The need to be eaten: Balanites wilsoniana with and without elephant seed-dispersal

Abstract

This study demonstrates that elephant seed-dispersal is vital for Balanites wilsoniana (Zygophyllaceae), a forest canopy tree with no other effective dispersers. Non-dispersed seeds suffered high mortality (84%) and low germination (3%). Elephants were reliable at visiting fruiting trees (46%), ingesting available seeds (55%) and dispersing the fruit crop (26%). Compared with unpassed seeds, seeds passed through an elephant's gut had improved germination (54.9% vs. 2.9%) and reduced time to germination (82 vs. 132 d). Given predation rates under parent trees, elephant gut passage is expected to improve absolute germination by 66%. When this change in germination is divided by germination of unpassed seeds, elephant gut passage is expect to improve relative germination by as much as 4000%. Although post-dispersal predation occurred in dung containing B. wilsoniana seeds, dispersed seedlings were abundant. Compared with seedlings under parent trees, dispersed seedlings had greater survival and height. Dispersed seedlings survived in a diversity of environments, thrived under high-light conditions, and were negligibly affected by post-dispersal density. Although elephants are not essential for B. wilsoniana germination, their actions greatly increase seedling establishment and move a large proportion of seeds. This high degree of effectiveness, together with very low non-dispersed seed survival, provides strong evidence that B. wilsoniana is dependent on elephants for its long-term persistence.