Ecology of the Silky Hairstreak butterfly and its attendant ant, a potential indicator of old growth forest

Description

The Silky Hairstreak butterfly (Pseudalmenus chlorinda) depends on three critical resources for its survival: (1) Acacia spp. for larval food, (2) an attendant ant Anonychomyrma biconvexa, and (3) large mature eucalypts growing in proximity of the acacia food plants which may be used as pupation sites by the butterfly. However, one aspect of this complex 4-way ecological association that is not well understood is the ecology of the attendant ant and its relationship with large eucalypts. The ants typically occur in large colonies comprising thousands of workers and they form conspicuous vertical foraging trails along the trunk that extend to the crown of the host tree. The ants seem to be always associated with very large trees, but the ages of these trees are unknown. It is also not clear where the ants nest. It is very likely that the host trees are hundreds of years old and the ants are arboreal. Clearly, from a conservation management standpoint the ecological requirement of the attendant ant and their dependency on mature old trees should be studied as a matter of urgency – the butterfly is likely to be an indicator of old growth forest or habitats supporting very old living trees, but there is a general lack of basic natural history. The susceptibility and loss of extant colonies of P. chlorinda to removal of mature trees through habitat loss (broad scale clearing) for the pastoral, timber and woodchip industries and frequent fires and has been well documented in Tasmania.

Research questions: How old are the host trees? What is the minimum age of host trees for ant occupancy? Where do the ants nest within host trees? How patchy are supercolonies of the ant within the landscape? What is the minimum density of trees required to support supercolonies of the ant within the landscape? Do ants survive fire and what is the time interval post fire until trees are suitable for recolonisation? And what is the level of occupancy of the butterfly among patches of suitable combinations of the attendant ant, eucalypt host tree and acacia larval food plant?

Field study sites: Brindabella Range, ACT; Tallaganda NP, NSW; Clyde Mountain, NSW; Mt Kembla, NSW.

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Dr Suzi Bond, Supervisor

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