Research overview: pollination, restoration and invasive alien plant species
Sjirk GeertsDepartment of Conservation, and Marine Sciences, Cape Peninsula University of Technology, PO Box 652, Cape Town 8000, South Africa.
Introduction
My aim is to explore the unknowns of the natural word at the scale of species interaction webs and I see this as an essential return to natural history science; biological science is currently focused on continental- and global-scale ideas, but we’re largely missing the radical ecological changes happening in our own backyards. My personal research agenda has been, simply put, focused on the “plant–animal interactions in novel ecosystems in the Cape and beyond”. On this foundation, I have built (and continue to build) my research consisting of two pillars 1) pollination and plant reproduction, and 2) alien plant invasive processes and management. These pillars are crowned with a link focused on bridging the research between them; for example, the sometimes unexpected pollinators in the novel range of alien plants (Geerts and Pauw 2009b) and the role of pollinators during plant invasions (Moodley et al., 2016; Adedoja et al., 2021; Geerts and Adedoja, 2021).
With the current global decline in pollinators, and the concurrent decline in plant species, pollination research is becoming increasingly important. However, studies outside Europe and North-America, and on groups beyond insects, are largely lacking. I study how pollination configures plant and bird communities in a biodiversity hotspot, the Cape Floristic Region of South Africa. I focus primarily on nectar feeding birds due to their disproportional importance in maintaining high plant diversity (more than 350 plant species dependent on only a few nectar feeding birds). Built on the understanding of plant-bird communities, I established how bird communities may disassemble in response to different pressures. For example, the effect of habitat fragmentation is becoming increasingly important as our urban areas expand. Fragmentation results in species-poor communities with specific nectar feeding birds lost in small urban fragments (Geerts and Pauw 2012 SAJB), including specialised pollinators in particular (Geerts and Pauw 2009a SAJB). Short-billed sunbirds are not substitutes, and fruit production could be increased by 35% in transformed landscapes if long-billed sunbirds are encouraged to return (and there are ways to do this!) (Geerts 2016).
In other research, I demonstrated that certain nectar feeding bird species avoid roads, and this challenges the idea that roadside verges act as biodiversity refugia and corridors in otherwise transformed landscapes (Geerts, 2011; Geerts and Pauw 2011a; Geerts et al. 2020; Mnisi et al., 2021; Geerts and Pauw, 2013).
I am also interested in bee–bird pollinator competition and fire–pollinator interactions. Human-driven impacts in the natural Cape landscapes include elevated bee populations (via beekeeping) and increased fire frequencies. Bees largely compete for the same food source as nectar feeding birds. Although beekeeping has increased honeybee abundance far above natural levels, nectar-feeding birds were not consistently affected (Geerts and Pauw 2011b; Geerts and Pauw 2009c). Fire is a frequent disturbance in fynbos communities. I have a longstanding concern that increased fire frequency will lead to declining populations of the endemic and iconic Cape sugarbird, with cascading effects on dependant plant species. In a before–and–after fire study and a burnt–unburnt transplant study, birds visited flowers in the “before fire” and “unburnt” areas only. The results are surprising given the large number of bird-pollinated plants flowering in the early post-fire vegetation. Previously, this issued had received little consideration and I provided clear management guidelines (Geerts et al. 2012 Journal of Ornithology); specifically, fire managers should attempt to retain enough mature vegetation to prevent declines in Cape sugarbird numbers. My field observations on nectar feeding bird communities has also led to some more theoretical questions, such as whether bird colour preferences are important in explaining, at least partly, the huge floral diversity at the Cape (Heystek et al. 2014). Also, does the lack of pollination translate into population declines? I have collected a 10 year dataset to address this question, and preliminary analyses demonstrate that missing pollinators can translate into plant population declines.
