Directed dispersal of Opuntia species in the Karoo, South Africa: are crows the responsible agents?

The densities of self-established non-indigenous prickly pears (Opuntia fic us-indica) in rangelands of the Karoo recorded on 3434 km of road transects , were on average 800 times greater below telegraph and transmission poles than away from poles, and 200 times greater next to wire fences along roads than in open rangelands. We suggest that this uneven establishment pattern is mainly due to the dispersal of Opuntia ficus-indica seeds by crows and other vertebrates (chiefly primates). Cape (Corvus capensis) and pied crows (C. albus) feed on fruits of Opuntia ficus-indica in the Karoo, and regurg itate pellets containing viable seeds of these plants below nest sites, roo sts and occasional perch sites. Crows use man-made structures such as wire fences, telegraph and transmission poles and windmills for perching, roosti ng and nesting. Crow pellets, containing regurgitated bones and viable seed s of Opuntia spp., and other alien and indigenous plants, are most common a round such perch sites. Crows are frequently associated with roads in the K aroo because they scavenge on road kills in the Karoo as well as feeding on fruits, insects and reptiles in road verges. We suggest that dispersal of seeds by crows is important for range extension and establishment of new Op untia populations. (C) 2000 Academic Press.