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Biological invasions | Disturbance | Herbivory | Aquatic macrophyte ecology | Freshwater biodiversity | Plant-animal interactions | Priority effects | Biotic resistance | Biological control | Invasive plant management | Restoration ecology | Aquatic ecology | Freshwater ecosystems | Wetlands | Conservation



2020 - present

Collaborators: Dr Samuel Motitsoe (CBC, Rhodes University), Prof. Julie A. Coetzee (CBC, Rhodes University), Prof. Martin Hill (CBC, Rhodes University), Samella Ngxande-Kosa (Technical staff), Prof. Ryan Wasserman (Department of Zoology, Rhodes University), Dr Chad Keates (Department of Zoology, Rhodes University), Lulutho Mancunga (Master student), Getrude Tshithukhe (PhD student/Technical staff), Sive Kolisi (Master student/Technical staff), Sonwabise Maneli (Master student/Technical staff)

Invasive plant species are among the major threats to biodiversity, strongly negatively affecting ecosystem structure and functioning. The control of invasive plants is often seen as beneficial but following removal, the plant communities do not always re-established nor do ecosystems recover. Active restoration of native plant communities is recognized as a strategy to limit invasions. However, restoration attempts which most commonly add species at the same time in a single seed mix or propagules have shown only moderate success. These unsatisfying results are often due to a failure to account for priority effects. Priority effect is the effect of species on the survival, growth or reproductive success of other species depending on the order and timing in which they arrive at a site.  Such priority effects occur either because the early-arriving species reduce the resource amount available for late-arriving species (niche pre-emption) or because the early-arriving species modified the niches available for the species arriving later (niche modification). ‘Being first’ however does not guarantee success and many factors may affect the strength of priority effects such as the overlap between competitive species in resource needs and the impact a species has on resource levels. Surprisingly, priority effects have only recently been considered for restoration practices and remains little explored, especially in freshwater systems. Therefore, the aim of this project is to evaluate the importance of priority effects of native plant species and the factors affecting its strength and direction in order to enhance ecological resistance of freshwater ecosystems to avoid plant reinvasions and secondary invasions.  In addition, phytoplankton, periphyton, zooplankton, macroinvertebrate, amphibian and microbial communities will be monitored as well as water quality parameters. This will allow us to assess how other community parameters and abiotic variables change during invasive plant control management and native vegetation re-establishment. For this, a whole-pond manipulation experiment has been set up (n=5). This project will advance our understanding on how we can guide restoration efforts in a way to maximize the likelihood of desired species establishment by strengthening native plant species priority effects to curb future invasions. 

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2021 - present

Collaborators: Dr Samuel Motitsoe (CBC, Rhodes University), Matthew Paper (CBC, Rhodes University), Prof. Julie A. Coetzee (CBC, Rhodes University), Prof. Martin Hill (CBC, Rhodes University), Getrude Tshithukhe (CBC, Rhodes University), Hlumelo Theo Mantshi (Honours student)

Invasions of freshwater bodies by floating vegetation, including giant salvinia (Salvinia molesta) threaten biodiversity globally. The large, dense mats formed by these plants can reduce light and nutrient availability, change pH and other water quality parameters. These plants are also able to produce and release allelopathic compounds gaining a competitive advantage over other primary producers. The inhibition of aquatic plants, for example, decrease habitat complexity and complex habitats promote the diversity of species and availability of resources, hence negatively affecting higher trophic levels. A major incentive for managing invasive floating plant species is to alleviate or reverse these impacts on freshwater systems. Invasive plant control through classical biological control using host specific natural enemies have successfully reduced many of these invasions and it is often considered one of the first steps in enabling recovery of invader-dominated sites. After successful control, it is often expected the recovery of native assemblages assuming that ecological communities are resilient to invaders and such removal will allow natural communities to recover to a pre-invaded condition. However, management success is often not assessed beyond if the plant invader has been successfully controlled. Thus, little is known about the response of aquatic plant communities and other natural assemblages following this practice. To address this deficit, we investigate aquatic plant and associated organisms responses to biological control of Salvinia molesta by the weevil Cyrtobagous salviniae in five freshwater systems located in Western and Eastern Cape regions of South Africa. Our main goal is to determine whether native aquatic plants and associated organisms recover after invasive floating plant control and determine the environmental factors driving this recovery. To disentangle the factors by which floating invasive plant species and its control affect aquatic plant communities, structural equation modelling (SEM) will be used. SEM is a powerful tool to test and evaluate multivariate causal relationships in complex natural systems. This study can provide management and research recommendations to improve restoration outcomes. 

