Abstract 3 Introduction 3 Theoretical framework 5 Drivers 6 Pressures 7 State 8 Impact 9 Response 11 Multidisciplinary approach 11 Problem definition 12
Methods and data 14
Ecosystem services 14
Environmental Scenario analysis 16
Results 17
Scenario 1: ‘Wadden Sea: back to nature’ 18
Scenario 2: ‘Green management of the Wadden Sea’ 18
Scenario 3: ‘Free oyster flow: deregulating the Wadden sea’ 19 Scenario 4: ‘Tragedy of the commons: exploitation of the Wadden Sea’ 20
Conclusion 22
Discussion 23
Abstract
The scope of this research is determining what measures should be taken regarding non-native Crassostrea Gigas in the Wadden Sea, in order to maintain and possibly improve the quality of its ecosystem services. By using the DPSIR framework, structure is provided to the relevant theories that are found in the field of Biology, Earth Science, Business
Administration and Social Geography. With the use of the Environmental Scenario analysis four different scenarios were constructed and the effects Crassostrea Gigas had on the Wadden Sea ecosystem services were analysed per scenario, to determine what measures should be taken to maintain and possibly improve the ecosystem services. We present a policy-relevant advisory tool that looks at limiting the damage of Crassostrea Gigas to ecosystem services, while profiting its presence as much as possible. Actively managing Crassostrea Gigas populations while keeping in mind the ecosystem services of the area can go hand-in-hand with utilizing the Wadden Sea’s uniqueness and the presence of Crassostrea Gigas for financial gain. Creating a comparable scale to measure the quantitative influence of Crassostrea Gigas on different ecosystem services poses a challenge for further research.
Introduction
Throughout time, human use of earth’s ecosystem services have had increasing impact on the functioning of said systems (Steffen, Crutzen & McNeill, 2007). One such interaction that alters the ecosystem dynamics is the introduction of a non-native species, or a so-called invasive species. As a result of globalization over the past century, organisms are now easily transported between continents, causing lots of species to be introduced to new habitats (Vitousek et.al., 1997). Many cases of invasive species introduction are clear-cut in the sense that they are harmful to their respective invaded habitats. This leads to clear-cut policies being developed in regards to these cases; total extermination of the invader is desirable, but may be unfeasible (Herbert et.al., 2016).
Accidental introduction of non-native species may however bring unforeseen benefits to the involved ecosystem, which can change the desirable management of the species. Currently, this is the case along the maritime coastline of the North Sea, where an introduced oyster species called Crassostrea Gigas (further referred to as Crassostrea or Pacific oyster) has established widespread communities (Troost, 2010).
The non-native Crassostrea Gigas was introduced by imports done for cultivation activities on the German island of Sylt. While a lot of concern was raised by this, policy makers reasoned that Crassostrea Gigas would not be able to invade the Wadden Sea environment since it would be too cold for it to survive. However, a few warm winters later the first oyster establishments were observed (Schmidt, Wehrmann, & Dittmann, 2008). Surprisingly, the effects of this invasion were not necessarily detrimental to the ecosystem. For example, research done by Waser et.al. (2018) revealed that Crassostrea Gigas introduction caused a decrease in mortality due to predation in native blue mussel (Mytilus Edulis) populations. If
Crassostrea Gigas does not need to be eradicated from the environment, the question arises of what measures dó need to be taken.
The scope of this research is determining what should be done, regarding invasive oyster, in order to maintain and possibly improve the quality of ecosystem services provided by the Wadden Sea. By doing literature research, we want to create an overview of the
ecosystemic interactions brought about by invasive Crassostrea, on both biotic and abiotic level, as well as the implications of conservation/extermination of Crassostrea for the various ecosystem services and the people that use them.
Obtaining information on the aforementioned issues is crucial in developing an optimal strategy for dealing with North Sea Crassostrea populations. This constitutes the main goal of our research. We want to explore how different government perspectives on the invasion of Crassostrea Gigas in the Wadden Sea area will influence the ecosystem services it provides. The research question therefore is: “How can we limit damage caused by the Pacific oyster in the Wadden Sea, while profiting its presence as much as possible?”. The importance for the seafood export and production should also be considered, since currently oysters are produced at a scale 20 times that of mussels (Ruesink et.al., 2005). Proper investigation of this trade-off is scarce, but of great influence on policy making. The multiple angles on this matter, which usually characterize policy making, justify a
multidisciplinary analysis of this situation. This will be worked out by making use of a DPSIR-framework. To then explore the different trajectories of oyster-governance in the Wadden Sea, an interdisciplinary scenario-analysis will be constructed. By comparing different scenarios, we can get an overview of how ecosystem services are affected by different decision-making strategies and determine what actions should be taken to create an optimally favourable future situation.
Firstly, the relevant scientific field of the case study covers ecology, because of the ecosystemic effects brought about by invasive Crassostrea Gigas. Since the oyster was introduced for financial gain, the economical perspective is of importance as well. Finally, the Wadden Sea has a so-called ‘fan-community’ consisting of about 14 million people that in some way make use of the Wadden Sea (Sijtsma et al., 2019). Therefore, the social side of this problem is also important to consider.
To cover the concepts of ecological, social and economical workings the following disciplines will be investigated: biology, earth sciences, social geography and business administration. Research done on the effects of non-native oyster introduction in particular generally yields highly applicable conclusions, since oyster species have been introduced in aquatic systems in 73 countries/regions around the world (Ruesink et.al., 2005). Because of this widespread presence of invasive oysters, newly gathered scientific knowledge has widespread effects on conservation policies.
Before we get into analyzing the scenarios we will provide the theories relevant to the situation in the form of a DPSIR-framework. After this, the methods of constructing the scenarios and the scenarios themselves will be presented and analysed. We then discuss our work in the context of policy making and suggestions for further research. Finally, the research is shortly concluded.
