Plenty of research has been done trying to calculate how ecosystems are going to be hit by climate change, how much productivity in a particularly ecosystem will change, but until now not so much on how extinctions and biodiversity loss will change productivity.
The bad news is that this new research shows biodiversity loss alone, if worst-case scenarios play out, would be as bad as the impact of climate change.
Report lead author David Hooper from Western Washington University says, "Some people have assumed that biodiversity effects are relatively minor compared to other environmental stressors. Our results show that future loss of species has the potential to reduce plant production just as much as global warming and pollution." (Science Daily)
Key word in the quote is 'potential'.
The study found that at the low range of projections for plant species loss (1-20%), the impact on ecosystem plant growth will be low in comparison to other environmental changes. In the intermediate range of species loss projections (21-40%), the effect is "comparable in magnitude" to climate change. At the high end of species loss however (41-60%), "the impact of species loss ranked with those of many other major drivers of environmental change, such as ozone pollution, acid deposition on forests, and nutrient pollution."
Perhaps all of that is fairly intuitive, at least at the high end (remove that much biodiversity and the impact is likely to be large), but now we have some sort of quantification of it.