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2020 - 2022

Collaborators: Norwegian Institute for Water Research (NIVA), Norwegian University of Life Sciences (NMBU), Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB), University of Rennes, UMR ECOBIO (ECOBIO), Universidade Federal do Paraná (UFPR)

Mass development of aquatic macrophytes (water plants) in rivers and lakes is a worldwide problem, and substantial resources are spent annually on removal of macrophytes. This approach, however, does not address the causes of the mass development and is not sustainable. Macrophyte stands either quickly grow back, or the removal causes other problems to surface (e.g. the mass development of algae or cyanobacteria). Macrophyte mass developments have known negative effects, but well-developed macrophyte stands also provide many ecosystem services, including nutrient and carbon retention (= purification of water), as well as providing shelter and nursery habitat for many organisms (= affecting biodiversity). The ecosystem services provided by macrophytes are often poorly known to the public or to water managers. Consequently, management decisions, despite being costly, are generally based on a prevailing intuitive negative perception rather than a rational knowledge-based decision. The specific regional reasons for macrophyte mass development are still poorly understood, likely because there is typically a combination of factors which together cause nuisance growth (multiple pressures). This makes analysis of causes of nuisance growth at a particular site challenging. Also, there is a lack of standardized before-after-control-impact (BACI) studies on the direct and indirect costs of macrophyte removal (= loss of ecosystem services provided by macrophytes) across multiple sites. This greatly hampers the possibility to generalize results, and makes giving general management advice difficult. In our project, we aim to address the following questions: 1) Which combination of natural conditions and pressures leads to undesired mass development of macrophytes? 2) What are the direct and indirect consequences of macrophyte removal for ecosystem functions and services? Which consequences of macrophyte removal are site-specific, and which are general? In collaboration with key stakeholders, we will execute a set of “real-world experiments” in a harmonized BACI design across six case studies in five countries (Norway (2), Germany, France, South Africa, Brazil).



June 2015 - June 2019

Supervisor: Prof. Ellen van Donk (NIOO, Utrecht University)

Daily supervisor: Prof. Liesbeth S. Bakker (NIOO, Wageningen University)

Co-supervisor: Dr Casper H. A. van Leeuwen (Netherlands Institute of Ecology (NIOO))

In this project, I focus on biotic resistance and the role of species interactions in reducing the success (colonization and performance) of alien species invasions. The major aim is to determine whether tropical and temperate native freshwater species communities can provide biotic resistance to alien plant invasions and to understand the underlying mechanisms in freshwater ecosystems. To study this, I used tropical and temperate submerged plant species and an aquatic generalist herbivore as a model system. My approach is a combination of mesocosm experiments and published evidence to answer my two main research questions:

I- Can native communities provide biotic resistance to alien plant invasions in freshwater ecosystems?

II- Which mechanisms are underlying biotic resistance to aquatic plant invasions?

I consider native community susceptibility to invasion (invasibility) as well as alien plant attributes which increase their likelihood to establish and potentially become invasive (invasiveness).



March 2013 - March 2015

Supervisor: Prof. Francisco de Assis Esteves (Universidade Federal do Rio de Janeiro (UFRJ))

Co-supervisor: Dr Anderson da Rocha Gripp (Universidade Federal do Rio de Janeiro (UFRJ))

Wetlands are the largest natural methane (CH4) source and the vegetated littoral areas are the major contributors for CH4 release from sediment to the atmosphere. Although the effects of herbivores on biomass removal, growth and reproduction of emergent macrophytes have been well documented, their effect on plant-mediated CH4 fluxes, especially by insects, remains unknown. We performed a mesocosm experiment in which we simulated the damage caused by herbivorous insects and manipulated the density of damaged culms of Eleocharis equisetoides (4 levels — 0, 20, 50 and 100%) measuring the corresponding CH4 emission, concentration and potential production in the sediment. We hypothesized that an increased percentage of culms with simulated herbivory would be associated with increased CH4 fluxes from sediment toward the atmosphere. Simulated herbivory positively affected CH4 emissions, but only under high herbivory pressure. The average CH4 flux from mesocosms with 50% and 100% damaged culms was 3.5 higher than those with intact or low levels of damage. These results indicate that physical damage on macrophytes affects gas transport within the plants. A field survey in our studied system revealed that plant biomass consumed by herbivores is relatively low. This result highlight that insects may have a disproportional effect on CH4 emissions, i.e., a very small damage (low biomass removal), when performed in many culms (50% and 100% of damaged culms treatments), may substantially increase CH4 fluxes. In summary, our findings bring a new perspective to the influence of herbivory on CH4 and carbon cycling, especially regarding the role insects might play.

Research: Research
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