Theoretical framework
In order to gain a broad, and yet clear, understanding of all theories and concepts involving the invasive Pacific oysters, this theoretical framework consists of a DPSIR framework. The DPSIR framework is a multidisciplinary tool to analyse, provide and communicate knowledge on several causal factors of a specific environmental system. Drivers - Pressures - States - Impacts - Responses are different for many stakeholders and ecosystems, making it a multidisciplinary approach. This framework draws on knowledge from different disciplines but stays within their boundaries and is therefore not interdisciplinary. In this particular
framework four disciplines will be discussed: social geography, business administration, biology and earth sciences.
Drivers are defined as global trends that can not be influenced on a small scale. Typical examples are climate change or the economic situation. Drivers lead to pressures, the interactions, which originate from human actions, that influence the state of a system. The state refers to the current environmental conditions of the resource, which in this case is the Wadden Sea. The impacts are fairly straightforward and stand for the consequences and effects observed as a result of the force exerted on the system’s state by pressures. Finally, the response encompasses the political and societal measures that are now being taken to counter the possible implications of the drivers and the pressures.
Figure 1. DPSIR Framework main findings. Every component of the framework has been applied to the case of invasive oyster and their representatives in the Wadden Sea area are shown.
Drivers
Economic Treadmill
To have a better understanding of what drivers are active in this case study we have to go into history to find out how these drivers were created. Until approximately the end of the nineteenth century, the subtidal beds of the native European oyster Ostrea edulis supported a flourishing fishing business. However, its overfishing resulted in a firm decline in the native oyster population as the demand for oysters grew rapidly (Hagmeier and Kändler 1927). The fishermen are therefore an actor in the ‘Economic Treadmill’.
The ‘Economic Treadmill’ theory describes how a constant search for economic growth results in an advanced economy being stuck on a so called “treadmill”. The well-being of the citizens of these advanced economies is not improved by the amount of economic growth, but the impact of this pursuit results in massive unsustainable environmental damage. In this case the fishermen are incentivized to search for opportunities to grow their business.
Therefore the oyster cultivation sector started to import large amounts of seed oysters to fulfill the demand of the oyster.
Introduction of the Pacific oyster
The flat oyster was in critical conditions in the winter of 1962/1963 during the outbreak of the oyster disease. In the 70’s the Pacific oyster was introduced to the Oosterschelde to restore oyster farming. Unlike the flat oyster, this Pacific oyster is less susceptible to oyster disease (Troost, 2009). The first sign of the successful establishment of Pacific oysters in the
Wadden Sea were in 1975, a year with sea temperatures that were relatively high. In the summer of 1976 the sea temperatures stayed above the 20 degrees level and therefore a high embedding success rate was recorded.
The warm summers of 1982 and 1986 resulted in a large amount of Pacific oyster spatfall (Diederich, 2005).The spread of the Pacific oyster in the northern Wadden Sea began 5 years after the first German oyster farm had started its business off the island of Sylt in 1986. From that moment on, the population increased slowly but eventually grew
exponentially from the mid 1990 onwards. Only at the beginning of the new millennium did it become clear that the Pacific oyster had spread through the entire Dutch Part of the Wadden Sea (Troost, 2009).
Environmental and climatic conditions
In the first stage of non-native species invasion, colonization, a high tolerance to a wide range of environmental conditions should be present. The invasion to other habitats of the Pacific oyster causes a change in conditions, to which the Pacific oyster has to adapt. (Reise, K. & Beusekom, K. 2008; CL, Sabine. 2004; Sakai, A. et al. 2001).
The imports done in purpose of sustaining the oyster businesses and social aspects, led to a successful establishment in the wild of the Pacific oyster. Although Pacific oysters originally
occur at latitudes from 30* South and 48* North, the oysters were able to successful establish at higher latitudes of 52* in the Netherlands (Troost, K. 2010). According to Carrasco and Baron (2010), the occurrence of the Pacific oysters between the specific latitude is changing due to climate change affecting water conditions. The study of Carrasco and Baron (2010) about the influence of water temperature on the establishment of the Pacific oyster resulted in a prediction of potential future distribution areas of this species. In their prediction they used a model with two variables: atmospheric temperature (AT) and surface water temperature (SST). As seabeds often are above sea level due to tides, atmospheric temperatures have to be taken into account. The following temperatures resulted from their model; the temperatures are expressed in degrees Celsius: AT: - Warmest month: 15 - 31 - Coldest month: -23 - 14 SST: - Warmest month: 14 - 29 - Coldest month: -2 - 20
On the basis of these temperatures, Bodvin et.al. (2012, 2013) concluded that the establishment and expansion of oyster to other water bodies is likely to occur with ocean warming. According to Wrange et.al. (2009), higher latitudes with relatively low water temperatures were restricting further spread, but these temperature-mediated limits of the habitat were spread out more towards North and South, which promoted the successful establishment of the Pacific oyster in the Netherlands.
Pressures
Ecosystem engineering
The Pacific oyster introduction puts pressure on the Wadden Sea ecosystem by changing the structure of the sea bed it settles on. It usually establishes itself on already existing beds of the local mussel (Mytilus edulis) and turns it into mixed bivalve-beds. The Mytilus-beds are soft and more open while the mixed bivalve-beds have a stronger structure and are more densely populated (Reise, Buschbaum, Büttger, & Wegner, 2017). This change increases the diversity of the landscape and thus increases the habitat heterogeneity. This change creates more niches for species to fulfil and will therefore lead to a change in species composition and an increase in biodiversity (Davidowitz & Rosenzweig, 1998). The change may however pressure the avian community since the Mytilus-beds are important foraging grounds for different bird species and a change in species composition of these beds can put pressure on the food availability (Markert, Esser, Frank, Wehrmann, & Exo, 2013).
Intraspecific competition
The introduction of Crassostrea Gigas put pressure on native Mytilus Edulis populations in two ways: competition for food resources and competition for living space. It is important for
the ecosystemic interactions that M. Edulis is not excluded from its habitat, so this pressure is also a direct pressure to the wellbeing of the Wadden Sea ecosystem.
