In 2001, an endemic coffee species, Coffea kihansiensis, was discovered in the Kihansi River gorge in the Udzungwa Mountains of Tanzania. The entire range of this species is estimated at about 17 ha between 775 and 950 m. The habitat was shaped by huge amount of spray from an 800-meter-high waterfall which stabilized temperature and humidity.
A hydroelectric project completed in 1999 diverted over 90% of the flow of the river, severely altering the spray-dependent ecosystems and threatening a range of plant and animal taxa, many of which are unique to this gorge. Prior to the diversion, average temperature and humidity was 70° F and 77% respectively; in 2007 after diversion it was 75° F and 69%. The environmental assessment performed prior to the project initiation only looked impacts on habitat inundated by the reservoir. It wasn’t until 2001, after completion of the diversion, that monitoring of species at risk from the loss of spray was initiated. This was when Coffea kihansiensis was discovered.
A just-published three-year study (2007-2009) looked at the health of the coffee trees in the gorge. Because the species was only discovered after completion of the project, pre-diversion parameters are not available. However, field observations made in the immediate years after diversion (2000-2003) found no evidence of insect or other parasitic infestation. In the recent study, over half of the sampled coffee trees had either insect damage (aphids, beetles, or crickets/grasshoppers) or a heartwood-boring insect larva that was usually lethal. The worst infestations were at sites closest to the river which had formerly been constantly drenched in spray. This suggests that the stress of increased temperatures and decreased humidity is making the coffee susceptible to these pests.
Since the general ecology of the area has changed dramatically, it is possible that conditions are now also favorable to the increase in pests; perhaps they were not even present prior to diversion. At least one fruit-eating bird has declined in the gorge, which may have negative effects on the dispersal of Coffea kihansiensis seeds in the area. The reduced flow of the river has also changed the water chemistry and quality. For example, dam releases have resulted in pulses of pesticides from upstream maize farms. The long-term effects of these changes on soils, nutrient dynamics, and the coffee are still not known.
Sadly, Coffea kihansiensis is not the poster-child for the environmental damage caused by this project. The Kihansi Spray Toad (Nectophrynoides asperginis) was first described in 1998. It was also endemic to the gorge and dependent on the spray, with a population estimated at nearly 20,000 in a 2-ha range. By 2004, after the diversion, fewer than 5 individuals were found, and it is now extinct in the wild. Fortunately, a group was taken into captivity in 2001, and after much trial and error is now being bred in several zoos.
Ironically, the hydro project was funded by the World Bank and several other development agencies, and now the World Bank is funding much of the spray toad preservation efforts. The World Bank also funded an elaborate, gravity-fed sprinkler system in the gorge designed to mimic the spray from the waterfall. The system has failed at least once due to silt clogs, and it is not known if the toads can be re-introduced in the area because the vegetation changed significantly when the area dried out.
Nobody can begrudge a developing country an opportunity to produce much-needed clean energy. But when can we learn it is less expensive — monetarily and for the environment — to perform due diligence and minimize our impact?
Davis, A., and Mvungi, E. 2004. Two new and endangered species of Coffea (Rubiaceae) from the Eastern Arc Mountains (Tanzania) and notes on associated conservation issues Botanical Journal of the Linnean Society 146:237-245. DOI: 10.1111/j.1095-8339.2004.00328.x
Rija, A. A., Mwamende, K. A., and Hassan, S. N. 2011. The aftermath of environmental disturbance on the critically endangered Coffea kihansiensis in the Southern Udzungwa Mountains, Tanzania. Tropical Conservation Science 4:359-372. See summary here.