Another major anthropogenic influence is invasive alien plant species. This kicked-off with an interest in the potential novel interactions that alien plants may have in the ecosystem. For example, I found that Nicotiana glauca (Solanaceae) a native to South America that is an alien invasive plant in South Africa is incorporated into the native pollination community in a most spectacular way: sunbirds adapt to a hummingbird-like, hovering behaviour to obtain nectar. As a result, alien invasive plants greatly increased the abundance of nectarfeeding birds (Geerts and Pauw 2009), and in turn these pollinators enhanced the invasion. Interactions with pollinators are thought to play a significant role in determining whether plant species may become invasive, and ecologically generalised species are predicted to be more likely to invade than more specialised species. To test this, colleagues and I assessed the floral biology and pollination ecology of N. glauca, which is a global invader. In regions where specialised bird pollinators are available, birds increase seed production. Surprisingly, in areas without pollinating birds, nectar resources are rarely exploited. We concluded that N. glauca is a successful invasive species, despite its specialised pollination system, because it has the phenotypic plasticity to become more frequently self-pollinating in foreign areas (Ollerton et al., 2012). I expanded on this research theme by exploring the role of plant
reproduction at the early stages of invasion and coupled this with the management aspect of emerging invasive plant species. This culminated in publications on emerging horticultural invasive plants (Le Roux et al., 2010, Geerts et al., 2013a; Geerts et al 2017; Afonso et al., 2022; Afonso 2021), cultivated indigenous species such as Honeybush (Shaw, 2023), floricultural species (Geerts et al. 2013b; Datta et al., 2020) and species introduced for agriculture (Geerts et al. 2016a) and using technology to map IAP’s (Duncan et al., 2023).
A major aim of invasion ecology is to identify characteristics of successful invaders. Most plant groups that have been studied in detail (e.g. pines and acacias) have a high percentage of invasive taxa. We examined the global introduction history and invasion ecology of Proteaceae a large plant family that have been widely disseminated by humans, but with few known invaders (Moodley et al. 2013). This review highlighted that propagule pressure is the most important determinant of naturalization. While some traits showed a consistent influence at introduction, naturalization and invasion, others appear to only influence a single stage, and some have contrasting effects at different stages. Trait-based analyses therefore need to consider different invasion stages separately. As such there is considerable value in seeking the correlates and mechanisms underlying invasions for particular taxonomic or functional groups (Erckie et al. 2022; Erckie, 2017; Hirsch et al., 2020; Moodley et al. 2014; Matthys et al., 2022; Satishchandra and Geerts, 2020; Wansell et al., 2022).
From this research foundation, my attention has recently shifted on how to prevent future invasions and to restore or manage altered ecosystems (Ngwenya et al., 2022). The former is illustrated by a presentation entitled: “The role of phytopathogenic fungi in limiting plant invasions: The case of Australian Banksia (Proteaceae) in South Africa”, and “Bird and plant assemblages’ recovery following removal of an alien tree invader from the riparian habitat of the Berg River, South Africa” (Qongqo et al., 2022).
From the beginning of my scientific career I realised the ultimate role of society in science and nature conservation. Initially I addressed science and conservation issues in newspaper articles and radio interviews, but recently became more proactive. This has naturally extended my involvement into the research side of citizen science and this has showcased at a recent conferences: Fynbos Forum 2015 (van der Colff, Geerts, Ebrahim, Raimondo: The value of volunteers in long term data collection of threatened plant species) and SAAB 2017 (van der Colff, Ebrahim, Marinus, Kotze, Raimondo, Geerts: Population dynamics of the critically endangered Euryops virgatus Highlighting the value of long-term monitoring by citizen scientists).
