1. Mimosa Mimosa pigra was an uncommon shrub on the Kafue Flats until the river was dammed in 1977 at Itezhi-tezhi to regulate the flow for hydropower production. The first Mimosa infestation was reported from Lochinvar National Park in 1982 and by 2005 at least 29 km2 of the floodplain had been invaded. Mimosa is now reported to be spreading widely on the Flats, most recently within Blue Lagoon National Park, but detailed information on its occurrence is lacking.
2. Mimosa thickets develop in wet areas heavily used by herds of the endemic Kafue lechwe, Kobus leche kafuensis. At Lochinvar, lechwe have been forced out of the Park by Mimosa encroachment and now face greater risks of disturbance, poaching, competition with cattle, nutritional stress, loss of condition and disease.
3. Before 1977 there were about 94,000 lechwe on the Flats; when the flood was regulated numbers fell and stabilised at about 49,000 during the 1980s and 1990s. Since then numbers have again fallen and in 2015 fewer than 30,000 remained.
4. A successful pilot control project during 2007-2009 cleared Mimosa from 800 ha at Lochinvar but work stopped at the end of the project when government support was withdrawn. Reinvasion of cleared areas has now taken place.
5. Government (Department of National Parks and Wildlife and Zambia Environment Management Agency) has no co-ordinated strategy for dealing with the problem. Biocontrol of Mimosa is feasible and would lower the recurrent costs of control, while the restoration of a more natural flood that would reduce Mimosa seedling establishment is resisted by the power utility ZESCO.
6. Survival of the Kafue lechwe and two national parks, Lochinvar and Blue Lagoon, is at a critical juncture.
Invasive Alien Species (IAS) are second only to habitat destruction in threatening biodiversity conservation worldwide. On the Kafue Flats, flood regulation for hydropower production in the 1970s initiated the destruction of one of the most productive floodplains in Africa. This is now compounded by the invasion of Mimosa Mimosa pigra, a native of South America, and considered amongst the World’s worst 100 invasive alien species (Global Invasive Species Database, 2016). This review summarises information on the Mimosa invasion: it’s history, causes, impact and management.
Invasion: the Occurrence of Mimosa on the Flats
Mimosa is a much branched, prickly shrub that can form dense thickets 3 – 6 m high in moist areas of seasonally flooded grasslands (IUCN, 2016). It reproduces via buoyant seed pods that can spread long distances in flood waters. Plants mature quickly and may set seed in their first year of growth. The seeds are extremely hardy and may remain viable in the soil for over 15 years until dormancy is broken by changes in temperature. Habitat disturbance by large herbivores facilitates Mimosa invasion of floodplains (Buckley et al., 2004, 2007).
The first botanical survey of the Kafue Flats in the late 1960s found Mimosa was uncommon on the floodplain, sparsely distributed along the river bank and in subsidiary drainage channels (van Rensberg, in FAO, 1968). It was not recorded in a botanical survey of Lochinvar National Park in the early 1970s (Douthwaite and Lavieren, 1977).
The first infestation at Lochinvar was found on the floodplain near Mulindi tree in 1982 following an exceptionally high flood in 1981 (G. Howard in lit.). It covered about 0.1 ha but in the following year, two small thickets were found near Mulindi tree and others reported near the high floodline at Chunga and on the Nampongwe stream (KBRC, 1984).
The infested area at Lochinvar extended to c. 1 km2 in 1986 (Thomson, 1986), at least 25 km2 in 2003 (Mumba and Thompson, 2005), and at least 29 km2 in 2005 (Thomas, 2007), representing an average compound growth rate in excess of 20% p.a.
No comprehensive survey of the extent of Mimosa on the Flats has been carried out. However infestations have been reported near Maala (2003) (M. Mumba and P. Leonard, pers. com.), on the North bank in Blue Lagoon National Park (2008) (Shanungu, 2009) and around Namwala and immediately below the Itezhi-tezhi dam (Shanungu, in lit., 2016).
Figure 1. Map of the Mimosa infestation at Lochinvar on 13 June 2005 (Thomas, 2007). Note the limited survey area.
In 1968, van Rensberg (FAO, 1968) concluded that “the floods alone prevent bush encroachment” on the Kafue Flats. However the flood regime was changed during the late 1970s following completion of the Kafue Gorge hydropower scheme in 1977. The scheme consists of a dam downstream, at the head Kafue Gorge, which raised the minimum flood level on the Flats, causing permanent flooding of the Chunga lagoon at Lochinvar (Fig. 1), and a flood storage reservoir at Itezhi-tezhi, upstream, which reduced the height of the flood downstream. Thus the water table on the Flats has been raised but depth of flooding reduced, reducing the area of floodplain but creating conditions more favourable for the establishment of Mimosa seedlings (Fig. 2) (Mumba and Thompson, 2005; Thomas, 2007; Berhanu, 2007).
Figure 2. Mean monthly discharges at Nyimba (pre- and post Itezhi-tezhi) (Mumba, 2005).
Impacts of the Invasion
No comprehensive environmental assessment of the Mimosa invasion has been made. The shrub is unpalatable to browsing stock and forms impenetrable barriers to their movement. At Lochinvar the growth of thickets has significantly reduced the area of open, herb-rich watermeadows, favoured by lechwe and a wide range of waterbirds (Blaser, 2013). Lechwe are now largely excluded from the floodplain edge in the Park, and traditional lekking sites used during the rutting season have been lost (Shanungu, 2009). The thickets are little used by nesting birds (Shanungu, 2008) and fewer bird species, in smaller numbers are seen on the floodplain between thickets than in areas free of them (Shanungu, 2009; Schmidt, 2014).
