The direct removal of adult trees by logging affects tree recruitment in tropical rain forests. However, secondary effects of logging, such as loss of vertebrate seed dispersers may also affect tree recruitment. We studied the recruitment and spatial distribution of five tree species namely Balanites wilsoniana, Celtis zenkeri, Chrysophyllum albidum, Cordia millenii and Ricinodendron heudelotii in Kibale, Budongo and Mabira Forests in Uganda. These forests have been subjected to varying degrees of disturbance leading to changes in their vertebrate seed dispersers. Vertebrate frugivores of the five tree species were identified. Three 1-ha plots were established around adult trees of the same five species in each forest and the distance from the juveniles to the nearest adult conspecific was measured to generate a recruitment curve. Frugivore visitation rates were high in the less disturbed Budongo and Kibale (2.2 and 1.6 individuals h−1 respectively) compared with the highly disturbed Mabira (0.9 individuals h−1). In the frugivore-impoverished forest, 70–90% of juveniles established beneath adult conspecifics, whereas in the less-disturbed forests juveniles were established up to 80 m from adult conspecifics. Shade-tolerant species capable of recruiting beneath adult conspecifics appeared to maintain their populations without dispersal. Consequently, disturbances leading to significant loss of vertebrates may alter tree recruitment and spatial distribution with consequences for long-term population viability of shade-intolerant tropical trees.

This study demonstrates that forest elephants (Loxodonta africana) play a key role as seed dispersal agents for the upper canopy forest tree, Balanites wilsoniana. Seed that passed through elephants had a much greater probability of germinating (50.7%) than seeds from fruits eollected directly from the tree (3%). Of 515 seeds that were collected from under parent trees, 89.1% were rotten at the time of collection, 2.6% had been eaten by forest pigs, 2.8% had been killed by squirrels, and 0.7% had been killed by insect damage. The remaining 4.8% of the seeds were still alive. In 150 hours of focal observations on B. wilsoniana trees, two fruits were seen to be destroyed by squirrels, no fruits were seen to be consumed by potential dispersers, and no evidence of scatter hoarding was found. Experimental seedling stations placed out under parent trees and away from parent trees indicated no significant effect of dispersal on seedling survival. Although elephants are not essential for the successful germination of B. wilsoniana seeds, they do greatly increase probability of germination and play a vital role in their dispersal, suggesting that this is a highly interdependent plant-animal seed dispersal system.