Aim The biophysical impacts of invasive Australian acacias and their effects on ecosystem services are explored and used to develop a framework for improved restoration practices.

Location  South Africa, Portugal and Chile.

Methods  A conceptual model of ecosystem responses to the increasing severity (density and duration) of invasions was developed from the literature and our knowledge of how these impacts affect options for restoration. Case studies are used to identify similarities and differences between three regions severely affected by invasions of Australian acacias: Acacia dealbata in Chile, Acacia longifolia in Portugal and Acacia saligna in South Africa.

Results  Australian acacias have a wide range of impacts on ecosystems that increase with time and disturbance, transform ecosystems and alter and reduce ecosystem service delivery. A shared trait is the accumulation of massive seed banks, which enables them to become dominant after disturbances. Ecosystem trajectories and recovery potential suggest that there are important thresholds in ecosystem state and resilience. When these are crossed, options for restoration are radically altered; in many cases, autogenic (self-driven and self-sustaining) recovery to a pre-invasion condition is inhibited, necessitating active intervention to restore composition and function.

Main conclusions  The conceptual model demonstrates the degree, nature and reversibility of ecosystem degradation and identifies key actions needed to restore ecosystems to desired states. Control and restoration operations, particularly active restoration, require substantial short- to medium-term investments, which can reduce losses of biodiversity and ecosystem services, and the costs to society in the long term. Increasing restoration effectiveness will require further research into linkages between impacts and restoration. This research should involve scientists, practitioners and managers engaged in invasive plant control and restoration programmes, together with society as both the investors in, and beneficiaries of, more effective restoration.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1472-4642.2011.00816.x

Impacts of invasive Australian acacias: implications for management and restoration

There has been a dramatic growth in research on biological invasions over the past 20 years, but a mature understanding of the field has been hampered because invasion biologists concerned with different taxa and different environments have largely adopted different model frameworks for the invasion process, resulting in a confusing range of concepts, terms and definitions. In this review, we propose a unified framework for biological invasions that reconciles and integrates the key features of the most commonly used invasion frameworks into a single conceptual model that can be applied to all human-mediated invasions. The unified framework combines previous stage-based and barrier models, and provides a terminology and categorisation for populations at different points in the invasion process.

A proposed unified framework for biological invasions.

Mediterranean-type ecosystems have provided ecologists with some of the most scientifically-rewarding opportunities to formulate and evaluate hypotheses about large and small-scale ecological phenomena. Comparison of mediterranean-type climate ecosystems in different parts of the world has not only permitted a strong test for ecological convergence, but also critical understanding of key ecophysiological and population processes.

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Ecology and biogeography of mediterranean ecosystems in Chile, California, and Australia

The invasion of natural ecosystems by alien plants is a serious environmental problem that threatens the sustainable use of benefits derived from such ecosytems. Most past studies in this field have focussed on the history, ecology and management of invasive alien species, and little work has been done on the economic aspects and consequences of invasions. This paper reviews what is known of the economic consequences of alien plant invasions in South Africa. These economic arguments have been used to successfully launch the largest environmental management programme in Africa. Ten million hectares of South Africa has been invaded by 180 alien species, but their impacts are not fully understood, although they are undoubtedly significant. The indications are that the total costs of these impacts are substantial. Selected studies show that invasions have reduced the value of fynbos ecosystems by over US$ 11.75 billion; that the total cost of invasion would be about US$ 3.2 billion on the Agulhas Plain alone; that the net present cost of invasion by black wattles amounts to US$ 1.4 billion; that invasions by red water fern have cost US$ 58 million; and that the cost to clear the alien plant invasions in South Africa is around US$ 1.2 billion. These few examples indicate that the economic consequences of invasions are huge. One of the unique aspects of invasive plant control programmes in South Africa has been the ability to leverage further benefits (mainly through employment) for the expensive control programmes from the government's poverty relief budget. This has made it possible to allocate substantial funding to a programme that would otherwise have struggled to obtain significant support. Biological control of invasive species also offers considerable benefits, but is often the subject of debate. We believe that, at least in the case of many invasive alien plant species in South Africa, biological control offers one of the best, and most cost-effective, interventions for addressing the problem.