With regards to competition for resources, Crassostrea have a competitive advantage over Mytilus because they cause more turbulence in the water, which leads to more food particles coming their way (Troost, 2010). Troost (2010) also found that Crassostrea use their food intake more efficiently, but according to Riera, Stal & Nieuwenhuize (2002) Crassostrea have different diet compositions than Mytilus. This means that the two species do not compete for the same food resource and therefore Crassostrea can not outcompete Mytilus on food resources alone.
The competition for living space has favoured Crassostrea, since they have been able to overgrow native Mytilus populations. At first, this might have smothered the native mussel beds but the oyster coverage provides native Mytilus with shelter to disturbances (Reise et.al., 2017). The trade-off is that Mytilus give up the best spots for food gathering to Crassostrea. Coexistence is possible due to this interaction that is similar to a symbiosis.
Hitchhiking species and pathogens
Transportation of Crassostrea around the world for cultivation purposes has lead to several unwanted introductions of hitchhiking species (i.e. algae, small invertebrates and pests) into marine environments (Carlton, 1992). The North Sea ecosystem too is pressured by these unwanted introductions, since they can lead to changes in ecosystem composition, altering the state of the marine environment.
The extent of hitchhiking species introduction was described by Ruesink et al. (2005), who found that roughly 40% of all invasive species in the North Sea were hitchhiking species. Among these are aquatic plants, algae and oyster pests. Since plants and algae are primary producers in ecosystems, establishment of an invasive one can alter ecosystem dynamics. Invasive pests are a great pressure to the ecosystem as well, since they can infect native Mytilus populations, which generally have lower resistance to pests than Crassostrea (Goedknegt et al., 2016).
State
In the current state of the Wadden Sea ecosystem the Pacific oyster has spread to almost every corner of the Wadden Sea, as can be seen in the species distribution model (SDM) (figure2). This is due to the fact that almost every part of the Wadden Sea is a suitable habitat for the Pacific oyster. The variables which best explain the settlement of the Pacific oyster are minimum surface temperature of the water and amount of phytoplankton in the water (figure3).
The minimum surface temperature explains the for now limited expansion to the coast of Norway. The Pacific oyster is also more likely to spread to regions where there is more phytoplankton in the water and thus a bigger food resource is available. By filtering the water of phytoplankton the Pacific oyster lowers the total biomass of phytoplankton in the water (Asmus & Asmus, 1991). This, at first, lead to a direct competition with Mytilus edulis over
food resources and caused a decrease in its population. However a balance has been reached and Mytilus edulis can co-exist with the Pacific oyster (Reise et.al., 2017).
The Wadden Sea mostly consists of loose sediment, but the places where the Pacific oyster has settled have been changed into harder, more reef-like structures (Reise et.al., 2017). The harder substrate is washed away less easily than the loose sediment and therefore has led to a decrease in erosion (Piazza, Banks, & La Peyre, 2005). However this change in geomorphology has also led to the loss of certain habitat types and replacement of hard oyster reefs. The replacement of a variety of habitats into hard oyster reefs made the Wadden Sea more homogeneous overall (Zwerschke et.al., 2018).
The import of the Pacific oyster facilitates the arrival of hitchhiker species. For now the import of the Pacific oyster has led to a limited amount of hitchhiker species being introduced into the Wadden Sea. It must be noted that it is very difficult to track the origin of hitchhiker species.
Figure 2. Occurrence points of C. gigas in the Wadden Sea area (Data retrieved from www.gbif.org)
Figure 3. Best environmental predictors for the distribution of C. Gigas
Impact
The Oyster reefs deliver critical ecosystem functions, so much so that several efforts to restore oyster ecosystems around the world have been taken. The native European mussels create natural reefs and thereby provide a rare hard substrate in the Wadden Sea that is dominated by soft sediments. Aside from providing a fishery commodity, oysters can provide a number of necessary ecosystem services such as water filtering, shorelines protection and providing a habitat for associated species. This results in an approximate economic value of $5500–$99,000 per hectare per year excluding the oyster harvest value. (Grabowski et.al., 2012). This economic value is highly debatable. This is because there is incomplete
scientific understanding of how ecosystems function. In practical terms this means that values of ecosystems are likely to be depended on the context and therefore the questions if this is transferable to compare service valuation across different environmental conditions (Ruffo & Kareiva 2009). The importance of quantifying the value associated with ecosystem services will result in a better ability to allocate limited resources to manage ecosystems effectively.
Furthermore, the economic impact of the introduction of the Pacific oyster in the Wadden Sea ecosystem can be divided into two categories: the use value and non-use value. Use value can best be described as all the tangible features of a commodity which can satisfy human requirements, desires or needs, or another useful purpose. Contrary to that is the non-use value. This is the value that people assign to economic goods that they will never use. Because the non-value category consists of ethical standards, it is often a point of discussion how much this category provide value without being traded in the market place and therefore is much more difficult to measure (Heal, 2000).
Ecological impact
In low abundances the Pacific oyster increases local biodiversity, due to the increase in habitat heterogeneity that the oyster causes (Markert et.al., 2010). However, if the Pacific oyster becomes more dominant this effect is counteracted due to the fact that the Pacific oyster makes it surroundings more homogeneous. The Pacific oyster then decreases habitat heterogeneity on a regional scale, e.g. the different sites where the Pacific oyster is found are more alike in terms of habitat composition and thus species composition. Consequently the biodiversity on a regional scale decreases (Zwerschke et.al., 2018; Green & Crowe, 2014).
The transition from a Mytilus-bed to a mixed bivalve-bed leads to a change in species composition of these areas. This can affect the avian community because they forage on these beds (Markert, Esser, Frank, Wehrmann, & Exo, 2013). Especially birds who feed solely on Mytilus edulis are impacted negatively (Waser et.al., 2016a). This is due to the competition between the Pacific oyster and Mytilus edulis for food resources. Birds which have a more diverse diet were hardly impacted, if impacted at all by the change in species composition (Waser et al., 2016; Markert et.al., 2010; Scheiffarth, Ens, & Schmidt, 2007). Some species even adapted to the new species composition by changing their diet composition (Markert et al., 2010).
