Bachelor's degree programme in Environmental Sciences, 42 pages (first reviewer)

The Wadden Sea of the North Sea is an important resting area for a large part of the global migratory bird population. In addition, the Wadden Sea National Park, with its unique conditions, serves as a breeding ground and feeding ground for a wide range of species. Many of these species are highly specialised in the breeding sites typical of the Wadden Sea, such as salt marshes, beach, dune, ditch and tidal flat landscapes. In some cases, the existence of entire populations is therefore dependent on the integrity and availability of such areas.

As a compensatory measure for the construction of the EUROPIPE I and II natural gas pipelines through the Wadden Sea National Park, an area of around 218 hectares on the East Frisian North Sea island of Langeoog, the Langeooger Sommerpolder, was selected by the responsible authorities as a renaturalisation area. The Langeooger Sommerpolder was diked in 1936/37 with an approx. 5.5 km long summer dyke, primarily to utilise the original salt marsh as agricultural pastureland. As part of the renaturalisation measures, the summer dyke was removed and partially relocated in 2004. Since then, the original salt marsh flora and fauna have been able to gradually recolonise the Langeoog summer polder.

As part of a trilateral monitoring programme of the Common Wadden Sea Secretariat (CWSS) and the European Fauna-Flora-Habitat Directive (FFH Directive), the bird populations along the North Sea coast are recorded every year through regular water and wading bird counts, as well as seasonal breeding bird mapping. With the help of breeding bird mapping data from 1997 to 2015, a data-based assessment of the renaturalisation measure was carried out in this paper based on five selected, typical salt marsh breeders. Data on grazing, tide levels and vegetation mapping were also used as site and influencing factors.

Despite the predominantly negative breeding population trends, the renaturalisation measure was classified as beneficial for the avifauna in the summer polder. The decline in breeding populations is taking place on a larger scale, as the breeding populations at the Flinthörn reference site suggest. The improved habitat quality of the summer polder after 2004 presumably plays an important buffer function for Langeoog's breeding population.

Bachelor's degree programme in Environmental Sciences, 58 pages (first reviewer)

In 2002, the summer dyke in the Langeoog salt marsh was removed as part of a compensatory measure for the laying of a natural gas pipeline in the area of the Lower Saxony Wadden Sea National Park. As a result, the polder area has been subject to the influence of the tides again since 2003/2004, with the aim of restoring a near-natural salt marsh ecosystem. To evaluate the success of the dyke removal, eleven permanent squares and six plaggen areas were mapped using the LONDO method (1976) in August 2015. The indicator values, such as salt and moisture content, were determined for each permanent area (according to ELLENBERG 1992) and categorised according to plant communities (TMAP code) and salt marsh zones. An adapted evaluation method was developed from various existing evaluation procedures, with which the success control can be carried out using permanent squares. The evaluation method developed, the vegetation development of the individual permanent plots and possible factors influencing succession were critically discussed in the context of this work.

The current vegetation surveys of the permanent squares from 2015 show different species compositions, but the salt marsh plants of the lower salt marsh and the pioneer zone predominate. These include, for example, the Atriplex portulacoides type and the Salicornia stricta type (according to TMAP), which indicate saline and moist soil conditions. The observation of the vegetation development revealed an unexpected picture. By 2009, the salt marsh vegetation had mainly developed into lower and middle salt marsh. Further sedimentation was assumed, which would lead to soil formation and altered abiotic conditions (reduced flooding frequency and duration). A higher ground surface would potentially lead to the development of the middle or higher salt marsh zone. However, some vegetation developments were recorded in 2015 that are characterised by backward (regressive) succession.

The assessments of the occurring species based on the ecological parameters and the determined ground heights resulted in a predominantly positive evaluation. The measure also appears to have a positive effect on the number of species, as the saturation index increases in the years considered. The varying elevation relief within a permanent area and the characteristic dynamic flooding regime make an exact evaluation of the renaturalisation success difficult. Overall, however, positive succession trends were observed.

