Number of non-indigenous species over time
In 2020, 79 non-indigenous species with established populations were identified in the Belgian part of the North Sea and adjacent estuaries (Figure 1). The arthropods (Arthropoda) represent 39% of these (31 species). This diverse group includes crabs, copepods, barnacles and small shrimp-like crustaceans. Algae and weeds (15%; 12 species) and molluscs (Mollusca) (10%; 8 species) follow at a considerable distance.
Figure 1: The number of non-indigenous species in the Belgian part of the North Sea and adjacent estuaries classified according to higher species levels.
Five species appear to have been present on Belgian territory since the 19th century. The number of new introductions has been increasing steadily throughout the 20th century (until the 1990s) with five or fewer new species per decade. In the year 1990, 33 non-indigenous species were known in the study area. However, the 1990s saw a sudden increase in the number of introductions with 23 new species in just ten years. Despite the fact that the number of new species introduced since 2000 seems to be declining steadily, it is still well above the level of before the 1990s (Figure 2).
Figure 2: The number of new non-indigenous species in the study area per decade (species introduced before 1901 are presented together). The bar graph shows the number of new introductions per decade, the line graph shows cumulatively the total number of established non-indigenous species at a given time.
Worldwide, international shipping plays a major role in the unintentional spread of organisms outside their area of origin. The fact that the Flemish seaports are located along one of the busiest maritime shipping routes therefore increases the risk of new introductions via maritime transport. The strong increase in the number of non-indigenous species from the 1990s onwards could at first sight be explained by the increase in shipping traffic to the Flemish seaports, which in terms of tonnage of goods is twice as high in 2019 as in 1990 and almost three times as high as in 1980 (Merckx 2020a). However, the number of shipping movements experienced a decrease of -15% over the same period (1980-2019), so that the increase in tonnage of goods is the result of increasingly larger seagoing vessels. However, the 'intercontinental' character of the shipping (e.g. in the port of Antwerp) has experienced a strong increase since 1990, with maritime transport to and from Asia (in volume) increasing sevenfold in the last three decades and goods transport with America and Africa doubling (Merckx 2020, personal communication).
Another possible explanation for the sudden increase in the number of non-indigenous species after 1990 - which occurs mainly in arthropods, algae and seaweed - is the increased frequency of biological sampling and the development of monitoring campaigns at sea, whether or not in the context of European regulations and international or regional obligations. For example, the number of species records in the GBIF (Global Biodiversity Information Facility - www.gbif.org/tools/observation-trends) for the study area show an increase since the 1970s and a sharp acceleration since 1990 (Figure 3). These data indicate a more intensive biological sampling in the study area, which also increases the chances of discovering new non-indigenous species. As a result, for some species the first observation may be later than the actual year of introduction.
Figure 3: The number of new non-indigenous species in the study area per decade, categorised by species groups. The grey area diagram shows the number of species records in GBIF for the 'Animalia' kingdom in the Belgian part of the North Sea and adjacent estuaries, and is an indicator of the intensity of biological sampling.
Areas of origin
The areas of origin of the established non-indigenous species in the study area are very diverse. Some species occur naturally in several regions while the natural range of some species is uncertain. As a result, the sum of all species across regions of origin is greater than the actual number of species (Figure 4).
Figure 4: The number of non-indigenous species in the study area by region of origin. The sum of the different regions exceeds the actual number of species because some organisms are linked to more than one region of origin
The regions of origin of the non-indigenous species present in the study area are mainly the Northwest Atlantic and the Northwest Pacific (Figure 4). Together they represent over half of the exotic species and account for 22 (26%) and 25 species (29%) respectively. From the North West Pacific, mainly arthropods and algae and weeds have been introduced in Belgian waters, whereas from the North West Atlantic, the main species groups are arthropods and molluscs. The Pacific Ocean as a whole forms the natural habitat for half (50%) of the non-indigenous species in the Belgian part of the North Sea and adjacent estuaries, while the entire Atlantic Ocean accounts for 31% of the species. For a dozen species their natural range is still unclear.
Introduction pathways
There are many ways in which marine species can be introduced outside their natural range. By far the most important introduction vectors for the non-indigenous species in the study area are shipping (Maes et al. 2018) (ballast water, fouling on hulls, in cargo) and aquaculture/live import (Figures 5 and 6). In addition, introductions take place through the construction of canals between regions that were initially separated by physical barriers, which encourages the further spread of species beyond their area of origin. Non-indigenous species can also be deliberately introduced through the stocking or planting of exotics, as is the case with the established non-native vascular plants in the study area.
Figure 5: Overview of possible ways of introduction at higher species level. The sum of the different species groups is higher than the actual number of species because there is often uncertainty about the primary means of introduction (several possibilities) or because some species were introduced several times separately from each other from the area of origin.
Figure 6: The relative importance of each of the vectors responsible for the primary introduction for the non-indigenous species in the study area. Shipping is considered here as a single vector and includes transport via ballast water, hull fouling and cargo. The sum of the modes of introduction exceeds 100% because there is often uncertainty about the mode of primary introduction (several possibilities) or some species have been introduced several times separately from the area of origin.
Depending on the area of origin, the importance of a certain vector varies. For example, all exotic species originating from the Pontokaspian region (Black Sea, Caspian Sea) were introduced as a result of digging canals between the Black Sea and Western Europe. In this way, organisms were able to spread naturally westwards via connected rivers and on barges. Introductions from the Atlantic Ocean, in turn, were mainly via ballast water transport, while from the Pacific Ocean aquaculture/live import played a very important role alongside shipping.
Despite the fact that international shipping trade has gained in importance in recent decades, this vector also played an important role (in relative terms) in the spread of species outside their area of origin as early as the 19th century and the first half of the 20th century (Figure 7). Both transport in ballast water and hull fouling have been occurring for a long time. The International Convention for the Control and Management of Ship's Ballast Water and Sediment (BWMC) (see Policy and Legislation) aims to significantly reduce or prevent the spread via ballast water within the framework of international maritime transport in the future.
Figure 7: Overview of modes of introduction as a function of time. The sum over the different decades is higher than the actual number of species because there is often uncertainty about the mode of primary introduction (several possibilities) or because some species were introduced several times separately from each other from the area of origin.