Arne Balder Roderich Witt

Plant invasions can have significant negative impacts on human wellbeing, especially in low- and middle-income countries where people are directly dependent on natural resources (Shackleton et al. ...

Alien plant species are widely utilised for ornament, agro‐forestry, or even for restoration and are more likely to be introduced multiple times and disseminated widely across a landscape. Here we report on the native and alien species used for hedging purposes in rural and peri‐urban eastern Africa and undertake a preliminary assessment to determine whether the use of alien species may be contributing to current invasions. We undertook roadside surveys and recorded the presence of all species used as hedges or barriers, excluding vines or climbers. We recorded 110 such plant species (taxa recorded at genus level excluded) but only report on 54 species recorded at five or more localities, and provide a more detailed analysis of the ten most widely used species. Nine of the 54 species grown as hedges were native to eastern Africa and 45 were alien, of which 73% were known to be invasive. Native Euphorbia tirucalli and the alien species Dovyalis afra , Lantana camara , Agave sisalana , Caesalpinia decapetala , Cascabela thevetia , and Tithonia diversifolia were the most widely grown hedge plant species. Lantana camara was recorded as invasive in 38% of grid cells (~55 × 55 km), followed by T. diversifolia (24%), C. decapetala (13%), and A. sisalana (9%). The study supports findings that propagule pressure in the form of alien plants used as hedges may contribute to their spread. However, species such as D. afra and C. thevetia contradict these findings, lending credence to the important role of species traits as drivers of plant invasions. Propagule pressure, species traits, and the characteristics of the recipient environment all contribute to plant invasions; no generalisations can be made.

Background: Lists of non-native invasive species are needed for the development of strategies and plans to control them. Objectives: This paper describes the development of an initial inventory of non-native plants based on roadside surveys, and reviews the advantages and challenges associated with the methodology. Methods: We conducted surveys in 11 southern and eastern African countries and listed the presence of non-native plants at just under 107 000 roadside localities between 2008 and 2019. Localities were georeferenced and used to create a database of the distribution of 708 non-native plant species. Results: The survey effort varied between countries, with sampling taking place fairly comprehensively in smaller countries (e.g., Eswatini, Malawi and Rwanda were well covered, with >90% of half-degree grid cells reached), while coverage was acceptably high in Uganda, Kenya and Tanzania (78–49%). In other countries, coverage was less. The greatest number of non-native species was recorded in Kenya (430), followed by Tanzania (341), Malawi (313) and Zambia (240). The most frequently recorded species was Lantana camara, with 5 566 records, followed by Senna siamea (3 526), Eucalyptus spp. (3 409), Ricinus communis (3 261) and Cascabela thevetia (3 156). Our inventories made a significant contribution to the regional database, constituting 58% of all plant observations in GBIF across the 11 countries in 2018. Several field guides have been produced based on these lists, and they provide a useful resource where none existed until recently. Conclusion: Alternative methods exist for the development of lists, including remote sensing and citizen science, and each has advantages and drawbacks. Roadside surveys are not comprehensive as they are limited to roadsides and subject to several potential biases because inconspicuous species are excluded, and observers have different levels of competence. We conclude nonetheless that roadside surveys, undertaken by experienced observers during the course of other duties that involve widespread travel, can provide a relatively inexpensive and very useful initial inventory, especially in lower- and middle-income countries where the capacity to conduct other survey options is lacking.

Cacti have been introduced to many parts of the world for food, fodder, and ornamental purposes. Many of these have become invasive, impacting negatively on human and animal health, biodiversity, and pasture production. Opuntia stricta is invasive in several countries in Africa. Studies in Laikipia County, Kenya, have shown that this invasive plant has significant negative impacts on livelihoods and biodiversity. We estimated the costs of current invasions to livestock production in Laikipia and regionally in eastern Africa. Using an eco-climatic model, we extrapolated potential costs to sub-Saharan Africa on the assumption that O. stricta is likely to at least partially invade all areas that are climatically suitable. Areas invaded by O. stricta in Laikipia prevented access by livestock and wildlife to an average of 408 g/m ² of forage, valued at USD 0.12/m ² . Based on this finding, we estimate that the current cost of O. stricta invasions is over USD 5.67 and USD 168 million in Laikipia and in eastern Africa respectively. A conservatively estimated area of 682,000 km ² (2.3% of sub-Saharan Africa) is at different levels of risk of invasion. Using plausible spread rates of 5–15% annually and scenarios of area at risk of invasion, we estimate that costs associated with the loss of forage in sub-Saharan Africa could grow to a mean present value of USD 77 billion over 50 years (range 10–300 billion assuming discount rates of 3 and 5%). The introduction of the biological control agent, Dactylopius opuntiae ‘stricta’ biotype, has already reduced these potential impacts significantly in South Africa and Kenya.

Biological invasions remain one of the greatest threats to biodiversity and livelihoods, and are predicted to increase due to climate change and globalization. In this study, we produced a comprehensive checklist of alien plants in Nigeria from online flora databases, herbarium records, published field surveys, and questionnaires administered to botanical gardens. The resulting alien flora was classified into naturalized, invasive, and cultivated plants. We then fitted a random forest model to identify the attributes which facilitate the naturalization of alien plants in Nigeria. We also used separate chi-squared tests to investigate if the frequency of these attributes is significantly different between the naturalized and invasive plants. The results include 1,381 alien plant taxa, comprising 238 naturalized, 190 invasive, and 953 cultivated species. The naturalized and invasive plants (428 species) are from 91 families, with Fabaceae and Poaceae having the highest representations. The random forest model showed that life forms and local economic uses were the most important drivers of alien plant naturalization in Nigeria. Chi-squared tests revealed a non-random distribution of life forms, higher frequencies of naturalized plants from the Indomalaya and the Neotropics, greater introductions during the British colonial rule, and that naturalized species are mostly used for medicinal, ornamental, food, or animal fodder purposes. Naturalized and invasive plants were recorded in all regions of Nigeria and are mostly found in urban and agricultural landscapes. This baseline information can support further ecological studies and conservation actions in Nigeria.

Koedoe - African Protected Area Conservation and Science

UNESCO World Heritage Sites (WHS) are areas of outstanding universal value and conservation importance. They are, however, threatened by a variety of global change drivers, including biological invasions. We assessed the current status of biological invasions and their management in 241 natural and mixed WHS globally by reviewing documents collated by UNESCO and IUCN. We found that reports on the status of biological invasions in WHS were often irregular or inconsistent. Therefore, while some reports were very informative, they were hard to compare because no systematic method of reporting was followed. Our review revealed that almost 300 different invasive alien species (IAS) were considered as a threat to just over half of all WHS. Information on IAS management undertaken in WHS was available for fewer than half of the sites that listed IAS as a threat. There is clearly a need for an improved monitoring and reporting system for biological invasions in WHS and likely the same for other protected areas globally. To address this issue, we developed a new framework to guide monitoring and reporting of IAS in protected areas building on globally accepted standards for IAS assessments, and tested it on seven WHS. The framework requires the collation of information and reporting on pathways, alien species presence, impacts, and management, the estimation of future threats and management needs, assessments of knowledge and gaps, and, using all of this information allows for an overall threat score to be assigned to the protected area. This new framework should help to improve monitoring of IAS in protected areas moving forward.