The current impact of hitchhiking species on the North Sea ecosystem is relatively
unexplored. As of now, no major shifts in ecosystem structure have been observed and no oyster pests have been introduced. It is known that introduced parasites can greatly damage native bivalve populations. NRC (2004) described the devastation caused by the H. nelsoni parasite among Crassostrea Virginica populations in the western U.S. This parasite was introduced by hitchhiking on Crassostrea imports and then established itself on native Crassostrea.
The introduction of Crassostrea in the North sea has lead to a new ecosystem structure where the native and non-native species coexist instead of outcompeting one another. This ecosystem structure is not necessarily undesirable, but according to Verlaque,
Boudouresque & Mineur (2007) unmonitored oyster imports occur frequently. These constant imports of Crassostrea could lead to an undesirable system if they are not monitored well and checked for any unwanted hitchhikers.
Response
The social metabolic approach
To better understand the decision making and the impact of exploitation of the ecosystems, the theory of the ‘social metabolic approach’ was developed within the social sciences. This approach emphasizes the interchange of materials and energy between the human society and the natural systems, highlighting the importance of the socio-ecological interrelationship. To this end, additional attention is given to the dynamics that regulate and regenerate
ecosystems (Longo & Clark, 2016).
This approach has been relevant in response measures taken to address the drivers as well as the pressures. As early as 1982, a trilateral pact was established between the
Netherlands, Germany and Denmark (Turnhout et.al., 2008). A Joint Declaration was agreed upon by the three countries, in which they stated that they would coordinate their activities in the region and that they would share the responsibility for the Wadden Sea area, which would eventually be declared a UNESCO World Heritage site in 2009. However, the Joint Declaration was not legally binding. Subsequent policy has been directed at the
conservation of the area. To achieve this goal, the Wadden Sea conservation and
management scheme is directed at the preservation of the functioning of the ecosystem and its natural services, whilst simultaneously making sustainable use of the Wadden Sea area within this framework (Enemark, 2005). A ban on mussel fisheries in certain areas of the Wadden Sea, coordinated between the Netherlands, Germany and Denmark are intended to sustain the biodiversity in the region and in this respect this counters the pressure of
intraspecific competition between the mussels and the Pacific oysters (Nehls et.al., 2009). Also, there have been permits given out to fish on the wild Pacific oysters in the Dutch part of the Wadden Sea (Fryslân, 2018), which inhibits oyster population growth if done on a large scale. Import regulations concerning the Pacific oyster could be considered as countering measures to the drivers behind the spread of the Pacific oyster, which is the commercial oyster cultivation (Enemark, 2005).
Multidisciplinary approach
This research has been done with a multidisciplinary approach. The threats but also opportunities of invasive species in general have societal, economic, environmental and biological implications. In order to create a clear and understandable advisory report, the invasion of these species is examined from different disciplines and the multidisciplinary overlap, as well as where the different disciplines compliment each other lays at the foundation of this paper.
Biology Earth Science Economics Sociology
Biology x The specific
environmental qualities that affect the spread of invasive species The economic drive behind the introduction of exotic species The effect of policy on the containment of exotic species
Earth Science The effect the invasive species have on the environment (e.g. water quality) x Economic motives behind certain environmental compositions The effects policy and human agency on environmental degradation Economics The effect of the
spread of invasive species on the fishing industry The alteration in the environment and their financial consequences (e.g. damaging mussel beds) x Active pursuit of capitalistic values (e.g. economic growth imperative) Sociology The impact that
is caused by invasive species on policy concerning invasive species The effects of the alteration of the environment on the human society (e.g. uncertainties surrounding preservation fishing grounds) The effect of economic alteration for the fishing
communities
x
Table 1. Cross table of the interaction between the different disciplines.
This cross table highlights the complexity of this case study and underscores the relevance of a multidisciplinary approach. For each of the disciplines the intersection with other disciplines is shown, resulting in the formulation of smaller topics of interest.
Problem definition
Currently, the state of the Wadden Sea ecosystem does not allow for Crassostrea Gigas populations to be exterminated from the environment. The lack of a proper competitor, its
short colonization time and the fact that the oyster is spread across the entire Wadden Sea make its removal practically and scientifically impossible (Herbert et.al. 2016). The
persistence of Crassostrea has been known for quite some time, with Reise et.al. (2005) drawing the conclusion that oyster can not be feasibly eradicated from the Wadden Sea. Due to its high densities, total removal would be so extensive that it could damage other parts of the ecosystem. Because of this, different ways of managing this invasive species must be employed.
Little research has been done on what can be done to actively manage the oyster
populations, despite its persistence. Even rarer is research done on strategies to exploit the non-native oyster populations to its fullest potential. The particular knowledge gap of
designing optimal coping strategies regarding invasive Crassostrea is addressed in this research, with the research question being: “How can we limit damage caused by the Pacific oyster in the Wadden Sea, while profiting its presence as much as possible?”. Since
Crassostrea is not necessarily detrimental to the Wadden Sea ecosystem and its services, the optimal strategy would go beyond the usual measures taken when a species invades an ecosystem, which consists of inhibition and extermination.
Since the Wadden Sea is a complex ecosystem, composed of many different interactions between species and habitats, it is insufficient to look at individual habitats in terms of policy development. The different habitats of the Wadden Sea are all connected to each other in some way via ecosystemic interactions and geological forces that function on multiple scales, a feature of the system called its connectivity (Cadenasso, Pickett & Grove, 2006; Sheaves, 2009). This characteristic is typical of complex problems and makes it difficult to determine the correct measures that should be taken to improve the situation, since improving the situation in one sub-area might make matters worse somewhere else.
Another indicator that the invasion of Crassostrea in the Wadden Sea is a complex subject is the vulnerability to disturbances the system harbours, which was exposed when the oyster invaded the area and altered the ecosystemic interactions and food web composition. This factor should be handled with care and requires perspectives from multiple disciplines to ensure the system remains balanced.
