My Research

My research has focused on the plant–animal interactions in novel ecosystems in the Cape. On this

foundation my research is built on two pillars namely pollination and invasive alien plant (IAP) species and restoration (IAP management and restoration go together in my view) and the interaction between these. A third pillar which since my PhD and up till recently taken a back seat, namely plant demography, but is coming to the forefront again now that some of our long-term datasets (some 10 year datasets) are becoming complete.  My research spans a broad range of topics within these overarching themes. This is both an outflow of my broad interest in any ecological question related to the cape flora, and requests from part time industry employed students bringing their questions and expertise to do a research project of their interest.

Pollination research

With the current global decline in pollinators, and the concurrent decline in plant species, pollination research is becoming increasingly important. I have largely focused on anthropogenic impacts on pollination. For example 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 2009, Geerts 2016). This led to a community project, in which we create stepping stones for pollinators in urban areas (Mnisi, Geerts et al 2021 Biological Conservation). Together with this, my long standing interest in how pollinator communities are shaped, and in particular plant species and nectar feeding birds was published a few years ago (Geerts et al 2020, Ecological Research). From this I started to explore biome scale climate change disruptions of pollination, and the potential mismatch in mutualist partners under different climate change scenarios. A postdoc did these analyses in which we modelled mismatched under climate change for 71 Fynbos Proteaceae species and their mutualistic partner and pollinators, nectar feeding birds. We have addressed reviewer comments for this manuscript that should soon be published in the Journal of Biogeography. More recently I have branched out away from fynbos and renosterveld, since pollination in the forests of the CFR are largely unknow (only detailed paper available is from the 1930’s!). This work is driven by a tree climbing postdoc, and the first pollination manuscript is nearly ready for submission, whilst a broader review on forests and mutualisms have been submitted to Annals of Botany.

On the more applied side, and on request from the Overberg Renosterveld Conservation Trust, we studied whether there is conservation value in small isolated renosterveld habitat fragments from a plant-pollinator-perspective (Hauber et al 2022).  Current we have addressed reviewer comments on a paper in South African Journal of Botany) applied research includes determining the pollinators of commercial Cyclopia species (Honeybush) as part of a project to prevent genetic contamination of wild honeybush. This project is funded by the Honeybush industry and required us to determine the pollinators of honeybush in plantations and in the wild. Subsequently determine whether there is pollen transfer between wild and cultivated honeybush (playing with minute radio trackers on carpenter bees), determine the potential for hybridisation and based on all this information we developed a Honeybush planting protocol. The planting protocol has been workshopped at conferences (such as MEDECOS 2022) and Honeybush farmer meetings (AGM 2022 and others) and has developed into a practical tool to safeguard genetic diversity of our Honeybush.

I have submitted a funding application to the NRF to continue with the Honeybush research. The main focus will be to continue with pollination aspect, with a particular focus on the threatened honeybush species but also expand into the social component and evaluate community honeybush farming initiatives. For the more theorical pollination aspects, pollination network responses to anthropogenic pressures such as fragmentation, climate change, changes in fire frequencies (see my papers on fire ecology; Geerts 2020; Geerts et al 2012 etc). All of which are important topics to inform conservation of the Cape flora, but also closely align with the research agendas of a number of UCT academics. Which in fact is the university closest to me, but surprisingly even though I have had discussion with a number of academics, published together and submitted funding proposals together, the active research collaboration is the most limited of all the Western Cape universities (including NMU). 

Invasive alien plant (IAP) and restoration research

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. With a number of students, I have 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, 2014, Geerts et al 2013, Qongqo et al 2022) and Myrtaceae (Morapi in prep.). My applied research on IAPs has focused on the impacts and best management approaches for already widespread invaders (see for example Afonso 2020, Erckie et al 2022, Duncan et al. 2023), but mostly on emerging invaders (see for example Afonso et al 2022, Geerts et al 2013, 2016, 2017, Matthys et al 2022, etc.). Future work include using Inaturalist, a citizen science platform, to inform new alien plant naturalisations. This is modelling intensive and driven by a postdoc and Cang Hui.  Recently I  become interested in the role of horticulture (Datta et al 2020), and this resulted in a PhD student, Duran Chetty, under my supervision, in which we explore the reasons for, and the importance of, sterile cultivars in alien horticultural species.  

From this research foundation, my attention has recently shifted on how to prevent future invasions and to restore or manage altered ecosystems. This includes the problem of secondary invaders in restoration (Nsikani et al 2020, Geerts et al 2022) but also considering the status quo and the way forward in a African restoration context from which a paper titled: ‘UN Decade on Ecosystem Restoration: key considerations for Africa” (Nsikani et al 2023) was the outcome of a collaborate effort.

More practical, my research informs management at in various ways, such as the role of fire in restoring fynbos vegetation (Ngwenya et al 2023), scaling up restoration (Ngwenya unpublished PhD thesis), testing different restoration approaches and determining the most cost effective methods (Retief et al. in which we are addressing reviewers comments for Restoration Ecology at the moment), but also exploring optimal restoration practices and in a large scale field experiment testing minimum seed amounts required for optimum restoration success (M student Thabang). Our findings so far show that by even using less than half the suggested number of seeds, restoration outcomes are similar. Which implies that since seeds are the limiting factor in these systems, restoration areas can be doubled.

Other current research includes assessing 10 years of detailed plot data over an entire fire cycle to determine optimal fire frequency for renosterveld vegetation (M student Penelope), testing treatments to limit phytophthora impacts on silver trees (M student Thamsanqa), and exploring impacts of invasive alien trees on mammal communities (M student Tashreeqah).

Lastly, I was involved in writing the guidelines for lowland fynbos restoration (Holmes et al 2022).

Pollination combined with invasive alien plant/restoration research

The pillars of pollination and IAP’s/restoration are also bridged in my research. For example 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. 2013 SAJB), floricultural species (Geerts et al. 2013 Austral Ecology) and species introduced for agriculture (Geerts et al. 2016 Biological Invasions). the role of pollinators during plant invasions (for example: Moodley, Geerts et al. 2016 Plant Biology), a review on the role of mutualisms in invasions in South Africa (Le Roux, Geerts et al 2020), and how pollination influence invasions (for example: Wansell, Geerts et al 2022).  More recently I am exploring how multiple mutualistic partners (in my case below and above ground mutualisms) prevent or enhance invasions (Geerts and Le Roux in prep). Due to the lack of guidelines, I have ventured into the field of the role of pollination in active restoration (Louw, Nsikani and Geerts in review Plant Biology).

Plant demography research

Plant demography research, in particular on long term datasets, is largely lacking in the CFR. That whilst there are so many plant species in decline, and long-term data sets are available for some. As part of my PhD thesis, I explored plant demography. Subsequently I have a draft manuscript that is based on 10 years of plant monitoring data at one population of a Cape geophyte, on which a snapshot with 2 years of data for 34 other populations is based. What is novel is that I have pollination data for these as well. The demographic data in itself will provide practical guidelines on whether projects such as reintroducing eland into small urban reserves has an influence on sun loving geophytic species that are out shaded by shrubs in the absence of fire/herbivores. From close collaborations with CREW, long term datasets for a number of threatened species are available. My goal here is to apply matrix modelling to highly threatened fynbos plant species (such as Euryops virgatus with about 2000 or so individuals left), to target the most effective conservation interventions. For example, for Euryops virgatus we have marked plants permanently, marked seedlings permanently, planted seeds, germinated seeds in the greenhouse and planted seedlings in the field etc. since 2013. This provides important conservation information by itself, but combining all these variables into demographic models will provide important insights into the effectiveness of the different conservation strategies.