Geostatistics for Ecology: An Application of the Variogram to Radio-Telemetry Data

Abstract

Radiotelemetry data provide useful ecological information on the biology of animals and their interactions with the environment that they inhabit. Many methods have been developed for analysis of such data for spatial information, on home range size and utilisation density. However, no generally accepted method is currently available specifically for determining the temporal independence of this type of data for statistical analyses of activity rhythms. Activity patterns have generally been analysed by ecologists with the assumption that the data are statistically independent, or by subjectively fixing an independence interval based on attributes of their ranging behaviour. Although some good approximations of the activity patterns can be obtained in these ways, we underline the need for a functionally correct method of estimating the independence interval. Here we use semi-variograms to estimate the minimum interval required for the readings to be sequentially independent. This geostatistical tool is applied for the analysis of data on activity of Chilean foxes (Pseudalopex culpaeus) and Chacoan peccaries (Catagonus wagneri). Data were collected in the field by radio-tracking over 24-hour periods, with readings on activity state taken every 15 min. The spatial dimension in which the theory of geostatistics lies has been transferred into the time dimension, so that the correlation interval is expressed in time units (minutes). Time of independence as estimated by the variogram was 110 mins for culpeo foxes, while data on Chacoan peccaries indicated that they have a defined period of activity and time series analysis may be more appropriate for this kind of data.