Vagility of P32‐Labeled Isopods in Grassland

The nocturnal activity of Armadillidium vulgare in California grassland suggests that this cryptozoan may be very vagile, but earlier attempts to study its lateral movements failed because of its secretive habits. In the present study, assimilated radiophosphorus was used to label isopods and make the recovery of hidden individuals possible. In each of four experiments, cohorts of about 1,000 radioactive animals were released in a grassland area, and the movements and dispersal of the tagged individuals were followed for as long as 25 days. Two of the experiments were conducted during the rainy season and two during summer drought. Activity of the labeled animals, as reflected in their dispersal rates, was greater in summer than in winter. A maximum dispersal rate of 13 m in 12 hr was observed in one of the summer experiments, as contrasted to a maximum of only 10 m in six days in one of the winter experiments. The greatest dispersal distance observed was 25 m (after 20 days in one of the winter experiments). The dispersal patterns can be described by the equation Y = a + c/r, where Y is a measure of the density of labeled animals at distance, r, from point of release, and c is a proportionality constant. A geometrical analogue for this equation, plus a modified dispersal experiment with Porcellio scaber, shows that c/r accounts for density decrease due to the dilution of tagged animals by the increasing area into which they move the constant a is interpreted as affording a relative measure of dropouts, individuals which disappear from the dispersal pattern. Foraging appears to account for the activity of Armadillidium during the winter. In the summer, on the other hand, isopods must search for shelter from desiccative daytime conditions following nightly forages. The search for shelter in addition to the search for food appears primarily responsible for the greater dispersal rate observed during the summer. The physical environment seems to interact importantly with food availability for these isopod populations.