When an enemy of an enemy is not a friend: Tri-trophic interactions between kākā, puriri moths and makomako trees

Predators can indirectly structure local plant communities by altering the diversity and behaviour of herbivores. These ‘trophic cascades’ can be seriously disrupted by the local extinction of top predators. They can also be restored by the subsequent re-introduction of top predators by conservationists. Here, we investigated trophic cascades involving kākā, puriri moths and their host trees. New Zealand kākā (Nestor meridionalis, Nestoridae) are large parrots that were extirpated from most of its range in the 20th century. Unlike most other parrots, kākā forage primarily by digging through the vascular cambium of trees in search of wood-boring insects. They regularly consume puriri moths (Aenetus virescens, Hepialidae), a giant species of ghost moth whose larvae tunnel into the heartwood of trees for protection. Puriri moths are host specific and frequently attack makomako (Aristotelia serrata, Elaeocarpaceae), a sub-canopy tree species. We conducted a field experiment in Zealandia, a nature reserve to which kākā were reintroduced over a decade ago, to test the hypothesis that kākā would increase tree fitness by removing larval parasites (i.e. herbivores). Contrary to our expectations, results revealed an additive, negative effect of both the predator (kākā) and parasite (puriri moths) on plant fitness. Therefore, the tri-trophic interactions restored by the reintroduction of kākā into Zealandia appear to be unique, as the actions of the predator decreased fruit and flower production, rather than increasing plant fitness, as typically reported for trophic cascades.