Nederlands
Plant strategies under herbivore pressure (8 Topics)
1. How tasty are water plants? Physicochemical determinants and ecological implications of macrophyte palatability to water birds
Key words
Anti-herbivore defences; Freshwater macrophytes; Plant-animal interactions; Water birds.
General context
Herbivory plays an important part in the abundance and geographical distribution of plant species. Plant palatability is therefore regarded as a key functional trait. Some plant species can effectively decrease their palatability, and therefore reduce herbivore consumption through (1) morphological adaptations and (2) deterrent chemical compounds. Such species should have the ability to persist in habitats where herbivore pressure is high, even under resource-limited conditions. In contrast, highly palatable species should be restricted to habitats with either low herbivore pressure or where productivity is sufficiently high to allow plants to compensate for biomass losses.
Project outline
The aim of this study is to determine the palatability of twenty common aquatic macrophyte species to waterfowl, and to relate their palatability to the traits of the plant. For this purpose, a standardized palatability trial using coot (Fulica atra) will be developed. In parallel, both physical (e.g. tissue toughness) and chemical (e.g. C:N ratio, phenolic content) attributes of the plant species will be measured in the laboratory. The palatability values will also be compared to the ones previously collected with invertebrate grazers, and related to the distribution of the plant species along nutrient and disturbance gradients.
Duration (indicative)
6 Months starting as soon as possible.
Contact
Marcel Klaassen (e-mail: m.klaassen@nioo.knaw.nl; tel.: +31 (0)294 239 317).
2. Effect of invertebrate herbivores on the structure of macrophyte communities
Key words
Community ecology; Freshwater macrophytes; Plant-animal interactions; Seedlings.
General context
In freshwater ecosystems, it is considered that the effect of invertebrate grazers on the structure of macrophyte communities is marginal, as the biomass they consume is generally low in comparison to vertebrates such as waterfowl. Numerous studies have shown that, except crayfish, invertebrates have actually little influence on the fitness of mature macrophytes. But surprisingly, virtually nothing is known about invertebrate herbivory on seedlings in freshwater habitats, whereas it has been recognized as an important process in terrestrial habitats. By consuming selectively the seedlings of the most palatable species, invertebrate grazers may have a strong qualitative effect on freshwater macrophyte communities, despite a low absolute biomass consumption.
Project outline
This study aims at determining the effect of invertebrate herbivory during the regenerative phase on the ultimate structure of macrophyte communities. The following hypotheses will be tested: (1) the presence of invertebrate grazers during the regenerative phase has an effect on the ultimate composition of plant communities, (2) this effect increases with invertebrate density, and (3) pioneer plant species with small propagules are the most affected by invertebrate grazing. To test these hypotheses, sediment will be sampled at the end of winter from ditches chosen for their diversified aquatic vegetation. Sediment samples will be put in mesocosms under laboratory conditions to allow the development of plant propagules. The grazing pressure will be modulated by setting various densities of pond snails (Lymnaea stagnalis). The development of aquatic vegetation in mesocosms will be monitored during 3-4 months. At the end of this period, the plants will be harvested, sorted by species, and their biomass will be determined. The effect of invertebrate herbivory will be assessed by comparing the biomass of the different plant species under the different grazing pressures. The sensitivity of the species to grazing will be related to their mode of reproduction (sexual or vegetative) and to the size of their propagules.
Duration (indicative)
6 Months, starting as soon as possible.
Contact
Marcel Klaassen (e-mail: m.klaassen@nioo.knaw.nl; tel.: +31 (0)294 239 317).
3. Coexistence of plants under disturbance? Modelling plant competition in disturbed habitats
Key words
Competition; Plant-animal interactions; Spatial grid-based model; Disturbance regime.
General context
The question why plants coexist has been of continuing interest to ecologists. Interactions between plants alone can not fully explain why some species co-exist. Other processes, such as herbivory and environmental fluctuations by disturbances seems to be of curcial importance. Herbivory might promote coexistence by providing local refuges for the inferior competitor by removing the superior competitor. Alternatively, when a more abundant species is more susceptible to predation, competing species may coexist because the predator provides an advantage to the minority. Disturbances share some characteristics with herbivory, they are local and potentially reduce competition by decreasing local density. Furthermore, species might differ in their susceptibility to damage. Thus, disturbances may support coexistence of otherwise mutually exclusive species by favouring a weaker competitor or by weakening a stronger competitor.
