Estimates of the Number of Foraging Ants with the Lincoln-Index Method in Relation to the Colony Size of Formica polyctena

Abstract

In a study of the role of ants in an ecosystem their foraging activity is one of the parameters needed for qualifying their influence on the populations of other animals, like aphids, prey, enemies, etc. To this end a reliable method of estimating the numbers of ants living in a nest is essential because it enables the investigator to measure the response of a nest to several environmental factors like weather, food supply and other ant populations. Estimates can be made by applying the Lincoln-Index method (Chew 1959), but hitherto it was not known to what extent the numbers calculated reflect the real size of the colony in some species. To establish this relation for the Red Wood Ant Formica polyctena Forst we performed a series of laboratory and field experiments, the report of which is found in this paper. In estimating the size of ant colonies by using the Lincoln-Index method Brian, Hibble & Stradling (1965), Nielsen (1970) and Stradling (1970) came to satisfying results for nests of Myrmica rubra (L.) and Lasius alienus Forster, respectively, by marking ants captured inside the nest. Other authors like Ayre (1962), Chew (1959), and Golley & Gentry (1964) found that their estimates were far too low, varying between 3 and 20% of the real colony size. In their experiments with Camponotus herculeanus L., Formica fusca (L.) and Pogonomyrmex badius Latr. these authors captured the ants to be marked outside the nest or from its surface. In this study of Formica polyctena we aimed at quantifying the relation between the estimates based on ants marked on the tracks and the real size of the entire colony, to make the method useful for field practice. In our opinion marking of ants taken from the track has to be preferred to marking of ants taken from the nest for the following reasons. In the field, and also in the laboratory, ant hills vary greatly in size, which interferes with standardizing the method of picking up ants from the nest, especially where there is division of labour in the nest. Moreover, digging out the easily excitable ants from the nest material disturbs the animals as well as the nest structure, which may upset the purpose of the measurements. Marking on the track, however, causes much less disturbance; the ants can be marked one by one with a small dot of paint upon the abdomen and/or with a tiny ring around the petiolus. Although release of the marked animals often causes them to walk around nervously for a while, and some communicate excitedly with their fellow workers, they seem to resume their pursuits within a short time; some do not seem to be concerned at all. Golley & Gentry (1964) suggest that only a small part of the whole colony takes part in foraging; 'this is at least indicated when marking is done during one or two days'.