Acacia trees as model systems

Acacia trees are the dominant trees of African and Middle Eastern savannas and are considered to have coevolved with the large mammalian herbivores in this biome. We studied the effects of large mammalian herbivores on the establishment of young Acacia raddiana and A. tortilis in desert regions of Israel, A. reficiens in Namibia and A. mellifera in South Africa. Seed accumulation under trees was high when large herbivores were excluded. Seed germination was facilitated by gut passage through ungulates. Under severe browsing pressure, growth in juvenile trees was delayed. There was no evidence of effects on the long-term demography of Acacia trees, because ungulates increased seedling densities by enhancing germination of seeds. Contrastingly, bush encroachment by Acacia trees affects the biodiversity of millions of hectares of southern and eastern Africa. Field experiments show that rainfall amount and frequency, coupled with specific soil nutrient levels, rather than grazing or fire, may drive this phenomenon. As might be expected of plants that coevolve with large herbivores, Acacia trees are well defended against herbivory. Furthermore, these defences have been shown to be induced by herbivory. Unlike their Australian relatives, African Acacias are well defended both chemically and physically. Thus, the presence of both types of defenses demands an explanation. Experiments conducted in Israel and Kenya indicate that, while defences are inducible, there is no trade-off between types of defence in Acacia trees. Rather, individual optimization results in some trees investing heavily in both types of defences and other trees investing little in either.