Who fell the mango tree?


​Keystone species

In the mango tree story, the Creature understood that the absence of the tiger affected other species in the forest. Species, which have a significant impact on how an ecosystem functions, are called keystone species. The tiger is a large keystone species, but keystone species can also be very tiny - such as the blue mussel (Mytilus trossulus). Keystone species can be animals, fungi, microbes or plants.

In Finland an example of a keystone species is the common aspen (Populus tremula). It is an important tree for forest organisms. A large number of different species feed and live off living or dead aspens. Both young and old aspen, dead ones and ones that are alive, house and feed a large variety of species that are dependent on the aspen.

Butterfly larvae climb on top of aspens and eat their leaves. Beetle larvae live inside the trunk, eating the wood matter. The bark and branches of an aspen offer nutrition to moose and mountain hare during the winter. Various kinds of lichen and moss live on the surface of the tree trunk. Some fungi live in symbiosis with the aspen, giving nutrients and water to the tree, receiving sugars in return. Wood-decaying fungi eat the old aspen and woodpeckers drill holes into its trunk. Later on, for example a flying squirrel can nest inside the very same holes.

Examples of species that thrive in the vicinity of aspens:
  • Moss and lichen: Neckera pennata, common orange lichen (Xanthoria parietina), Phlyctis argena
  • Fungi: Leccinum albostipitatum, Cortinarius lucorum, Aspen bracket (Phellinus tremulae), Phellinus populicola
  • Butterflies and moths: Poplar admiral (Limenitis populi), Aspen hawk-moth (Laothoe amurensis)
  • Beetles: Saperda carcharias, Rusticoclytus rusticus, Cucujus cinnaberinus
  • Birds: Black woodpecker (Dryocopus martius), Grey-headed woodpecker (Picus canus), Great spotted woodpecker (Dendrocopos major)
  • Mammals: Moose (Alces alces), Mountain hare (Lepus timidus), Siberian flying squirrel (Pteromys volans)

Another example of a keystone species in Finland is the blue mussel, which attaches itself to the rocky bottoms of the Baltic Sea. The blue mussel is less than four centimetres in height, but it is a very common animal that offers habitats for many other species living in the sea. It’s possible for algae and bay barnacle to stick themselves to the shell of the mussel. Tiny creatures find hiding places inside the empty mussel shelves. Blue mussels also provide nutrition for many birds and fishes, for example the common eider, the long-tailed duck and the flounder feed on them.
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It’s important to remember, that the term keystone species is a concept introduced by humans, and it has its problems. Researchers do not agree on what species can be called keystone species. Organisms are referred to as keystone based on different justifications. We are yet to describe all species and their interactions, so it may well be that many species important for their habitat haven’t been identified yet, or the importance of a species is still not understood. When we say that an organism is a keystone species, we also often forget that under some circumstances one species can be more important than the other.  Every species has an intrinsic value, which does not depend on, for example, its benefit for humans.

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References:
* Cottee‐Jones H.E.W. & Whittaker R.J. 2012. perspective: The keystone species concept: a critical appraisal. Frontiers of Biogeography 4: 117–127.
* Keto-Tokoi P. & Siitonen J. 2021. Puiden asukkaat - Suomen puiden seuralaislajit. Gaudeamus.
* Tieteen termipankki: https://tieteentermipankki.fi/wiki/Biologia:avainlaji
* Viitasalo M. ym. (toim.) 2017. Meren aarteet - Löytöretki Suomen vedenalaiseen meriluontoon. Gaudeamus.
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