After the initial oyster invasion, the system showed another key feature that complex problems have in common, which was its adaptiveness. Oyster and mussel formed a coexistence and avian communities changed their dietary needs, which ultimately lead to a balanced Wadden Sea ecosystem.
The situation becomes even more complex when the opportunity of financial gain from Crassostrea is taken into consideration. Designing an optimal strategy to meet this requirement as well as retaining the Wadden Sea ecosystem in its natural state calls for collaborative efforts from both environmental and economical stakeholders as well as the community of people who use the Wadden Sea in some way. We tackle the demand for cooperation of different fields of science by approaching the Wadden Sea case in a multi disciplinary manner.
The objective of this research is an advisory tool for policy makers regarding invasive oyster. Different scenarios will be analysed to assess what should and should not be done in order
to optimally retain the Wadden Sea and its ecosystem services in the context of the invasive oyster, as well as profit its presence as much as possible. The outcome of the scenario analysis is not only relevant to the Wadden Sea area, but to other areas invaded by Crassostrea as well. We have chosen to use this particular method because the optimal strategy for dealing with the oysters is not yet fully clear and looking at the outcomes of different trajectories of driving forces will advance our knowledge of the situation, which allows for more educated policy making.
We want to estimate what the effects will be of different trajectories that could take place and how this affects the ecosystem services of the Wadden Sea in order to determine the most favourable scenario. Consequently, the decisions in policymaking that create the most favoured scenario will be identified and can serve as policy-relevant advice.
Methods and data
Ecosystem services
From the DPSIR analysis it was made clear that the Pacific oyster has a significant impact on the Wadden Sea and ecosystem. To clearly illustrate what the different scenarios will lead to, we must look at the way oysters impact the ecosystem services for every scenario. Firstly, we need to identify the ecosystem services the Wadden Sea provides to its users. This was done by Hattam et.al. (2015), who made a general list of marine ecosystem
services and their indicators for a different case study, which we applied to the Wadden Sea area. Not all of the ecosystem services identified in their research are affected by the Pacific oyster and are thus not relevant to our research. Therefore, we made a selection of
ecosystem services from the previously mentioned list, including only those relevant to our research.
The following services of the Wadden Sea are influenced by Crassostrea: food provisioning, biological control, tourism and recreation, migratory and nursery habitat, flood protection and cultural heritage. An overview of the different ecosystem services with their indicators and relevance to the Wadden Sea can be found in.
Ecosystem service description indicator Relevance to the Wadden Sea
Food provisioning Available marine flora and fauna for human consumption
Populations of fish or
shellfish The Wadden Sea has population i.e. mussel and oysters which can be used for human consumption
Biological control Conservation of population dynamics and pest control
Frequency of pests and change in biodiversity
The Wadden Sea is a stable ecosystem with low
frequency of pests Migratory and nursery
habitat Habitat provisioning for migratory and resident species
Extent of a suitable
habitat The Wadden Sea provides a place for migratory and resident birds to forage and nurse
Tourism and recreation
Opportunities for tourism and recreation
Number of recreational beaches, quality of water
The Wadden Sea is an important area for tourists and residents to recreate
Cultural heritage Maintenance of
cultural heritage Mudflat area The Wadden Sea is a protected UNESCO world heritage site because of its unique mudflat landscape
Flood protection Protection against high
tides Soil elevation, erosion rates The Wadden Sea creates a buffer zone between deep sea and mainland,
decreasing flooding risks Table 2: overview of Wadden Sea ecosystem services impacted by Crassostrea Gigas.
To assess the extent to which the ecosystem services listed above are influenced by Crassostrea Gigas, we first use the available literature to determine whether the impact of oysters on a service is positive, negative or neutral. It is essential to acquire this knowledge first, because it forms the basis of the decision-making regarding policies for dealing with Crassostrea Gigas. This is important to the scenario analysis, in which the qualitative impact (positive/negative) of the oyster to ecosystem services will be estimated for each scenario in comparison to the current state of these services. The effects are visualized in table 3 and their scientific background is explained in the section below.
Ecosystem service Oyster impact
Food provisioning +/-
Biological control -
Migratory and nursery
habitat +/-
Tourism and recreation +/-
Cultural heritage -
Flood protection +
Table 3: indication of qualitative impact of oysters on ecosystem services
Wild mussel populations decreased since the arrival of Crassostrea, meaning less wild mussel stocks are available for fishing (Troost, 2009). A new opportunity is however created since wild oysters stocks can now be fished and the combination of these two consequences makes the impact on food provisioning a neutral one.
In terms of biological control, the oyster had a positive impact on biodiversity locally by creating more niches for different species to fulfill, but it also made the species composition of the Wadden Sea generally more homogeneous which led to an overall decrease in
biodiversity of the Wadden Sea area. The oysters also decrease pest control, since they can facilitate the spread of non-native pests that can be introduced via oyster imports for
cultivation, which makes for an overall negative impact on biological control.
We have seen that oyster introduction caused migratory bird populations to adapt their diet, maintaining a balanced habitat for nursery and foraging. Since the ecosystem found its balance again after Crassostrea invaded, oyster impact on this service is neutral.
For tourists or residents visiting the Wadden Sea, the sharp oyster beds might lead to cut feet and can decrease the amount of visitors to sites with oyster beds. The oyster creates the opportunity to organize oyster foraging tours though, which attracts tourists to the area. These two effects combined lead to a neutral impact on tourism.
