Are all forms of defence lost on islands? Persistence of a defensive mutualism in six island colonists from New Zealand

The loss of defence hypothesis posits that island colonizers experience a release from predation on the mainland and subsequently lose their defensive adaptations. However, while support for the hypothesis from direct defensive traits is abundant, far less is known about indirect defensive traits. Leaf domatia are cave-like structures produced on the underside of leaves that facilitate an indirect defensive interaction with predaceous and microbivorous mites. I tested the loss of defence hypothesis in six domatia-bearing taxa inhabiting New Zealand and its offshore islands. No support for the loss of defence hypothesis was found. Changes in domatia investment were instead associated with changes in leaf size—a trait that has been repeatedly observed to undergo rapid evolution on islands. Overall results suggest that not all types of defence are lost on islands.