The number of species in an environment is dependent on the biodiversity of the habitat. Many species are well adjusted to life in cities, but some other species have retreated. The number of naturally occurring plant species found in Helsinki is just over 1,000.
The Helsinki inner city has little natural fauna, and the share of weeds and other one-year introduced plants grows as urbanization progresses. The new species benefit from an unbalance in the environment.
At the same time, native species diminish. For example, Helsinki has surprisingly few plant species that only occur in forests. The forest plants of Helsinki mostly consist of deciduous species that are better adjusted to changes in the environment than the species of rugged forests.
Wetland plants suffer from lack of habitat. However, plants typical of meadows survive in various parts of Helsinki. They benefit from the ample light in their habitat and even from soil worn by feet.
Wear is, however, detrimental to plants that grow on rocks. The eutrophication of the inner bays is manifest as disturbances in the shoreline vegetation, but the situation in terms of vegetation is fairly good on islands.
Today Helsinki is home to 46 mammal species. The most frequent mammals in the city are the hare, squirrel, hedgehog and fox.
The bird species of Helsinki are many and varied, because the city area has a wide variety of suitable environments for birds. About 60 species of fish have been detected in the Helsinki sea area.
On the basis of observations, reptiles and frogs have retreated, and the individuals of many species are few. No official estimate exists of the number of insect species in Helsinki.