Browsing by Author "Le Maitre, David"

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    Grasses as invasive plants in South Africa revisited : patterns, pathways and management
    (AOSIS Publishing, 2017) Visser, Vernon; Wilson, John R. U.; Canavan, Kim; Canavan, Susan; Fish, Lyn; Le Maitre, David; Nänni, Ingrid; Mashau, Caroline; O’Connor, Tim; Ivey, Philip; Kumschick, Sabrina; Richardson, David M.
    Background: In many countries around the world, the most damaging invasive plant species are grasses. However, the status of grass invasions in South Africa has not been documented recently. Objectives: To update Sue Milton’s 2004 review of grasses as invasive alien plants in South Africa, provide the first detailed species level inventory of alien grasses in South Africa and assess the invasion dynamics and management of the group. Method: We compiled the most comprehensive inventory of alien grasses in South Africa to date using recorded occurrences of alien grasses in the country from various literature and database sources. Using historical literature, we reviewed past efforts to introduce alien grasses into South Africa. We sourced information on the origins, uses, distributions and minimum residence times to investigate pathways and patterns of spatial extent. We identified alien grasses in South Africa that are having environmental and economic impacts and determined whether management options have been identified, and legislation created, for these species. Results: There are at least 256 alien grass species in the country, 37 of which have become invasive. Alien grass species richness increased most dramatically from the late 1800s to about 1940. Alien grass species that are not naturalised or invasive have much shorter residence times than those that have naturalised or become invasive. Most grasses were probably introduced for forage purposes, and a large number of alien grass species were trialled at pasture research stations. A large number of alien grass species in South Africa are of Eurasian origin, although more recent introductions include species from elsewhere in Africa and from Australasia. Alien grasses are most prevalent in the south-west of the country, and the Fynbos Biome has the most alien grasses and the most widespread species. We identified 11 species that have recorded environmental and economic impacts in the country. Few alien grasses have prescribed or researched management techniques. Moreover, current legislation neither adequately covers invasive species nor reflects the impacts and geographical extent of these species. Conclusion: South Africa has few invasive grass species, but there is much uncertainty regarding the identity, numbers of species, distributions, abundances and impacts of alien grasses. Although introductions of alien grasses have declined in recent decades, South Africa has a potentially large invasion debt. This highlights the need for continued monitoring and much greater investment in alien grass management, research and legislation.
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    Impacts of alien plant invasions on water resources and yields from the Western Cape Water Supply System (WCWSS)
    (Water Research Commission, 2019) Le Maitre, David; Gorgens, Andre; Howard, Gerald; Walker, Nick
    A key motivation for managing invasive alien plant (IAP) species is their impacts on streamflows, which, tor the wetter half of South Africa, are about 970 m(3).ha(-1).a(-1) or 1 444 mill.m(3).a(-1) (2.9% of naturalised mean annual runoff), comparable to forest plantations. However, the implications of these reductions for the reliability of yields from large water supply systems are less well known. The impacts on yields from the WCWSS were modelled under three invasion scenarios: 'Baseline' invasions; increased invasions by 2045 under 'No management; and under 'Effective control' (i.e. minimal invasions). Monthly streamflow reductions (SFRs) by invasions were simulated using the Pitman rainfall-runoff catchment model, with taxonspeci tic mean annual and low-flow SFR factors for dryland (upland) invasions and crop factors for riparian invasions. These streamflow reduction sequences were input into the WCWSS yield model and the model was run in stochastic mode for the three scenarios. The 98% assured total system yields were predicted to be +/- 580 million m(3).a(-1) under 'Effective control', compared with +/- 542 million niLa under 'Baseline' invasions and +/- 450 mill. m(3).a(-1) in 45 years' time with 'No management'. The 'Baseline' invasions already reduce the yield by 38 mill.m(3).a(-1) (two thirds of the capacity of the Wemmershoek Dam) and, in 45 years' time with no clearing, the reductions would increase to 130 mill. m(3).a(-1) (capacity of the Berg River Dam). Therefore I A P-related SFRs can have significant impacts on the yields of large, complex water supply systems. A key reason for this substantial impact on yields is that all the catchments in the WCWSS are invaded, and the invasions are increasing. Invasions also will cost more to clear in the future. So, the best option for all the water-users in the WCWSS is a combined effort to clear the catchments and protect their least expensive source of water.