Master's programme Landscape Ecology, 71 pages (first author)

Crassula helmsii is a semi-aquatic invasive neophyte from New Zealand. It was first recorded at Norderney in the Wadden Sea National Park Lower Saxony in the early 2000-years. Since then it is spreading rapidly into the Südstrandpolder and the Meierei-Wiesen. There are no scientific studies about the habitat requirements and dispersal possibilities of Crassula helmsii at the coastal zone of North West Germany. Therefore, this thesis takes a closer look on the distribution, the habitat requirements and the dispersal possibilities of Crassula helmsii on Norderney through habitat modelling. The extent of the Crassula helmsii populations in the study area was recorded to determine the study area. 51 presence points of Crassula helmsii and 50 absence points were generated using random stratified sampling in GIS. These sample points have been studied in greater detail to specify the habitat requirements. For these requirements several generalised linear models were generated, which contained different combinations of environmental parameters. The best model included the parameters conductivity, illuminance and bank characteristics. Crassula helmsii prefers flat unshaded banks with a conductivity of less than 2000 µS/cm. Higher conductivity leads to less shading tolerance and results in dropping probabilities of occurrence.

The model shows that most of the possible habitats in the study area are already inhabited by Crassula helmsii, but there are several new possible locations for Crassula helmsii within and probably even beyond the study area. Plant parts and seeds are likely to be transported by deers, horses, birds and humans over short and long distance.

In the future, further spreading can be prevented by appropriate measures. Such measures should not aim for biomass reduction, since earlier studies have shown little or no effect, but rather focus on the adjustement of important environmental factors. The habitat model can also be applied to the other East Frisian islands.

Bachelor's programme: Physical Geography at the Johann Wolfgang von Goethe University, Frankfurt Main, 31 pages (second reviewer)

The establishment of S. inaequidens on Wangerooge can be regarded as certain. It has completed the four stages of the INVASS model and is colonising new sites in its new ecosystem. Fortunately, it does not currently appear that its colonisation is putting the indigenous species under competitive pressure. The Federal Agency for Nature Conservation lists the species as potentially invasive. This view can only be partially accepted. Its dispersal mechanisms clearly correspond to those of a successful invader. The large amount of aeolian dispersed seeds enables the plant to be carried far and wide on the islands by the strong wind and to adapt quickly to varying environmental conditions. Furthermore, its hemerochory guarantees that it can spread to almost every corner of the earth. Nothing is known about its predators in the study area. It is eaten by a few generalists such as the brown hare, some phytophagous insects, snails and slugs. Its habitat requirements in terms of moisture and nitrogen are low and it lives pluria-pollacanth (Söchting Zwerger 2016).

It builds up relatively small amounts of biomass and is very light-affine. These Ellenberg light values are reflected in its occurrence in the dunes of Wangeroog at the edge of the shrub layer. The lack of competition ensures that S. inaequidens colonises free natural niches on the East Frisian Islands and retreats to ruderal sites with few or no competitors when more competitive species appear.

The question therefore increasingly arises as to how S. inaequidens affects the indigenous flora and other neophytes present there. Detailed data on the abiotic and biotic requirements of the plant are needed to answer this question adequately. Precise data on nitrogen requirements and salt tolerance could help to better predict the distribution vectors on the East Frisian Islands and assess their competitive behaviour in relation to the indigenous vegetation. There is also the question of whether the genome of the species may have changed during the colonisation process in Central Europe, leading to an adaptation to the prevailing climate (Böhmer 2001, Heger & Böhmer 2005). Furthermore, an investigation of the competitive behaviour of indigenous species of dune vegetation is necessary. In order to be able to intervene in time, S. inaequidens should exert a displacement effect. Some studies have already shown that mowing indirectly promotes the plant (Söchting & Zwerger 2016). Although fertilisation ensures the rapid closure of disturbed areas, it is harmful to the indigenous flora in the near-natural ecosystems of the dunes. Accordingly, a suitable method for controlling the species is still lacking.