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Onderdeel van Invasieve exoten


Monitoring types

The overview of the various regulations, directives and guidelines and their information needs given in chapter 2 indicates that a single monitoring approach cannot satisfy all information and monitoring requirements. On the contrary, different monitoring types are needed to answer the various questions and these can be linked to the stages and barriers as described by Blackburn et al. (2011) in their unified framework for biological invasions (Figure 2).

1. Prevention monitoring
Prevention monitoring aims at detecting alien species before they become established in a new environment. Generally, vectors for transport of alien species are being targeted with this kind of monitoring (Transport Stage). Monitoring results can instantly be used to avert the spread of alien species. Examples are ballast water monitoring and the monitoring of alien species in shellfish transports in the Netherlands, which is based on comparing alien species present among the mussels of the export region and alien species present in the import region (Bleker 2012). Monitoring takes place when necessary, e.g. when shellfish are being prepared for transplant, thus generally at irregular intervals. Relevant policy documents:

2. Evaluation of prevention monitoring
This kind of monitoring would be conducted to assess the effectiveness of regulations aimed at stopping new alien species, and to maximise the likelihood of early detection of new invaders. Evaluation of prevention monitoring would be used to control whether preventative measures have been successful, and would take place at the (potential) introduction stage of an alien species. Ideally, this kind of monitoring would be preceded by prevention monitoring, but may also simply monitor the success or failure of general measures taken to avoid the introduction of alien species. An example could be the monitoring of Wadden Sea mussel seed plots and their surroundings before and after seed mussels have been transplanted from other locations. In practice, this monitoring type is similar to early detection monitoring described below, but is mentioned separately here, as it would always be preceded by some measure of prevention which would not necessarily be the case for early detection monitoring.

3. Early detection monitoring
Early detection monitoring aims to detect new alien species during the introduction or early establishment stage, thereby increasing the chances of a cost-efficient and successful eradication of the newly introduced species. Monitoring is usually focused on the most invasion-prone habitats (hotspots) for new introductions. In the case of an ‘alert list’ and extensive knowledge of species’ traits, habitat preferences etc., the monitoring of hotspots might be sufficient, whereas a broader approach might be more appropriate in the event that such knowledge is lacking. The rapid assessments of alien species performed in the Netherlands (Gittenberger et al. 2009, 2012 and 2014) and in Germany (Buschbaum et al. 2012, Lackschewitz et al. 2015) serve the purpose of early detection. Existing monitoring programmes that create species lists can also be used for this purpose, as proposed for example by Andersen et al. (2014) for Danish marine waters. Relevant policy documents:

4. Population dynamics monitoring
Population dynamics monitoring has the objective of describing the detailed population dynamics of an alien species over time and spatial scales, from its introduction and establishment to its (potential) later stage of further spreading. This kind of monitoring aims to help explain the mechanisms underlying the population dynamics. It cannot answer any questions with regard to the impacts of alien species (see 5. Impact monitoring). Examples include the monitoring of the population dynamics of Crassostrea gigas in the East-Frisian Wadden Sea in Germany (Wehrmann et al. 2006). Relevant policy documents:

5. Impact monitoring
Impact monitoring is the systematic identification and evaluation of the potential effects an alien species may have on its environment. Scientific experiments will usually precede this kind of monitoring in order to determine the presence and mechanisms of impacts. With the help of monitoring, these will then be quantified in the field to determine their extent. Hence, impact monitoring will not solely be directed at the alien species themselves, but also at variables they affect. This kind of monitoring can be combined with monitoring of the population dynamics of an alien species. Generally, evidence of impacts of alien species on marine ecosystems is weak, as only a minority of reported impacts have so far been inferred via experiments, not based solely on subjective judgments or correlations (Katsanevakis et al. 2014). Six alien species have been described as having already had or about to have effects on the composition of the existing biota in the Wadden Sea: cord-grass (Spartina anglica), Japanese seaweed (Sargassum muticum), bristle worm (Marenzelleria viridis), American razor clam (Ensis americanus), American slipper limpet (Crepidula fornicata) and Pacific oyster (Crassostrea gigas) (CWSS 2010).

