Africa Invaded: Prosopis

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The genus Prosopis, commonly known as mesquite, includes more than 40 species, most of which are indigenous to an area ranging from Argentina to the southern United States. Several species have become invasive in Africa and other parts of the world, particularly the sub-tropical Prosopis glandulosa and P. velutina and the tropical P. juliflora and P. pallida. These species have been widely introduced as a source of fuelwood, fodder and shade, and are also used for sand stabilisation, soil improvement, or for hedges to contain livestock.

Impenetrable thickets

Prosopis are fast-growing, nitrogen-fixing trees that are tolerant of arid conditions and saline soils. They are valued as a source of fodder because the seed pods are a nutritious food for livestock when ripe. However, green pods are bitter and can poison livestock in large quantities, while the foliage is unpalatable due to the high tannin content.

Although individual prosopis occur as small trees, invading populations tend to form dense, impenetrable thickets made up of shrubby, multi-stemmed plants that provide minimal shade and produce fewer pods. The thickets reduce grass cover, so they limit natural grazing and hence stocking density. They also restrict the movement of livestock and obstruct their access to water, since they frequently invade watercourses. Long tap roots
allow the plants to reach deep water tables, so invasive prosopis may deplete vital groundwater reserves in water-scarce environments. Furthermore, the thickets impact on biodiversity by excluding indigenous vegetation and associated animal life.

The success of prosopis species as invaders is largely attributable to the massive number of seeds produced – about 60 million per hectare per year – and their efficient dispersal. Some seeds are carried far from their source by flowing water, especially during floods, but on a more local scale livestock and wild animals disperse the seeds after feeding on the pods. The hard-coated seeds are softened during their passage through the digestive tract, which enhances their germination, while the animals’ droppings provide a ready supply of nutrients for the developing seedling. If the seeds fail to germinate immediately they may lie dormant in the soil for up to 10 years. Destruction of sur rounding vegetation and exposure of the soil often stimulates mass germination of the soil seedbank, resulting
in a sudden infestation.

A win-win solution

Prosopis is considered a valuable asset in many arid regions of Africa, where few other trees could survive, so eradication of this alien invader is generally not an option. A possible solution to the conflict of interests surrounding prosopis is to control invasive populations and manage them as agro-forestry plantations.

Apart from providing fodder and fuelwood, prosopis trees may yield hard and durable timber that can be used to make furniture and parquet flooring, while the protein-rich pods can be used in the manufacture of various food products. Unfortunately, the shrubby, multi-stemmed plants typical of invasive thickets generally only yield small pieces of lower-quality wood, with a large amount of wastage. Nevertheless, the wood may be suitable for making handles for appliances, brushes and tools, as well as charcoal and wood chip products.

In South Africa, prosopis is being controlled through an integrated approach incorporating mechanical, chemical and biological control methods. Where feasible, control costs are offset by commercial exploitation of wood generated from clearing operations. Control of prosopis is especially difficult because the plants can re-grow from vegetative buds just below ground level. These buds sprout new shoots if the above-ground parts of the plant are damaged, with the result that a small shrub may become a dense bush if attempts at control are inefficient. The plants are therefore felled close to the ground, preferably below the point of branching, after which an appropriate registered herbicide is sprayed on both the cut surface and surrounding bark.

Biocontrol

Two biocontrol agents – Algarobius prosopis and Neltumius arizonensis – are currently available in South Africa for prosopis control, after being introduced from Arizona in the United States. Both are seed-feeding beetles that reduce the invasiveness of prosopis plants, without affecting their useful attributes. The female beetle lays its eggs on the prosopis seedpods, and when a larva hatches it chews its way through the pod and into the seed. Here it feeds for several weeks, destroying the seeds’ ability to germinate in the process. The larva then pupates in the seed, but not before it has tunnelled
up to the surface of the pod and made a circular trapdoor. When the adult beetle emerges from the pupa a few days later it is able to push out the trapdoor and escape from the seed pod.

Algarobius prosopis                     Neltumius arizonensis

Since the beetles are able to fly long distances, they have dispersed widely within areas invaded by prosopis. However, their effectiveness as biocontrol agents is compromised by the fact that livestock and game eat most pods before the larvae have had a chance to colonise them. Additional biocontrol agents that will attack immature pods, as well as other reproductive organs such as flowers and flower buds, are therefore being explored. A number of fungi that infect prosopis are also being investigated, to assess their potential for development as mycoherbicides.

Reference: Matthews S. & Brandt K.   Africa Invaded: The growing danger of invasive alien species Global Invasive Species Programme 2004

   
 
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