The value of dump sites for monitoring biological invasions in South Africa.
Biological invasions are a major threat to natural systems, as well as to human wellbeing and livelihoods. An association of alien plant species with dump sites has received limited research attention in South Africa, and this creates a loophole for effective monitoring and management. We surveyed dump sites from 79 towns/localities in the Free State, Gauteng, and the Northern Cape and North West provinces of South Africa for presence of alien plant species. We recorded 206 alien plant species classified into 50 families, and the dominant families included Cactaceae (18.5%; N = 767), Asteraceae (12.1%), Solanaceae (10.5%), and Fabaceae (8.2%). Significant numbers of herbaceous species (33.0%; N = 206), and succulents (13.6%) had been introduced from North and South America and Europe, while woody species (39.8%) came from different regions of the world. We detected significantly fewer (i.e., 3.9% N = 206) emerging alien plant species in national regulations' category 1a suggesting they may be restricted to their introduction locations, while 8.8% were species not listed in the national regulations, thus suggesting that they should be listed. The species' richness was significantly and inversely correlated with environmental temperatures of each town/locality. In support of the study prediction, the dump sites in Free State Province had the highest number of alien plant species that are unknown in the main provincial species pool (i.e., 71); Northern Cape, 42 species, while the North West and Gauteng Provinces were equal in having 10 unknown species. We conclude that a context-specific management policy is urgently required for biological invasions in dump sites as a way of reducing further invasions and their impact, and that dump sites should be considered as monitoring points in South Africa.