A study of bird-plant interactions in New Zealand

Throughout their evolutionary history, animals have interacted with and evolved alongside plants. There is a long tradition of studying mutualistic relationships between plants and animals. However, the study of mutualistic relationships still present substantial challenges. For instance, the role of dual mutualists in the rebuilding of degraded ecological networks is poorly understood. Additionally, while syndromes have been studied for years, support for the role of fruit and flower colour and their association with animals are still being debated. Finally, even after decades of study, it is still unclear how colours are adaptive in fruit colour polymorphisms and the conditions that lead to them being maintained in nature. It is the goal of this thesis to investigate these understudied aspects of ecology using model species in New Zealand.

To better understand how dual mutualists influence network properties in New Zealand, I compared how network properties have changed with reintroduction of native bird species. Rate of restoration of network properties was directly impacted by the order of reintroduction. Reintroducing dual mutualists can accelerate the rewiring of networks, but their loss will also have a two-fold impact on networks.

Next, I examined how fruit reflectance properties influence frugivore behaviour. Results were consistent with a growing body of research that suggests that fruit reflectance properties are unrelated to frugivore behaviour. On the other hand, research with large scale study systems are uncovering links between fruit colour and frugivore behaviour. Support remains equivocal and may be dependent on scale.

Thirdly, I examined whether sexual selection theory can predict flower traits by looking at how a suite of flower traits such as nectar production, flower conspicuousness and flower size are related. Flower traits such as nectar production were correlated with flower size and flower hue. These traits varied between gender consistent with sexual selection theory.

Lastly, I investigated how fruit colour in a fruit-colour polymorphic plant can be attributed to both biotic and abiotic conditions. ‘Red’ fruits were more conspicuous than ‘white’ fruits and found at elevations that coincide with higher densities of avian frugivore. On the other hand, ‘white’ fruits were more common at higher elevations, reflected more UV and were better able to protect their seeds from UV damage.

Together, these studies broaden our understanding of animal-plant interaction in New Zealand and the importance of dual mutualistic bird species to ecological networks. It provides evidence to a growing body of research on the role of colour in syndromes. Additionally, the results offer new insights into how different conditions influence fruit colour across an elevation gradient.