Norman A. Slade

TESTING FOR INDEPENDENCE OF OBSERVATIONS IN ANIMAL MOVEMENTS

Many analyses of animal movements assume that an animals 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 test may provide a means of examining how an animals use of space is affected by its internal clock.

Influence of sampling interval on estimates of home-range size

Accurate estimation of home-range size often requires large numbers of observations. Radiotelemetry and direct observation are capable of yielding large sample sizes in a short period of time, but observations collected using a short sampling interval often are autocorrelated (i.e., not independent). We examined the effect of autocorrelation on six measures of home range and found that positive autocorrelation resulted in underestimation of home-range size. In long-term studies of movement, sampling intervals should be chosen so that autocorrelation between successive observations is negligible. If home-range estimates must be obtained in a relatively short period of time, collection of autocorrelated data may be unavoidable; under these circumstances nonstatistical measures of home-range size are more appropriate than statistical mea-

APPARENT SIZE AS THE DETERMINANT OF PREY SELECTION BY BLUEGILL SUNFISH (LEPOMIS MACROCHIRUS)

Copyright by the Ecological Society of America. This is the publishers version, also available electronically from http://www.jstor.org/stable/1935055.

AN EMPIRICAL TEST OF USING COUNTS OF INDIVIDUALS CAPTURED AS INDICES OF POPULATION SIZE

Abstract Simple counts of individuals are commonly used in studies of mammalian populations. However, such counts are known to routinely underestimate population size. When used, counts are assumed to be proportional to population size. We tested the validity of that assumption by analyzing capture–recapture data on 5 species of rodents trapped at a single site during monthly sessions from 1973 through 1993. For each 3-day trapping session, we estimated numbers of animals of each species residing on our trapping grid with selected-model and interpolated-jackknife estimators from the program CAPTURE, the modified Lincoln–Petersen estimator, and the Jolly–Seber estimator. We tested for proportionality by fitting a regression line, constrained to pass through the origin, of estimated population size to each of 3 “counts”: numbers of individuals, numbers of captures, and minimum number known alive (MNKA). We then tested validity of these equations by predicting estimated population size from counts from 1994 through 1998. In general, counts were proportional to estimated numbers within a species, so counts were indices of density and yielded similar patterns of population fluctuations. However, regression coefficients, which reflected probabilities of capture, varied among species. Consequently, counts were not appropriate for interspecific comparisons of abundance, even when trapping protocols were invariant. Probabilities of capture also are likely to vary among sites, trapping protocols, and perhaps by seasons, so conditions for counts as valid indices of population size are restrictive.

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 1964-71 to determine the role of behavior in population regulation. In 1968 the population was reduced experimentally to about one-half the previous density. The study area consisted of a lawn area where resident squirrels more than replaced themselves, and an area of mixed shrubs and grasses where they did not. Surplus squirrels from the lawn raised the population density off the lawn. Also, habitat suitabilities varied with population densities. Hence, densities in the two areas were not proportional to habitat preferences. Before the reduction the population density fluctuated widely but the mean change for 5 yr was approximately zero. A potentially high rate of increase was curtailed by disappearance of juveniles and yearlings; juveniles tended to disperse from the natal burrow. The principal changes following the reduction were increases in percentage of yearling squirrels breeding and of juveniles remaining on the study area. Litter size and proportion of adult females breeding did not change significantly, but losses during hibernation, due in part to badger predation, decreased. Dispersal of squirrels from the study area played a key role in population regulation. Dispersers probably suffered higher mortality than sedentary squirrels, but dispersal was not necessarily maladaptive for the individual. Dispersers may have been unable to produce young at high densities; thus emigration offered the only opportunity for reproduction.