Other research areas include fire ecology (Geerts, 2021; Hurzuk and Geerts, 2017), scorpions (Visser and Geerts, 2021; Visser and Geerts, 2021; Visser and Geerts, 2020; Visseret al., 2020), scale insects (Satishchandra and Geerts, 2020), secondary invasions (Nsikani et al., 2020; Geerts et al., 2022), fragmentation (Hauber et al., 2022), restoration (Nsikani et al., 2022; Holmes et al., 2022; Geerts et al., 2022), termites (Geerts et al., 2016b), genetic tools (Nitin et al., 2022), tourism (Mangachena et al., 2023) and birds (Mangachena 2017; Mangachena and Geerts 2017, 2019) or urban invaders (Afonso et al., 2020)
Student projects are disseminated in papers but also in theses, whether 4th year mini theses (see for example Matthys 2019) or masters (see for example Qongqo, 2018; Wansell, 2021; Glatthaar, 2022).
References
Adedoja, O., Erckie, L., Boatwright, J.S., van Wyk, E. and Geerts, S., 2021. An invasive alien Proteaceae lures some, but not all nectar‐feeding bird pollinators away from native Proteaceae in South African fynbos. Plant Biology, 23(6), pp.915-922.
Afonso L., Esler, K., Gaertner, M., Geerts, S. 2020. Comparing invasive alien plant community composition between urban, peri-urban and rural areas; the city of Cape Town as a case study. In: Pramit et al. (eds) Urban Ecology: Emerging Patterns and Social-Ecological Systems. Chp 13, pp 221-236. Elsevier.
Afonso, L., 2021 Urban plant invasions: how to classify, prioritize and manage invasive alien plants; Cape Town as a case study. Masters thesis Stellenbosch University Stellenbosch
Afonso, L., Esler, K., Gaertner, M. and Geerts, S., 2022. The invasive alien Hypericum canariense in South Africa: Management, cost, and eradication feasibility. South African Journal of Botany, 146, 685-694.
Datta, A., Kumschick, S., Geerts, S., Wilson J.R.U. 2020. Questions that need to be answered to identify safe cultivars of invasive plants. Neobiota 62: 81–97.
Duncan, P., Podest, E., Esler, K.J., Geerts, S., Lyons, C., 2023. Mapping Invasive Herbaceous Plant Species with Sentinel-2 Satellite Imagery: Echium plantagineum in a Mediterranean Shrubland as a Case Study. Geomatics 3 (2), 328-344.
Erckie, LNK., 2017. Impacts and control of alien Proteaceae invasion in the Western Cape Province, South Africa. Masters thesis, University of the Western Cape, Cape Town.
Erckie, L., Adedoja, O., Geerts, S., van Wyk, E., Boatwright, J. S. 2022. Impacts of an invasive alien Proteaceae on native plant species richness and vegetation structure. South African Journal of Botany 144: 332-338.
Geerts, S., Mangachena, JR., Nsikani, M. 2022. Secondary invaders in riparian habitats can remain up to 10 years after invasive alien Eucalyptus tree clearing. South African Journal of Botany, 146: 491-496.
Geerts, S., Mangachena, J.R. and Nsikani, M.M., 2022. Secondary invaders in riparian habitats can remain up to 10 years after invasive alien Eucalyptus tree clearing. South African Journal of Botany, 146, pp.491-496.
Geerts, S., Adedoja, O. 2021. Pollination and reproduction enhance the invasive potential of an early invader: the case of Lythrum salicaria (purple loosetrife) in South Africa. Biological Invasions. 23, 2961–2971.
Geerts, S , 2021. Protea maturation rates and fire return intervals in a mediterranean ecosystem: testing the rules of thumb at a local scale. International Journal of Wildland Fire, 30(12), pp.971-977.
Geerts, S., Coetzee, A., Rebelo, T., Pauw, A. 2020. Pollination structures plant and nectarfeeding bird communities in Cape fynbos, South Africa: implications for the conservation of plant-bird mutualisms. Ecological Research 35: 838-856.
