Factors Affecting Mourning Dove Use of Water in Artificial Catchment Basins in a Dryland Farming Area of Utah

The mourning dove population of the Howell-Blue Creek Watershed in northern Utah was studied by Norman Slade in an effort to determine why many more doves frequented certain of the 20 fiberglass catchment basins installed in the area. More doves used basins on the west side of the valley, probably as a result of temperature differences. More doves used those basins surrounded by more land in summer fallow and with fewer basins nearby. Areas in sagebrush were used for nesting. The number of doves drinking in a particular hour was affected by the presence of predators or antagonists but not by light rain, cloud cover, temperature, wind velocity, or amount of space available for drinking. Frequency of drinking was highest in the early morning and late evening, particularly in late summer

Randomization tests using standard statistical software

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Statistical Detection of Density Dependence from a Series of Sequential Censuses

The use of simple linear regression to estimate slopes of plots of N(t + 1) against N(t) as a test of density dependence has been criticized because such data violate the assumption of negligible measurement error in the independent variable and because they represent a time series rather than independent pairs of points. Of the several alternatives which have been suggested, ordinary and standard major axes and the coefficient of first-order autoregression behave in accordance with the logic of detecting density dependence in such plots. The power of the test of the slopes' being equal to 1 d epends on the magnitude of density-dependent and independent (random) influences and on the type of error, measurement or environmental. However, slopes of major axes appear to be unbiased estimators of the true slopes, when sequential population estimates include values sufficiently displaced from equilibrium conditions. If data follow a purely autoregressive process, density dependence can be detected without such displacement

Population Ecology of Uinta Ground Squirrels

The behavior and ecology of Uinta ground squirrels (Spermophilus armatus) at the Utah State University Forestry Field Station northeast of Logan, Utah were studied from 1964-1971 to determine the role of behavior in population regulation. From 1965 to 1968, data were collected to establish population norms. In 1968, the population density was reduced experimentally to help elucidate regulatory processes. This paper compares the dynamics of the population before and after the population reduction. Before the density reduction, juvenile survival was lower than that of yearlings and adults. The disappearance of juveniles was associated with their tendency to disperse from the natal burrow while still small. The litter size of yearlings was smaller than that of adults. The principal changes following the reduction were an increase in percentage of yearling squirrels breeding and a decrease in dispersal of juveniles. Litter size and proportion of adult females breeding did not change significantly. Dispersal of squirrels from the study area was the principal regulatory factor. Losses during hibernation, presumably to badger predation, also decreased after the reduction. Several conclusions were drawn. The importance of anyone of the complex of factors, which determined the dynamics of the population, varied with place, time, and population density. The value of habitat was affected by population density as well as the physical environment. Density pressures seemed to have more effect on subordinate than on dominant squirrels. Surplus animals from the prime habitat raised the population density in the surrounding area. Dispersing animals probably suffered higher mortality than those which stayed, but this did not imply that dispersal was maladaptive for the individual. (58 pages

The Use of Regression for Detecting Competition with Multicollinear Data

Monte Carlo simulations were used to demonstrate that regression methods could be successfully used to estimate competition coefficients with collinear data, but only under conditions that may be difficult to meet with ecological data. Ordinary least squares performs well when estimating coefficients associated with noncollinear predictor variables. Stepwise regression and maximum eigenvalue least squares reduce collinearity by deleting information that, even though not statistically significant, may be important in accurately estimating interaction. We propose that apparent competition (Holt 1977) and the failure of regression techniques to detect competition when it is known to exist experimentally may be due to the omission or lack of measurement of critical elements of the community matrix

Some Comments on Niche Analysis in Canonical Space

Discriminant functions have been used to identify axes of niche separation, and dispersion of locations on these axes have, in turn, been interpreted as representing niche width and specialization. We discuss the influence of sampling scheme and choice of niche measures on the results of an analysis conducted in canonical space. Specifically, habitat variables measured at random locations should provide a more representative measure of habitat available than would recording habitat only where animals are observed. Species presence information alone tends to bias the mean habitat toward the one where the most common species is found. If niche specialization is to be measured by distance from the overall mean habitat, each sampling station should receive equal weighting. The most straightforward measure of habitat breadth seems to be the variance or standard deviation of canonical scores. The mean squared distance from the species centroid can then be used as a comparable multidimensional breadth measure. Finally, we suggest that niche overlap be measured via a joint probability density function rather than by area of overlap of concentration ellipses

Uinta Ground Squirrel Demography: Is Body Mass a Better Categorical Variable Than Age?

