GERBILS UNDER THREAT OF OWL PREDATION - ISOCLINES AND ISODARS

We trained barn owls to fly over 2-ha field enclosures containing popu lations of Gerbillus allenbyi. Each 2-ha plot was divided into two equ al parts by a fence with gates allowing easy passage of the gerbils. W e varied the number of gerbils in the enclosure and the number of owl flights on each side of the dividing fence. Gerbil foraging activity r esponded within two hours to the difference in number of owl flights o ver the subplots. The greater the difference, the more the gerbils shi fted their foraging to the subplot with fewer flights. Gerbils did not reduce their total foraging effort but redistributed it between subpl ots. In the absence of owl flights, gerbils divided their foraging eff ort equally between the matched subplots. Thus, they appear to seek an ideal free distribution in the two halves of each plot. Therefore, ba sed on the gerbils' distribution of activity in the presence of differ ent numbers of owl flights over the two subplots we estimated the shap es and slopes of the gerbil victim isoclines in the middle and right h and side of the 'gerbil-activity vs number of owl flights' state space . The isoclines were parallel straight lines with slope equal to -0.57 . We believe this to be the first estimate of a victim isocline for a population of vertebrates in the field. Using the isodar method of Mor ris, we also determined that the fitness cost of each additional gerbi l, i.e., per capita intraspecific competition among the G. allenbyi, i s constant. It does not depend on either owl flight frequencies or G. allenbyi densities. The isodar also shows that the fitness cost of add ed owl flights does not vary with G. allenbyi population densities.