Geerts, S., Rossenrode, T., Irlich, U.M., Visser, V. 2017. Emerging ornamental plant invaders in urban areas; Centranthus ruber in Cape Town, South Africa as a case study. Invasive Plant Science and Management 10 (4): 322-331
Geerts, S., Mashele, B.V., Visser, V. and Wilson, J.R.U. 2016a Lack of human assisted dispersal means Pueraria montana var. lobata (kudzu vine) could still be eradicated from South Africa. Biological Invasions 18(11): 3119-3126.
Geerts, S. 2016. Can short-billed nectar thieving sunbirds replace long-billed sunbird pollinators in transformed landscapes? Plant Biology 18 (6): 1048–1052.
Geerts, S., van der Linden, J., van der Linden, E. 2016b. The ecology and foraging behaviour of the harvester termite, Baucaliotermes hainesi in semi-arid grasslands in the northwestern interior of South Africa. Insectes Sociaux, 63(3): 457-465.
Geerts, S., Botha, P.W., Visser, V., Richardson, D. M. and Wilson, J.R.U. 2013a. Montpellier
broom (Genista monspessulana) and Spanish broom (Spartium junceum) in South Africa: an assessment of invasiveness and options for management. South African Journal of Botany, 87: 134-145.
Geerts, S., Pauw, A. 2013. Pollination structures plant and nectar-feeding bird communities in Cape Fynbos, South Africa South African Journal of Botany 86 (109), 161.
Geerts, S., Moodley, D., Gaertner, M., Le Roux, J. J., McGeoch, M., Muofhe, C., Richardson, D. M. and Wilson, J.R.U. 2013b The absence of fire can cause a lag phase the invasion dynamics of Banksia ericifolia (Proteaceae). Austral Ecology 38, 931–941.
Geerts, S., and A. Pauw. 2012. The cost of being specialised: pollinator limitation in the endangered geophyte Brunsvigia litoralis (Amaryllidaceae) in the Cape Floristic Region of South Africa. South African Journal of Botany 78: 159–164.
Geerts, S., Malherbe T., Pauw, A. 2012. Reduced flower visitation by nectar-feeding birds in response to fire in Cape fynbos vegetation, South Africa. Journal of Ornithology 153 (2): 297-301.
Geerts, S. 2011. Assembly and disassembly of bird pollination communities at the Cape of Africa (Ph.D thesis). Stellenbosch University, Stellenbosch
Geerts, S., and A. Pauw. 2011b. Farming with native bees (Apis mellifera subsp. capensis) has varied effects on nectar-feeding bird communities in South African fynbos vegetation. Population Ecology 53:333–339.
Geerts, S., and A. Pauw. 2011a. Easy technique for assessing pollination rates in the genus Erica reveals road impact on bird pollination in the Cape fynbos, South Africa. Austral Ecology 36, 656–662.
Geerts, S., and A. Pauw. 2009a. Hyper-specialization for long-billed bird pollination in a guild of South African plants: the Malachite Sunbird pollination syndrome. South African Journal of Botany 75:699-706.
Geerts, S., and A. Pauw. 2009b. African sunbirds hover to pollinate an invasive hummingbird-pollinated plant. Oikos 118:573-579.
Geerts, S., and A. Pauw. 2009c. Does farming with native honeybees affect bird pollination in Cape fynbos? South African Journal of Botany 2 (75), 401-402
Glatthaar, C.B. 2022. Barking up the wrong tree: characterizing farmers, farms, and a behavioural framework regarding livestock guardian dog use in South Africa. MSc thesis, Cape Peninsula University of Technology, Cape Town.
Hauber, S.J., Maier S. L., Adedoja, O., Gaertner, M., Geerts, S. 2022. Mixed effect of habitat fragmentation on pollinator visitation rates but not on seed production in Renosterveld of South Africa. South African Journal of Botany 146: 48-57.
Heystek, A., Geerts, S., Barnard, P., Pauw, A. 2014. Pink flower preference in sunbirds does not translate into plant fitness differences in a polymorphic Erica species. Evolutionary Ecology 28: 457-470.
