School of Animal, Plant and Environmental Sciences (ETDs)

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Characterizing microclimates as potential thermal buffers for reptiles, birds, and mammals in an arid-zone environment
(University of the Witwatersrand, Johannesburg, 2023) Warner, Grace M.; Alexander, Graham J.; Fuller, Andrea
Aridification, increases in air temperatures and frequency of extreme weather events, such as heat waves, are predicted to intensify under climate change, suggesting dire consequences for dryland animals. Microhabitats may buffer dryland animals against the changing climate, yet microclimatic data remains scarce from Africa and arid regions. The distribution, abundance, and thermal buffering capacity of five microhabitat types (burrows, nests, rock crevices, tree hollows and vegetation) were investigated across three habitat types (dunes, plains, and mountains) in the Kalahari. The distribution and abundance of microhabitats were assessed via strip transects, while thermal buffering capacity was evaluated by comparing miniature black globe temperatures from 70 representative sheltered microhabitats with 12 localised exposed-site microhabitats. The data generated have provided a baseline measure of the abundance and distribution of microhabitats in the Kalahari and have produced a year-long fine-resolution microclimatic thermal dataset. Microhabitat types and abundance were found to differ per habitat type and across topographic gradients. Abiotic drivers, such as soil type and fire frequency, along with drivers such as biotic community composition, were highlighted as potential causes of these microhabitat distribution patterns. Burrows were the best buffered microhabitat across all tested temporal and spatial gradients, providing an average maximum decrease of -29.5 ˚C during the day, and an average maximum increase of 20.1 ˚C during the night. Well-buffered microhabitats were rare within the field-site, whereas some of the poorest buffering microhabitats (Vegetation) were abundant and widespread. Thus, competition for well-buffered microhabitats may increase under climate change, whilst the risk of traversing large distances between microhabitats may become too great for smaller animals as exposure risk to extreme temperatures increases. Furthermore, birds and large mammals may likely be at a high risk of extreme temperature exposure, as their size and structural adaptations may restrict them from access to well-buffered microhabitats. Therefore, the need for the concurrent assessments of microhabitat distribution, abundance and buffering capacity in future species vulnerability assessments is highlighted, as it is the combination of these factors that are likely to impact population viability under climate change.
Eco-art for a transformative climate culture
(University of the Witwatersrand, Johannesburg, 2022) Warrington-Coetzee, Hannelie; Voge, Coleen; Manderson, Lenore
Human”‘development” since the industrial revolution has unequivocally attributed to a code red of climate disasters, according to the recent IPCC Sixth Assessment Report of the Intergovernmental Panel on Climate Change report (IPCC, 2022a). To selectively unlearn or de-grow the unsustainable industrial culture crisis, humanity needs seriously to consider and act to transform (e.g., through ecocitizenship) in support of governments, scientists, and other civic actions in our journey to sustainable futures. Using a transdisciplinary praxis approach, artists can provide various forms of transformative possibilities, including transgressive interventions at a grassroots level, deliberately designed to provoke and inspire change. This research interrogates how eco-art deeply engages audiences to identify the key characteristics of such potentially radically transformative artworks. Humans have a narrowing window to transform our relationship to the Earth’s resources and reverse or slow temperature rise. The ‘near term’ (2022 – 2030) will determine and define the extent of various climate transformation pathways (IPCC, 2022, p. 7). Relational eco-art creates spaces for meaningful dialogue to design opportunities for transcendence to ecological citizenship. Art also holds a potential revolutionary connection tool that can unite science and society in incidences of immersion and change to spur further creations and change. These works, which visually articulate diverse types of knowledge, are described in the literature as situated in the fecund middle, a hidden third zone in which components are rhizomatically connected. Here I use fecundity to refer to intellectual productivity. The study aimed to establish which characteristics in eco-art can contribute significantly more to sustainable eco-cultural development and what form and opportunities such transformative interventions manifest. The ways in which artists position their work to contribute to cultural climate change adaptation is interrogated in transdisciplinary praxis. In doing so, I interrogate previously published work of a select group of artists. This is not an inquiry into the methodology of transdisciplinary research, but, drawing on the artists’ praxis, I argue and expand on how the value of collaboration of scholars and artists working in these liminal spaces can reach new audiences. Two datasets are interrogated to analyse the approaches in art related to the environment, one eco-art characteristic derived from the author’s public art praxis, and the second, a broader interrogation of 50 international eco-artworks. By comparing and contrasting the two datasets, characteristics of intentionality, often built into the artworks, were identified. Six key characteristics were isolated, described and further interrogated to discover how they may create opportunities for society to build and foster potential cultural climate change solutions
