Abandoned homestead gardens as a source of plant invasions in rural South Africa

Abstract

Alien invasions are a global concern, with impacts on native biodiversity. Invasive species may be introduced to new regions for many reasons, with ornamental horticulture being one of the major pathways for the entry of potentially invasive plant species. To prevent future alien invasions potentially invasive species should be detected early after their introduction. Newly introduced/naturalized alien plants can be identified during field surveys, and alien species with larger potential range sizes can be identified with the use of species distribution modelling techniques (SDMs). The aim of this study was to investigate if alien ornamental species were persisting and escaping from abandoned gardens in Limpopo and Mpumalanga provinces and to estimate their invasion potential. The first objective was to test which species have escaped cultivation from gardens since abandonment, and if these species’ distance of spread was related to the taxonomic group, NEM: BA category, reproduction mode, or growth form. The second objective was to test if potential range size of species was correlated with their distance of spread, or if potential range size differed between NEM: BA listed and non-listed species, and between spreading and non-spreading species. Alien ornamental plant species were recorded from 13 abandoned gardens in north-eastern South Africa. The family Proteaceae had the highest average maximum distance of spread and the families with a high number of alien ornamental representatives recorded were Solanaceae, Rosaceae, Lauraceae, Euphorbiaceae, and Bignoniaceae. Species’ maximum distance of spread from abandoned gardens was positively related to species’ NEM: BA status, with species that are required to be controlled showing the greatest spread distances. Species’ mode of reproduction and growth form were not related to spreading distances. Ensemble modelling of the potential distribution of the ornamental plants recorded from the abandoned homesteads showed that the ability of a species to invade larger areas is not related to species NEM: BA status or whether species were spreading or not. No significant positive correlation was found between the species average maximum distance of spread and the predicted potential range size. Potential range size maps were overlaid to generate species richness maps per group (NEM: BA spreading, NEM: BA not spreading, non-listed spreading, and non-listed not spreading), and areas of high potential richness were similar between the four groups of species. These results are at least partly contingent on the data for this study having been collected from areas with relatively high rainfall, and therefore may not be representative of more arid areas in South Africa. In conclusion, several invasive alien ornamental plants were found escaping cultivation from abandoned gardens to surrounding natural vegetation. The species which are currently listed on NEM: BA were found to have spread considerable distances although non-listed species were also were also recorded to spread. As a result, further studies need to investigate abandoned gardens in other biomes and climatic zones to better understand which species can persist and spread from abandoned gardens.