Wood ant colouration as an ecological indicator for the level of disturbance in managed coniferous forests

The search for ecological indicators of population well-being in natural and managed ecosystems is a crucial aspect of effective biomonitoring, conservation and nature protection. In long-term monitoring programs environmental stress has a measurable effect on naturally selected traits, such as body shape or size. However, changes in colouration provide information about early warning responses. The red wood ant Formica aquilonia is ecologically and territorially dominant among wood ant species in European boreal coniferous forest, and possesses variable red-brownish melanin-based cuticular colouration. F. aquilonia gynes, which are unfertilized queens, exhibit colour variability on the head, propodeum and abdomen, yet only head colour features allow setting clearly visible and symmetrical (left/right) classes of morphs. We studied phenotypic colour variability, melanisation and fluctuating asymmetry of colour patterns in faces of F. aquilonia gynes from natural (forest interiors) and disturbed (clear-cut zones and forest edges) habitats in Finland. We defined five variable, clearly visible and symmetrical (left/right) colour morphs of gynes' faces. Individuals of totally dark morph were present only in disturbed habitats. General analyses of melanisation degree showed, that gynes from disturbed habitats were significantly darker compared to those from forest interiors. Individuals from the same nest tend to have a similar degree of melanisation. The mean darkness of face was highest among individuals from forest-edge nests and the lowest from forest interior nests. In all habitat types the darkness of face increased with an increase in head width. Increase of cuticular melanisation in F. aquilonia gynes in disturbed habitats could be explained with the theory of thermal melanism and stress-induced immune defence. Although some amount of fluctuating asymmetry among left/right symmetrical colour variations on faces of F. aquilonia gynes was found, it was affected neither by habitat type nor by head width. Melanisation degree of red wood ants have a potency to be used as ecological indicator for the level of disturbance in managed coniferous forests and fluctuating asymmetry of colour variations in ants might be studied furtherly in cases of more severe environmental concerns.