Extreme weather events and human health in South Africa: implications for climate services
(University of the Witwatersrand, Johannesburg, 2022) Manyuchi, Albert Edgar; Erasmus, Barend; Wright, Caradee; Vogel, Coleen
Extreme weather events (EWEs), defined as ‘weather events that are rare at a particular place and time of the year’, have increasingly been affecting many countries worldwide (IPCC 2014). The increased occurrence and intensity of EWEs, among other factors, has increased public interest and demand for climate information. More detail is needed on EWEs and how they can be more effectively coupled to climate services in Africa. This study brings a compendium of empirical evidence, conceptual clarity and transdisciplinary approaches to policymakers, researchers and practitioners dealing with these crucial issues. The main aim of this study is to explore the human health effects of EWEs, particularly heat and the delivery of climate services for health in Africa. Through a systematic review, the study examines the potential impacts of heat on human health in Africa. Using a case study approach heathealth effects are evaluated including the status of climate services in the Agincourt subdistrict of South Africa. A novel systems theory-based conceptual framework and an inclusive analytical framework are employed to explore climate services delivery within the context of climate change adaptation. The study produces two main findings. First, EWEs particularly heat, potentially affects human health in Africa by changing mortality and morbidity patterns. In the Agincourt sub-district in particular, heat may be associated with adverse health effects on vulnerable populations, including inter alia the elderly, children and outdoor workers. Despite this, Africa-specific heat-health interventions and policy suggestions are scarce. Second, climate services are generally in embryonic stages of development and climate services for health in Agincourt subdistrict are non-existent. Within the South African context, the main barriers to delivery of climate services for health are paucity of interagency coordination and gaps in meteorological data. One of the key conclusions of the study is that African countries can promote development of climate services to adapt to EWEs such as heat. Heat, and heatwaves, for example, are already affecting populations, and are projected to increase in frequency and intensity with concomitant adverse effects on health outcomes on vulnerable population groups. The three key recommendations from this study are as follows. First, African policymakers and practitioners must avoid a reductionist approach to analysis of EWEs, especially heat impacts and start viewing these as harmful to human health. Therefore, governments must prioritise interventions, create institutions and formulate policy measures to deal with the health effects of heat within their national adaptations plans. Second, African countries must prioritise policy mixes that promote climate services in general and climate services for health in particular. And finally, further policy-science research that generates empirical evidence for African policymakers and practitioners engaged in international negotiations and programmes for climate change adaptation and climate services should be encouraged.
Factors affecting mammal utilisation of non-wildlife railway underpasses within the Greater Kruger, South Africa
(University of the Witwatersrand, Johannesburg, 2023) de Villiers, Hannah; Parrini, Francesca; Teren, Gabriella
Railways are an integral component of sustainable transport systems, but cause significant impacts to biodiversity, primarily through wildlife-train collisions and the habitat fragmentation created in the landscape. Crossing structures (CSs) for wildlife are the key mitigation measure for these adverse impacts, which will be critical throughout Africa, where 55,000 km of new railway lines are predicted to cut through hundreds of protected areas. For the first time in southern Africa, mammal usage of railway underpasses that were not designed to facilitate wildlife movement was studied using camera traps in the Greater Kruger National Park, South Africa. This study demonstrates that 70% of the terrestrial medium and large mammals species present in the area used at least one non-wildlife CS during the study period, with a total of 1,823 usage events by 33 species. Twelve medium to large African mammal species used underpasses regularly, improving the permeability of the railway line for these species. A viaduct design was characterised by almost four times the usage rate of a typical box culvert and was used by twice as many species, including megaherbivores and mesoherbivores, whereas box culverts supported a smaller number of species, of which the majority were predators. Species most affected by rail collisions in the study area were mostly herbivorous and seldom used non-wildlife underpasses. These results indicate that a railway line with only drainage culverts and the occasional viaduct is not effective in providing adequate safe crossing structures for megaherbivores and ungulate mesoherbivores. Contrary to expectations, vegetation and environmental factors influenced mammal usage of box culverts more than structural dimensions. Ensuring that there is high visibility through the structure, with less herbaceous cover and more woody cover outside of entrances, is likely to enhance culvert usage by most trophic groups. In a savanna context where faunal diversity is high and no single species is a target for mitigation, a range of underpass designs including larger viaducts, located in different types of vegetation cover, thereby combining the range of requirements of different guilds, is recommended.
