Complex agroforestry systems against biotic homogenization: The case of plants in the herbaceous stratum of cocoa production systems

Summary

In addition to their potential against deforestation and climate change, agroforestry systems may have a relevant role in biodiversity conservation. In this sense, not only species richness per se, but also community composition, including the distribution range of the species, should be considered. The latter is especially relevant in the current context of biotic homogenization, where the presence of geographically widely distributed species is increasingly frequent in detriment of native species.
By studying plant species of the herbaceous stratum of cacao production systems, we can evaluate, at a narrow local scale (>10 ha), the potential role of agroforestry systems and management intensity in diversity conservation and against biotic homogenization. This study was performed in an experimental trial in Bolivia, where five production systems representing a gradient of management intensity were compared: two monocultures and two agroforestry systems under conventional and organic farming containing a planted cover crop layer, and a complex successional agroforestry system with no external inputs. All species in the herbaceous stratum were identified and classified according to their geographical range. An indicator species analysis was implemented to identify species linked to a specific production system.
In total, 171 species were recorded, 42% of which were strictly herbaceous while the rest of them were woody regrowths, mainly found in the successional agroforestry system (an average of 40 species compared to 16 species in the other systems). Total species richness was higher in the successional agroforestry system compared with the other production systems. No main differences were found between the other systems, that is, monocultures and conventionally managed systems did not have less species than agroforestry and organically managed systems. However, community composition did change following the management intensity gradient. In addition, we found that widely distributed species, including some exotic species, were associated to intensive management, i.e. monocultures and conventional systems with high solar exposure levels and/or glyphosate application. Conversely, successional agroforestry and organic systems harbored species with a geographical distribution range restricted to the Neotropics or South America. According to our results, promotion and support for the adoption of cocoa organic and agroforestry systems, as a counterbalance to the common intensively managed plantations, could contribute to both biodiversity conservation and the minimization of biotic homogenization.