There are no clear-cut examples of impact monitoring in the Wadden Sea. To our knowledge, the research into the effects of the Pacific oyster on the native blue mussel, Mytilus edulis, (e.g. Diederich 2005, 2006, Eschweiler & Christensen 2011) and the monitoring of the Pacific oyster, including its occurrence on blue mussel beds in parts of the Wadden Sea (e.g. Wehrmann et al. 2006, Büttger et al. 2014) are the closest to impact monitoring to date, when applying the more rigorous demands on impact monitoring that Katsanevakis et al. (2014) suggest. They see the need to shift from approaches that offer only weak evidence (e.g. non-experimental based correlations) to measurements or experiments (e.g. manipulative experiments) that ‘offer both strong evidence of an impact as well as an estimate of the magnitude of the impact’. Relevant policy documents:

This short overview exemplifies the point that each monitoring type fulfils a different task and can help to answer different questions. Often, monitoring types get intermingled, as in the case of population dynamics monitoring and impact monitoring. Hence the objective (what do we want and need to know to answer the question we have?) needs to be carefully chosen and defined.


Requirements for a trilateral monitoring strategy
Article 5(2) of the MSFD states that Member States sharing a marine region shall cooperate to ensure that required measures are taken. Article 8(3) further states that member States shall cooperate to ensure that ‘assessment methodologies are consistent across the marine region or subregion’ and ‘transboundary impacts and transboundary features are taken into account’. This conforms to the aim of the European Regulation to protect native biodiversity and ecosystem services for the whole of the European Union (including coastal regions such as the Wadden Sea).

The Netherlands, Germany and Denmark currently differ in approaches to the monitoring of alien species in the Wadden Sea. Discussions between the Wadden Sea neighbouring countries concerning a trilateral strategy that guarantees a common approach led to the formulation of the Strategic Framework for dealing with Alien Species in the trilateral Wadden Sea. In order to facilitate a common approach, some aspects need to be safeguarded:

  • A trilateral monitoring approach should suit national interests and approaches in terms of feasibility and costs, thereby making use of existing national monitoring programmes.
  • The monitoring performed in each of the three countries must produce reliable data that will ideally be able to answer the questions posed by the MSFD and the EU Regulation on the Prevention and Management of the Introduction and Spread of Invasive Alien Species.

Strengths and shortcomings

Strengths and shortcomings of current alien species inventories and other (long-term) monitoring programmes
WaLTER employs the definition by Lindenmayer & Likens (2010) for long-term ecosystem monitoring, which reads ‘Repeated field-based empirical measurements are collected continuously and then analysed for at least 10 years’. Not all monitoring types described in the previous section meet this definition, as prevention monitoring and its evaluation usually take place when necessary, thus at irregular intervals (e.g. during ballast water discharge or when preparing the transplant of shellfish species). In the following (sub)chapters, focus will thus be put on the analysis and recommendations for early detection, population dynamics and, to a lesser extent, impact monitoring of alien species (types 3-5 in the previous section) as these could be part of a long-term basic monitoring programme as advised by WaLTER.

To date there has been no long-term monitoring of alien species in the trilateral Wadden Sea according to the above definition by Lindenmayer & Likens (2010). The longest running programmes, the SETL-project in the Netherlands and the rapid assessments performed by Alfred Wegener Institute researchers in the German Wadden Sea, only began in 2006 and 2009, respectively. Nevertheless, all explicit inventories of alien species in the Wadden Sea will be considered here, as they could form the basis of a long-term monitoring programme for alien species in the future. There are also a number of long-term monitoring programmes being established that could contribute to accumulating the necessary knowledge on alien species for management purposes. Table 2 gives an overview of relevant monitoring programmes and inventories that could become (part of) a future long-term monitoring programme for alien species. It includes details on their suitability in answering the monitoring questions and information needs stemming from policies and guidelines, namely with regard to early detection and monitoring of population dynamics.

The table shows that differences exist between the inventories/monitoring programmes with regard to investigated taxa, extent to which Wadden Sea national areas were investigated (NL, D, DK, respectively), and inclusion of hotspots. Some conclusions that can be drawn from the table are:

  • Not surprisingly, none of the inventories/programmes survey all of the taxa listed in the table. It must be stressed that the list of taxa in table 2 is not conclusive, as there are alien species groups that have not yet been monitored (in much detail) and therefore do not show up in the table, e.g. microorganisms or parasites.
  • The spatial extents of inventories and programmes vary, from good or very good coverage of the whole extent of the respective national Wadden Sea area (as is the case for the inventories by GiMaRIS, AWI, and the SIBES project), to single sampling locations for fish (NIOZ fyke net), phytoplankton (NIOZ jetty monitoring, Danish NOVANA monitoring programme), or single tidal flat areas (Balgzand monitoring). However, the latter single monitoring locations in the Dutch Wadden Sea are situated at the Southern ‘gateway’ to the Wadden Sea and may potentially function as early warning systems should new alien species arrive with currents (or other vectors) from further south. Also, alien species found here can be expected to show up further north in the future, should their larvae be capable of spreading via currents.
  • Only the dedicated alien macrobenthos inventories by GiMaRIS (2009, 2011), GiMaRIS and NIOZ (2014), AWI (since 2009), the SETL project and the surveys performed under the Joint HELCOM/OSPAR Guidelines sample clearly known hotspots of alien species, such as marinas or mussel beds. It is not known whether there are hotspots for endobenthic alien species in soft sediments or where these would be, and a search for alien species therefore resembles a ‘blind poking’ leading to ‘chance findings’. However, it is possible to visually search for soft sediment-related species living on the sediment (epibenthos), such as algae or crustaceans.
  • In Denmark, neither the NOVANA nor the fish monitoring programme can currently provide sufficient information on alien species for the Danish Wadden Sea (see also Andersen et al. 2014). The NOVANA monitoring programme includes only two monitoring stations for phytoplankton and for eelgrass, respectively. Information on other taxa is not collected. The fish monitoring programme that Andersen et al. (2014) suggested to include in a monitoring programme of alien species in Danish marine waters does not collect any data on fish in the Danish Wadden Sea. This would have to be amended.
  • None of the inventories or monitoring programmes are equally suited for early detection and for uncovering the population dynamics of an alien species. This, of course, lies in the nature of the alien species inventories/rapid assessments as these are ‘aimed to detect as many alien species as possible, combining an efficient use of given resources in manpower and available time with the highest gain of information’ (Buschbaum et al. 2012). On the contrary, monitoring programmes like the Balgzand intertidal transect programme or the SIBES project are set up to monitor long-term changes in populations of Wadden Sea macrobenthic fauna in detail, and are therefore much more time-consuming. In order to judge whether the ‘good environmental status’ of alien species under the MSFD is met, questions about the trends in abundance or the temporal occurrence of alien species in the Wadden Sea have to be answered. It is debatable whether the statement of pure ‘presence/absence’ as achieved in alien species inventories/rapid assessments is good enough to comply with the MSFD. In any case, the table illustrates that the current MWTL programme for macrozoobenthos in the Wadden Sea and the Eems-Dollard is neither suited for early detection, nor for uncovering the population dynamics of alien species, as the time intervals between sampling are too large.

The recent dedicated alien species inventories carried out in the Netherlands and Germany both give a good indication of the presence of alien benthic macrofauna and -flora on hard substrates at a national Wadden Sea scale. Differences between the Dutch and the German inventories exist with regard to 1) the inclusion of soft sediment habitats in the German rapid assessment, and only hard substrates in the Dutch inventory, 2) the larger number of sampling locations in the Dutch inventory compared to the number of sampling locations in the German assessment, and 3) the frequency of inventories performed, which is on an annual basis in Germany, while inventories have been done irregularly in the Netherlands (2009, 2011, 2014). If these inventories are to form the basis of a future trilateral monitoring programme for alien species, it will be necessary to harmonise the methodologies employed. This will enable a concerted control of the target setting under the MSFD, for which indicators are currently being developed in Germany (inclusion of soft sediment locations, agreement on number of sampling locations, a minimum of annual inventories).

One shortcoming of both these inventories is that that they looked at benthic macroflora and macrofauna only, thereby neglecting possibly relevant smaller sized alien species (such as phyto- or zooplankton). Buschbaum et al. (2012) restricted their inventory in the German Wadden Sea to macrobenthos because alien micro- and meiobenthos are not known for the Wadden Sea. However, this can also be attributed to a reduced search effort for species in these groups and thus incomplete knowledge (Reise et al. 2006). Furthermore, the inventories focused on benthic species, thereby neglecting pelagic taxa such as fish and jellyfish, which can also contain important invasive alien species. However, with regard to pelagic taxa, it can be argued that these can also arrive in the Wadden Sea via natural drifting and it is very difficult, if not impossible, to assess whether they have been transported via vectors for alien species that can and should be managed. It has been suggested to deal with them at the scale of the entire North Sea (Buschbaum et al. 2012). Further trilateral discussions on how to deal with these species groups therefore seem necessary.

