The Lake Turkana Basin, while being a hot and dry area, includes several river systems and the 6,000 km2 + lake Turkana. Three rivers feed into the lake: The Turwel, Kerio and Omo. The Omo, from the highlands of Ethiopia contributes about 90 % of the lakes’ waters. The Kerio River comes from the south, from streams originating in parts of the Cherangani Hills and north-western Kenyan Highlands. The rivers and deltas close to TBI are home to a rich diversity of insect and bird life.
After a long journey through the dry country the Kerio reaches the lake. We visited the Kerio Delta to look at some of the freshwater ecology issues and biodiversity. To get to the water we had to walk through a thick green tangle of prickly Prosopis bushes.
This is an invasive species that was first introduced to the area some 25-30 years ago. Prosopis, more commonly known as mesquite. A short tree/shrub, not striking in any aspect, has swiftly and silently colonized vast stands of Kenya, and indeed East Africa’s arid rangelands.
Eight species are currently placed in the genus Prosopis, originating primarily from Central and South America (Prosopis alba, P. chilensis, P. glandulosa, P. juliflora, P. tamarugo), with P. africana native to the Sahelian margins and P. cineraria found in parts of Afghanistan, India, Iran and Pakistan.
Several of the above species have been introduced and managed in different parts of Kenya over the last few decades. Of these, the main villain has proved to be Prosopis juliflora, originating in the drylands of the Americas. It is exceptionally drought tolerant, can live on the most marginal of soils, and tolerates strongly saline conditions as well as seasonal waterlogging. It is Prosopis juliflora that has taken over vast areas of the Kerio Delta at Lake Turkana.
We found that the Prosopis is in the process of swamping the natural beds of Typha bulrushes and aquatic grasses that normally serve as nurseries for fish and help oxygenate the water. Beneath the Prosopis little survives as the trees rot and the bitter tannin-filled leaves fill the water.
Managing the Prosopis will be a big challenge in the future especially as it spreads through larger and larger areas. Total eradication is not really feasible. Prosopis spreads rapidly into areas that have been overgrazed, a sad reality over much of Kenya’s drylands. Seed dispersal, an oft-overlooked aspect, is rapidly effected by browsing goats, other wild ruminants and hares. The thicket-forming growth habit and deep-roots make it extremely difficult to remove once established even over a few square metres.
The one thing that Prosopis does do is provide forage for many different bee species, including both wild bees and honeybees
Where the Prosopis opened up to more natural rushes and reeds we started seeing interesting birds and insects. The most dramatic sighting was of a large, majestic Goliath Heron.
One of the most interesting insect behaviours that we were able to watch was mate-guarding by damselflies. We had discussed this in class and it was exciting to see how diligently the male Cherry-eyed Damselflies held on to their mates to keep them from taking off and mating with other males!