Limited ant co-occurrence and defensive mutualism in Acacia plants in a West African savanna

Our understanding of the role of fire and effect of ant species
composition, beyond their diversity and abundance, on the effectiveness of
mutualism defense is limited. Most of our knowledge of ant-plant defense
in tropical Africa is biased toward East African savannas which have
richer soil, higher primary productivity and a more diverse arthropods and
mammal community than West African savannas. We assessed the
diversity of ant species associated with Acacia species in the Pendjari
Biosphere Reserve in the Dahomey Gap, and their impacts on elephant
damage. Elephant damage, ant diversity and abundance were
measured in stands of five Acacia species. Eleven ant species were
identified in the Acacia stands. The composition of these ant
communities varied across Acacia species. Pair of ant species
co-occurred in only 2 % of sampled trees, suggesting a strong competitive
exclusion. 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 defense may mask strong
physical and chemical defense mechanisms of Acacia trees against
elephant damage. Ant assemblages in West Africa, unlike those in the more
productive East Africa, are particularly species-poor. However, there is a
convergence between these two regions in low rate of ant co-occurrence
which might indicate strong competitive exclusion. Our study suggests that
such low ant species richness while limiting the efficacy of mutualism in
controlling megaherbivore damage may mask a strong defense syndrome.