Project outline
In this project, we attempt to identify the role, disturbances play in mediating plant coexistence. For gaining a general understanding of the processes and their interactions, we will model the competition of two hypothetical plant species in a spatially explicit habitat. Disturbances in the model occur also spatially explicit, i.e. their consequences will be locally restricted. Furthermore, as disturbances may vary in their frequency, intensity and spatial extent, specific disturbance regimes will be developed and their impact quantified.
Duration (indicative)
6 Months, starting date to be arranged.
Contact
Marcel Klaassen (e-mail: m.klaassen@nioo.knaw.nl; tel.: +31 (0)294 239 317).
4. Genetic differences in leaf toughness and phenolic content of fennel pondweed (Potamogeton pectinatus): an adaptation to invertebrate grazing?
Key words
Anti-herbivore defences; Evolutionary ecology; Freshwater macrophytes; Plant-animal interactions
General context
Previous research in our group have shown that different clones of fennel pondweed (Potamogeton pectinatus) exhibit significant differences in leaf toughness and phenolic content. These traits are usually considered as major determinants of plant palatability to generalist invertebrate herbivores, and are therefore expected to play an important role in the fitness of the plant.
Project outline
Different clones chosen for their contrasted leaf toughness and phenolic content will be grown under laboratory conditions. Their palatability will be measured through the consumption rate of the pond snail (Lymnaea stagnalis) in a standard feeding assay. Moreover, their fitness will be measured in mesocosms under various herbivore pressure and nutrient availability. It is hypothesized that (1) the clones with high leaf toughness and phenolic content will be better competitors in the presence of herbivores, and that (2) their competitive advantage will increase with herbivore pressure and nutrient scarcity.
Duration (indicative)
4 Months, starting as soon as possible.
Contact
Marcel Klaassen (e-mail: m.klaassen@nioo.knaw.nl; tel.: +31 (0)294 239 317).
5. Regrowth ability of fennel pondweed (Potamogeton pectinatus): an adaptive response to herbivory?
Key words
Evolutionary ecology; Freshwater macrophytes; Herbivory tolerance; Plant-animal interactions
General context
Herbivory exerts a selective pressure on plants and shape both the genetic structure of populations and the species composition of communities. Some plants can resist herbivory through chemical or mechanical adaptations that prevent grazing. But poorly defended plants are not necessarily maladapted to herbivory if they can replace biomass losses following grazing by active regrowth and therefore tolerate herbivory.
Project outline
The present study aims at assessing the ability of fennel pondweed (Potamogeton pectinatus), a freshwater macrophyte, to compensate for biomass losses, in relation to environmental conditions and timing of losses. For this purpose, several clones of P. pectinatus will be grown in the laboratory, under different conditions of nutrient and light availability. These clones will be submitted to different intensities of biomass losses, at different moments of their life-cycle. In each case, the capacity of the plant to compensate for biomass losses will be assessed by comparing its final biomass to the one of reference undamaged plants. The following hypotheses will be tested: (1) there is a genetic determinism in the regrowth ability of fennel pondweed; (2) the regrowth ability is favoured by light and nutrient availability; (3) the regrowth ability is higher when damages occur at the middle of the life-cycle. The results will be replaced in the context of interactions between fennel pondweed and herbivorous birds.
Duration (indicative)
6 Months, starting as soon as possible.
Contact
Marcel Klaassen (e-mail: m.klaassen@nioo.knaw.nl; tel.: +31 (0)294 239 317).
6. The impact of Greylag geese on the expansion of reed (Phragmites australis) vegetation
Keywords
Plant-Animal Interactions; Geese; Helophytes.