The UNESCO world heritage state of the Wadden Sea is negatively affected by oyster presence, since the characteristic mud banks are transformed into strong oyster beds. Finally, oysters improve flood protection, since more waves are damped and less erosion occurs because of the strongly structured beds they form (Borsje et.al., 2011)
Environmental Scenario analysis
An environmental scenario analysis offers a framework to get better insights from a range of disciplines and to study complex interactions between environmental developments and socio-economic factors. A scenario analysis is a way of analyzing future events by
considering alternative possible outcomes. It is one of the main forms of future projection, it does not try to show one exact picture of the future. It instead presents several alternative future developments and it does not rely on extrapolation of the past or the extension of past
trends. Furthermore it does not rely on historical data and does not expect past observations to be valid in the future. Scenario exercises have in history proven to be useful to assess the prospects of the future developments within uncertain and complex systems. In each of the scenarios it normally combines pessimistic, optimistic and more or less probable
developments. However, all aspects of scenarios should be plausible in the future. (Hsia, 1994).
A scenario analysis on the possible outcomes of different trajectories of driving forces was done to deduce which strategies should be employed per scenario. Different scenarios can emerge when different events of change take place. This change is mediated by driving forces, which are elements that cannot be controlled by humans, i.e. public opinion, the stock market or the weather.
The identification of the driving forces in a problem situation rely on two key assumptions: first, they can not reasonably be influenced by human actions. Secondly, the driving forces are independent from one another. Put differently, a force that is driven by another force is not the true driving power of a changing situation, since something else drives it. In most scenario analyses, two driving forces are present, yielding four different scenarios. The forces driving change regarding the situation of oysters in the Wadden Sea are defined by the social metabolic analysis.
Results
Scenario 1: ‘Wadden Sea: back to nature’
This first scenario occurs when conservation of the Wadden Sea world heritage area is valued more than the economic opportunities of Crassostrea Gigas farming. Progressive green thinking leads to the ban of wild oyster fisheries and the imports of the Pacific oyster. The ban on imports results in expansion of the population, as the carrying capacity has not yet been reached and the population is not restricted in its growth by wild oyster fisheries. The fisheries are banned from fishing the Pacific oyster in the region in order to decrease pressure on the ecosystem services. Society wise, the values of the ecosystem of the Wadden Sea should be made clear to the citizens and visitors, in order to ensure safe and green handling of the visitors and stakeholders of the area (Gabrielson, 2008).
The ecosystem services affected by this scenario are clear to draw. The Pacific oyster competes with the native mussel, but they can coexist very well. The overall population of bivalves on which we feed ourselves is not harmed. Therefore, this scenario is no danger or threat to the food provisioning service of the Wadden Sea.
Although the Pacific oyster introduction increased the biodiversity with the creation of more niches to establish for other species, the overall Wadden Sea species composition is becoming more homogeneous. The expansion of the population due to restrictions on fisheries will result in less biodiversity in the Wadden Sea.
Since oysters could carry pests and diseases, their imports are potentially harmful to native bivalve populations. However, in this scenario the imports and introductions are banned, as the Wadden Sea should be conserved and the economic value of the oysters is neglected. Therefore, the chance of putting pests or diseases into the environment, which can be dangerous for other species, is almost none. The biological control of the area under this scenario will be positive. The value of the conservation of the Wadden Sea together with fish restrictions and ban on oyster imports will not result in changes for the service as a migratory and nursery habitat as the bird species are not affected by the population
As the economic value of the Pacific oyster is neglected, foraging tours and tourism of oysters is not actively stimulated. A tourist opportunity does arise from the fact that the Wadden Sea is world heritage and tourists might visit it to experience its uniqueness. The oyster and other species are not disturbed and experience no harm or pressure from humans. This allows the growth of the Pacific oyster population to increase, meaning more formation of strong oyster beds, enhancing the damp of waves and increasing the flood protection service of the Wadden Sea area (Borsje et.al., 2011).
This scenario will put less pressure on the ecosystem services of the Wadden Sea, without the economic exploitation of the Pacific oyster.
Scenario 2: ‘Green management of the Wadden Sea’
The green management of the Wadden Sea is a scenario in which the Netherlands is most likely right now. This scenario is based on the balance between green management of the Wadden Sea and on the other hand the profit of the economic opportunities of the presence of the Pacific oyster.
Oyster businesses are able to import the oysters to sustain the business and to grow the population, after which they are fished. However, these imports are thoroughly documented and controlled on pests and diseases, in order to prevent harm to other species in the Wadden Sea. The imports stimulate the growth of the already existing population, in order to get a balance between the oysters for fishing and the growing population.
For the oyster fisheries, a limited amount of permits should be given to ensure a green way of exploitation of the oysters. Fisheries with sustainable techniques and efforts could get permits, on which the Netherlands is experimenting nowadays.
The growth of the oyster population is not only beneficial for the oyster fisheries, it brings also a lot of opportunities to the tourism sector. Foraging tours and boat tours are ways to create value out of the existence of the oyster. Also with this opportunity, a limited amount of tourism tours should be allowed, preventing the region from pressures such as pollution from the tourists.
The influence on the ecosystem services provided by the Wadden Sea are clear for this scenario. The oysters are imported by the oyster fishing businesses for economic reasons. The oyster population balances due to import and fishing, provides us with food. The fishing industry creates therefore use-value out of the oysters, resulting in a positive influence on the food provisioning ecosystem service of the Wadden Sea.
In this scenario, the imports of oysters will be regulated and controlled on pests and
diseases. Therefore, the biological control service of the Wadden Sea remains intact in this scenario, unless a lack of the controlling system occurs and pests or diseases accidently enter the ecosystem. The tourism and recreation service will be improved, as there are foraging tours and boat tours to observe and pick up oysters in small amounts. It’s a unique site of the environment and the value of oysters has been used in this scenario. Although this recreation and tourism increases, it is managed in a sustainable manner in order to sustain the ecosystem and its services. The UNESCO world heritage state will remain as it is nowadays, by keeping the environment a high priority.