Hirsch, H., Allsopp, M., Canavan, S., Cheek, M., Geerts, S., Geldenhuys, C.J., Harding, G., Hurley, B.P., Jones, W., Keet, J.-H., Klein, H., Ruwanza, S., van Wilgen, B.W., Wingfield, M.J. & Richardson, D.M. 2020. Eucalyptus camaldulensis in South Africa – past, present, future. Transactions of the Royal Society of South Africa, 75(1): 1-22.
Holmes, P.M., Esler, K.J., Geerts, S., Ngwenya, D.K., Dorse, C., van der Merwe, J., Retief, K., Hall, S.W. and Grey, P., 2022. Guidelines for Restoring Lowland Sand Fynbos Ecosystems. SANBI. [online]. Available from: http://opus.sanbi.org/jspui/ handl e/20.500.12143/ 8382
Hurzuk, AAH and Geerts, S. 2017. Permanent Protea (Proteaceae) monitoring plots and fire frequency in Fynbos. South African Journal of Botany 100 (109), 335
Satishchandra, K.N, Geerts, S. 2020. Modeling the distribution of the invasive alien Cycad Aulacaspis Scale, Aulacaspis yasumatsui (Hemiptera: Diaspididae) in Africa under current and future climate scenarios. Journal of Economic Entomology 113(5), 2276–2284.
Le Roux, J. J., Geerts, S., Ivey, P., Krausse, S., Richardson, D. M., Suda, J., Wilson, J.R.U. 2010. Molecular systematics and ecology of invasive Kangaroo Paws in South Africa: management implications for a horticultural important genus. Biological invasions 12:3989
4002.
Mangachena, J.R., 2017. Response of Bird Assemblages to the Invasion and Clearing of Alien Trees in the Western Cape, South Africa. Dissertation, Cape Peninsula University of
Technology, South Africa.
Mangachena, J.R. Geerts, S. 2017. Invasive alien trees reduce bird species richness and abundance of mutualistic frugivores and nectarivores; a bird’s eye view on a conflict of interest species in riparian habitats. Ecological Research 32(5), 667-676.
Mangachena, J.R. Geerts, S. 2019. The absence of keystone indigenous trees inhibits bird recovery up to a decade after invasive tree removal from riparian habitats. Acta Oecologica 101, 103483.
Mangachena, JR, Geerts, S., Pickering, CM. 2023. Spatial and temporal patterns in wildlife tourism encounters and how people feel about them based on social media data from South Africa. Journal of Outdoor Recreation and Tourism, 100642.
Matthys, C., 2019. Distribution, invasive potential and management of Callistemon rugulosus (Link) Craven (Myrtaceae) in South Africa. BTech Thesis, Cape peninsula university of Technology, Cape Town
Matthys, C., Jubase, N., Visser, V. and Geerts, S., 2022. Distribution of Melaleuca rugulosa (Schlechtendal ex Link) Craven (Myrtaceae) in South Africa: Assessment of invasiveness and feasibility of eradication. South African Journal of Botany, 148, pp.228-237.
Mnisi, B.E., Geerts, S., Smith, C., Pauw, A., 2021. Nectar gardens on school grounds reconnect plants, birds and people. Biological Conservation 257, 109087.
Moodley, D., Geerts, S., Richardson, D. M. and Wilson, J.R.U. 2016. The importance of pollinators and autonomous self-fertilization in the early stages of plant invasions: Banksia and Hakea (Proteaceae) as case studies. Plant Biology 18(1): 124–131.
Moodley, D., Geerts, S., Rebelo, T., Richardson, D. M. and Wilson, J.R.U. 2014. Site-specific conditions influence plant naturalization: The case of alien Proteaceae in South Africa. Acta Oecologia 59: 62-71.
Moodley, D., Geerts, S., Richardson, D. M. and Wilson, J.R.U. 2013. Different traits determine introduction, naturalization and invasion success in woody plants: Proteaceae as a test case. PLoS ONE 8(9): e75078.
