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

Abstract

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 defence mechanisms, such as physical defence. The resource availability hypothesis (RAH) predicts that plant defence investment is dependent on resource availability and plant's life history. However, the effect of resource availability on plant investment in defence is mediated through biotic pressure. We tested the effects of soil qualities and browsing pressure on the physical defence 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 defence for trees in the floodplain. Furthermore, A. sieberiana trees produced less fruit in the floodplain than in the plateau. Contrary to the predictions of the RAH, 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. This physical defence was probably induced to cope with the episodic but high levels of herbivory observed in this environment. 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, and fertility) in shaping plant physical defence can be outweighed by high herbivory pressure.