Scenario 3: ‘Free oyster flow: deregulating the Wadden sea’
The third scenario that is described within this analysis refers to a possible situation where, within a conservative environmental skepticism framework, the Wadden Sea area is
considered of non-use value. This most likely would refer to the ‘option value’, which attributes to the value being placed on the exploitable wildlife (e.g. the Pacific oyster and other bivalves) in regards to them being available and being used by future generations, or the ‘existence value’, concerning the economic value that is placed on the Wadden Sea, reflecting the benefit people experience by the sheer existence of the Wadden Sea. The conservative environmental skepticism framework will however probably mean that there will be little to no governmental support, which is possible because the international conservation plan of the Wadden Sea that has been agreed upon is not legally binding. Support for the option value could come from the fisheries themselves, as they might halt their activities when they prove to be not sustainable enough. In this situation the
preservation of exploitable ecological systems might be more desirable than immediate exploitation.
The existence value of the Wadden Sea could increase, concerning the special status that the area and its wildlife hold as a UNESCO World Heritage site. There could be a general concern within the public when it comes to the existence of the Wadden Sea and its biodiversity. However, because of the conservative environmental skepticism stance of the government, this environmental preservation should come from NGOs, who lack the means of the state.
These changes would affect the ecosystem services in several ways. A possible reluctance of the fisheries to continue the exploitation of the ecosystem, will affect the food provisioning service the Wadden Sea provides. Economic exploitation of the ecosystem would not be pursued, resulting in the decline of the overall yield. This means a somewhat immediate economic decline for the sectors involved, due to the dependence on a continued yield. The change of stance of the government to conservative environmental skepticism would implicate that there would be less biological control on the Wadden Sea area. The Trilateral Cooperation would probably come to a halt, which means that the Wadden Sea area and the spread of the Pacific oyster would be less effectively monitored. An accurate assessment of the competition between the Pacific oyster and other bivalves becomes more complicated. Combined with the given that the carrying capacity of the Pacific oyster population has not yet been reached, will probably result in the further increase of the Pacific oyster population. Nevertheless, this would not differ much from the current impact the Pacific oyster has on the ecosystem services.
This likely increase in the Pacific oyster population might benefit the tourism sector, especially when it is in relation to an increase in existence value. Tourist tours could be arranged, to fulfill the desire of the people to experience the Wadden Sea as a world heritage site. In this sense, the existence of the Pacific oyster could still be economically exploited. If the mudflats are considered to be the cultural heritage, the increase in the Pacific oyster population would affect this ecosystem service negatively, due to the increase in oyster bed density.
Finally, the increase in the Pacific oyster population, as a result of them not being physically exploited, would mean the expansion of the oyster beds, which in turn would mean an increased flood protection.
Scenario 4: ‘Tragedy of the commons: exploitation of the Wadden Sea’
The last scenario refers to a possible future situation where the Wadden Sea is being exploited, within the framework of conservative environmental skepticism. The use value is deemed important, to the point that economic exploitation is seen a more essential than preservation or sustainability. This would probably result from an overall distrust in environmental research, concerning the capacity of the Wadden Sea area to sustain exploitation, or a general indifference to the implications that large scale exploitation can have on the long term.In this possible future situation, governmental regulations concerning imports of exploitable wildlife (e.g. oysters and mussels) would cease to exist, to maximise the benefits that could
be gained through cultivation and fishing. Fisheries would have more freedom to operate in the Wadden Sea than they currently have. Effective cooperation between the Netherlands, Germany and Denmark to conserve the natural systems of the Wadden Sea would be deemed less important than the individual gain that is to be had from exploitation. This results in the gradual dissolvement of the trilateral cooperation in the face of economic competition. However, the Wadden Sea area would still experience some degree of
protection as a UNESCO World Heritage site. This is a fact that could not be ignored by the involved countries, resulting in limitations to the complete exploitation of the Wadden Sea. This scenario would lead to a certain degree of imbalance in the ecosystem services. At first, the increased amount of exploitation would lead to an increased food output from the
Wadden Sea area. Areas that are currently protected and restricted could be exploited, and larger amounts of fish and other animals could be taken out at once, due to less regulations. However, this seems not to be sustainable. On the long run, populations would get depleted. Also, sustained exploitation would result in the inability of the ecological systems to recover (Giebels et.al., 2013). This would eventually lead to the decline in the food output.
The deregulation on imports of exploitable fauna might have the effect of importing more hitchhiking species and diseases, making the ecosystem more susceptible to disturbances. This would be complemented by the lack of governmental monitoring and the resulting lack of knowledge concerning the state of the ecosystem.
Additionally, this damaging of the ecosystem of the Wadden Sea would have a negative effect on the current migratory services that the Wadden Sea area provides. Migratory birds would see their food sources shrink, making the Wadden Sea a less appealing habitat. The Wadden Sea could, to a certain extent, still provide for tourism however. The area would not immediately lose its natural ecosystem and there would still by some degree of existence value. Tours might still be organized, especially when it comes to such exploitation as
through, for example, oyster picking. Yet, eventually this also would result in the decline of the habitat of such bivalves, the damaging of the oyster and mussel beds, and the
consequent loss of tourist opportunity.
Also, the damaging of the oyster and mussel beds would affect the buffer zone between the deep sea and the mainland. This effectively decreases the coastal protection against high tides and flooding.
Table 4: Overview of the different impacts from scenario 1 - 4 on ecosystem services. Red indicating a negative impact, green indicating a positive impact, red / green meaning positive impacts as well as negative impacts and gray indicating no impacts at all.
In the table (4) shown above an overview is given of the impacts the different scenarios have on the ecosystem services. As we can see scenario 1 only puts pressure on the Wadden Sea by affecting its status as cultural heritage. However in this scenario all economic benefits that can be gained are left aside. In scenario 2 only food provisioning is partly pressured by the Pacific oyster but this is compensated by new opportunities for fisheries. Scenario 3 almost pressures all ecosystem services by not controlling or maintaining the Pacific oyster or the Wadden Sea in general. It only has a positive effect on the flood
protection by the creation more hard substrate. Finally scenario 4 puts pressure on the more biocentric ecosystem services namely the biological control and the function as a migratory and nursery habitat of different bird species. However in this scenario maximum economic profit is gained by exploiting the Pacific oyster as much as possible.
