Territorial behavior in the speckled wood butterflies <i>Pararge xiphia</i> and <i>P. aegeria</i> of Madeira: a mechanism for interspecific competition

Abstract

Two species of speckled wood butterfly occur in Medeira. Pararge xiphia is endemic and is very similar in morphology, behaviour and general ecology to P. aegeria which was first recorded on the island in 1976. We collected behavioural data on the males of the two species at sites where the indigenous laurel forest meets non‐native forest and agriculture. It is in these areas that the two species are found together in particularly high densities. Male Pararge butterflies defend sunlit areas of vegetation and attempt to exclude other butterflies. If a territorial intruder is a conspecific or the other Pararge species, extended chases or spiraling flights may take place. Interactions between the two Pararge species are longer and more likely to be escalated than those between either species and a range of other butterfly species. Pararge aegeria is more aggressive in its territorial behaviour than P. xiphia and the latter may be suffering more from the interactions. The results demonstrate that the two species are competing for space and therefore, that territorial behaviour could be a mechanism by which interspecific competition could be taking place. Any adaptive explanation for the interspecifc territoriality remains speculative but this recent and probably natural, colonization may provide an excellent opportunity to examine the role of interspecific competition in structuring communities. The arrival of P. aegeria on Madeira has created an almost unique natural experiment, the study of which will potentially avoid many of the problems traditionally associated with the study of competition.