The impact of invasive fish and invasive riparian plants on the invertebrate fauna of the Rondegat River, Cape Floristic Region, South Africa

Abstract

Invasive fish and plants are widespread in the rivers and riparian zones of the Cape Floristic Region of South Africa and represent potential threats to the highly endemic freshwater fauna. We investigated the impact of invasive smallmouth bass (Micropterus dolomieu) and black wattle (Acacia mearnsii) on assemblages of macroinvertebrates and fish in a cobble-bed foothill river, the Rondegat River, in the Cederberg mountains, by sampling from sites representing four invasion conditions: no invasion; invasive trees; invasive fish; invasive trees and fish. Invertebrate assemblages were sensitive to invasion from bass and/or wattle: in the presence of A. mearnsii the abundance of some cobble-dwelling taxa were reduced, while those of particle-feeding mayflies and chironomids increased, reflecting increased leaf litter and sediment produced by acacias. In the presence of bass the abundance of most invertebrate taxa, particularly Chironomidae and Baetidae, increased, whereas the algal-grazing Heptageniidae and Elmidae were moderately reduced. These changes in invertebrate assemblages, coupled with data on the biomass and diet of fishes, indicate that smallmouth bass, in contrast to indigenous fish, do not exert a strong influence on the abundance of their invertebrate prey. Bass remove indigenous insectivorous fish predators, thereby reducing predation on invertebrate prey, with consequent food-web effects that influence the invertebrate community assemblage.

Keywords: Acacia mearnsii; aquatic macroinvertebrates; food webs; invasive species; Micropterus dolomieu; trophic control

African Journal of Aquatic Science 2008, 33(1): 51–62