Ngwenya, D.K., Holmes, P.M., Geerts, S., Esler, K.J. 2023. Scaling up restoration efforts by simulating the effects of fire to circumvent prescribed burns when preparing restoration sites in South African fynbos ecosystems Austral Ecology 48 (1), 158-181
Nitin, K. S., T. S. Masehela, A. K. Chakravarthy, and S. Geerts. 2022 Management of Pests Using Genetic Tools in Africa, In: A. K. Chakravarthy (ed.), Genetic Methods and Tools for Managing Crop Pests. Chp 10, P303-326. Springer
Nsikani, M. M., Geerts, S., Ruwanza, S., Richardson, D. 2020. Secondary invasion and weedy native species dominance after clearing invasive alien plants in South Africa: status quo and prognosis. South African Journal of Botany 132: 338-345.
Nsikani, M. M., Anderson, P., Bouragaoui, Z., Geerts, S., Gornish, E. S., Kairo, J. G., Khan, N., Madikizela, B., Mganga, K. Z., Ntshotsho, P., Okafor-Yarwood, I., Webster, K. M. E., & Peer, N. (2022). UN Decade on Ecosystem Restoration: key considerations for Africa. Restoration Ecology, e13699. https://doi.org/10.1111/rec.13699
Qongqo, A 2018 Introduction pathways of phytopathogenic fungi and their potential role in limiting plant invasions: the case of Banksia spp. (Proteaceae) in the Cape Floristic Region. MSc. Thesis. Cape Peninsula University of Technology (Cape Town).
Qongqo, A., Nchu, F., Geerts, S. 2022. Relationship of alien species continues in a foreign land: The case of Phytophthora and Australian Banksia (Proteaceae) in South African Fynbos. Ecology and Evolution, 12 (7).
Ollerton, J., Watts, S., Connerty, S., Lock, J., Morgan, L., Wilson, I., Schueller, S., Nattero, J., Cocucci, A.A., Izhaki, I., Geerts, S., Pauw, A. and Stout, J.C. 2012 Pollination ecology of the invasive tree tobacco Nicotiana glauca: comparisons across native and non-native ranges. Journal of Pollination Ecology, 9(12): 85-95
Shaw, K., 2023. Quantifying pollen transfer between cultivated and wild Cyclopia species in South Africa. MSc thesis, Cape Peninsula University of Technology, Cape Town.
Van der Colff, D., Ebrahim, I., Marinus, E., Kotze, D., Raimondo, D., & Geerts, S. 2017. Population dynamics of the Critically Endangered Euryops virgatus: Highlighting the value of long-term monitoring by citizen scientists. South African Journal of Botany, 100(109), 372-373.
Visser, and Geerts. 2021. Sexual dimorphism and static allometry in the South African scorpion Opistophthalmus karrooensis. Arachnology 18 (9), 1057–1063
Visser, J., Geerts, S. 2021. Static allometry and sexual dimorphism in the Striped Lesserthicktail Scorpion Uroplectes lineatus. Arachnology 18 (7), 700-707
Visser, J., Geerts, S. 2020. Describing sexual dimorphism and fine scale spatial distributions in the Drab Thick-tail Scorpion, Parabuthus planicauda. African Zoology 55 (3), 250-256.
Visser J.H., Geerts S., Jansen van Vuuren, B. 2020. Phylogeographic Patterns in a SemiLithophilous Burrowing Scorpion, Opistophthalmus pallipes, from South Africa. Zoological Science 38(1):36-44.
Wansell, S.N.L., 2021. The invasion ecology of Pontederia cordata L.(Pontederiaceae) in South Africa . Dissertation, Mmasters thesis, Rhodes University.
Wansell, S N L., Geerts, S., Coetzee, J.A. 2022. Where are the seeds? Lack of floral morphs prevent seed production by the tristylous Pontederia cordata in South Africa. Ecology and Evolution 12 (10): e9366