I went to a talk last week about the dynamics of biotic communities on coffee plants on a 100 year-old organic shaded coffee plantation. The biotic interactions were fascinating, and ever increasing as the speaker elaborated on her research. The speaker was Ivette Perfecto from the University of Michigan. Here's the gist:
Arboreal ants live in nests in the shade trees and come down to tend scales (coffee herbivores) on the coffee plants and eat the honeydew produced by the scales. At first glance, it's boo ants.
The ants are parasitized by a decapitating fly that kills ant nests, leaving aggregates of the arboreal ant nests rather than a dense blanket of nests across the plantation. So the ant parasites hedge ant populations and thereby the scale populations too.
An adult beetle is a coffee herbivore and is predated upon by the arboreal ants. So, in the presence of the adult beetles, a good level of ants is beneficial. Yay ants. However, young beetle larvae are covered in fuzz that fumbles up ant mandibles, allowing beetle larvae to live in refuges near the ants' scale farms. Thus, ants are both helping and hindering beetle populations.
Additionally, coffee beetle borers dive into the fruit and eat the seed. Ants camp out by the bored hole to predate on them. Yay for ants again.
Finally, coffee rust (a leaf fungus) is present in Central America but has not started to threaten a crop collapse (as it did in Sri Lanka, which now grows tea). This is seriously scary stuff. A different white ring fungus grows on the tended scales and harms the coffee plants and is possibly transmitted by the tending ants. So it's boo for ants and scales again, except that the presence of this white fungus appears to stymy the growth of the more dangerous coffee rust. So, it's actually yay for ants again.
Surely this story only gets larger. As with any ecosystem, boundaries are fuzzy and dynamics are like ripples. However, it is a complete enough story to see that the shade trees harbor a rather useful biotic control, namely the ants. Farmers are disposed to see them tending harmful scales and dislike them. Educational efforts are being made to communicate the benefit of this intertwined system that acts like a rubber band web, pulling the system back to stability after perturbations. And especially the resilience it offers to the greatest perturbation: the threat of collapse from coffee rust.
Natural systems are unique in that their complexity creates an effective buffer against disturbance. (One striking note is that this plantation has acquired such a complex biotic system in only a century.) A shaded plantation is by definition more complex in its flora. Cacao, a shade crop, might also reveal such dynamism in both plantations and natural forests.
When I asked the speaker, she said that cacao research lags coffee research, but has begun in earnest in the last 5 years or so. I will definitely keep an ear to the field and report back!