Colonisations and extinctions at multiple spatial scales: a case study in monophagous herbivores

Many plant—insect interactions show spatial structure at a hierarchy of scales. We investigate population turnover at different spatial scales in two specialist insect herbivores that feed on tansy, Tanacetum vulgare, in the Tvärminne archipelago in southwestern Finland. Colonies of both aphid species form on individual ramets and in addition to the level of ramets, individuals cluster at the levels of the genet, groups of plants, and islands in the archipelago. Colonisations and extinctions were observed at all spatial levels studied. Mean persistence times in both species ranged from less than five weeks at the level of the ramet to less than 20 weeks at the level of islands, such that aphids over-wintered on only a fraction of the islands in the archipelago. Our field study suggests that aphids on tansy form a hierarchically structured metapopulation where longer-term persistence is only possible at the level of the archipelago.