Browsing by Author "Richardson, D M"

Now showing 1 - 6 of 6
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    A decision-making framework for restoring riparian zones degraded by invasive alien plants in South Africa
    (2005) Holmes, P M; Richardson, D M; Esler, K J; Witkowski, E T F; Fourie, S
    Riparian habitats in many parts of South Africa are severely degraded by invasive alien plants, especially trees. These invasions reduce water yields from catchments and affect riverine functioning and biodiversity. Initiatives are under way countrywide to clear alien plants from watercourses and surrounding catchments. Current understanding of key processes that regulate riparian functioning and define options for restoration is rudimentary. We review the impacts of riparian invasions and identify factors limiting the recovery of natural vegetation following alien clearance. We propose a framework of strategic interventions for optimizing restoration success. The framework identifies abiotic and biotic barriers to restoration at the scales of catchments and local reaches. In highly transformed catchments, interventions at the reach scale may fail if important barriers at the catchment scale are not addressed. The extent to which propagule supply and microsite conditions inhibit vegetation recovery is unknown. We also know little of the relative importance of dispersing vegetative propagules, dispersing seeds and soil-stored seed banks in vegetation dynamics, particularly after severe disturbances such as dense invasion by alien plants. The importance of geomorphological and hydrological factors in mediating recovery of riparian vegetation has not been adequately explored for all climatic areas in South Africa. More research is needed to determine the influence of different alien species and clearing treatments on the recovery of riparian vegetation. The literature strongly suggests that in highly alien-transformed catchments, the re-introduction of riparian species is required to promote recovery and suppress re-invasion. However, such interventions are unlikely to be widely implemented unless the cost: benefit ratios are favourable.
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    A proposed classification of invasive alien plant species in South Africa: Towards prioritising species and areas for management action
    (2004) Nel, J L; Richardson, D M; Rouget, M; Mgidi, T N; Mdzeke, N; Le Maitre, D C; van Wilgen, B W; Schonegevel, L; Henderson, L; Neser, S
    Many invasive alien plant species in South Africa are already well-established and cause substantial damage, while scores of others are at the early stages of invasion (only recently introduced and/or entering a phase of rapid population growth). Management programmes must target well-established invaders, but must also give appropriate attention to emerging problems. Protocols for objectively prioritizing species in the two groups for management action are lacking. To this end, we describe the objective derivation of two lists of invasive alien plants in South Africa, using available quantitative data and expert knowledge on current patterns of distribution and abundance, life-history traits, and (for emerging invaders) estimates of potential habitat. ‘Major invaders’ are those invasive alien species that are well-established, and which already have a substantial impact on natural and semi-natural ecosystems. ‘Emerging invaders’ currently have less influence, but have attributes and potentially suitable habitat that could result in increased range and consequences in the next few decades. We describe the derivation of lists that contain 117 major invaders (categorized into groups based on geographical range and abundance) and 84 emerging invaders (categorized into groups based on current propagule-pool size and potentially invasible habitat). The main lists, and groupings within them, provide a useful means for prioritizing species for a range of management interventions at national, regional and local scales
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    A rapid assessment of the invasive status of Eucalyptus species in two South African provinces
    (2004) Forsyth, G G; Richardson, D M; Brown, P J; van Wilgen, B W
    Gum trees, or eucalypts (Eucalyptus species), have been targeted for invasive alien plant clearing programmes in many parts of South Africa. This has caused some dissatisfaction where the species concerned also have useful characteristics, and stakeholders contend that some of these useful species are not invasive. A rapid assessment of the invasive status of Eucalyptus species at 82 sites in South Africa (54 in the Western Cape and 28 in Mpumalanga) indicated that only Red River gum (E. camaldulensis) and flooded gum (E. grandis) are clearly invasive. Surveys were not undertaken in parts of the Western Cape known to be invaded by spider gum (E. lehmannii); the invasive status of this species is well known and is not contested. Red River gum has transformed long stretches of rivers and its importance as a major weed has been underestimated in previous reviews of alien plant invasions in South Africa. Most other species were naturalized. We recommend that projects aimed at clearing eucalypts should focus on riparian areas and nature reserves (where all eucalypts have deleterious effects), but that clearing projects outside these areas should only target species known to be invasive until such time as the invasive status of the other eucalypts (notably sugar gum, E. cladocalyx, and karri, E. diversicolor) can be ascertained with a greater degree of confidence.
