. This system is also equipped with a thesaurus. The system has been tested and examined in several ways. The outcome of these independent tests and investigations confirm the usefulness of this center for information transfer. The reference center can be helpful in providing overviews on and structuring of environmental information in the Netherlands. The results of the activities have been presented in several ways and were described extensively in a number of reports.