Heat-related health impacts: the responses of local governments to current and emerging heat extremes in the Gauteng province
(University of the Witwatersrand, Johannesburg, 2024) Riley, Bradley; Vogel, Coleen
In recent years, particularly during the last few summer seasons across the globe, there has been growing attention to the impacts of heat on human health due to unprecedented extreme heat and heatwave events. Cities and urban areas are especially at risk due to enhanced urbanisation and anthropogenic activities, leading to increased heat-related deaths and hospitalisation. Although many cities globally have developed adaptation approaches to address the current and emerging heat risks, heat adaptation strategies and planning in an African context have been poorly recognised despite the continent warming at double the rate of global temperatures. The high vulnerability of African cities due to poverty and poor socio-economic conditions, coupled with these temperature extremes, puts African societies at the forefront of future heat-related health impacts. South Africa is one African country that has recognised these growing heat risks and provided frameworks in their national and municipal policy and planning instruments. However, implementing and mainstreaming heat action across institutional structures remains a challenge. This study investigated the extent of current heat responses within three metropolitan municipalities in the Gauteng Province, South Africa, in the cities of Tshwane, Ekhuruleni and Johannesburg; to advance heat action within the local government and institutional structures of the province. In this mixed-methods research design, local officials from the municipality’s Environmental Health Department in each metro were interviewed, and a perception-informed assessment of vulnerability to heat-related health impacts was developed using Principal Component Analysis. Through interviews with the local Environmental Health Officials, it was revealed that extreme heat is not a priority within the Department, as heat action is neither mandated nor budgeted for, leading to a misalignment between policy and implementation. The ability of the Department to unlock its full potential for heat action is constrained by many challenges, highlighted by three central themes: (1) Resource capacity and the ripple effects, (2) Policy-implementation gap and (3) Interdepartmental coordination. Despite this, the Department has the potential to unlock many opportunities for future heat responses, highlighted by four central themes: (1) Existing systems: connecting the dots, (2) Power in unity: embracing a collaborative approach, (3) Mandated heat action and (4) Specialised heat action units. Additionally, this study demonstrated how a heat vulnerability assessment tool could be created using heat-specific socio-economic variables. This reveals the spatial distribution of vulnerability throughout the metro’s and highlights high levels of vulnerability within rural and informal settlements due to poverty, lack of access to resources and services and poor living conditions in these areas. Such an approach can offer valuable insights into the populations and areas most susceptible and at risk of heat-related health impacts, which can aid in developing targeted heat action responses and plans. Finally, this study contributes
Modelling current and future distributions of Warburgia species at continental (Africa) and local (South Africa) scales Samista Kim
(University of the Witwatersrand, Johannesburg, 2024) Rooplal, Samista Kim; Thompson, D.I.; Glennon, K.L.; Witkowski, E.T.F.
Warburgia is a genus of trees and shrubs that is greatly valued in Africa for its use in traditional medicine. The genus contains four species, one of which has two subspecies: Warburgia elongata, W. salutaris, W. stuhlmannii, W. ugandensis subsp. longifolia and Wugandensis subsp. ugandensis. Individuals are harvested primarily for their bark, which contains pharmacological compounds that are used to treat various ailments. Due to the high demand, species within the genus are overharvested throughout their range and have consequently become threatened by extinction. Warburgia salutaris, the pepper-bark tree, is the only species of the genus that naturally occurs in South Africa. Like its congeners, Wasalutaris is heavily exploited for its bark and has been listed as IUCN Endangered in South Africa. This dissertation, therefore, assesses the distributions of Warburgia species in eastern and southern Africa to identify new, potentially suitable areas to increase population numbers to aid in the conservation of the genus. The aim of the first part of the study was to assess the eastern and southern African distributions of Warburgia species. Species distribution models (SDMs) were created for the four Warburgia species and two subspecies, and the geographic distributions and key environmental predictors were identified for each taxon. Environmental niche analyses were also performed to understand whether the two subspecies of W. ugandensis should be considered as a single species in accordance with the ecological species concept. The second part of the study aimed to assess how the current South African distribution of W. salutaris will be affected by future climate change. An SDM was produced to assess the