The decline in wildlife on the floodplain led to the closure of the Star of Africa tourist camp at Chunga with an estimated loss of 50,000 US$ revenue p.a. (Post, 2007; Southern Times, 2008).
Outside the Park, lechwe are more vulnerable to human disturbance and hunting by poachers; they may also be in competition with cattle for grazing and at greater risk of nutritional stress, loss of condition and increased vulnerability to disease, especially bovine TB (Shanungu et al., 2015).
Following completion of the Kafue hydropower scheme the lechwe population fell from about 94,000 in the early 1970s to stabilise at around 49,000 from 1981-1999 (Table 1). However the results of three censuses conducted since 1999 suggest either the mortality rate has increased or recruitment fallen and the population is in steep decline. In 2015 fewer than 30,000 lechwe remained. It is possible that the encroachment of Mimosa thicket onto the most heavily utilised lechwe pastures (cf. Buckley et al., 2004, 2007) is now the ultimate factor driving the decline.
Table 1. Kafue lechwe population estimates based on aerial censuses (data from Bell et al., 1973 and Shanungu et al., 2015).
The lechwe is a ‘keystone species’, grazing perennial grasslands and creating open, herb-rich watermeadows and areas of bare mud. These open areas once supported some of the highest concentrations of ducks, geese and large variety of other waterbirds in southern Africa (Douthwaite 1977, 1978). Though numbers are now much diminished a further decline of the lechwe will have further consequences for waterbird populations.
A pilot project to control Mimosa was implemented in Lochinvar National Park between 2007 and 2009. The project was supported financially by the IUCN, CABI, UNEP, GEF and Zambian government as part of a larger UNEP-GEF Project, ‘Removing Barriers to Invasive Plant Management in Africa’. The project was supported because it would benefit local tourism, fishermen and pastoralists, as well as wildlife and biodiversity preservation. It had the strong support of local MP Highvie Hamdudu (Post, 24 September 2007).
Work involved manual bush clearing and burning, and treatment of the germinating seedlings with the herbicide glyphosate (Kiff and Boateng, 2012; Shanungu, 2008, 2009). Eight hundred of the 3,000 ha infested with Mimosa in the Park was cleared. The cost of clearing 31 ha in 2007 amounted to $1,100 per ha of which labour cost $640 per ha (Shanungu, 2008).
Project implementation in Zambia was “thorough, forward thinking and efficient” (Kiff and Boateng, 2012) and during the life-time of the project government committed significant additional funding of some $625,000 to expand the area to be cleared. The project increased political recognition of the importance of IAS issues resulting in the inclusion of stiffer penalties within the new Environmental Management Act No. 12 of 2011 for the importation and introduction of invasive alien species. Damage to biological diversity now carries a maximum fine of K90,000 (£5,500) or imprisonment up to a term not exceeding five years or both. IAS issues were also included in the Fifth National Development Plan, with the introduction of targets for the Ministry of Tourism and Natural Resources to reduce Mimosa infestation. However without the driver of continued project activity and advocacy, plans to continue and expand control operations costing $450,000 over five years were dropped at the end of the project. With control halted Mimosa re-colonised the cleared areas and by 2014 the infestation was worse than before (Schmidt, 2015).
The final pilot project evaluation concluded that “while external financial support can be effectively leveraged to start interventions, methods of cost recovery and improved productivity from the areas cleared are crucial for long-term sustainability of control” (Kiff and Boateng, 2012). These conditions cannot foreseeably be met on the Flats where resources are largely “open access” and incentives for management lacking. Control methods involving significant recurrent costs, as used in the pilot project, are therefore not an option.
It is known that biological control enhances other control options and can lead to significant cost reductions for Mimosa management (Shanungu, 2009). In Australia, a total of 15 agents have been released to date. Two moths Carmenta mimosa and Neurostrota gunniella, are currently inflicting severe damage, defoliating plants and reducing both seed production and seed banks, which favours competing vegetation (Heard, 2012). Fire can penetrate up to 100 m in Mimosa stands affected by the biocontrol agent Carmenta mimosa (Paynter 2005).
Biological control measures were recommended by CABI, one of the pilot control project sponsors. This option offers lower recurrent costs after potential control agents have been screened and released. However biocontrol is not supported by ZAWA, the antecedent of the Department of National Parks and Wildlife, which has jurisdiction over all National Park areas. Consequently this option is not at present under consideration.
Given the importance of habitat disturbance in allowing Mimosa to spread, Buckley et al., (2007) concluded that management of an invasion solely focussed on the removal of Mimosa will fail unless either disturbance of unoccupied sites or opportunities for Mimosa recruitment in unoccupied disturbed sites is reduced. On the Kafue Flats this may require the restoration of a more natural flood regime that would force lechwe to range more widely, reducing the intensity of habitat disturbance, and also create less favourable conditions for the establishment of Mimosa seedlings.
Long-standing attempts to restore the productivity of the Flats through environmental flow releases from the dam at Itezhi-tezhi have been made, supported by WWF, TNC and other organisations, but have been resisted by ZESCO, the power utility.
Regardless of the technical feasibility of control or financial provision, there is thus an institutional impasse on how to proceed that only government can resolve. The environmental state of the Flats, including the two national parks of Lochinvar and Blue Lagoon, and the survival of the Kafue lechwe, is at a critical point (Shanungu et al., 2015). Without significant management intervention it is possible the spread of Mimosa will drive the Kafue lechwe to extinction in the wild.
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