We compared the relative impact of age and body mass on fecundity and survival in a population of Uinta ground squirrels, and modeled the population using bodymass classes as stage categories in Lefkovitch stage transition matrices. Body mass was a better predictor of survival than was age, and was nearly as good a predictor of litter size. Mass-based stage transition matrices provided results similar to age-based transition matrices, but also indicated that larger and subsequently more fecund young were a consequence of a population reduction. We believe that mass-based analysis of ground squirrel populations is a viable alternative to traditional age-based analyses. Body mass is easy to measure in small-mammal populations and is linked to a variety of life-history characteristics

The Effect of Sigmodon Hispidus on Spatial and Temporal Activity of Microtus Ochrogaster: Evidence for Competition

Spatial and temporal activities of cotton rats, Sigmodon hispidus, and prairie voles, Microtus ochrogaster, in an old-field habitat in northeast Kansas, USA were studied for 39 mo. During a major portion of the S. hispidus reproductive season the species were spatially segregated but co-occurred the rest of the year. Following a local extinction of cotton rats, prairie voles established a resident population in habitats formerly occupied by cotton rats that was greater than or equal to the vole population in areas which had never been occupied by cotton rats. Comparison of vole population size before and after the loss of cotton rats indicated that cotton rats produced a negative effect on vole population size during the period of spatial segregation but not during the period of spatial co-occurrence. Comparison of vole activity patterns indicated that, with the loss of cotton rats, voles shifted their activity during times when the species had co-occurred but not during periods when they were spatially segregated. Studies of behavioral interactions in a large (4 m x 4 m) arena with varying densities of vertical wires indicated that during single species tests voles and cotton rats preferred the densest areas. Their diel activity patterns were broadly overlapping. In the presence of nonreproductive cotton rats, voles continued to use the same areas as during the control tests but shifted their temporal activity reducing overlap with cotton rats. Interspecific encounters resujted in voles avoiding cotton rats even in the absence of overt aggression. In the presence of reproductive cotton rats, voles shifted their arena usage toward areas least used by cotton rats. Total nocturnal activity by voles was the same as during the control tests but was restricted to hours when cotton rats were least active. Reproductive cotton rats showed higher levels of interspecific aggression than did nonreproductive cotton rats. Voles in the presence of reproductive cotton rats showed increased frequency of serious wounds and deaths, and decreased mass when compared to voles in the presence of nonreproductive cotton rats. We conclude that cotton rats produce a negative effect on prairie vole populations but that the interaction is restricted to the cotton rats' reproductive season. Coexistence of the species appears to rely on the seasonality of the interaction, habitat heterogeneity, and the wider habitat tolerance of prairie voles

The Importance of Statistical Power When Testing for Independence in Animal Movements

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Testing For Independence of Observations in Animal Movements

Many analyses of animal movements assume that an animal's position at time t + 1 is independent of its position at time t, but no statistical procedure exists to test this assumption with bivariate data. Using empirically derived critical values for the ratio of mean squared distance between successive observations to mean squared distance from the center of activity, we demonstrate a bivariate test of the independence assumption first proposed by Schoener. For cases in which the null hypothesis of independence is rejected, we present a procedure for determining the time interval at which autocorrelation becomes negligible. To illustrate implementation of the test, locational data obtained from a radio-tagged adult female cotton rat (Sigmodon hispidus) were used. The test can be used to design an efficient sampling schedule for movement studies, and it is also useful in revealing behavioral phenomena such as home range shifting and any tendency of animals to follow prescribed routes in their daily activities. Further, the testmay provide a means of examining how an animal's use of space is affected by its internal clock