Environmental factors influencing the distribution of Opuntia stricta, an invasive alien plant in the Kruger National Park, South Africa

AbstractOpuntia stricta (Cactaceae), an alien weed, has invaded an area of more than 35 000 ha in the Skukuza region of the Kruger National Park [KNP]. The distribution of the plant and features of the environment were used to identify biotic and abiotic factors which may be affecting the distribution and density of the plant. A Canonical Community Analysis revealed that none of the environmental factors that were monitored influenced the distribution or abundance of O. stricta in KNP. There were no apparent natural barriers that might limit the occurrence of O. stricta within the KNP. The study provides insight into the relationship between the invader and host environment, showing that there is a high probability that, unless there is appropriate intervention, O. stricta will eventually colonise the entire KNP

Patterns of alien plant distribution in a river landscape following an extreme flood

AbstractThe availability of suitable patches and gaps in the landscape is a crucial determinant of invasibility for alien plants. The type and arrangement of patches in the landscape may both facilitate and obstruct alien plant invasions, depending on whether alien species perceive the patches as barriers. In February 2000 tropical weather systems caused an extreme flood with an estimated return interval of 90 to 200 years in the Sabie River, South Africa. The impact of the 2000 flood on the Sabie River landscape provides an array of patches that may provide suitable resources for the establishment of alien plants. This study examines the distribution of alien plants in relation to patchiness of the Sabie River landscape. Our hypothesis was that if certain patches in the river landscape do not represent environmental barriers to alien plant invasion, alien species will occur preferentially in these patch types. The Sabie River within Kruger National Park [KNP] was divided into six patch types (zones, channel types, elevations, geomorphic units, substrates and flood imprint types). We then examined the distribution of native and alien woody and herbaceous density and species richness in patches. The density and species richness of alien plants in the Sabie River in KNP is very low when compared to the density and species richness of native plants. Some patches (bedrock distributary and braid bar geomorphic units) contained higher density and richness of alien plants compared to the other patches examined, indicating that these locations in the river landscape offer the resources necessary for alien plant establishment. Individual alien species are also associated with different parts of the river landscape. Failure of large numbers of alien plants to establish after the 2000 flood is most likely due to a combination of factors—the plant specific barriers imposed by landscape patchiness, the high abundance and richness of native vegetation leading to competition, and for some species certainly, the clearing by the management (Working for Water) programme

A cross-scale approach for abundance estimation of invasive alien plants in a large protected area

Efficient management of invasive alien plants requires robust and cost-efficient methods for measuring the abundance and spatial structure of inva- sive alien plants with sufficient accuracy. Here, we present such a monitoring method using ad hoc presence-absence records that are routinely collected for various management and research needs in Kruger National Park, South Africa. The total and local abundance of all invasive alien plants were estimated using the area-of-occupancy model that depicts a power-law scaling pattern of species occupancy across scales and a detection-rate-based Poisson model that allows us to estimate abundance from the occupancy, respectively. Results from these two models were consistent in predicting a total of about one million invasive alien plant records for the park. The accuracy of log-transformed abundance estimate improved significantly with the increase of sampling effort. However, estimating abundance was shown to be much more difficult than detecting the spatial structure of the invasive alien plants. Since management of invasive species in protected areas is often hampered by limited resources for detailed surveys and monitoring, relatively simple and inexpensive monitoring strategies are important. Such data should also be appropriate for multiple purposes. We therefore recommend the use of the scaling pattern of species distribution as a method for rapid and robust monitoring of invasive alien plants in protected areas. Not only do these approaches provide valuable tools for managers and biologists in protected areas, but this kind of data, which can be collected as part of routine activities for a protected area, provides excellent opportunities for researchers to explore the status of aliens as well as their assemblage patterns and functions

Defining optimal sampling effort for large-scale monitoring of invasive alien plants: a Bayesian method for estimating abundance and distribution

