Supervising team: Guntram Weithoff, Ralph Tiedemann, Jana Eccard, Jana Petermann (external)
Workplace: University of Potsdam
0. Brief expected profile of PhD student
We are looking for a highly motivated candidate holding a Master’s degree in ecology or in a related field. The candidate should have expertise in aquatic/plankton ecology. The position requires the willingness to perform large experimental studies both in the field and in the laboratory. A good conceptual and statistical background is desirable. Further valuable skills are knowledge in molecular genetics and taxonomy as well as scientific writing and working in a team.
1. Short Abstract
In this subproject the role of dispersal (movement) and local adaptation in shaping zooplankton meta-communities will be investigated. In a landscape with isolated ponds, zooplankton dispersal is typically limited and relies for example on wind dispersal of resting stages or animal vectors such as birds. The role of wind dispersal attains particularly importance when ponds dry out and active life stages are no longer present. Thus, the sediment of dry ponds serves as an egg bank for successful recolonization (= insurance) after re-wetting and also as a source for effective wind dispersal. As a consequence, resting stages might play an important role in maintaining biodiversity in temporary habitats which will be investigated in this project. Another aspect of dispersal limitation is that increasing isolation of habitats favors the potential for local adaptation. Populations that prevail in certain sites are likely better adapted to their environment than population from other sites (local vs. foreign) or better adapted to their site than to other sites (home vs. away). Common garden experiments are a suitable tool to estimate the relative degree of local adaptation among different actual living populations and sites. Conceptually, this holds for both, within and between species´ adaptations. The combination of these two experimental approaches will improve our understanding of the factors that shape zooplankton meta-communities. This subproject is closely related to subproject no. 07 in sharing data, cultures and performing experiments.
2. Background and previous work
The study will be performed using a set of permanent and temporary ponds in the agricultural landscape of the Uckermark. Within this landscape, hundreds of small, natural ponds (kettle holes) are scattered in areas of different land use such as crop, meadows, forests etc. From those, a subset of 19 ponds has been investigated during the first three years of the project. Field studies on the seasonal biodiversity pattern of these meta-communities reveal a strong seasonal species turn-over but also a high spatial species turn-over among ponds at the same sampling time. Overland dispersal and sub-sequent colonization of experimental mesocosoms was found and demonstrated the capability of some species to rapidly disperse (=species-specific). Wind dispersal was measured by using wind socks, i.e. collecting propagules with a fine mesh. The results from these studies suggest that dispersal is of potential importance to maintain biodiversity in ponds. Furthermore, the ability of dispersal appears to be species-specific. To complement those studies, the hatching success of zooplankton from dry sediment was quantified; this revealed that relatively small amounts of dry sediment harbor a number of viable resting stages of a variety of species. Combining results from these studies, two main components of dispersal related meta-community features have been identified and will be investigated in more detail.
The two key objectives/ hypotheses are as follows:
1) Resting stages in dry sediments are an effective resource for colonization and potentially compete with dispersed propagules after re-wetting.
2) Species (strains) are locally adapted to their home site i.e. they have a higher fitness at home than away and a higher fitness than foreign species (strains). If fitness differences are low, dispersal may play an important role in species distributions, if they are high, dispersed individuals likely encounter superior competitors.
4. Outline of work program
To investigate the first key objective, two meta-community experiments will be performed.
I) The role of sediment on the colonization of mesocosms will be investigated. Therefore, a number of mesocosms with and without sediment will be set-up and filled with pond water and, then, sampled for several weeks to determine the community dynamics. Results from this experiment will reveal the relative importance of the egg bank in dry sediments to the community dynamics.
II) In a second experiment, sediments from different sites will be collected and used to mimic wind dispersal by adding sediments from different ponds to one selected model pond sediment. The subsequent analysis of the community dynamics will reveal the role of wind dispersal compared to the local founder egg bank.
The second key hypothesis will be tested by using common garden experiments. Water samples will be taken, the zooplankton removed by sieving them and then distinct species found in the samples will be given to sieved water (still containing algal food) from that and several preselected other ponds. Depending on the simultaneous presence of species, experiments will be run for several species. Based on the results from these experiments, the relative local adaptation can be estimated.
To complement these two key hypotheses, the wind dispersal by zooplankton resting stages can be optionally quantified using a wind tunnel, or, individual movement behavior can be analysed using advanced video-filming techniques.
5. Linkage to ‘BioMove’ hypotheses, objectives and concepts
Resting stages in zooplankton are typically the product of sexual reproduction (in cladocerans and rotifers) and lead to a decline in the actual growth rate since the more effective parthenogenetic way of reproduction is stopped. This may lead to a trade-off such that an early switch to sexual reproduction reduces the growth rate (and abundance) of a species, but increases the number of resting stages in the egg bank. Species that have a lower propensity for sexual reproduction reach higher abundances, but have a lower contribution to the egg bank. By this trade-off, it is hypothesized that resting stages and the variability to produce them can act as a stabilizing factor for co-existence.