Wood ants are often absent on islands of the Gulf of Finland, even when seemingly suitable habitats are available. Their absence may partially be explained by the lack or rarity of ant species suitable as host during colonisation through temporary parasitism. To search for the limits of living conditions on islands, given colonisation constraints are overcome, we artificially established wood ant colonies on several islands constituting a series from suboptimal to extremely harsh living conditions. The case reported here showed that a barren < 0.2 ha islet, with aphids on its single pine tree the only permanent and relatively rich food source, has allowed the existence of an artificially introduced Formica polyctena Först. colony for 22 years. The ambient living conditions are probably close to the limit for the species, as evidently the colony does not produce sexual offspring. Thus the sustained existence of the colony is dependent on adoption of fertile gynes originating in colonies that live in more optimal conditions. We suggest that the polygynic social mode of the inherently polycalic F. polyctena is the key for its sustained existence on the islet, as polygyny together with receptivity to new, even alien queens keeps the colony alive in a sink habitat insufficient for production of own sexual offspring.
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