The term ‘invasive species’ is most commonly associated with species which have been introduced to new areas by human activity (such as Eurasian species in Australia), but also applies to species that have been able to expand to new areas within their native ranges due to human activity. Activities such as road building, agriculture and mineral extraction have been shown to enable some species to expand their ranges in such a way, generally at the expense of other species.
The Sicklebush, Dichrostachys cinerea is a deciduous, woody Leguminous shrub or small tree native to the Sahel and Sudan regions of Africa and the southern part of the Arabian peninsula. It produces seed pods with a strong smell that are attractive to browsing animals, which eat the pods and spread the seeds to new areas. When established in an area it is very hard to get rid of, as it is a tough, thorny woody plant which produces networks of persistent underground roots from which new plants can grow, as well as large numbers of seeds which can survive in the soil for at least five years before germinating. The plant has become established around boreholes (artificial water-holes) in cattle farming areas in many parts of Africa, where it modifies the environment, replacing open grassland with a dense shrubby woodland, which is often then invaded by larger trees excluded from grassland by grazing animals, eventually resulting in the replacement of the original grassland with mature forests.
Due to the difficulty of removing Dichrostachys cinerea once it becomes established, efforts to control the plant currently revolve around the manual collection of seed pods before they become ripe, in order to prevent the plant from spreading further. However this is extremely labour intensive and of limited effectiveness, making gaining a better understanding of the plant in order to establish more effective methods of controlling it a priority for agricultural scientists in affected parts of Africa.
In a paper published in the journal Nature Conservation on 8 August 2014, Clarice Mudzengi of the Department of Research and Specialist Services at the Makoholi Research Institute, Shakkie Kativu of the Department of Biological Sciences at the University of Zimbabwe, Everson Dahwa, also of the Department of Research and Specialist Services at the Makoholi Research Institute, XavierPoshiwa of the Department of Animal Science at the University of Zimbabwe and Chrispen Murungweni of the Department of Animal Production and Technology at the Chinhoyi University of Technology describe the results of a study of the impact of Dichrostachys cinerea on soil chemistry and herbaceous plant diversity on cattle rangelands on a farm to the north of Masvingo in eastern Zimbabwe. The area is located at a mean altitude of 1163 m above sea-level and has a semi-arid climate, with recorded temperatures ranging from 13.3˚C recorded in June to 21.8˚C recorded in October. The soil is generally poor in nutrients, particularly nitrogen, sulphur and phosphorus, being dominated by granite-derived sandy loams.
Mudzengi et al. found 26 species of grasses and herbaceous plants living in areas that had been colonized by Dichrostachys cinerea, compared to 32 species in areas that had not. However this was not just a simple loss of biodiversity, as eight of the species found in invaded areas were not found in the uninvaded areas, suggesting that the presence of Dichrostachys cinereawas also facilitating the spread of these species.
The soil in areas where Dichrostachys cinerea had become established had raised levels of nitrogen, phosphorous and potassium compared to uninvaded areas. As a Legume Dichrostachys cinerea is known to fix nitrogen in the soil, but this is clearly not the only factor present. Mudzengi et al. suggest that the raised nutrient levels may be associated with the layer of leaf-litter established around the deciduous shrubs (which returns nutrients to the soil), or with competitive exclusion of plants using high levels of nutrients. The establishment of new Dichrostachys cinerea patches did not seem to be affected by soil chemistry.
The invasive plants found in Dichrostachys cinerea patches but not in uninvaded areas are predominantly Grass species typically found on disturbed ground, and often very tolerant of low nutrient levels. This suggests that these invasions are facilitated by the exclusion of other plant species which might otherwise outcompete them, rather than directly due to the modification of the soil chemistry.
Plants being excluded by the presence of Dichrostachys cinerea may have problems adapting to the altered soil chemistry, or may be excluded by reduced light levels available beneath the plants. Dichrostachys cinerea produces a fairly dense canopy, greatly reducing the amount of light to plants growing beneath it. In addition it produces a layer of leaf litter, which further reduces the light available to new seedlings; this is particularly efficient in excluding plants with small seeds that store little in the way of nutrients and which need to start photosynthesizing soon after germination (this includes most herbaceous plants with wind-blown seeds).
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