Exotic plant species invade diversity hot spots: the alien flora of northwestern Kenya

We analysed the distribution of native and alien plant species across 20 ec ogeographic zones of northwestern Kenya. The source pool for the majority o f aliens was Europe and America. Thus, the source pool has a biogeographic bias which explains the low proportion of aliens in the tropics: most speci es in the European or American source pool are not well adapted to tropical conditions. As expected, native and alien plant species showed an area eff ect. Correcting for this area effect, species rich zones showed a higher pr oportion of alien plant species in their flora. At the analysed scale, spec ies richness of native plant communities does not increase the resistance t o invasions and alien plant species invade diversity hotspots. Compared to the other ecogeographic zone, the urban area around Nairobi showed an incre ased richness in alien and native plant species. This is very similar to fi ndings in Europe, although the history of urbanisation is much shorter in K enya. The species turnover between zones (beta-diversity) shows a similar p attern in native and alien plant species. Within a very short time scale th e alien plant species mapped the biogeographic patterns of natives, althoug h the geography of human activities influences the propagule pressure.