Plant biodiversity declines with increasing coffee yield in Ethiopia’s coffee agroforests

1. Tropical agroforestry systems provide farmers with resources for their
livelihoods, but are also well recognized as refuges for biodiversity.
However, the relationship between yield and biodiversity might be negative
in these systems, reflecting a potential trade-off between managing for
increased yield or biodiversity. The potential for synergies will depend
partly on the shape of the biodiversity-yield relationship, where a
concave relationship suggests a faster decline of biodiversity with
increasing yields than a linear or convex shape. 2. We studied the
relationship between biodiversity (plant species richness and composition)
and coffee yield along a gradient of management in southwestern Ethiopia,
coffee’s native range. We inventoried species richness and community
composition of woody plants, herbaceous plants and bryophytes at 60 sites.
We also measured coffee management-related variables and assessed coffee
yield for three consecutive years at each site. 3. Species richness of
woody plants had a concave relationship with coffee yield, i.e. tree
richness declined fast initially before levelling out at higher yields,
whereas there was no relationship between coffee yield and species
richness of herbaceous plants or bryophytes. Species composition of woody
plants, herbaceous plants and bryophytes all had a concave relationship
with coffee yield. 4. From a methodological perspective, we found that
multi-year data on yield was necessary to reliably assess the relationship
between biodiversity and yield, and that the number of coffee shrubs or
coffee dominance were poor proxies for yield when trying to capture the
biodiversity-yield relationship. 5. Synthesis and applications: The
concave relationship between biodiversity components (species richness and
composition) and yield suggests that there is a strong conflict between
the goals of increasing production and conserving biodiversity. However,
it is important to recognize that this pattern is largely driven by the
very low-yielding sites in natural forests. Here, even minor
intensification of coffee management seems to rapidly erode biodiversity.
Along the rest of the productivity gradient, there was generally no
negative relationship between yield and biodiversity, implying
opportunities for developing strategies for increasing yields without
biodiversity loss.