Browsing by Author "Canavan, Susan"

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    Assessing the interaction between history of usage and plant invasions : Bamboo as a case study
    (Stellenbosch : Stellenbosch University, 2018-12) Canavan, Susan; Wilson, John R.; Le Roux, Johannes J.; Richardson, David Mark; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: Studies in invasion science often focus on the biological or environmental implications of invasive alien species. However, biological invasions are inherently due to the humanmediated dispersal of species; this means that there needs to be a greater focus on the socioeconomic context of why species have been introduced by humans, and how humans have created environments or situations to foster the success of alien taxa. This dissertation explores the interaction of human usage and plant invasions using bamboos as a case study, with the intention of answering the following questions: Why are some species selected by humans over others? How does this influence invasion success, impacts, and the cultural role of alien species in receiving environments? I found that, like many other alien taxa, the transfer of bamboos globally has been nonrandom. Large-statured Asiatic species and those with a greater number of cultivars are more likely to have been introduced outside of their native ranges. The species with high introduction rates are the bamboos that are found to be invasive; this supports the notion that propagule pressure is a universally important factor in facilitating invasions. However, in contrast to many other studies, biogeographic status (the native status) of bamboo was not a strong predictor of the type and magnitude of impacts. Impacts are rather explained by certain human activities (disturbance such as logging and clearing, increasing temperatures related to climate change, and the promotion of bamboo monocultures in mixed forests for commercial purposes). As such, the management of highly competitive native species should be considered in conjunction with the management of invasive alien species in forest ecosystems where bamboos are present. The competitive nature of bamboos that leads to impacts can be applied to other tall-statured grasses. Specifically, rapid growth rate and the capacity to accumulate biomass (a function of height) allow many tall grasses to form monospecific stands, accumulate dense and deep litter mats, reduce light availability and alter fire and nutrient-cycling regime. Naturalisation rates are greater in tall-statured grasses compared to other grasses, and the pathways of introduction are generalizable (e.g. for addressing environmental problems). Tall-statured grasses are a useful functional group for predicting high-risk taxa and for making generalised management plans. iv Looking at South Africa as a case-study, I found 26 alien species of bamboo recorded as introduced, with populations of several species widely naturalised around the country. I also found bamboos to be an inherently difficult group to identify to the species level, emphasizing the caution that must be taken with regard to future introductions. Amongst the public there is a complex tapestry of perceptions towards bamboos related to (1) a long history of introduction, and multiple introduction events, where bamboos have become culturally significant for some groups of people, and (2) the realisation that they can cause problems. In conclusion, many bamboos can be classified as synanthropic species in that they have benefited from human activity or the habitats that humans create around them. Studying bamboos has provided further insights into how social and economic imperatives are shaping a new biota at a global level.
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    The global distribution of bamboos : assessing correlates of introduction and invasion
    (Oxford University Press on behalf of the Annals of Botany Company, 2016-12-23) Canavan, Susan; Richardson, David M.; Visser, Vernon; Le Roux, Johannes J.; Vorontsova, Maria S.; Wilson, John R. U.
    There is a long history of species being moved around the world by humans. These introduced species can provide substantial benefits, but they can also have undesirable consequences. We explore the importance of human activities on the processes of species dissemination and potential invasions using the Poaceae subfamily Bambusoideae (‘bamboos’), a group that contains taxa that are widely utilised and that are often perceived as weedy. We (1) compiled an inventory of bamboo species and their current distributions; (2) determined which species have been introduced and become invasive outside their native ranges; and (3) explored correlates of introduction and invasion. Distribution data were collated from Kew’s GrassBase, the Global Biodiversity Information Facility and other online herbarium information sources. Our list comprised 1662 species in 121 genera, of which 232 (14 %) have been introduced beyond their native ranges. Twelve (0.7 % of species) were found to be invasive. A non-random selection of bamboos have been introduced and become invasive. Asiatic species in particular have been widely introduced. There was a clear over-representation of introduced species in the genera Bambusa and Phyllostachys which also contain most of the listed invasive species. The introduction of species also correlated with certain traits: taxa with larger culm dimensions were significantly more likely to have been moved to new areas; and those with many cultivars had a higher rate of dissemination and invasion. It is difficult to determine whether the patterns of introduction and invasion are due simply to differences in propagule pressure, or whether humans have deliberately selected inherently invasive taxa. In general, we suggest that human usage is a stronger driver of introductions and invasions in bamboos than in other taxa that have been well studied. It is likely that as bamboos are used more widely, the number and impact of invasions will increase unless environmental risks are carefully managed.
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    Grasses as invasive plants in South Africa revisited : patterns, pathways and management
    (AOSIS Publishing, 2017) Visser, Vernon; Wilson, John R. U.; Canavan, Kim; Canavan, Susan; Fish, Lyn; Le Maitre, David; Nänni, Ingrid; Mashau, Caroline; O’Connor, Tim; Ivey, Philip; Kumschick, Sabrina; Richardson, David M.
    Background: In many countries around the world, the most damaging invasive plant species are grasses. However, the status of grass invasions in South Africa has not been documented recently. Objectives: To update Sue Milton’s 2004 review of grasses as invasive alien plants in South Africa, provide the first detailed species level inventory of alien grasses in South Africa and assess the invasion dynamics and management of the group. Method: We compiled the most comprehensive inventory of alien grasses in South Africa to date using recorded occurrences of alien grasses in the country from various literature and database sources. Using historical literature, we reviewed past efforts to introduce alien grasses into South Africa. We sourced information on the origins, uses, distributions and minimum residence times to investigate pathways and patterns of spatial extent. We identified alien grasses in South Africa that are having environmental and economic impacts and determined whether management options have been identified, and legislation created, for these species. Results: There are at least 256 alien grass species in the country, 37 of which have become invasive. Alien grass species richness increased most dramatically from the late 1800s to about 1940. Alien grass species that are not naturalised or invasive have much shorter residence times than those that have naturalised or become invasive. Most grasses were probably introduced for forage purposes, and a large number of alien grass species were trialled at pasture research stations. A large number of alien grass species in South Africa are of Eurasian origin, although more recent introductions include species from elsewhere in Africa and from Australasia. Alien grasses are most prevalent in the south-west of the country, and the Fynbos Biome has the most alien grasses and the most widespread species. We identified 11 species that have recorded environmental and economic impacts in the country. Few alien grasses have prescribed or researched management techniques. Moreover, current legislation neither adequately covers invasive species nor reflects the impacts and geographical extent of these species. Conclusion: South Africa has few invasive grass species, but there is much uncertainty regarding the identity, numbers of species, distributions, abundances and impacts of alien grasses. Although introductions of alien grasses have declined in recent decades, South Africa has a potentially large invasion debt. This highlights the need for continued monitoring and much greater investment in alien grass management, research and legislation.