Biotic pressure outweighs the influence of resource availability on physical defense investment in a tropical tree

Plants have long-lasting and complex interactions with herbivores, including insects and mammals. In response to high herbivory rates, plants either tolerate biomass loss or develop several defense mechanisms, such as physical defense. The resource availability hypothesis predicts that plant defense investment is dependent on resource availability and plant’s life history. However, the effect of resource availability on plant investment in defense is mediated through biotic pressure. We tested the effects of soil qualities and browsing pressure on the physical defense and reproductive investments in Acacia sieberiana at the Pendjari Biosphere Reserve in West Africa. We selected six populations, including three in the Pendjari River floodplain where soil moisture is high but with high elephant browsing pressure, and three populations on the plateau in the hunting zone where soils are drier and relatively poorer with a lower density of elephants. We found greater investment in physical defense for trees in the floodplain. Furthermore, Acacia sieberiana trees produced less fruit in the floodplain than in the plateau. Contrary to the predictions of the resource availability hypothesis, we found more and longer thorns in populations in the richer floodplains than on the plateau. This was linked to higher elephant browsing pressure in the floodplains. Surprisingly, the negative influence of thorn number and size on the likelihood of elephant damage was observed only in the rich floodplains and not in plateau sites. Altogether, our study demonstrates that the influence of resource availability (soil moisture, pH, fertility) in shaping plant physical defense can be outweighed by high herbivory pressure.