The functional ecology of the pale-leaf mistletoe in the arid zone

Abstract

Mistletoes exert a pervasive influence over many aspects of an ecosystem but the positive effects of mistletoe have historically been underappreciated. This is because of the nature of mistletoes as hemiparasites, and how an overabundance of mistletoe can debilitate the functioning of its host. However, different host species respond to different mistletoe infections in different ways. In this thesis, I sought to understand more about the functional role of the pale-leaf mistletoe (Amyema maidenii) that parasitises mulga trees (Acacia aneura) in arid zone Australia. Firstly, I used the host quality hypothesis as a framework to evaluate the relative abundance of the pale-leaf mistletoe in mulga trees. Secondly, I conducted an experimental mistletoe removal study to test the theory that mistletoes can influence the microclimate of its host, thereby facilitating habitat use of a free ranging animal.

The first study revealed that mistletoes aggregate on taller mulga trees, where the negative effects of mistletoe infection are more likely to occur. In the experimental removal study, my results evidence a novel pathway for how mistletoes can modulate the microclimate beyond its own physical structure, creating a distinctly cooler understorey microclimate at the host-scale. In trees where mistletoe was present, kangaroos (red kangaroo – Osphranter rufus) also preferentially rested beneath these trees when compared to trees where mistletoe was removed. I conclude that the pale-leaf mistletoe is an important constituent within arid zone Australia but this is likely mediated by different mechanisms. Results from my first study suggest that the pale-leaf mistletoe may have an important negative role as they aggregate on taller trees, thereby increasing the likelihood that they may debilitate the functioning of taller host trees. On the other hand, results from the second study provides further support for the theory that mistletoes function as a keystone resource, structuring species interactions and positively influencing ecosystem functioning. More broadly, results from the second study emphasise the prevalence of positive interactions within ecological communities and how they can occur in unexpected ways and between organisms that never come into direct contact.