The genus Acacia comprises some 1500 species, and close to 1000 of these are indigenous to Australia, where they are commonly known as wattles. Many other Acacia species naturally occur in Africa, spreading to other parts of the continent where they are considered serious invaders. Because of the vast number of acacia species, only a few are highlighted below, with a focus on some of the more serious Australian invasive species introduced to South Africa, highlighting a variety of negative consequences deriving from these introductions.
Loss of water and land
Alien acacias generally have higher water requirements than the indigenous vegetation they replace, so infestations in catchment areas and along watercourses reduce runoff and hence river flow. This not only has detrimental impacts on riverine and wetland ecosystems, but ultimately translates to less water in dams for agricultural, industrial and
Impenetrable thickets along watercourses block access of people and livestock to water, and obstruct the flow of rivers – particularly during floods, when fallen trees create logjams and blockages that cause further flood damage. Dense stands of acacias also reduce the productive potential of land by taking over agriculturally valuable areas, and heighten the risk and intensity of fire by increasing the fuel load. Very hot fires destroy the seeds of indigenous species, compromising post-fire regeneration.
Invading the Cape Floristic Region
Alien acacias also cause a loss of biodiversity by outcompeting indigenous species and disrupting natural ecosystem functioning. The Cape Floristic Region – world renowned for its rich biodiversity – is particularly vulnerable in this regard. The indigenous fynbos plants are adapted to nutrient-poor sandy soils, but acacias are nitrogen-fixing plants that increase nitrate levels in the soil. Many indigenous species cannot survive in the enriched soils surrounding acacias, allowing the alien invaders to form bland monocultures. These spoil the Cape Floristic Region’s natural beauty and detract from the tourism experience. Furthermore, the absence of ground-cover in acacia thickets may result in increased soil erosion.
Australian acacias that have become invasive in the Cape Floral Kingdom include rooikrans (A . cyclops), Port Jackson (A. saligna), long-leaved wattle (A. longifolia), black wattle (A. mearnsii) and blackwood (A. melanoxylon), as well as golden wattle (A. pycnantha), which is Australia’s national floral emblem. Ironically, an African acacia, A. nilotica, is one of Australia’s worst invasive weeds!
Rooikrans was originally introduced for drift-sand control, but is now well established in coastal and lowland fynbos throughout the Cape Floristic Region. Along the coast, the plant usually takes the form of a low shrub, but further inland it occurs as a tree with an average height of 3 metres, although it can grow as tall as 8 metres. Rooikrans is commonly used for firewood, and farmers value it as a source of stockfeed. The foliage is eaten by game and goats, while the pods can be fed to cattle.
Rooikrans produces massive quantities of seeds, which are widely dispersed by birds. Seeds can also be spread by coastal sand blown inland or taken from infested areas by birds and used for building.
Like all invasive acacias, the seeds can remain viable in the soil for long periods prior to germination. However, rooikrans is one of the few species that can be effectively controlled using mechanical methods alone – since the plant does not coppice, it is not necessary to treat the stumps of felled rooikrans with herbicides.
Nevertheless, biological control offers the most sustainable control method in the long term. The acacia seed weevil Melanterius servulus was first released for the biological control of rooikrans in 1994. During spring, when mating occurs, the adult beetles feed on rooikrans flowers and developing seeds. The female beetles then chew small holes through the walls of developing pods and lay their eggs singly on young seeds, so that the larvae will have a ready food supply after they hatch. The larvae burrow into and feed on the seed tissue, destroying the developing seeds in the process.
After about six weeks they chew their way out of the pods and drop to the ground, where they pupate in the soil until they emerge as adult beetles six to eight weeks later (December-March). The beetles spend the remainder of the year over-wintering under bark, becoming active in spring to begin the cycle again. By destroying the viability of seeds, seed weevils not only reduce the rate of spread of rooikrans and other invasive acacias, but also limit regeneration from the soil seedbank after mechanical and chemical clearing.
Port Jackson was introduced to South Africa for sand stabilisation and as a source of tannin, used in the leather industry. Once it was discovered that black wattle produced tannin of a superior quality, the Port Jackson plantations were neglected, and the species started spreading uncontrollably. It is now a serious invader in the coastal areas of the Cape Floral Region, and has also penetrated into the interior, especially along river valleys.
Control of Port Jackson is difficult, because the plant coppices after being cut down or burnt, and new seedlings can continue germinating from the soil seedbank for many years. Control is therefore best achieved through a combination of mechanical, chemical and biological methods.
Biological control of Port Jackson has to date relied mainly on the gall rust fungus Uromycladium tepperianum, introduced from Australia. The fungal spores are dispersed by wind and rain, and when germinating on a Port Jackson plant they send thin filaments into the plant to extract nutrients from the cells.
Eventually the fungus causes galls to develop on the branches and foliage of the Port Jackson plant. These are the spore-producing structures of the fungus, and they further drain the host plant of nutrients that would normally be used for growth and reproduction. A severe infestation of the gall rust fungus
will kill the host tree by predisposing it to other stress factors, such as drought stress.
The acacia seed weevil Melanterius compactus has recently been introduced as an additional biocontrol agent for Port Jackson.
The black wattle is an evergreen tree normally 5-10 metres tall, although it can reach a height of 15 metres. It is commercially cultivated for its tannin-rich bark, which is used in the leather industry, and also for its timber – a source of woodchips, firewood and building material. However, the species has spread from plantations and is now widespread in South Africa. It invades fynbos, grasslands, forest gaps and roadsides, but is especially prolific along watercourses.
Black wattle is a vigorous resprouter, so felled trees will coppice unless the stump is treated or the entire plant is removed. Large trees are usually felled as close to the ground as possible, and the stump treated with a registered herbicide. Seedlings and saplings can be pulled out by hand when the soil is damp, but chemical control is often preferable if growth is very dense as large-scale uprooting results in soil disturbance, which promotes the germination of wattle seeds. However, it is important that selective herbicides are used where grasses are present, and that diesel-based herbicides are not used along watercourses, so as to avoid contaminating the water.
The seed-feeding weevil Melanterius maculatus is available for the biological control of black wattle, while two gall-forming midges are being considered as supplementary biocontrol agents. In addition, an indigenous fungus has been registered as a mycoherbicide. Applied as a cut-stump treatment, it kills the stumps and any regrowth within a year.
Like rooikrans and Port Jackson, the long-leaved wattle was originally introduced to stabilise drift-sand, but it failed to establish on coastal dunes. Instead it spread rapidly along the drier mountain slopes and became a major threat to mountain fynbos in the Cape Floristic Region. It has also invaded forest and grassland habitats elsewhere in South Africa, and commonly occurs along rivers.
Fortunately, the long-leaved wattle has been successfully brought under biological control. This has been achieved with two biocontrol agents that reduce both the rate of invasion and the density of existing infestations.
The bud-galling wasp Trichilogaster acaciaelongifoliae lays its eggs in immature flower buds on the plant. After the eggs hatch, the feeding larvae secrete chemicals that cause the buds to develop into round, fleshy galls. These not only prevent seed production in the affected buds, but also deprive other plant parts of nutrients and water. As a result, the plant grows more slowly and becomes susceptible to environmental stresses such as drought. Where water is not a limiting factor the reduction in seed production is not as marked, so in these areas biocontrol by the bud-galling wasp is supplemented by the acacia seed weevil Melanterius ventralis.
Reference: Matthews S. & Brandt K. Africa Invaded: The growing danger of invasive alien species Global Invasive Species Programme 2004