Conclusion
To conclude, an answer can be given on the research question: “How can we limit damage caused by the Pacific oyster in the Wadden Sea, while profiting its presence as much as possible?”. First of all we used the DPSIR framework to give structure to our main theories to identify the Drivers, Pressures, State, Impact and Responses. For the drivers we identified financial gain and willingness to preserve the ecosystem, this resulted in the introduction of a
non native species as the main pressure. The state can best be identified as the current situation of the ecosystem. The impact that the ecosystem is undergoing is the introduction of pests and the loss of biodiversity. This eventually results in a social metabolic analyses as the response. After that a scenario analysis on the possible outcomes of different
trajectories of driving forces was done to deduce which strategies should be employed per scenario.
From the first scenario it can be concluded that progressive green thinking leads to the ban on wild oyster farming businesses and the imports of non-native oyster species for
cultivation. The imports and introductions are banned, as the Wadden Sea should be conserved and the economic value of the oysters is neglected. Therefore, the chance of putting pests or diseases into the environment, which can be dangerous for other species, is almost none.
In the second scenario there is a balance between green management of the Wadden Sea and on the other hand the profit of the economic opportunities of the presence of the Pacific oyster. Oyster fisheries are able to import the oysters to sustain the business and to grow the population the imports of oysters will be regulated and controlled on pests and diseases. For the third scenario the conservative environmental skepticism entails that there will be few to none governmental support, The increase in the Pacific oyster population, as a result of them not being physically exploited, would mean the expansion of the oyster beds, which in turn would mean an increased flood protection.
The last scenario, the increased amount of exploitation would lead to an increased food output from the Wadden Sea area. Areas that are currently protected and restricted could be exploited, and larger amounts of fish and other animals could be taken out at once, due to less regulations. However, this seems not to be sustainable. On the long run, populations would get depleted. Also, sustained exploitation would result in the inability of the ecological systems to recover (Giebels et.al., 2013). This would eventually lead to the decline in the food output.
Comparing the scenarios we conclude that the second scenario is the most ideal for limiting damage caused by the Pacific oyster in the Wadden Sea, while profiting its presence as much as possible. In this scenario there is a balance between financial gain and ecosystem preservation. The financial gains are made in the tourism sector and businesses are able to import the oysters to sustain their businesses. The ecosystem preservation side of this scenario consists of biological control of the Wadden Sea and businesses are legally forced to document the amount of oysters they import. In the first scenario there is less pressure on the ecosystem services but the economic opportunities are not exploited and are therefore not in balance which makes it a less ideal scenario.
Discussion
Ecological impacts
As of now it is still not entirely certain the impact the Pacific oyster has biologically as well as socio-economically. There has been done a lot of research on how the Pacific oyster
impacts biodiversity locally (Troost, 2010) (Markert, Esser, Frank, Wehrmann, & Exo, 2013) (Markert, Wehrmann, & Kröncke, 2010). Much less has the impact of the Pacific oyster on regional biodiversity been researched, especially in regards to the Wadden Sea specifically (Green & Crowe, 2014). This can make it hard to precisely predict the long term impact the Pacific oyster will have on biodiversity. It is clear that an increasing abundance will
eventually lead to the homogenization of the Wadden Sea, and therefore lead to a decrease in biodiversity. The increase in harder substrate caused by this homogenization can also have an effect on the overall geomorphology of the Wadden Sea. This has to be brought to the attention of policy makers considering the role of the Wadden Sea as UNESCO cultural heritage. As for the hitchhiker species no pests have been introduced into the Wadden Sea up until this point, but further import of the Pacific oyster will undeniably lead to more
hitchhiker species. To minimize the effect hitchhiker species have, import should be suspended or regulations have to be more strict to prevent establishment outside of the cultivation area. All these effects are currently not detrimental to the ecosystem or its services. However future spread and growing abundances do put more pressure on the ecosystem and its services and should therefore be minimized as much as possible.
Economic impacts
In this paper a broad assessment has been done about the economic gain that can be ascribed to the Pacific oyster. The overall amount that can be gained is still very uncertain but it is important to notice that there is profit to be gained from the invasion of the Pacific oyster. In order to do this ways have to be explored in which exploitation can be done without the further disturbance of the ecosystem. Sustainability is usually not the most profitable way of exploiting a resource. Therefore the government may need to promote sustainable fisheries in order for a market to grow. This is the case if sustainable oyster fisheries are deemed not profitable enough. Sustainable fisheries on the Pacific oyster will likely not have a significant impact on the current population size due to their limited exploitation ability but will rather provide an alternative to oyster cultivation. This can compensate for the possible regulations on oyster cultivation and lead to a policy in which every stakeholder is accounted for.
Scenarios
In most papers regarding the invasion of the Pacific oyster only a brief advice is given on how to deal with this invasive species. In this research four specific scenarios are given for policy makers to base their decision on. The multidisciplinary approach that was taken to make these scenarios plays a key factor in sketching the most complete image of the situation. This helps policy makers to interpret all of the factors involved in the situation. However the scenario matrix only explores the most extreme sites of the driving forces. The reality is not as black and white as is described in the scenarios. It is up to policy makers to make a balanced decision between conservation of the ecosystem and the exploitation of the Pacific oyster.
There are a few recommendations for further research. Firstly the specific impacts can be explored in more detail. It is for now still unclear at what degree the homogenization of the Wadden Sea takes place. Furthermore the economic losses due to loss in the mussel
population have not been appointed and the economic gains that can be linked to the Pacific oyster are highly uncertain. A quantification of these gains and losses can help our
understanding of the impacts of the Pacific oyster. Secondly the scenarios that were made are qualitative analyses and thus bring a degree of uncertainty. These scenarios can be made quantitative and would help eliminate a portion of the uncertainties. Lastly a
comparative study can be done on the ways of wild fishing on the Pacific oyster. This can aid policy makers in making an educated decision on which way oyster fishing should be done in order to minimize ecological disturbance and maximize profit.
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