ONGOING AND PAST PROJECTS

I am mostly interested in understanding how alien species establish, spread and impact biodiversity and ecosystem structure and functioning. More recently, how we can use this ecological theory to guide restoration efforts and management practices toward the desired outcomes. I use aquatic plants, organisms associated and their interactions as a model system. I use a combination of mesocosm and field experiments, field sampling and quantitative tools to address research questions.  

Areas of interest

Biological invasions | Aquatic macrophyte ecology | Plant-animal interactions | Priority effects | Biotic resistance | Herbivory | Biological control | Restoration ecology | Aquatic ecology | Freshwater ecosystems | Wetlands

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ACHIEVING THE GOAL "MORE NATIVES, LESS INVASIVES": PRIORITY EFFECTS AS A POTENTIAL TOOL FOR RESTORATION

2020 - present

Collaborators: Dr Samuel Motitsoe (CBC, Rhodes University), Prof. dr. Julie Coetzee (CBC, Rhodes University), Prof. dr. Martin Hill (CBC, Rhodes University), Samella Ngxande-Kosa (CBC, Rhodes University), Prof. dr. Ryan Wasserman (Department of Zoology, Rhodes University), Dr Chad Keates (Department of Zoology, Rhodes University)

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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DOES BIODIVERSITY RECOVER AFTER BIOLOGICAL CONTROL OF INVASIVE AQUATIC PLANTS? A STRUCTURAL EQUATION MODELLING APPROACH

2021 - present

Collaborators: Dr Samuel Motitsoe (CBC, Rhodes University), Matthew Paper (CBC, Rhodes University), Prof. dr. Julie Coetzee (CBC, Rhodes University), Prof. dr. Martin Hill (CBC, Rhodes University),

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MADMACS: MASS DEVELOPMENT OF AQUATIC MACROPHYTES - CAUSES AND CONSEQUENCES OF MACROPHYTE REMOVAL FOR ECOSYSTEM STRUCTURE, FUNCTION, AND SERVICES

2020 - present

Collaborators: Norwegian Institute for Water Research (NIVA), Norwegian University of Life Sciences (NMBU), Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB), University of Rennes1, 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).

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BIOTIC RESISTANCE TO ALIEN PLANT INVASIONS IN TROPICAL AND TEMPERATE FRESHWATER ECOSYSTEMS

June 2015 - June 2019

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

Daily supervisor: Prof. dr. 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).

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INFLUENCE OF HERBIVORE DAMAGE ON METHANE EMISSIONS FROM EMERGENT AQUATIC MACROPHYTES

March 2013 - March 2015

Supervisor: Prof. dr. Francisco de Assis Esteves (Federal University of Rio de Janeiro)

Co-supervisor: Dr Anderson da Rocha Gripp (Federal University of Rio de Janeiro)

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.