Lauren J. Chapman
Department of Biology
April 16, 2008
The world’s fresh waters have suffered some of the most intense human impacts over the past century. The result has been a massive reshaping of fish communities, with the pace of change quickening in the tropics. In Africa, fish catches have declined drastically; forests are being rapidly cleared; and water pollution has become a serious threat. In addition, introductions of exotic species have lead to unprecedented changes in freshwater communities, a most notable example being Lake Victoria, East Africa, the world’s largest tropical lake. After it was fished down in the first half of the 20th century, Lake Victoria became home to a series of introduced food fishes, including the predatory Nile perch. A dramatic increase in Nile perch in the 1980s coincided with the disappearance of many species including ~40% of Lake Victoria’s evolutionary treasure -- its 500+ species of haplochromine cichlid fishes. Other fishes persisted with Nile perch at much reduced numbers. Some of these species have started to recover over the past decade in response to intensive fishing of Nile perch; and a few species have actually flourished with the invader. Understanding mechanisms of persistence, resurgence, and prosperity of some species in response to the Nile perch invasion is critical to conservation of remnant faunas and sustainability of Africa’s largest inland fishery.
In this lecture, I review key lessons learned from the Lake Victoria region that reveal both the vulnerability and resilience of native and introduced species in the face of environmental change. First, I discuss mechanisms that have fostered persistence of native species, in particular the use of structural and physiological refugia. Second, I explore cascading effects of Nile perch on the food web of waters that it has invaded. Third, I consider changes in the morphology and ecology of species in response to decades of human-induced perturbations. The future holds many challenges for the Lake Victoria region, but the basin offers a glimpse of what the future may have in store for many of the world’s fresh waters, and highlights the importance of human predators as integral forces in shaping aquatic systems.