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    An investigation of riparian vegetation recovery following invasive alien tree clearing in the Western Cape
    (2008) Blanchard, Ryan; Holmes, P M; Richardson, D M
    Riparian zones are dynamic, as a result of varying levels of disturbance from natural flooding regimes, and this makes them particularly susceptible habitats to invasion by alien plants. In South Africa, particularly the Fynbos Biome, closed-stand invasions by alien Acacia and Eucalyptus species have been able to develop within riparian areas. Their impacts on water resources and biodiversity have been countered by manual clearing in order to protect the valuable ecosystem services provided by intact riparian zones, as well as the biodiversity of indigenous communities. The Working for Water programme is tasked with the important role of controlling invasive alien plants with an assumption that indigenous vegetation will recover naturally. Current management objectives are to reduce above ground biomass of invasive alien plants by labour intensive means, after which indigenous vegetation is usually left to recover without further intervention. However, it is unclear to what extent natural recovery can be achieved. The main aims of this study were to ascertain the nature of riparian vegetation recovery, as well as determine which clearing treatment was most successful in promoting recovery. This was achieved by focusing on: 1) the recovery of species composition and biodiversity, 2) recovery of vegetation structure (assumed to be a surrogate for ecosystem function) and 3) whether a particular clearing treatment best promoted indigenous riparian vegetation recovery. Reference sites (control), as determined by Prins et aI., (2004), were compared to alien impacted sites in order to analyse variation among vegetation variables. Three initial clearing treatments were identified, namely: Fell Only (trees are felled and slash left on site), Fell & Remove (slash is removed from the riparian zone) and Fell & Bum (the slash is left for six months to a year before it is burnt).
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    Quantifying threats to biodiversity from invasive alien plants and other factors : a case study from the Cape Floristic Region
    (2004) Latimer, A M; Silander, J A; Gelfand, A E; Rebelo, A G; Richardson, D M
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    The effects of past management practices for invasive alien plant control on subsequent recovery of fynbos on the Cape Peninsula, South Africa
    (2004) Parker-Allie, F; Richardson, D M; Holmes, P M
    Integrated control aimed at reducing impacts of alien woody plant invasions on biodiversity is underway in many parts of the Cape Floristic Region. However, the kinds of control measures applied may themselves affect the recovery of the natural vegetation. In view of this, we investigated the effects of past management practices, viz: ‘bulldozing and aerial-spraying’, ‘aerialspraying’, ‘boom-spraying’ and ‘foliar-spraying’ aimed at clearing invasive alien woody plants on subsequent fynbos recovery in the Table Mountain National Park. Changes in soil (pH and depth) and vegetation (species cover, richness, diversity, evenness) properties, the total cover of species representing different dispersal guild, regenerative mode, and life form categories were compared between the differently treated and control plots at three different sites in the reserve. Only partial recovery of fynbos was observed in the ‘boom-sprayed’ and one of the ‘foliar-sprayed’ plots where measured species richness was significantly lower than that in control plots. However, marked changes in community structure were observed following ‘boom-spraying’ and ‘foliar-spraying’ at one site where a significant reduction in long-lived obligate reseeding species and an increase in the graminoid component was measured, though high fuel loads with possible post-fire erosion may also have aggravated the effects of foliar-spraying. Growth form structure changed the least following ‘aerial-spraying’ and ‘foliar-spraying’. Multivariate analysis of plant functional types indicated a greater similarity between the control plots than the differently treated plots, implying a treatment effect on subsequent fynbos recovery. However, past land use and residual effects of the alien woody invaders may also have contributed to the differential recovery in the treated plots. It is concluded that re-introduction of species in the under-represented guilds may speed up fynbos recovery. The recommendation is a comprehensive seed mix containing the major guilds, and an initial seed mix of fast-growing indigenous species to stabilise the soil.