current distribution of W. salutaris in South Africa and identify its key predictor climate variables. The SDM was then extrapolated into the future (2070) using two climate change scenarios, RCP 4.5 and RCP 8.5, which are greenhouse gas emission scenarios that predict future climates under a probable and extreme scenario, respectively. Ecological niche analyses were also used to assess the degree to which W. salutaris’ climatic niche will change in response to the two climate change scenarios. Results showed that the distributions of Warburgia species are restricted and primarily influenced by climatic variables that likely impact their seeds’ and seedlings’ sensitivity to water stress and desiccation. Ecological niche modelling results show that the climatic niches of W. ugandensis subsp. longifolia and W. ugandensis subsp. ugandensis are identical and should therefore be considered as a single species according to the ecological species concept. The South African distribution of W. salutaris is fragmented and restricted. The species’ current and future distribution is influenced by its sensitivity to frost and the fact that it produces seeds that are susceptible to drying out. SDMs predicted that the distribution of W. salutaris will shrink in eastern Limpopo and in parts of Mpumalanga, but increase in eastern KwaZulu-Natal by 2070 under both climate change scenarios. Overall, this species was predicted to contract from the Indian Ocean coastal belt and grassland biomes and expand into the savanna biome. This study has expanded our knowledge of the distributions and environmental drivers of Warburgia species. It was found that in general, Warburgia species have restricted ranges that are likely governed by their sensitivity to desiccation at the seed and seedling stages. Climate change is anticipated to negatively impact the populations of many plant species, especially those in Sub-Saharan Africa. While the environmental niche of W. salutaris will remain stable, its geographical distribution was predicted to expand further in the savanna biome along the eastern coast of South Africa in response to climate change. Results from this study support pursuing different conservation techniques, including propagating Warburgia populations around the Great Lakes of Africa and Mt. Kenya and in western Limpopo and eastern KwaZulu-Natal for W. salutaris only. This study therefore emphasizes the importance of using SDMs as a baseline to inform effective conservation efforts for important medicinal plant species
Quantifying and Mapping Urban Ecosystem Services in Johannesburg, South Africa
(University of the Witwatersrand, Johannesburg, 2024) Friemond, Joseph
Modern cities face a wide range of challenges such as flooding and heat stress, which are driven by urbanisation and exacerbated by the impacts of climatic change. The ecosystem services provided by green spaces in cities have become a crucial element in addressing these challenges by supporting climate change mitigation and adaptation. The first step in maintaining and improving the supply of these services is their quantification and mapping. However, large knowledge gaps exist in South Africa and Johannesburg relating to the provision of urban ecosystem services. This study aimed to quantify the supply of three important urban ecosystem services (carbon storage, runoff retention and cooling) and map their distribution across the wards of Johannesburg. Carbon storage was quantified through field sampling of four urban forest types (roadside trees, parks, gardens and nature reserves) and the use of biomass equations. InVEST's urban flood risk mitigation model was used to quantify runoff retention, while cooling was quantified by deriving land surface temperatures from Landsat satellite imagery, which were then used as inputs for a cooling indicator. All three services were mapped across the wards of Johannesburg and then normalised for comparison. The results revealed that Johannesburg's urban forest stores 2.4 million tonnes of carbon, with significant differences in carbon storage between forest types. Johannesburg’s ecosystem services provide great value in mitigating urban challenges, retaining 20.9 million m3 of runoff during a 50 mm storm, and providing cooling services across most of the city. However, the supply of these services is unequal, with large spatial disparities between the northern and southern regions in the city. Numerous wards receive critically low supply of these services, making them vulnerable to the impacts of climatic change. The northern- central wards have optimal supply of all three services, highlighting synergies between services. Ultimately, these three services have immense value in the Johannesburg context and play key roles in supporting the city’s climate change mitigation and adaptation, through the multi-functional delivery of ecosystem services from urban green infrastructure. By mapping these services at the ward scale, our findings can be used to accurately inform authorities and decision makers of priority areas for intervention, as well as key areas for conservation
South African Podocarpaceae distribution interpreted from a physiological and population genetics perspective
(University of the Witwatersrand, Johannesburg, 2023-09) Twala, Thando Caroline; Fisher, Jolene T.; Glennon, Kelsey L.
