Are island plants really poorly defended? No support for the loss of defense hypothesis in domatia-bearing plants

1. The loss of defense hypothesis posits that island colonizers experience a release from predation on the mainland and subsequently lose their defensive adaptations. While support for the hypothesis is abundant, it has never been tested in domatia-bearing plants. Leaf domatia are cave-like structures produced on the underside of leaves that facilitate a defensive mutualism with predatory and fungivorous mites. I tested the loss of defense hypothesis in six domatia-bearing taxa inhabiting New Zealand and its offshore islands. No support for the loss of defense 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 demonstrate that not all types of defense are lost on islands, suggesting a higher-resolution approach is needed when studying the evolution of defense on islands.