First official record of a naturalised population of Mimosa albida Humb . & Bonpl . ex Willd . var . albida in Africa

Mimosa albida var. albida is a woody shrub indigenous to Central and South America that is well adapted to disturbed habitats. This paper is the first formal report of this species outside of cultivation in Africa. A total of 61 plants were recorded along a 1.5km arc of the Mkhomazi River in KwaZulu-Natal, South Africa. Efforts are underway to eradicate the population.


Introduction
Mimosa albida Humb.and Bonpl.ex Willd. is a New World woody shrub or small tree in the family Fabaceae, subfamily Mimosoidea (Barneby 1991;Bessega an Fortunato 2011).It has a relatively wide natural distribution, from western Mexico to Bolivia (Rudd 1968;Seymour 1980;McVaugh 2005;EOL 2014;Tropicos 2012;USDA 2014).It is a frequently encountered weed species in Mexico on pastures (Esquada-Esquivel and Tosquy-Valle 2007;Esquada-Esquivel et al. 2009) and fallow land (Jiménez-Ferrer et al. 2008).In Honduras it forms part of the undergrowth in Pinus oocarpa stands (Clewell 1973) and has also been recorded as a wetland plant from Gautamala (Scott and Carbonell 1986).It is one of the dominant shrub species on the northern coast of Peru (Valdivia and Aragón 1979).For a full species description see Barneby (1991).
There  (Randall 2012) and is considered naturalised in the Galapagos (Tye 2001) and invasive in Reunion (Tassin et al. 2006).It is a prolific seed producer (Barneby 1991), coppices readily (Jiménez-Ferrer et al. 2008) and can be an indicator of disturbance (McCorkle 1968;Dorado et al. 2005).This paper reports on the first record of Mimosa albida var.albida outside of cultivation in Africa and provides a basis for future management.

Methods
M. albida var.albida was first encountered in April 2010 as a single shrub along the perennial Mkhomazi river in the Highover Wildlife Sanctuary, KwaZulu-Natal province (S29.91430E30.10012).This area forms part of the Eastern Valley Bushveld which is a semi-deciduous savannah with a mean annual summer rainfall of 773 mm and mean annual temperature of 17.8ºC (Mucina and Rutherford 2006).Mucina and Rutherford (2006) list 26 woody species, nine succulents, four herbs and 14 grasses as the most characteristic of this vegetation type, with Chromolaena odorata (L.) R.M.King and H.Rob., Lantana camara L., Caesalpinia decapetala (Roth) Alston and Achyranthes aspera L. being the most abundant invasive plant species in this area.In November 2011, a thorough search along the riverbank that could be covered on foot, detected 61 plants along a 1.5 km arc of the Mkhomazi.Before cutting each plant at the base, GPS co-ordinates were taken as well as plant height and diameter in two dimensions.Herbarium specimens were collected and submitted to the KZN herbarium (NH; Accession number NH0137189).
A map of potential distribution throughout South Africa was generated with MaxEnt v 3.3.3(Phillips et al. 2006) for Mimosa albida at the species level (Figure 1).The default settings were used except random seed was selected and the subsample replicate run type.The random test percentage was set to 25 and the number of replicates to ten.Occurrence data from the Global Biodiversity Information Facility (GBIF 2014) were used and records falling in the ocean or missing GPS co-ordinates were removed with a script in R (V.Visser, pers.com, 2013; R Core Team 2014).
Following Loiselle et al. (2008), eight environmental variables, that are relatively uncorrelated, were downloaded at the 2.5 arc minute resolution (Hijmans et al. 2005) and extracted for the indigenous range only to form an environmental layer mask.Using this information a projection was made of potential distribution for South Africa.

Results and discussion
M. albida var.albida had a clumped distribution (Figure 2) with forty-nine plants occurring as a single patch on a section that is part of the western bank every winter, but becomes an island during the rainy season.One plant was found upstream of this point and the remaining 11 plants scattered further downstream.The plants were robust, thorny shrubs with an average height of 1.2 metres.On a subsequent site visit in March 2012, all plants had resprouted as multi-stemmed Table 2. Categorisation of the Mimosa albida population along the Mkhomazi River in KwaZulu-Natal Province, South Africa as selfsustaining (C3) according to the scheme propose by Blackburn et al. (2011), following the interpretation of Wilson et al. (2014).The source of the introduction is not known and there no obvious putative source, but the observed population along the Mkhomazi River is clearly not planted and stretches over >1km.As only one population has been recorded the species cannot be currently classified as E. shrubs and 15 to almost their original height and were about to produce flowers.It is possible that most of these plants were present in 2010 but were only detected during the survey.This population is therefore at the C3 stage of invasion according to the scheme proposed by Blackburn et al. (2011) as interpreted by Wilson et al. (2014; see Table 2) with a self-sustaining population in the wild that is reproducing.The source of the current population is unclear, and as such it might well have dispersed further.
The map of potential distribution indicates that M. albida is suited to the warmer, summer rainfall region, from the mouth of the Great Kei river northward to the Soutpansberg mountain area, near the 23 rd degree latitude line (Figure 1).The two regions with the highest projected suitability for M. albida are the coastal region between the Great Kei and Umtamvuna rivers and the Soutpansberg mountain area.The section between Great Kei and Umtamvuna rivers is approximately 300 km long and encompasses nine vegetation types, the conservation status of three of these is vulnerable: Ngongoni veld, Pondoland-Ugu sandstone coastal sourveld and the Transkei coastal belt (Mucina and Rutherford 2006).Similarly, the Soutpansberg mountain bushveld vegetation is classified as vulnerable (Mucina and Rutherford 2006).
Of the eight bioclimatic variables included, precipitation seasonality and annual mean temperature were the most important in predicting M. albida's distribution (Table 1).Although less accentuated, a large part of the lowveld is also indicated as suitable habitat southward to Barberton, which stands to reason considering that the lowveld is also a summer rainfall area with a relatively high annual mean temperature.In addition M. pigra L. has been recorded from the lowveld, near Tzaneen, as a frequent weed and M. pudica L. var.hispida Brenan in disturbed places along the east coast, from the Little Amanzimtoti river to Port Durnford (Henderson 2007).
It is most likely that M. albida var.albida was imported as a garden ornamental, although it is not currently listed as such for South Africa (Glen 2004), nor do residents in the direct vicinity of Highover Wildlife Sanctuary recognise the species.The Natural Heritage Site status (site no.44) of this nature reserve is an additional factor making the plant's presence a matter of conservation concern.
Considering M. albida var.albida's weedy status in its native range (Esqueda-Esquivel et al. 2009) and invasive status elsewhere in the world, as well as the results of the distribution model, the species is likely to become invasive in South Africa.Further studies are planned to assess whether it warrants listing as an invasive species under the National Environmental Management: Biodiversity Act (2004) and associated regulations, and whether monitoring and control measures would be needed in the country or whether eradication is possible.

Figure 2 .
Figure 2. Known distribution of naturalised Mimosa albida var.albida in South Africa.The numbers correspond with numbers in Appendix 1; all plants have been cut.

Table 1 .
Analysis of bioclimatic variable contributions to the MaxEnt distribution model.