Podocarpaceae (podocarp) are the most diverse conifer family with a Southern Hemisphere distribution. Podocarps occur in Afrotemperate and Afromontane forests at high elevations that are cool and humid. Podocarps once dominated the forest canopy but due to their slower growth rate and photosynthetic rates they have are in competition withangiosperms which have faster growing and higher photosynthetic rate. Due to the competition between podocarps and angiosperms, Bond (1989) proposed that podocarps were excluded to nutrient poor and unfavourable environments due to their limited competitive ability. However, podocarps persist under the forest canopy until conditions become favourable. This power dynamic shifts with climate oscillations where podocarps distributions expand and dominate when conditions become cooler. Owing to podocarps being the most diverse and widespread conifers they make for a good study system. This thesis focuses on understanding the climatic variables driving the current and future distribution of podocarps, how their seedling physiology may influence their ability to recruit and establish under climate change, and how this can influence their ability to disperse in their South African distribution. Ensemble species distribution modelling was used to characterise the current and future distribution of podocarps and identify the climatic variables that influence their distribution. The current and future environmental niche was quantified using environmental niche modelling. I found that variables predicting rainfall seasonality were the most important at determining the distribution of podocarps in South Africa. Afrocarpus falcatus and P. latifolius were predicted to have the largest geographic distribution, with P. henkelii and P. elongatus having restricted distributions. Both A. falcatus and P. latifolius were predicted to occur in the Limpopo, Mpumalanga, KwaZulu-Natal, Eastern Cape and Western Cape provinces of South Africa. Podocarpus henkelii was predicted to occur in the KwaZulu-Natal and Eastern Cape provinces. Podocarpus elongatus is endemic to the Western Cape Province. All four podocarps were predicted to expand to higher altitudes (up the escarpment) under climate change and contract in its coastal distribution. Although P. elongatus was predicted to occupy the smallest geographic distribution it was predicted to have the widest environmental niche than the other species, which was predicted to contract under climate change. The environmental niche of P. latifolius and P. henkelii was predicted to remain stable. Afrocarpus falcatus, P. latifolius, and P. henkelii showed niche conservatism, however, P. elongatus under RCP 4.5 → current and the RCP 8.5↔ current niche comparisons showed niche divergence. Podocarpus elongatus was predicted to expand to an environment it currently does not occupy. Ecophysiological and morphological experiments were conducted to understand how podocarp seedlings respond to drought and elevated temperatures. The experiments indicated that P. henkelii seedlings were more drought and heat tolerant than A. falcatus seedlings. Conditions are predicted to become hotter and drier in some parts of South Africa, and this study has shown that P. henkelii seedlings will be able to tolerate these conditions better than A. falcatus seedlings. Furthermore, this suggests that the distribution of P. henkelii is not constrained by its physiology but rather by other mechanisms such as competition, reproductive biology, and/or shade tolerance. Microsatellites were used to inform us about possible podocarp dispersal patterns in A. falcatus, P. latifolius and P. henkelii in South Africa. The results suggested that podocarp populations in South Africa were shown to have higher genetic diversity than other podocarps globally, however these results may be due to the limited number of microsatellites used in this study, smaller population sizes in comparison to other studies and methods used to measure population structure and diversity. As expected, the geographically widespread species (A. falcatus and P. latifolius) are more diverse than the geographically restricted P. henkelii. Geographically distant A. falcatus and P. henkelii populations showed higher differentiation than geographically proximal populations. In P. latifolius South African populations, there was strong isolation by distance. Although the distribution of podocarps is disjunct, there is dispersal between populations. Podocarps are resilient to climate change as was demonstrated by the work in this thesis, and by their paleodistribution expanding and contracting with climate oscillations. In this thesis I considered climate, ecophysiology and genetics as determinants of podocarps distribution. Under climate change, podocarps are predicted to expand to higher elevations to track favourable climatic conditions. Seasonal drought is the most important climatic determinant of podocarp distribution. The ability of these species to tolerate drought and heat stress suggests that the seedlings might be able to tolerate short periods of drought and heat stress, however prolonged exposure may lead to seedling mortality, but populations will then be maintained by adults. Populations show evidence of gene flow, indicating they will be able to persist through changing climates, as they have done in the past. This thesis has highlighted that the factors constraining podocarp distributions might be demographic, and future works should investigate the role of fire in podocarp seedling establishment and longevity, as well as their interactions with angiosperm competitors.
