The cost of interspecific competition in two gerbil species

1. It has been shown that the two common granivorous gerbil species Gerbillus allenbyi and G. pyramidum that coexist in the sand dunes of the Israeli Negev show temporal partitioning in their time of activity. The bigger species, G. pyramidum, aggressively displaces the smaller species from early night-time. We examined the change in the activity pattern of G. allenbyi in pure and mixed populations in two 1-ha field enclosures. 2. We confirmed the temporal pattern reported by Ziv et al. (1993) and Kotler et al. (1993). 3. We also measured how much energy it takes (in g of millet seeds) to compensate for the costs, associated with interference and exploitation competition, by adding millet seeds to 18 seed trays/enclosure. We added 1, 3, 5, 7 or 9 g of seeds/seed-tray. In each seed-tray we mixed the seeds in 2 L of sand. 4. It took 3-5 g of seeds/seed-tray (1·8-3 g seeds/day/ha/individual) to completely overcome the competition by G. pyramidum. This is the cost of the interference and exploitation competition from G. pyramidum (in the currency of millet seeds). 5. The result suggests that there is a trade-off between interference competition and food to which gerbils respond behaviourally.