The economic consequences of alien plant invasions: examples of impacts and approaches to sustainable management in south africa

The primary cause of soil degradation in sub-Saharan Africa (SSA) is expansion and intensification of agriculture in efforts to feed its growing population. Effective solutions will support resilient systems, and must cut across agricultural, environmental, and socioeconomic objectives. While many studies compare and contrast the effects of different management practices on soil properties, soil degradation can only be evaluated within a specific temporal and spatial context using multiple indicators. The extent and rate of soil degradation in SSA is still under debate as there are no reliable data, just gross estimates. Nevertheless, certain soils are losing their ability to provide food and essential ecosystem services, and we know that soil fertility depletion is the primary cause. We synthesize data from studies that examined degradation in SSA at broad spatial and temporal scales and quantified multiple soil degradation indicators, and we found clear indications of degradation across multiple indicators. However, different indicators have different trajectories—pH and cation exchange capacity tend to decline linearly, and soil organic carbon and yields non-linearly. Future research should focus on how soil degradation in SSA leads to changes in ecosystem services, and how to manage these soils now and in the future.

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The State of Soil Degradation in Sub-Saharan Africa: Baselines, Trajectories, and Solutions

Field experiments were conducted on wheat to study the effects of foliar-applied iodine(I) alone, Zn (zinc) alone, and a micronutrient cocktail solution containing I, Zn, Se (selenium), and Fe (iro...

Simultaneous Biofortification of Wheat with Zinc, Iodine, Selenium, and Iron through Foliar Treatment of a Micronutrient Cocktail in Six Countries.

The effect of fire on an Acacia longifolia seed bank in the south-western Cape

SYNOPSIS Acacia mearnsii is an important invasive plant in most of the higher rainfall areas in South Africa. The production of long-lived water-impermeable seeds results in the establishment of large soil-stored seed banks. One way of controlling these seed banks is by felling the plants and burning them to destroy or to stimulate seeds to germinate, so that the resulting seedlings may be controlled. In order to save costs, the hypothesis that the plant population can be burned without felling beforehand, was investigated. A wildfire that swept through a standing population of A. mearnsii plants provided an opportunity to investigate the effects on the population. The mortality of the pre-fire population, the subsequent establishment of seedlings, mortality of seedlings and the spread of the population after the fire were investigated. It was found that the above-ground population increased by a factor of ca. 21,8 due to coppicing of young plants and the mean distance of spread from the edge of an infest...

Is Burning a Standing Population of Invasive Legumes a Viable Control Method? Effects of a Wildfire on an Acacia mearnsii Population

Apart from the last 40 to 50 years, weed control in crops has always been a labour-intensive and time-consuming mechanical practice. Relief came in the form of chemical compounds that could control weeds, in particular selective herbicides. The development of highly effective selective herbicides has resulted in farmers becoming increasingly reliant on chemical weed control. Continuous application of herbicides with the same mode of action, however, selected for resistant mutations within weed populations to become dominant, rendering the herbicides less effective. Herbicide resistant weeds currently occur in 187 weed species distributed over 60 countries worldwide. South Africa is no exception and resistance in 15 weed species to herbicides in seven modes of action groups has been verified. This situation is compelling farmers to consider alternative weed control strategies and cropping systems. The most effective strategy to manage resistant weeds would most probably be an integrated weed management program that does not rely on any one type of weed control method alone. Farmers will therefore have to adapt weed management systems and cropping systems to delay the onset of herbicide resistance or, if it is already present, to manage it effectively in order to keep future crop production profitable and sustainable.

Herbicide resistance in weeds - a threat to effective chemical weed control in South Africa.

SYNOPSIS The population dynamics studies of Acacia longifolia (Andr.) Willd. differed greatly in the absence or presence of fire. In the absence of fire few seedlings established under the living parent plant even though a large quantity of seeds is incorporated into the seed bank annually. Fire caused the destruction of the parent trees but resulted in the establishment of a dense, even-aged stand of seedlings. However, this treatment depleted the seed bank of this weed by 90%. The implications of these findings for integrated control are discussed.

P. J. Pieterse

Papers

Simultaneous Biofortification of Wheat with Zinc, Iodine, Selenium, and Iron through Foliar Treatment of a Micronutrient Cocktail in Six Countries.

The effect of fire on an Acacia longifolia seed bank in the south-western Cape

Is Burning a Standing Population of Invasive Legumes a Viable Control Method? Effects of a Wildfire on an Acacia mearnsii Population

Herbicide resistance in weeds - a threat to effective chemical weed control in South Africa.

The Population Dynamics of the Weed Acacia longifolia (Fabacae) in the Absence and Presence of Fire