The Wadden Sea’s natural values are related to the soft sediments and its mussel beds (Thematic report Natural values of tidal flats). While mussel beds have received attention in hard substrate inventories for alien species, soft sediments have received less attention in inventories dedicated to alien marine species, as the sampling of soft sediments is 1) more difficult to perform due to technical limitations, and 2) the endobenthic species to be sampled are invisible when in the ground, which complicates their discovery (‘chance findings’). For soft sediment sampling programmes, the area sampled and the abundance of organisms found are extrapolated to larger areas of relevance for processes in the Wadden Sea. The current soft sediment sampling programmes/projects (Balgzand, SIBES) offer the opportunity to detect alien species related to soft sediments in the first place (albeit eradication may not be possible) and, as they are located at the gateway to the trilateral Wadden Sea, can function as early warning systems as described above. They also allow for the monitoring of population dynamics. Both SIBES and the Balgzand intertidal transect programme have advantages and disadvantages with regard to efficiency and accuracy (abstract of report ‘A comparison between the SIBES and the Beukema/Dekker benthos sampling program at Balgzand’).

All methods being used in alien species monitoring have advantages and disadvantages with regard to different variables, including sensitivity, taxonomic resolution, and time and manpower required (table 3). Methods should be carefully chosen to account for various factors, such as the size of the area to be monitored (the larger the monitored location, e.g. a large port, the higher the chance of missing species), and the frequency at which the monitoring should be performed (e.g. zooplankton should be sampled monthly or bi-weekly to account for population dynamics, while longer lived biota can be sampled annually; for suggested sampling frequency requirements, see Lehtiniemi et al. 2015).

‘Citizen science’ and DNA technologies are two promising tools that could supplement ‘traditional’ monitoring approaches, such as rapid assessments.

Citizen Science

‘Citizen science’ and educational approaches
The invitation and active involvement of diving and fishing associations and NGOs, so called ‘citizen science’, can lead to the reporting of alien species otherwise gone unnoticed. The advantages of this approach are that it is public-oriented, the costs are relatively low and locations that would have otherwise gone unvisited are also being investigated. The disadvantage is that the smooth running of citizen science-aided projects depends on a well-planned commitment of coordinators, as well as experts willing to assist in the identification of newly found species and the subsequent data management. A general, major drawback can also be the over-eagerness of laypersons reporting alleged alien species sightings, thereby tying up valuable resources in agencies to check the accuracy of these sightings (pers. comm. S. Nehring, German Federal Agency for Nature Conservation, meeting of FAG Neobiota, Nov. 2014). By restricting the call for participation to report alien species to informed laypersons like the above-mentioned members of diving and fishing associations and NGOs, this problem can be reasonably controlled.

In the Netherlands, the ANEMOON Foundation (Dutch: Stichting ANEMOON) played an important role in recording new alien species in the last two decades (pers. comm. A. Gittenberger, GiMaRIS). The foundation cooperates closely with expert taxonomists, and thus most records are checked and validated within days to weeks after a species is first discovered. Of particular interest is the foundation’s project MOO (Dutch: ‘Monitoringproject Onderwater Oever’), a monitoring project in which volunteer divers collect biological population information on native and alien species in various marine areas in the Netherlands.

Another promising approach is the close cooperation with educational organisations and institutions with a marine focus. As these already work together with schools, the public, and sometimes also universities, they can help increase public awareness of the problem of bioinvasions. One such ‘educational monitoring’ example is the Marina Aliens Project in the UK, in which school groups examine the arrival and settlement of alien species in marinas via settlement plates (for this and further educational projects with a focus on alien marine species, visit Marina Aliens Project).

Table 1: Organisations and projects with a marine educational mission in the trilateral Wadden Sea area, which could act as awareness multipliers for the topic of alien species.

Country Organisation Internet address
Netherlands Waddenvereniging
Stichting Anemoon
Germany Schutzstation Wattenmeer
Verein Jordsand
Multimar Wattforum
Triateral Intern. Wadden Sea School

DNA techniques

Species-level identification is historically conducted visually, and more recently with the use of DNA barcoding. DNA barcoding is a taxonomic method that uses a short, species-specific genetic marker in an organism’s DNA to identify it as belonging to a particular species. It can be used for direct sampling of species whose taxonomic position is unknown (adding value to rapid assessments performed by experts).

