Ant mutualists stabilize megaherbivore-driven landscape change in a savanna ecosystem

Background/Question/Methods

Tree cover in savanna ecosystems is usually regarded as unstable, varying with rainfall, fire, and herbivory. In sub-Saharan Africa, elephants (Loxodonta africana) suppress the growth and survival of adult trees, thereby maintaining landscape heterogeneity by promoting tree-grass coexistence. In the absence of elephants, tree encroachment may convert savannas into closed-canopy woodlands; when elephants increase in abundance, intensified browsing pressure can transform savannas into open grasslands. The spines of savanna tree species are well-known defenses against browsing mammals. Another form of plant defense, co-evolved relationships with aggressive ants, usually is assumed to protect plants against insect herbivores. Using behavioral and large-scale ecological experiments, we show that ant symbionts effectively protect a common tree (the whistling-thorn tree [Acacia drepanolobium]) against elephants, thus reducing landscape change stemming from elephant activity over broad spatial expanses (~300 km²).

Results/Conclusions

In feeding trials and in the field, elephants avoided plants with ants and did not distinguish between whistling-thorn tree and a palatable congener in the absence of ants. Percent cover of the whistling-thorn tree in satellite imagery did not change significantly despite a near-tripling of elephant abundances between 2003 and 2008; in contrast, cover of trees not defended by ants differed by >14% inside versus outside large-scale elephant exclusion fences. Our findings indicate that ant symbionts deter herbivory by the largest extant land animal, thereby stabilizing tree cover in the face of strong variation in elephant abundance and highlighting the powerful role that mutualisms play in driving biomass accumulation in savanna ecosystems.