Effects of forest fragmentation on plant‐bird mutualisms in New Zealand lowland forests
Thesis DisciplineBiological Sciences
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
In New Zealand, there is concern about whether native bird densities have declined so much that pollination and seed‐dispersal services to plants are failing, especially in small forest remnants. I aimed to test whether there is a threshold area and isolation level under which a lack of interactions with birds adversely affected breeding systems of native plants. Through geospatial analyses I calculated effective patch areas and connectivity indices of 18 forest fragments on Banks Peninsula, Canterbury, ranging from 2.8 ha to 93.4 ha. There I measured bird densities using 5 minute counts and found that habitat connectivity (measured with the Harary index dH) rather than area affected bird densities and presence of native birds.
The bird‐pollinated, bird‐dispersed, gynodioecious native tree Fuchsia excorticata is prone to inbreeding depression, pollen limitation and seed limitation. Using the National Pollination Survey visual scoring method to measure pollen deposition on Fuchsia flowers, I found that female trees in 4 of 13 sites were suffering pollen limitation. Although self‐compatible hermaphrodites at those sites showed no pollen limitation (as predicted from earlier work), they were likely experiencing higher local selfing rates. Fruit removal was measured as an indicator of dispersal service in 10 sites, and removal rates increased with site connectivity and with densities of silvereyes (Zosterops lateralis). Fruits from female trees produced higher quantities of filled seeds (linked to > 80% germination) at sites with higher connectivity. Germination trials were set up to detect inbreeding depression using fruits from 11 sites, and the difference in growth performance between offspring of obligately cross‐pollinated females and self‐compatible hermaphrodites reduced with increasing connectivity, suggesting higher self‐pollination rates (and thus higher inbreeding depression) for hermaphrodites in isolated fragments.
Increasing patch area only improved pollination service in sites smaller than 15 ha, and had otherwise minor and mostly negative effects on plant‐bird mutualisms. Patch connectivity dH was the most important fragmentation parameter, positively affecting mutualist bird populations, dispersal service, and limiting inbreeding depression. Therefore, on Banks Peninsula even small reserves (< 5 ha) can have successful plant‐bird mutualisms if they maintain sufficient functional connectivity within the fragmented landscape.