Ant protection from elephant herbivory in West African Acacias

In African savannas, elephants (Loxodonta africana) are a major driver of ecosystem dynamics. Specifically, they impact trees by browsing foliage, stripping the bark, and even uprooting whole trees. However, certain tree species have formed effective relationships with ants to defend against this ‘elephant damage’. Some Acacia trees provide nest space and carbohydrate-rich nectar for ants, whilst the ants provide protection from vertebrate herbivory. In Africa, most evidence of this important mutualism comes from Acacia drepanolobium-dominated savannas in protected areas of East Africa, with few studies focussing on West African savannas.

More generally, we know relatively little about species interactions in most African ecosystems, and this clearly limits the ability of ecologists to conduct global analyses without criticism of a tropical bias. In their new study published in AoBP, Djogbenou et al. present one of the rare studies on plant–ant interactions in West Africa, studying Acacia–ant interactions in the Pendjari Biosphere Reserve in north-west Benin. The reserve covers an area of 4661.4 km². The vegetation of the reserve is dominated by savannas (tree, shrub and grass savannas) with islands of woodlands, dry forests and gallery forests along rivers.

In the study, Djogbenou et al. assessed the diversity of ant species associated with five Acacia species in the reserve and their impacts on elephant damage. They found only 11 ant species dwelling on the Acacia species studied. However, there was rarely more than one ant species found on any individual tree, with co-occurrence of more than one ant species found in only 2% of the sampled trees. Within this annually burnt environment, ants were rare on small trees.

The intensity of elephant-caused branch breaking did not vary between trees with ants and trees without ants, suggesting limited Acacia–ant mutualism. Such limited biotic defence may mask strong physical and chemical defence mechanisms of Acacia trees against elephant damage. Ant assemblages in West Africa, unlike those in the more productive East Africa, are particularly species-poor. The results of this study suggest this lack of diversity may limit the effectiveness of mutualism in controlling mega-herbivore damage.