- Newcomers on board
- > The immigration and spread of non-indigenous species is one of the drivers of species turnover. In many parts of the global ocean, newcomers have already altered established biological communities by preying on keystone species, displacing native species, or introducing diseases. But not everything is black and white. In the Wadden Sea of the south-eastern North Sea, newcomers and long-established species live in mutually beneficial coexistence.
Non-indigenous species: Curse and blessing
Non-indigenous species: Immigrations caused by human activity
An important driver of marine species turnover is the immigration and spread of non-indigenous species. These are organisms that have either been introduced into an ecosystem by humans or have been able to migrate on their own after humans removed barriers to their spread.
Non-indigenous species can migrate, for example, when the construction of a canal connects two marine areas that were previously separated. Species migrations also occur when climate change warms the ocean and warm-blooded species find suitable conditions in previously cold areas.
In contrast, researchers speak of alien species introductions when organisms are brought into new habitats by ships, when they are deliberately released by humans, or when they colonize new areas because they are kept in cages or net bags for food production and manage to escape – whether through accidents, storm damage or the release of larvae.
Non-indigenous species that spread into a habitat and disrupt the normal interactions of native animals, plants and micro-organisms are called invasive species. Such adverse effects occur, for example, when newcomers increase predation pressure on keystone species, displace native species or introduce diseases to which local marine life is vulnerable.
There are several examples of marine invasive species. However, their numbers are much smaller than on land because marine habitats are more open at their edges. When newcomers outcompete long-established marine species, the latter are more likely to migrate or retreat into niches than many species inhabiting terrestrial habitats, such as forests or meadows. Nevertheless, scientists fear that globalization may one day cause marine biological communities to become too similar world-wide, with the overall result that global biodiversity declines. Whether this will actually come to pass is a matter of debate, even among experts.
In species-poor marine areas such as the Wadden Sea in the south-eastern North Sea, the more than 100 known non-indigenous species have so far not displaced any of the native species. In fact, some of the invaders have actually streng-thened local ecosystems. For example, the colonization of blue mussel beds in the Wadden Sea by introduced Pacific oysters has made the mixed reefs more resilient. When storms whip across the North Sea, the oyster-mussel beds tend to suffer less damage than beds formed by blue mussels alone.
Researchers have also been able to document examples in which non-indigenous species have replaced established species in their function. Although this has changed the species composition in the ecosystems concerned, importantly their range of functions has remained the same.
Nevertheless, everything humanly possible should be done to prevent the spread of non-indigenous species, as it is also clear that many of the long-term consequences of human-induced species migrations are not yet fully understood.
While intensive and, in many places, systematic research is now under way, it is often unclear which species have been present in the area under investigation for what length of time and whether that time span is long enough for them to be considered “native”. Moreover, it is still too rarely possible to compare the results of observations made in different regions.