This was something I had never considered until reading this article by Koptur and colleagues. These researchers studied the epiphytic fern Pleopeltis crassinervata in cloud forest of Veracruz, Mexico. This species is related to our ngārara wehi (Pyrrosia eleagnifoia) and was found to employ bodyguards to keep nasty herbivores away!
Pleopeltis crassinervata is one of many plants that produces nectar in glands outside of flowers (click here for more info). Nectaries on the leaves of this fern produce nectar that attracts ants.
The ants then repay their host for this tasty meal with defence services against hungry caterpillars. Koptur et al. (2013) found that plants with healthy, functioning nectaries had fewer caterpillars browsing on their leaves and that the bodyguard ants were often responsible for attacking, removing or killing them.
So that is how it is done in Mexico, now I wonder if any of New Zealand's epiphytes have similarly clever tactics?
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I work with NZ's native vascular epiphytes at the University of Waikato. I completed an MSc on epiphyte ecology and the shrub epiphyte Griselinia lucida and have recently published the Field Guide to NZ's Epiphytes, Vines & Mistletoes.