Interactions of Avian Frugivores and Invasive Trees in French Polynesia
- Author(s): Spotswood, Erica Noelle
- Advisor(s): Bartolome, James W.
- et al.
Invasive species pose a threat to the persistence of the sensitive endemic biotic communities on oceanic islands. The direct ecological, social and economic effects of invasions have been well documented and can be significant on both islands and continents. The indirect effects on the interactions between species has received less attention despite being of critical importance to the long term stability of ecosystems facing multiple threats from anthropogenic impacts. In this dissertation, I examined how seed dispersal relationships are modified by the presence of multiple introduced species on the islands of Tahiti and Moorea in the Society archipelago of French Polynesia. Additionally, I evaluated the risks associated with one of my research methods; the use of mist nets to capture wild birds.
I first evaluated how the local abundance of the invasive Miconia calvescens modifies seed dispersal relationships between birds and plants. The species is an invasive fruit-bearing tree that currently covers much of the island of Tahiti and is present at much lower densities on Moorea. I found that while the overall size of networks was similar across sites, networks on the highly invaded island of Tahiti were less diverse and less even because birds concentrated a greater proportion of their foraging on Miconia calvescens. There were fewer links between birds and native plants at highly invaded sites where birds switched their diets away from a broader range of fruit and insects. The endemic Grey-green Fruit Dove (Ptilinopus purpuratus) consumed native fruit in larger quantities and more total species than two introduced frugivores. This study demonstrates that the impacts of invasive fruit-bearing plants on seed dispersal networks depends in part on their abundance, and are likely to increase as a species becomes increasingly dominant in a community. Additionally, the dispersal of native plants continues to depend heavily on the single extant native fruit dove on these islands despite the presence of multiple introduced frugivores.
The impact of invasive plants on seed dispersal networks is the result of the cumulative effects of foraging decisions by birds. The available evidence suggests that birds choose which fruit to consume based on the complementarity between fruit traits and their own preferences as well as the relative abundance of fruit in a community. I used fruit choice experiments with captive Red-vented Bulbuls (Pycnonotus cafer) to uncouple fruit preferences from the effects of abundance to determine which operates more strongly on foraging decisions in birds. Birds showed both reliable and consistent preferences for some fruits over others and a strong response to abundance. However, when included simultaneously in the same experiment, the patterns of preference remained intact while the effect of abundance disappeared. Taken together, experimental and field data suggest that foraging decisions are highly context-dependent, and neutral models that consider only the relative abundance of fruit in the community are unlikely to provide reliable predictions about how seed dispersal networks will change in response to invasion.
In the final study of my thesis, I evaluated the risks associated with one of my primary research methods; the use of mist nets to capture birds. Mist nets are used widely for monitoring avian populations. While the method is assumed to be safe, very few studies have addressed how frequently injuries and mortalities occur, and no large-scale comprehensive evaluation has been conducted to determine the associated risks. In collaboration with several banding organizations, I quantified the rates of mortality and injury at 22 banding organizations in the United States and Canada and used capture data from five organizations to determine what kinds of incidents occur most frequently. I compiled a dataset including nearly 350,000 records of capture over a 22 year period and evaluated what makes birds most at risk to incident. I found that the risks varied among species and factors such as body mass and the number of previous captures were related to the probability of an incident. Additionally, I found that birds that were released back into the wild after an injury were recaptured at similar rates compared to birds that were released without an injury, indicating that injured birds survived in similar numbers as those released uninjured. This study fills a gap by providing the first comprehensive evaluation of the risks associated with mist netting and concludes that while overall risks are low, species and traits can predict a bird's susceptibility to incident. These results can be useful for organizations that use mist netting, and should be incorporated into protocols aimed at minimizing injury and mortality. Finally, I emphasize that projects using mist nets should monitor their performance and compare their results to those of other organizations.
I conclude that the indirect effect of species invasions are variable and depend in part on the abundance of the invasive species. Thus, consequences for seed dispersal networks are likely to be most pronounced at the highest levels of invasion. Despite the effects of abundance, birds also showed strong preferences for certain kinds of fruits. Thus, the integration of novel fruit into native seed dispersal networks is likely to depend not only on the relative abundance of fruit but also on the preferences for each fruit relative to others that are available. The Grey-green Fruit Dove is the sole disperser for several native plants, and the maintenance of viable populations of these plants is likely to depend on conservation of this protected and sensitive endemic species. Similarly, it is likely that the Fruit Dove also depends on native plants despite the integration of many exotic fruit into its diet. There is an urgent need for more research evaluating the habitat requirements and population dynamics of this frugivore in order to ensure the long term persistence of the species.