Nicky Allsopp

Plant invasions – the role of mutualisms

Many introduced plant species rely on mutualisms in their new habitats to overcome barriers to establishment and to become naturalized and, in some cases, invasive. Mutualisms involving animalmediated pollination and seed dispersal, and symbioses between plant roots and microbiota often facilitate invasions. The spread of many alien plants, particularly woody ones, depends on pollinator mutualisms. Most alien plants are well served by generalist pollinators (insects and birds), and pollinator limitation does not appear to be a major barrier for the spread of introduced plants (special conditions relating to Ficus and orchids are described). Seeds of many of the most notorious plant invaders are dispersed by animals, mainly birds and mammals. Our review supports the view that tightly coevolved, plant‐vertebrate seed dispersal systems are extremely rare. Vertebrate‐dispersed plants are generally not limited reproductively by the lack of dispersers. Most mycorrhizal plants form associations with arbuscular mycorrhizal fungi which, because of their low specificity, do not seem to play a major role in facilitating or hindering plant invasions (except possibly on remote islands such as the Galapagos which are poor in arbuscular mycorrhizal fungi). The lack of symbionts has, however, been a major barrier for many ectomycorrhizal plants, notably for Pinus spp. in parts of the southern hemisphere. The roles of nitrogen‐fixing associations between legumes and rhizobia and between actinorhizal plants and Frankia spp. in promoting or hindering invasions have been virtually ignored in the invasions literature. Symbionts required to induce nitrogen fixation in many plants are extremely widespread, but intentional introductions of symbionts have altered the invasibility of many, if not most, systems. Some of the worlds worst invasive alien species only invaded after the introduction of symbionts. Mutualisms in the new environment sometimes re‐unite the same species that form partnerships in the native range of the plant. Very often, however, different species are involved, emphasizing the diffuse nature of many (most) mutualisms. Mutualisms in new habitats usually duplicate functions or strategies that exist in the natural range of the plant. Occasionally, mutualisms forge totally novel combinations, with profound implications for the behaviour of the introduced plant in the new environment (examples are seed dispersal mutualisms involving wind‐dispersed pines and cockatoos in Australia; and mycorrhizal associations involving plant roots and fungi). Many ecosystems are becoming more susceptible to invasion by introduced plants because: (a) they contain an increasing array of potential mutualistic partners (e.g. generalist frugivores and pollinators, mycorrhizal fungi with wide host ranges, rhizobia strains with infectivity across genera); and (b) conditions conducive for the establishment of various alienalien synergisms are becoming more abundant. Incorporating perspectives on mutualisms in screening protocols will improve (but not perfect) our ability to predict whether a given plant species could invade a particular habitat.

Effects of grazing and cultivation on soil patterns and processes in the Paulshoek area of Namaqualand

Soils were sampled at three different sites within the Paulshoek area of the Leliefontein District in Namaqualand, South Africa. At one site the effect of heavy grazing on soil characteristics under shrubs and from open positions was compared to a more lightly grazed area. At this site a diverse shrub community was mostly replaced by a single species, Galenia africana, when overgrazed. In another area the effects of cultivation on soil properties was investigated. At the third site the effect of Galenia shrub size on soil properties was measured.Soils from the Paulshoek area of Namaqualand are generally infertile and very low in arbuscular mycorrhizal propagules. Patterns of nitrogen distribution at the landscape level are a result of nutrient enrichment associated with perennial shrubs. Arbuscular mycorrhizal infectivity, soil moisture and pH are also affected by shrubs. Only species capable of forming arbuscular mycorrhizas support populations of mycorrhizal propagules in their rhizosphere. Differences in plant available phosphorus under some shrubs indicate that processes associated with nutrient cycling are also affected by shrubs. Galenia tended to deplete soil moisture and increase soil pH more than other shrubs.Small stock production results in a depletion of soil nutrients at the landscape scale since larger areas of low nutrient soil develop when shrub density decreases following heavy grazing. Heavy grazing also affects soil properties indirectly through a change in shrub composition. Cultivation resulted in a decrease in soil nitrogen and organic matter, a loss which was still detectable after 20 years of fallow. Zones of nutrient enrichment and higher pH were found where Galenia established in the old field. When soil was sampled under different size Galenia shrubs a positive correlation was found between shrub diameter and nitrogen, available phosphorus and pH indicating that Galenia was capable of establishing patches in the soil with different properties rather than selecting such areas for establishment. Thus, Galenia is able to maintain soil patterns typical of Succulent Karoo although it also establishes conditions which probably ensure its success. Individual shrub species also affect soil differently with respect to some components such as mycorrhizas.

Soil nutrient ecology associated with Acacia sieberana at different tree densities in a South African savanna.

Abstract Three aspects of soil nutrient ecology in an Acacia sieberana savanna were studied. First, information was collected about the effects of the savanna tree species Acacia sieberana on soil fertility, in terms of isolated tree effects and in terms of tree density effects. Second, the influence of savanna trees on arbuscular mycorrhizal abundance was investigated, and third, arbuscular mycorrhizal dependence of the indigenous African savanna grass species Panicum maximum was tested. Organic matter content, pH, mineral nitrogen, total nitrogen, mineralisable nitrogen, resin extractable phosphorus, total phosphorus and mineralisable phosphorus were determined in soils from open and subcanopy positions and in soils from sites with increasing tree density. In bioassays Panicum maximum growth, phosphorus content, mycorrhizal infection levels and mycorrhizal dependence were studied. A positive influence of isolated trees, in this case Acacia sieberana, on soil fertility and mineralisation was found, causi...

How could herd mobility be used to manage resources and livestock grazing in semi-arid rangeland commons?

Pastoralists in Namaqualand, South Africa, use herd mobility to manage livestock and rangeland resources. However, their socioeconomic conditions and ecological landscapes are changing and we explore the options that are available for pastoralists to respond to these changes. This paper presents five possible scenarios for managing livestock in rangeland commons in semi-arid Namaqualand and outlines some of the major advantages and disadvantages associated with each scenario. Scenarios in response to drivers of change include (1) maintaining the status quo with an assumption that current mobility practices are adapted to local environments, (2) integrating new lands into the existing commons for use by mobile pastoralists, (3) using existing lands under existing mobile pastoralism conditions but introducing grazing reserves for use in times of drought, (4) amalgamating herds into larger units under the care of skilled herders, and (5) developing commercial-scale farms for single-occupancy owners because government might be pressured to further promote black commercial farmers to deracialise the commercial farming sector in South Africa. We consider these scenarios as starting points for discussions on future management options that pastoralists in Namaqualand may wish to consider as the managers of rangeland commons.

Adaptive management for complex communal rangelands in South Africa

Many of the intransigent problems facing the world arise in complex systems. In this paper, I propose that communal rangelands in South Africa be recognised as complex social–ecological systems and that one of the reasons that development initiatives have had little impact on improving livelihoods and rangeland condition is that interventions have been based on reductionist thinking that has failed to recognise non-linearities and uncertainties in the system. This complexity resides in ecological, social and economic components of the system, and is characterised by dynamics operating at different scales within and beyond the boundaries of the rangelands. People and the environment are vulnerable to change in these systems from factors such as changing climate, economics, governance arrangements, as well as disasters. Policy that promotes adaptability and resilience, and is itself responsive to changing dynamics, should be sought. Complex systems modelling with an inclusive group of stakeholders holds potential for realising such policy.

Staying Maasai? Livelihoods, Conservation and Development in East African Rangelands

2009, Springer, 233 Spring Street, New York, USA, 418 pages, hardcover, ISBN 978-0-387-87491-3. Price €79.95