1. Monitoring the abundance and spatial structure of invasive alien plant populations is important for designing and measuring the efficacy of long-term management strategies. However, methods for monitoring over large areas with minimum sampling effort, but with sufficient accuracy, are lacking. Although sophisticated sampling techniques are available for increasing sampling efficiency, they are often difficult to implement for large-scale monitoring, thus necessitating a robust yet practical method. 2. We explored this problem over a large area (c.20 000 km2), using ad hoc presence–absence records routinely collected over 4 years in Kruger National Park (KNP), South Africa. Using a Bayesianmethod designed to solve the pseudo-absence (or false-negative) dilemma, we estimated the abundance and spatial structure of all invasive alien plants inKNP. Five sampling schemes, with different spatially weighted sampling efforts, were assessed and the optimal sampling effort estimated. 3. Although most taxa have very few records (50% of the species have only one record), the more abundant species showed a log-normal species-abundance distribution, with the 29 most abundant taxa being represented by an estimated total of 2Æ22 million individuals, with most exhibiting positive spatial autocorrelation. 4. Estimations from all sampling schemes approached the real situation with increasing sampling effort. An equal-weighted (uniform) sampling scheme performed best for abundance estimation (optimal efforts of 68 records per km2), but showed no advantage in detecting spatial autocorrelation (247 records per km2 required). With increasing sampling effort, the accuracy of abundance estimation followed an exponential form, whereas the accuracy of distribution estimation showed diverse forms. Overall, a power law relationship between taxon density (as well as the spatial autocorrelation) and the optimal sampling effort was determined. 5. Synthesis and applications. The use of Bayesian methods to estimate optimal sampling effort indicates that for large-scale monitoring, reliable and accurate schemes are feasible. These methods can be used to determine optimal schemes in areas of different sizes and situations. In a large area like KNP, the uniform equal-weighted sampling scheme performs optimally for monitoring abundance and distribution of invasive alien plants, and is recommended as a protocol for large-scale monitoring in other protected areas as well.We are grateful to SANParks and the DST-NRF Centre of Excellence for Invasion Biology for financial support. C.H. acknowledges support from the NRF Blue Sky Programme. L.C.F. acknowledges support from the NFR Incentive Funds Programme. D.M.R. acknowledges support from the Hans Sigrist Foundation

Patterns of alien plant distribution at multiple spatial scales in a large national park: implications for ecology, management and monitoring

Aim Spatial scale is critical for understanding and managing biological invasions. In providing direction to managing alien plant invasions, much emphasis is placed on collecting spatially explicit data. However, insufficient thought is often given to how the data are to be used, frequently resulting in the incompatibility of the data for different uses. This paper explores the role of spatial scale in interpreting, managing and monitoring alien plant invasions in a large protected area. Location Kruger National Park, South Africa. Methods Using 27,000 spatially-explicit records of invasive alien plants for the Kruger National Park (> 20,000 km2) we assessed alien plant species richness per cell at nine different scales of resolution. Results When assessing the patterns of alien plants at the various scales of resolution, almost identical results are obtained when working at scales of quarter-degree grids and quaternary watersheds (the fourth level category in South Africa’s river basin classification system). Likewise, insights gained from working at resolutions of 0.1–0.5 km and 1–5 km are similar. At a scale of 0.1 x 0.1 km cells, only 0.4% of the Kruger National Park is invaded, whereas > 90% of the park is invaded when mapped at the quarter-degree cell resolution. Main conclusions Selecting the appropriate scale of resolution is crucial when evaluating the distribution and abundance of alien plant invasions, understanding ecological processes, and overationalizing management applications and monitoring strategies. Quarter-degree grids and quaternary watersheds are most useful at a regional or national scale. Grid cells of 1 to 25 km 2 are generally useful for establishing priorities for and planning management interventions. Fine-scale data are useful for informing management in areas which are small in extent; they also provide the detail appropriate for assessing patterns and rates of invasion.Centre of Excellence for Invasion Biolog

Invasion syndromes: a systematic approach for predicting biological invasions and facilitating effective management