A Bivariate Home Range Model with Possible Application to Ethological Data Analysis

Although home range and ethological data are believed to be intimately related, they have usually been analyzed separately for lack of a practical method of analysis or reasonable model relating them. We briefly review the calculation of some basic home range statistics and modified methods for estimating home range characteristics. A bivariate normal model shows most promise for such use. The additional calculation of eight points on the confidence ellipses for the location points, and for the activity center facilitates graphic representation of these probability ellipses. A standardized distance, which takes into account home range characteristics, is calculated. We believe this standardized distance may be related to familiarity within the home range, and as such could be correlated with behavior.

Effects of supplemental food on population dynamics of cotton rats, Sigmodon hispidus

Variation in resource abundance affects population dynamics by altering demographic processes and interactions among individuals in the population. For small mammals, food is likely to be a critical resource. Population densities should vary directly with food abundance, but the underlying demographic changes are more difficult to predict. We experimentally increased food available to populations of hispid cotton rats, Sigmodon hispidus, to examine how rates of recruitment and disappearance varied with food abun- dance. We expected supplemental food to increase winter survival and to increase recruit- ment, perhaps advancing the date of first reproduction in the spring. We also thought larger, behaviorally dominant animals might dominate the point sources of supplemental food, altering age or size ratios and producing an excess of transient animals. Supplemental food increased population densities but did not dampen pronounced sea- sonal fluctuations. Supplemented populations contained proportionately more juveniles and small adults than did controls; social behavior may have limited increases in numbers of large adults. Survival rates did not change; density increases were due to increased repro- duction and immigration. In supplemented populations, reproductive effort by females increased, but the proportion of reproductive females decreased. Reproductive females and nonreproductive animals of both sexes were less likely to be transients than were repro- ductive males. Transients constituted higher proportions of control populations because of successful settlement, primarily by juveniles and small adults, into supplemented areas. The strongly seasonal climatic conditions under which northern S. hispidus populations exist produce contrasting selective pressures which might favor season-specific foraging strategies. Our data support a scenario of territorial females and wandering males in the reproductive season, with females maximizing resource acquisition for production of off- spring. During winter, both sexes may restrict foraging time (and exposure to predators and weather) to the minimum required for survival.

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.

Home Range Indices for the Hispid Cotton Rat (Sigmodon hispidus) in Northeastern Kansas

Movement data collected during a 9-year live-trapping study of Sigmodon hispidus in northeastern Kansas were analyzed using three indices of home range size: 1) distance between successive captures; 2) mean squared distance from center of activity; and 3) S∞ Results obtained from the three indices generally were consistent. Reproductively mature adult and subadult males occupied larger home ranges than their nonreproductive counterparts. Reproductive females showed the same tendency, but the differences were not statistically significant. Adult males inhabiting a more sparsely populated abandoned pasture displayed larger home ranges than did those in an old field. These patterns of Sigmodon movements were comparable to trends reported in other geographic localities. Independence between successive capture locations, a requirement for the analysis of movement data using routine statistical techniques, was observed for 86% of all individuals.

Application of mark-recapture models to estimation of the population size of plants

Mark-recapture models have been widely used in ecology to estimate pop- ulation sizes of animals. In contrast, estimation of plant population size has usually been assumed to be much easier. However, detection of individuals is difficult for perennial plants, such as the rare prairie plant Meads milkweed ( Asclepias meadii), which does not produce aboveground parts every year and lives in dense vegetation where nonflowering stems are hard to observe. In these cases, a count of the number of plants observed in a particular year may greatly underestimate the true population size, just as a count of animals in traps does not adequately estimate the total number of animals in an area. Using a family of closed population models (CAPTURE), we applied mark-recapture methodology to estimate population size of A. meadii. Over a 4-yr period, a total of 129 patches (aggregated collections of stems) was observed, with 124 flowering in at least one year. In any one year, however, the number of flowering patches ranged from 15 to 105. Using model M th of CAPTURE with these data, the estimated number of patches capable of flowering was 219. Although the confidence interval is broad (95% confidence interval of 175-302), these results emphasize that the observed number of patches in any one year, or even over a 4-yr period, underestimates the actual population size.