Prof. Geerts

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Within the ecological field of plant-animal interactions, I am interested in pollination and the change and breakdown of pollination communities due to anthropogenic impact.

Another main research area involves alien invasive plants, and in particular emerging invasive plant species. I also have an interest in the long term impact of reduced pollination on plants. This is the case for some species in fragmented habitat. Whether these populations are heading for extinction, I try to answer with a demographic modelling approach.

Current projects include assessing the reproduction strategies of invasive plant species in South Africa with currently restricted distributions and determine whether there is a lag phase because plants are self-incompatible. Other research projects involve the restoration of pollinator migration routes by creating stepping stones in urban environments (

Sjirk Geerts Research Overview

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Selected publications

Geerts, S. Testing the rules of thumb at a local scale: Protea maturation rates and fire return intervals in South Africa Fynbos (submitted).

Bongani E. Mnisi, Sjirk Geerts, Ceinwen Smith, Anton Pauw. Nectar gardens on school grounds reconnect plants, birds and people (Submitted).

Sjirk Geerts, Opeyemi Adedoja. Pollination and reproduction at the early stages of a plant invasion: the case of Lythrum salicaria (purple loosetrife) in South Africa (submitted).

Visser and Geerts Sexual dimorphism and fine-scale distribution in a burrowing scorpion from South Africa, Opistophthalmus pallipes (Submitted).

Opeyemi Adedoja, Laimi Erckie, J. Stephen Boatwright, Ernita van Wyk, Sjirk Geerts. Invasive alien Proteaceae lure some, but not other nectar feeding bird pollinators away from native Proteaceae in South African fynbos (Submitted)

Visser, J., Geerts, S. (accepted). Static allometry, sexual dimorphism and a heterogeneous fine-scale spatial distribution in the lapidicolous Striped Lesser-thicktail Scorpion, Uroplectes lineatus.

Kanle Satishchandra Nitin and Sjirk Geerts. 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.

Datta, A., Kumschick, S., Geerts, S., Wilson J.R.U. (accepted) Questions that need to be answered to identify safe cultivars of invasive plants. Neobiota 62: 81–97.

Visser J.H., Geerts S. & Jansen van Vuuren, B. (accepted). Phylogeographic patterns in a semi-lithophilous burrowing scorpion from South Africa, Opistophthalmus pallipes. Zoological Science.

Visser, J., Geerts, S. 2020. Describing sexual dimorphism and fine scale spatial distributions in the Drab Thick-tail Scorpion, Parabuthus planicauda. African Zoology. 18:1–7.

Nisikani, 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.

Geerts, S., Coetzee, A., Rebelo, T., Pauw, A. 2020. Pollination structures plant and nectar-feeding bird communities in Cape fynbos, South Africa: implications for the conservation of plant-bird mutualisms. Ecological Research 2020: 838-856.

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.

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.

Novoa, A., Shackleton, R., Canavan, S., Cybèle, C., Davies, S., Dehnen-Schmutz, K., Fried, F., Gaertner, M., Geerts, S., Griffiths, C.,  Kaplan, H.,  Kumschick, S., Le Maitre, D.,  Measey, J., Nunes, A.L., Richardson, D.M., Robinson, T.B., Touza, J., Wilson, J.R.U. 2018. A framework for engaging stakeholders on the management of alien species. Journal of Environmental Management 205(1) 286-297.

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

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.

Geerts, S., Mashele, B.V., Visser, V. and Wilson, J.R.U.  2016. Lack of human assisted dispersal means Pueraria montana var. lobata (kudzu vine) could still be eradicated from South Africa. Biological Invasions DOI 10.1007/s10530-016-1226-y

Geerts, S., 2016.Can short-billed nectar thieving sunbirds replace long-billed sunbird pollinators in transformed landscapes? Plant Biology doi:10.1111/plb.12474.

Geerts, S.,van der Linden, J., van der Linden, E. 2016. 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.

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. 

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.

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. doi:10.1371/journal.pone.0075078

Geerts, S., Botha, P.W., Visser, V., Richardson, D. M. and Wilson, J.R.U. 2013. 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., Moodley, D., Gaertner, M., Le Roux, J. J., McGeoch, M., Muofhe, C., Richardson, D. M. and Wilson, J.R.U. 2013. The absence of fire can cause a lag phase—the invasion dynamics of Banksia ericifolia (Proteaceae). Austral Ecology 38, 931–941.

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.

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, A. Pauw. 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., and A. Pauw. 2011 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. 2011 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.

JJ Le Roux, S Geerts, P Ivey, S Krausse, DM Richardson, J. Suda, JRU Wilson 2010. Molecular systematics and ecology of invasive Kangaroo Paws in South Africa: management implications for a horticultural important genus. Biological invasions 12:3989–4002.

Geerts, S., and A. Pauw. 2009. 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. 2009. African sunbirds hover to pollinate an invasive hummingbird-pollinated plant. Oikos 118:573-579.

Book Chapters

Luca Afonso, Karen Esler, Mirijam Gaertner Sjirk Geerts. 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.

Le Roux J.J, Susanna Clusella-Trullas, Thabiso M. Mokotjomela, Mario Mairal, David M. Richardson, Lisa Skein, John R.U. Wilson, Olaf Weyl, Sjirk Geerts. 2020. Biotic Interactions as Mediators of Biological Invasions: Insights from South Africa. In: van Wilgen B., Measey J., Richardson D., Wilson J., Zengeya T. (eds) Biological Invasions in South Africa. Invading Nature - Springer Series in Invasion Ecology, vol 14 pp 387-427. Springer, Cham

Holmes, P.M., Karen J. Esler, Mirijam Gaertner, Sjirk Geerts. David M. Richardson, Sheunesu Ruwanza, Stuart A. Mlungele M. Nsikani. 2020. Biological invasions and ecological restoration. In: van Wilgen et al (eds) Biological Invasions in South Africa. Invading Nature - Springer Series in Invasion Ecology, vol 14 pp 665-700. Springer, Cha

John R. Wilson, Llewellyn C. Foxcroft, Sjirk Geerts; M. Timm Hoffman, Sandra MacFadyen, G. John Measey, Anthony Mills, David M. Richardson, Mark P. Robertson, Brian W. van Wilgen. 2020. The role of environmental factors in promoting and limiting biological invasions in South Africa. In: van Wilgen et al (eds) Biological Invasions in South Africa. Invading Nature - Springer Series in Invasion Ecology, vol 14 pp 355-385. Springer, Cham.