Published: 02 March 2020Our ability to predict invasions has been hindered by the seemingly idiosyncratic context-dependency of individual invasions. However, we argue that robust and useful generalisations in invasion science can be made by considering “invasion syndromes” which we define as “a combination of pathways, alien species traits, and characteristics of the recipient ecosystem which collectively result in predictable dynamics and impacts, and that can be managed effectively using specific policy and management actions”. We describe this approach and outline examples that highlight its utility, including: cacti with clonal fragmentation in arid ecosystems; small aquatic organisms introduced through ballast water in harbours; large ranid frogs with frequent secondary transfers; piscivorous freshwater fishes in connected aquatic ecosystems; plant invasions in high-elevation areas; tall-statured grasses; and tree-feeding insects in forests with suitable hosts. We propose a systematic method for identifying and delimiting invasion syndromes. We argue that invasion syndromes can account for the context-dependency of biological invasions while incorporating insights from comparative studies. Adopting this approach will help to structure thinking, identify transferrable risk assessment and management lessons, and highlight similarities among events that were previously considered disparate invasion phenomena.Ana Novoa, David M. Richardson, Petr Pyšek, Laura A. Meyerson, Sven Bacher ... Jasmin Packer ... et al

Prickly Pear. The Social History of a Plant in the Eastern Cape

The control of alien and invasive plants has long been the centre of much debate, driven by personal value systems and perceptions of benefits and costs. This is typified by the century-long controversy over the introduction, use and control of the prickly pear and, in many respects, this continues. The ‘prickly pear’, a name often used when generally referring to a wide range of Opuntia species (although scientifically applicable to Opuntia ficus-indica), was introduced into South Africa in the 1700s and became a plant of special significance in the Eastern Cape. Most people in the region had a personal association with the Opuntia, using the fruit for a wide variety of purposes and the plant itself for animal fodder and for hedges. Following its wide dispersal for these purposes, the rapid invasion and environmental transformation that was visible across large areas led to calls to control the plants. Thus began the debates on the costs and benefits of the miracle plant or major invader that continue into the present.Conservation Services, South African National Parks, and Centre for Invasion Biology, Stellenbosch Universit

Dispersal of <i>Dactylopius opuntiae</i> Cockerell (Homoptera: Dactylopiidae), a biological control agent of <i>Opuntia stricta</i> (Haworth.) Haworth. (Cactaceae) in the Kruger National Park

Chemical control efforts, the introduction of Cactoblastis cactorum and attempted releases of Dactylopius opuntiae Cockerell into the expanding infestation of Opuntia stricta in the Skukuza region of the Kruger National Park (KNP) have had limited suc- cess in preventing the spread and densification of 0. stricta. To boost the biological control component, a new strain of D. opuntiae was introduced into KNP during 1997. The new strain established readily and has destroyed large clumps of plants in the vicin- ity of the release site. A large-scale redistribution programme with D. opuntiae is now needed to exploit this biological control agent to the full. In order to match the frequency of manual releases with the natural rates of spread of the insects, surveys were conducted under field conditions to determine the dispersal abilities ofD. opuntiae, with regard to rate and direction of movement. Dispersal of D. opuntiae was found to be slow and restricted and that the insects need to be redistributed by placing them onto plants at approximately 10 m intervals to ensure that they become quickly and evenly distributed on the weed. This information will be crucial in the revision of the integrated management plan for 0. stricta in the KNP, in integrating the cochineal and other control mechanisms

Implementing invasive species management in an adaptive management framework

Adaptive management theory has attracted substantial interest in recent years, in natural resource management in general and also for invasive alien species management. However, whilst many theoretical and conceptual advances have been made, documented cases of practical applications are rare. Coupling invasive species management components with adaptive feedback processes is not without challenges, requiring a substantial change in the thinking and practice of all those involved. Drawing on a decade of experience in South African National Parks, we suggest an approach to implementing adaptive management for controlling invasive alien species. Whilst efforts have been made to advance components of the overall management strategy, the absence of a framework for decision making and feedback mechanisms, inflexibility in the system and shortcomings in the governance structure are all identified as barriers to learning and knowledge integration for the purposes of effective invasive alien species management. The framework provided here, encompassing documents, committees and processes, is aimed at addressing these shortcomings. Conservation implication: Adaptive management theory offers a robust tool for managing inherently complex systems. Its practical application, however, requires distilling the theory into useable functions. We offer a framework to advance implementation of strategic adaptive management for the control of invasive alien species using experiences gained from South African National Parks