Key species and key habitat types drive the Baltic Sea

Key species and key habitats mean species and species communities that have a broad impact on the functioning of their habitat and the ecosystem services provided by an area.

Suvi Kiviluoto

The author works as a researcher at the Finnish Environment Institute in the Sustainable Use of Marine Areas group.

The best known key species in the Baltic Sea is the bladder wrack, formerly known as the bladder seaweed. You might be somewhat surprised to find that the multi-species vascular plant communities along shallow summer-cabin shorelines also belong to key habitat types. Favourable habitats for many endangered species are created by protecting such key species and habitats.

Common species play an important role in ecosystems

When talking about the species and diversity of the Baltic Sea, attention is often focused only on endangered or alien species. This can give the impression that a species is important only when it is disappearing or threatening some function of the ecosystem.

However, most species in the Baltic Sea, in particular those making up the biological biomass, are anything but rare or alien. Indeed, the marine ecosystem can be viewed as a whole entity where the different parts support each other. The habitats formed from the common and dominant species, as well as the nature types, affect the state of the entire Baltic Sea. Therefore, their survival should also be safeguarded.

 Blue mussels filter seawater while covered by filamentous algae and snails.
Key species, such as blue mussels, maintain a large group of organisms.

Bladder wrack is a key species of the Baltic Sea

Key species are those which are often very common, and which maintain larger communities. The best known example is probably the bladder wrack, which turns bare rock surfaces into leafy forests.

Other species of algae grow under the shelter of the bushy bladder wrack, and this seaweed community provides shelter and food for a large variety of invertebrate animals. The shelter and abundant nutrition also attract a large number of juvenile fish. Thus, the influence of key species extends beyond the actual occurrence of that species in time, space, as well as species composition.

Key species and the natural habitat types they form occur in different environments. Wrack kelp and red algae communities are located on reefs and underwater rocky surfaces, while sandy and muddy bottoms, on the other hand, are brought to life by seagrass meadows and vascular plant communities. Many endangered species require the living conditions created by key species to survive. For example, in Finnish coastal waters, the endangered marine leaf beetle requires dense colonies of vascular plant species, such as fennel pondweeds or watermilfoils, to survive and reproduce.

 Two loose bushes of bladder wrack tumble on a seabed covered with filamentous algae.
On shallow rocky shore, green algae and bladder wrack are typical species that are the lifeblood of many small organisms.

Key habitats are formed around key species, which arise from the interaction of species

Key habitats are typically considered to be habitats maintained by one or two key species. They are permanent and maintain a large variety of other species.

The most easily observed key habitats are found in the vascular plant communities of shallow marine bays. They can be found in almost all naturally-occurring, as well as slightly modified soft bottom shore areas.

The vascular plant communities are typically characterised by the diversity of plant species and a variety of dominant species. In the more open parts of shallow bays, the predominant species are, in addition to various pondweeds, the horned pondweeds and the ditch grasses. Closer to the shore, space is increasingly occupied by water milfoils and hornwort.

Regardless of the predominant species, the whole community creates a habitat in which various organisms thrive, such as invertebrates, insect larvae, fish fry, as well as waterfowl and shorebirds.

Key habitat types are therefore characterised by the yield of ecosystem services independent of their individual species. For example, the vascular plant community is necessary for the production of juvenile perch. The spawning ribbon of perch can be wrapped around a reed or a branch protruding from the mud, and the tiny juveniles can easily hide in the protection of pondweeds, water milfoils, as well as ditch grasses.

When luring fish, anglers rarely consider this when they complain about their baits becoming entangled by water plants. 

The protection of key species and key habitats is crucial for the well-being of the entire Baltic Sea

Due to their ubiquity and diversity, many key species and habitats are currently outside the remit of environmental legislation. Any threats to key species and/or key habitats are mainly related to the changes affecting the entire coastline, such as increases in water turbidity following eutrophication.

In the Baltic Sea, key species maintain such a large proportion of ecological functions and ecosystem services that their survival should be safeguarded by any means necessary. Curbing the development of eutrophication and extending protected areas does more than merely ensuring the occurrence of endangered species. It would safeguard the functioning of the Baltic Sea, as well as its economic and recreational use.

The diverse species of vascular plant “jungles”, the seagrass meadows of sandy bottoms, the various algal communities on the rocky shores, and the blue mussel colonies filtering seawater on dimly-lit rocky seafloors are not only locally and intrinsically valuable but also for the state of the entire Baltic Sea.

 Blue mussels cover the bottom even climbing the vertical face of a large boulder.
Water-filtering blue mussel communities are important for the entire Baltic Sea area.