Ecology of Salt Marshes 40 Years of Research in the Wadden Sea

Ecology of salt marshes 40 years of research in the Wadden Sea Jan P. Bakker Locations Ecology of salt marshes 40 years of research in the Wadden Sea Texel Leybucht Griend Spiekeroog Terschelling Friedrichskoog Ameland Süderhafen, Nordstrand Schiermonnikoog Hamburger Hallig Rottumerplaat Sönke-Nissen-Koog Noord-Friesland Buitendijks Langli Jan P. Bakker Friesland Skallingen Groningen Tollesbury Dollard Freiston Contents Preface Chapter 7 04 59 Impact of grazing at different stocking densities Introduction Chapter 8 06 Ecology of salt marshes 67 Integration of impact 40 years of research in of grazing on plants, birds the Wadden Sea and invertebrates Chapter 1 Chapter 9 09 History of the area 75 De-embankment: and exploitation of enlargement of salt marshes salt-marsh area Chapter 2 Chapter 10 17 Geomorphology of 81 Concluding remarks natural and man-made salt marshes Chapter 3 Bibliography 25 Plants on salt marshes 88 Chapter 4 Species list 33 Vertebrate herbivores 96 with scientific names on salt marshes and English names Chapter 5 Colofon 43 Invertebrates 98 on salt marshes Chapter 6 49 Changing land use on salt marshes 2 Contents 3 Studies of the ecology of the Wadden Furthermore, the intense long-term Sea unavoidably touch upon the part field observations and experiments by Preface salt marshes play in this dynamic Jan Bakker and his colleagues provide coastal system. Considering the role a wealth of information on other of salt marshes inevitably leads to the structuring factors of salt-marsh eco- longstanding ecological research by systems. The interactions between Jan Bakker, now honorary professor vegetation characteristics and sedi- of Coastal Conservation Ecology at mentation rates, the impacts of atmos- the University of Groningen in the pheric deposition on vegetation Netherlands. In this booklet, he sum- succession, and the role of small inver- marises his main findings and views tebrates (such as insects and spiders) on the functioning and management on ecosystem functioning are just a of salt-marsh ecosystems founded on few examples. As Jan indicates, the a lifetime of expertise. long-term access to well-equipped field stations such as the ‘Herdershut’ For more than 2500 years man has on the island of Schiermonnikoog has embanked salt marshes or used them been (and remains) crucial for scientific for agricultural purposes. As a result, progress in salt-marsh ecology. As was most contemporary salt marshes show (and will be) the cooperation between clear signs of human activity such as scientists and nature managers, exem- grazing by livestock, and the presence plified by the joint Coastal Ecology of artificial channels or protective Expeditions and Workshops. structures, especially along the mainland coast of the Wadden Sea. Conservation Although much has been learnt, many of present salt-marsh ecosystem values questions on salt-marsh ecology remain partly relies on decisions regarding including their development over livestock densities and species, artifi- time, their role in carbon sequestration cial drainage and sedimentation fields. and their contribution to coastal pro- Such decisions should be guided by tection. These are important themes scientifically sound information on requiring attention within the pressing the interactions within salt-marsh context of climate change. Hence, ecosystems and between these ecosys- research on salt marshes still offers tems and their surroundings. various challenges for Jan Bakker and for the many that he has trained along In the 1970s, the state-of-the-art sci- the way. entific knowledge on salt-marsh ecology was compiled in a book entitled Katja Philippart ‘Wadden Sea – Nature area of the Board Member Ecology Waddenacademie Netherlands, Germany and Denmark’ by Abrahamse and co-authors. The present booklet by Jan Bakker on “Ecology of Salt Marshes” constitutes a new landmark, clearly illustrating the progress made by the salt-marsh research community during the past 40 years. For example, whereas not much was known about the impacts of grazing on salt marshes in the 1970s, present knowledge includes species-specific effects of grazing on vegetation and soil conditions by brown hares, barnacle geese, brent geese, sheep, cattle and horses, including biological interactions such as com- petition and facilitation. 