Browsing by Author "Keet, Jan-Hendrik"

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    Is invasion science moving towards agreed standards? The influence of selected frameworks
    (Pensoft, 2020) Wilson, John R. U.; Datta, Arunava; Hirsch, Heidi; Keet, Jan-Hendrik; Mbobo, Tumeka; Nkuna, Khensani V.; Nsikani, Mlungele M.; Pysek, Petr; Richardson, David M.; Zengeya, T.A.; Kumschick, Sabrina
    The need to understand and manage biological invasions has driven the development of frameworks to circumscribe, classify, and elucidate aspects of the phenomenon. But how influential have these frameworks really been? To test this, we evaluated the impact of a pathway classification framework, a framework focussing on the introduction-naturalisation-invasion continuum, and two papers that outline an impact classification framework. We analysed how these framework papers are cited and by whom, conducted a survey to determine why people have cited the frameworks, and explored the degree to which the frameworks are implemented. The four papers outlining these frameworks are amongst the most-cited in their respective journals, are highly regarded in the field, and are already seen as citation classics (although citations are overwhelmingly within the field of invasion science). The number of citations to the frameworks has increased over time, and, while a significant proportion of these are self-citations (20–40%), this rate is decreasing. The frameworks were cited by studies conducted and authored by researchers from across the world. However, relative to a previous citation analysis of invasion science as a whole, the frameworks are particularly used in Europe and South Africa and less so in North America. There is an increasing number of examples of uptake into invasion policy and management (e.g., the pathway classification framework has been adapted and adopted into EU legislation and CBD targets, and the impact classification framework has been adopted by the IUCN). However, we found that few of the citing papers (6–8%) specifically implemented or interrogated the frameworks; roughly half of all citations might be viewed as frivolous (“citation fluff”); there were several clear cases of erroneous citation; and some survey respondents felt that they have not been rigorously tested yet. Although our analyses suggest that invasion science is moving towards a more systematic and standardised approach to recording invasions and their impacts, it appears that the proposed standards are still not applied consistently. For this to be achieved, we argue that frameworks in invasion science need to be revised or adapted to particular contexts in response to the needs and experiences of users (e.g., so they are relevant to pathologists, plant ecologists, and practitioners), the standards should be easier to apply in practice (e.g., through the development of guidelines for management), and there should be incentives for their usage (e.g., recognition for completing an EICAT assessment).
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    Understanding the biodiversity impacts of invasive species : investigating changes in below- and above-ground mutualistic networks in response to invasions
    (Stellenbosch : Stellenbosch University, 2019-04) Keet, Jan-Hendrik; Le Roux, Johannes. J.; Ellis, Allan G.; Hui, Cang; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: Invasive non-native plant species threaten global biodiversity, and significantly impact on economic, agricultural, and ecosystem services. Specifically, invasive plants impact on native communities by altering ecological interactions between native species and by altering soil conditions, eventually impacting on whole ecosystems. For example, invasive nitrogen (N) fixing species such as legumes (Fabaceae) are some of best-known examples to cause such ecosystem-level impacts by elevating soil N content and altering soil bacterial community diversity and functionality. Considering that soil bacteria are essential for the health and diversity of plant communities, and ultimately to the functioning of ecosystems, such native system impacts ultimately lead invasive species in becoming ecosystem engineers, to the detriment of recipient environments. Considered a global biodiversity hotspot, South Africa’s Core Cape Subregion (CCR) is an area of international significance and is home to exceptional plant diversity. The generally strong link between above- and belowground community diversity implies that soil microbial diversity might mirror plant diversity in the CCR, e.g. like its unique fynbos vegetation. Despite this, virtually nothing is known about communities of CCR soils. Moreover, several invasive plants, notably Australian acacias, have severe impacts on CCR ecosystems. Thus, the aim of this thesis was to study the diversity and structure of CCR (fynbos) soil bacterial communities, and to investigate the impacts that invasive acacias have on them, together with impacts on soil nutrients, that ultimately lead to alteration in soil functioning. Furthermore, it is believed that the mutualistic associations that acacias form with nitrogen-fixing bacteria, known as rhizobia, might give them a competitive advantage when establishing, colonizing, and invading new environments. Thus, I also aimed to investigate whether differences in invasiveness between various acacias in South Africa can be explained by differences in the effectiveness of mutualistic rhizobial associations. To address the aims outline above, I made use of next-generation DNA sequencing (NGS) techniques and a paired design consisting of various sites with heavily acacia-invaded areas (as treatments) in close proximity to pristine, uninvaded fynbos areas. This allowed me to generate baseline data of the diversity and community composition of pristine fynbos soil bacterial communities, and how these relate to spatial and environmental attributes across different seasons. I then determined how invasive acacias alter fynbos soil bacterial communities, specifically in terms of community composition and diversity, and how impacts relate to the main spatial and environmental patterns of soil bacterial community turnover. Thereafter I investigated the impacts of acacias on soil chemistry and function (carbon, nitrogen, and phosphorus cycling), and determined what the links are between soil function, soil nutrient loads, and bacterial community composition, and whether acacia-induced changes translate into altered soil functionality. Finally, I shifted focus to differences between various acacia species in terms of their mutualistic rhizobial partnerships under field conditions, and asked whether there are differences in the rhizobial mutualistic associations and their effectiveness between widespread and invasive acacias, and localised non-invasive acacias. I found fynbos soils to be characterised by high bacterial diversities and unique bacterial assemblages characterised by specific dominant taxa. Turnover in pristine fynbos soil bacterial communities was mainly due to replacement, with little nestedness. Furthermore, turnover itself was largely driven by differences in abiotic soil conditions, specifically pH and NH4 + , as well as spatial separation. Together with these soil abiotic and spatial drivers, I found seasonality to play a significant role in shaping fynbos soil bacterial communities. Upon introducing the invasion component, I found acacias to significantly alter soil bacterial community composition, but not diversity, and that the presence of invasive acacias reduced the spatial variability across soil communities, such that community turnover could no longer be predicted by geographical distance, as was the case for pristine soils. This compositional change in bacterial communities was primarily driven by acacia-induced changes of soil pH and NH4 + . Furthermore, I found acacias to significantly increase levels of soil nitrogenous compounds (NO3 - , NH4 + , and total N), C and pH, and although such impacts were not consistent across all invaded sites, the direction of impacts were. Acacias significantly impacted on key aspects of soil functioning, as demonstrated by elevated activities of enzymes involved in nitrogen (urease) and phosphorous (phosphatase) cycling, but such impacts were site-specific. Changes in soil nitrogen and phosphorous content were correlated with changes in the activities of enzymes linked to their cycling, i.e. urease and phosphatase, respectively. For one of these enzymes (phosphatase), changes in soil bacterial community composition was correlated with enzymatic activity, suggesting that altered soil functionality is a direct result of acacia induced changes in soil nutrients, and an indirect result of alteration in bacterial community composition. Finally, I did not find any differences in richness, diversity and rhizobium community composition between localised and widespread invasive acacias in fynbos, and also did not find consistent differences in their ability to fix atmospheric nitrogen, except for some species by site comparisons, indicating differential symbiotic effectiveness between these species at specific localities. Thus, differential invasiveness of acacias in South Africa is likely linked to attributes other than mutualistic bacterial interactions, such as differences in propagule pressure, introduction pathways (e.g. forestry vs. ornamental) and intensity of plantings in the country.