General context
Greylag geese are becoming more and more abundant in The Netherlands. The increasing goose numbers can have consequences for the helophyte vegetation. Greylag geese forage on helophytes, especially reed, and can considerably affect reed expansion. The presence of reed is important as habitat for breeding marsh birds and it helps to improve water transparency through several mechanisms.
Project outline
In the area of Loenderveen two lakes lay next to each other: one has a peat sediment, the other is sandy. Greylag geese breed and moult in this area and may significantly reduce reed growth through foraging. In this study we want to answer the following questions: do Greylag geese reduce reed expansion? Does the sediment type matter for reed, i.e. is expansion more successful on peat or sand? Does selective grazing by Greylag geese on reed give the other abundant helophyte, cattail (Typha angustifolia), a competitive advantage? The study consists of fieldwork with goose exclosures combined with a transplantation experiment of reed in the field.
Duration (indicative)
4 - 6 Months.
Contact
Liesbeth Bakker (e-mail: l.bakker@nioo.knaw.nl, tel.: +31 (0)294 239 357)
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photo: Margreet Roos |
7. The impact of eutrophication and waterfowl grazing on aquatic vegetations and fresh water ecosystem functions
Keywords
Ducks; Ecosystem functions; Environmental stress; Field work; Plant-Animal interactions.
General context
Eutrophication has a strong impact on freshwater ecosystems: at low nutrient availability the water is clear and aquatic plants are abundant, whereas with increasing nutrient availability algae become dominant and aquatic plants disappear. Aquatic plants are consumed by ducks, geese and swans, which may also decrease aquatic plant abundance. Plants at low nutrient availability may be more vulnerable to grazing, whereas plants at high nutrient availability have more resources to re-grow after grazing. However, plants at high nutrient availability experience more competition with algae. We want to investigate whether there is an interactive effect of nutrient availability and grazing on aquatic vegetation and freshwater ecosystem functions.
Project outline
We will conduct a field experiment and an experiment using outdoor ponds. In the field we select three different lakes close to each other which can be ranked from nutrient-poor to nutrient-rich. We will establish exclosures in each lake to exclude waterbirds. We monitor the vegetation development within and outside of the exclosures. Additionally, we use potted plants which we introduce in the exclosures and the controls to measure the amount of herbivory and the growth of our plants in the different lakes. Separately we will conduct a large-scale outdoor experiment using our experimental pond facility at Loenderveen, close to our institute. We establish a vegetation consisting of several aquatic plant species in each pond and add nutrients to half of them. Then we will add ducks to half of the fertilized and unfertilized ponds and measure plant response as well as macrofauna (invertebrates) densities, water chemistry and abundance of algae. There are also possibilities to observe duck grazing behaviour at the ponds.
Duration (indicative)
4 - 6 Months.
Contact
Liesbeth Bakker (e-mail: l.bakker@nioo.knaw.nl, tel: +31 (0)294 239 357)
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8. The interactive effects of nutrients and light on plant palatability and decomposition rates
Keywords
Aquatic plants; Decomposition; Nutrient availability; Plant-animal interactions.
General context
Resource availability is often limiting plant growth. For aquatic plants the most important resources are nutrients and light. The availability of nutrients and light has not only a strong impact on plant growth rates, but also on plant palatability to herbivores. Herbivores generally select the most nutritious plants. However, plants produce anti-herbivore deterrents, meaning that the plant with the highest nutrient content is not always the most palatable. In terrestrial plants plant palatability is correlated with plant decomposition rates, but it is unclear whether this also works in aquatic plants.
Project outline
In this project we test the interactive effects of nutrient and light availability on the palatability and decomposition rates of aquatic plants. We will establish a greenhouse experiment where we will grow several aquatic plant species at low and high nutrient and light levels in a factorial design. Afterwards we will test the palatability of the plants in feeding trials using a generalist herbivore: the aquatic snail Lymnea stagnalis. Separately, we let plant material from the different treatments decompose under standard conditions and measure decomposition rates.
Duration (indicative)
4 - 6 Months.
Contact
Liesbeth Bakker (e-mail: l.bakker@nioo.knaw.nl, tel.: +31 (0)294 239 357)
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