Species in an environmental sample can be identified by comparing obtained sequences to a standard reference library of sequences from known organisms (NCBI, the American National Center for Biotechnology Information’s GenBank, and BOLD, the Barcode of Life Data Systems). This assumes that the genetic information stored in a gene reference bank is correct, i.e. that sequences are based on correctly identified species. Caution must also be taken to ensure that alien species are not misidentified as native species, e.g. when their DNA sequence is not yet known, but the closely resembling DNA of its native sister species is present in the reference bank. One disadvantage of any DNA-based technology is the current incomprehensiveness of gene reference banks, though it seems to be only a matter of time before this is rectified as gene sequences are constantly added, as also noted by Andersen et al. (2014).

An advantage of this technology is that early life stages of pelagic and benthic alien species and other small organisms (e.g. bacteria, parasites and viruses) can be identified with it. For several habitats (mainly terrestrial, freshwater and semi-enclosed water bodies), it has already been demonstrated that metabarcoded samples are taxonomically more comprehensive, much faster to produce, and at lower costs, compared to standard morphological identification, which requires individual identification of large numbers of specimens by scarce expert taxonomists (Ji et al. 2013). However, whether this also holds true for the Wadden Sea will need to be investigated in more detail.

Another promising method for alien species monitoring is the environmental DNA technology. This uses a forensics approach to detect alien species. Whereas other methodologies require direct observations of species or, as in the case of barcoding, a direct sampling of the species itself, eDNA technology can detect species ‘sight unseen’. eDNA was also named as one of the promising tools in the online WaLTER survey performed by the Radboud University (Vugteveen & Hanssen 2012/2013 a,b,c) and in the Danish proposal for the monitoring of alien species in Danish marine waters (Andersen et al. 2014).

This non-destructive technology is based on the principles that all aquatic species release genetic material into the environment (mucus, faeces, urine). These trace amounts of suspended eDNA can be collected in water samples, extracted and amplified. At a later step the presence of individual species can be detected through the recognition of diagnostic fragments. If the eDNA technology indicates the presence of an alien species, taxonomic experts will have to be contacted to verify the presence and assess the invasion extent. A review of the possible applications of eDNA for the detection of (invasive) species via eDNA can be found here.

The application of the eDNA technology is hampered by some characteristics of marine habitats, namely the extreme water volume to biomass ratio, the effects of sea currents and wave action on dispersion and dilution of eDNA, and the impact of salinity on the preservation and extraction of eDNA (Thomsen et al., 2012). eDNA persistence in aquatic systems varies per species, ranging from a day up to a month (Thomsen et al. 2012, Rees et al. 2014), making it impossible to obtain real-time information on an organism’s location. Furthermore, conclusions about abundances of (marine) alien species are still impossible.

However, a promising avenue is the use of eDNA in combination with alert lists, e.g. by controlling for alien species that are being listed on the alert list under the European Regulation on the Prevention and Management of the Introduction and Spread of Invasive Alien Species. e-DNA samples could potentially be taken within the framework of the regular statutory water sampling programmes in the Wadden Sea and the Eems-Dollard. Similar to the proposal by Andersen et al. (2014), taking additional water samples for e-DNA research could easily be achieved. The fact that eDNA could possibly derive from organisms further upstream does not per se constitute a disadvantage, as this would inform IAS managers in advance of the arrival of a known invasive alien species. The same applies to the fact that eDNA could derive from dead specimens, as it would alert IAS managers that the possibility of living specimens is present. Comtet et al. (2015) discuss these ‘false positives’, which are ‘likely to occur at high frequency in highly diffusive and dispersive habitats’, like seawater. The authors conclude that, as early detection is crucial to managing biological invasions, ‘an early detection of eDNA (…) is a ‘red flag’ indicating that the sample was in some way exposed to the organism (…). Additional protocols, a good level of sampling replication and a verified sequence database are then necessary but their implementation may be much simpler because the potential hazard is known’ (Comtet et al. 2015, p. 915). Also, the eDNA technology could be applied in (semi) enclosed areas such as marinas with no or hardly any current, and no ballast water management taking place (which would potentially release eDNA of dead organisms).