The influence of environmental change on the provision of cultural ecosystem services in selected rural villages, Limpopo Province
(University of the Witwatersrand, Johannesburg, 2024) Mokono, Khomotso; Twine, Wayne
Rural communities in communal areas in South Africa are vulnerable to significant environmental change, due to their heavy dependence on the local environment. The influence of local environmental change on the provision of ecosystem services (ES) has been well studied, but less attention has been paid to cultural ecosystem services (CES than the other categories of ES (provisioning, regulating, and supporting)) in the literature. This study fills this gap by focussing on the influence of environmental change on CES in rural communities. The study documents local environmental changes perceived by local communities, how these were perceived to impact the provision of CES, and how these perceptions differ across socio-demographic factors. Data were collected using key informant interviews (20) and survey questionnaires interviews (n=100). Survey respondents were stratified by village, gender, age group, and farmer type. The key findings were that residents identified a great variety of CES provided by the natural environment, including resources important in ceremonies, and as significant places for culturally important practices. These also included ES that would usually be classed as provisioning (e.g., wild foods) and supporting (e.g., grazing for livestock) services that had particularly important cultural value in addition to contributing to livelihoods. Furthermore, the residents identified significant changes to climate, soil, plants, animals, and water bodies, which were driven by local and external drivers. The most important finding was that the perceived influences of environmental change on the provision of cultural ecosystem services were mostly the changes influenced by climate. The village in which the respondents resided had a greater influence on the perception of CES and environmental change than other socio-demographic factors. This study provides valuable information and insight about local communities’ perceptions of cultural ecosystem services, and how they might be affected by environmental change.
The past, present and future of cactus invasions in South Africa in response to rising atmospheric CO2 and climate change
(University of the Witwatersrand, Johannesburg, 2023) Venter, Nicolaas; Byrne, Marcus
Cactaceae originate from the Americas and over the past 250 years have been introduced into South Africa and elsewhere for agricultural and horticultural purposes. Numerous species, including useful taxa, have become important invasive weeds that have serious socio-economic and environmental impacts. Fortunately, management strategies, in particular biological control, have been successful in controlling certain species. However, with rising atmospheric CO2 invasive cacti are likely to pose a renewed threat, whereby evidence shows that cactus species are responsive to CO2 fertilisation, which is likely to increase their vigour, mainly through improved water use efficiency (WUE). Additionally, studies show that plant quality declines with increasing CO2 which in general has negative effects on their arthropod herbivores. This study sought to determine the effect of CO2 on two functionally different invasive cactus species and how they may respond to predicted increases in atmospheric CO2. Opuntia stricta (a succulent shrub) is an obligate CAM photosynthetic species that invades grasslands and savannas across semi-arid to subtropical environments whereas Pereskia aculeata (a scrambling woody vine with well-developed leaves) is a CAM-cycling photosynthetic species that invades forest type habits across subtropical environments. Plants were grown at three CO2 concentrations that represented pre-industrial (sub-ambient - 250 ppm), current (ambient - 400 ppm) and future (elevated - 600 ppm) atmospheric CO2 conditions. Plants were also subject to suboptimal and optimal watering treatments for the duration of the experiments to determine the ameliorative effect of CO2 on productivity in response soil moisture deficits. In addition, an experiment was conducted on O. stricta to test the effects of the three CO2 concentrations on plant quality and the subsequent effect on the fitness and efficacy of its insect biological control agent, Dactylopius opuntiae. Growth and productivity of O. stricta and P. aculeata responded positively to increasing CO2, however the responses varied with CO2 concentration. Increasing atmospheric CO2 concentration from date of introduction to present possibly facilitated O. stricta invasion whereas this was less likely for P. aculeata. In both species WUE increased with increasing CO2. Under suboptimal watering, there was partial amelioration of productivity at 600 ppm CO2, but the plant traits that benefitted varied according to species. Plant quality declined for both species, most notably cladode nitrogen (N) content decreased, and carbon/nitrogen ratios (C/N) increased. When D. opuntiae were exposed to O. stricta grown at elevated CO2 (only tested on well-watered plants), D. opuntiae fitness was reduced which in turn reduced the rates of plant mortality due to the insect damage. Using the WUE results from O. stricta, a mechanistic species distribution model (SDM) created here predicted greater increases in its potential distribution in South Africa under climate change relative to the SDM that did not include WUE as a predictor variable. This suggests that improved WUE under future CO2 concentrations can offset the effect of declining rainfall in certain regions of South Africa. Overall, these results suggest that O. stricta and P. aculeata will show more vigorous growth and are also likely to expand their ranges into regions where rainfall currently limits their distribution. This expanded distribution may be further facilitated by reduced biocontrol agent efficacy as host plant quality declines. These findings suggest that management of these two species and other invasive cacti is likely to become more challenging with increasing atmospheric CO2 and climate change.

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