4 Introduction 5 generalized. We could deliberately start established in natural and man-made experiments involving the abandon- salt marshes? The establishment and Ecology of salt ment of grazing by livestock and nat- development of organisms can best be ural grazers such as geese and hare. discussed in terms of a natural marsh We could compare renewed livestock that reveals a time series without live- marshes grazing and mowing to mimic removal stock grazing. How does the soil affect of biomass without trampling the soil, plants in these series? How do soil and on Schiermonnikoog to understand the plants interact with vertebrate animals 40 years of research mechanisms behind observed changes. such as geese and hare and invertebrate We were fortunate to discover that some animals in these series? How are eco- barrier islands are free of hare, and thus systems of natural and man-made salt in the Wadden Sea could be compared with islands har- marshes affected by livestock grazing? bouring hare. We could learn from Another issue is the decreased area of salt marshes as a result of embankments. To what extent can a salt marsh be Introduction plants, animals with animals, and plants restored after the de-em- The existing scientific knowledge of with animals as part of a community. bankment of a summer- the Wadden Sea, including salt marshes, Moreover, organisms interact with abio- polder? Finally, I discuss was compiled into a book entitled ‘Wad- tic conditions as part of an ecosystem. aspects of interactions be- den Sea’ in the early 1970s.1 However, This system changes over time and is tween abiotic conditions not much was known about grazing of referred to as succession. When super- and organisms, and their salt marshes. The impetus for the research imposed on a salt-marsh ecosystem that impact on the manage- discussed in this book, was the question changes over time, the impact of live- ment of salt marshes. what would be the effect of renewed stock grazing or abandonment is felt. livestock grazing of parts of the salt Finally, being influenced by tides with The work presented here marsh of the barrier island of Schier- drainage by meandering creeks, salt started with people from monnikoog in the Netherlands. That marshes are considered natural. They the University of Gro nin- particular salt marsh became covered by can be found in the barrier conditions gen. Gradually, a net work vegetation of a tall grass species in many in the lee of dune systems in the Wadden of people collaborating locations after being abandoned for 13 Sea. However, many salt marshes are in work in salt marshes years. The conservation agency Dienst man- made, i.e. established from sedi- Crossing the tidal watershed emerged. This network der Domeinen (without monitoring mentation fields and artificial ditching of Langli, Denmark ex cluding livestock and grazing with included scientists from universities (Photo Dries Kuijper) data) believed that the tall vegetation along the mainland coast. different livestock species and various and research institutes in Denmark, was negative for plants and birds. The stocking densities established by col- Germany, the United Kingdom, France, agency therefore wondered whether the Unique field laboratory leagues on other sites, including sites in Belgium and the Netherlands. Members tall vegetation could be controlled by The question of the impact of renewed Germany and Denmark. We could also of conservation agencies were also in- renewed cattle grazing. cattle grazing can focus on that parti cu- learn from practices such as de-em- volved. Great efforts were made by a lar site. That would not, however, allow bankment for coastal defence and bio- series of PhD students. Most results In fact, many questions are hidden be- generalizations. It would also reveal diversity in the United Kingdom. We discussed here are published in peer- hind the first question. The fact that the limited insight into processes occurring could establish a unique experiment on reviewed journals. Many papers include entire place did not become covered by in salt-marsh ecosystems. Although the the biodiversity of salt marshes, inte- data collected by colleagues, and are the tall grass, could be due to different research started on the island of Schier- grating the impact of different livestock co-authored by them. We were fortu- abiotic conditions in the salt marsh, such monnikoog, it was soon extended to species at various stocking densities on nate that these colleagues allowed us to as elevation related to sediment input and other back-barrier marshes. Later, man- vegetation, breeding birds and inver- share their experiments and knowledge. soil type. However, after a prolonged made marshes along the mainland coast tebrates along the mainland coast of period of abandonment, could the entire were included in the international Friesland. site be covered by the tall grass? The Wadden Sea, and the United Kingdom. encroachment of the tall grass can affect We were fortunate that Schiermonnik- In the following chapters I will give an both plants and animals living in the salt oog is a dynamic island and offers a time impression of how the simple question marsh, such as breeding birds, winter- series, thus featuring a unique field of the impact of renewed grazing on staging geese, and hares, but also small laboratory. The large number of salt a salt marsh triggered a series of new invertebrates such as insects, spiders and marshes along the coast of the North Sea questions. What is known about the animals living on dead plant material. made it possible to test whether results de velopment of salt marshes in the All organisms may interact: plants with found on a single salt marsh could be 1.

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