Ecology and conservation of the Såli (Micronesian Starling, Aplonis opaca) in the presence of the invasive Brown Treesnake (Boiga irregularis) in Guam, Mariana Islands
| Autor: | Kastner, Martin |
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| Jazyk: | angličtina |
| Rok vydání: | 2024 |
| Předmět: | |
| Druh dokumentu: | Dissertation |
| Popis: | The world's islands are of singular importance from a conservation perspective because they hold a disproportionate share of global biodiversity. However, island ecosystems are fragile, and the introduction of invasive species poses an especially grave threat to their species. My research took place on the island of Guam, which has been particularly hard hit by invasive species in recent history. The introduction of the invasive Brown Treesnake (BTS; Boiga irregularis) to Guam in the mid-20th century caused the extinction of the vast majority of its avifauna. Of the remaining bird species, the native Såli (Aplonis opaca) is a forest starling now limited to breeding and roosting in certain urbanized areas on the island. Previous research has shown that although Såli adults appear to have high survival, their fledglings are highly vulnerable to predators. My research focused on refining our understanding of how BTS are impacting Såli ecology and survival, and how BTS management can benefit Såli conservation. In my first chapter, I conducted the first field study to investigate the influence of both prey and predator sizes on the frequency of ingestion success by snakes. I found that nearly half of ingestion attempts by BTS on fledgling Såli were unsuccessful, and that BTS routinely pushed the upper limits of their capacity to swallow prey. These results suggest that relatively small BTS pose a greater predation risk to birds than previously recognized, and broaden the size range of BTS that should be targeted in management efforts for avian conservation. In my second chapter, I investigated a potential life-history trade-off for Såli between the proximity of nest sites to foraging areas and the risk of fledgling predation by BTS. I found that Såli nesting closer to the forest make more yearly nesting attempts, but that fledgling survival was higher the farther their nest site was from the forest. Såli therefore balance potential fecundity with predation risk to fledglings in their selection of nest sites. In my third chapter, I examined the survival response of fledgling Såli to the suppression of BTS. I found that snake control did not lead to a change in fledgling survival from the pre-treatment baseline or in comparison to areas where snakes were not controlled, and that there was no shift from snake predation to other causes of mortality. Changes to the strategies or tools employed will therefore be required in order for future BTS control efforts within similar habitats to effectively improve avian survival. This body of work demonstrates that BTS are affecting Såli in unexpected ways, and offers new guidance on how to manage damaging invasive species for native species conservation. Doctor of Philosophy Islands are of high value important for conservation because they hold a more than their share of the world's animal and plant species. However, island ecosystems are fragile, and the introduction of damaging exotic species is especially threatening to species that live there. My research took place on the island of Guam, which has been particularly hard hit by damaging non-native species in recent history. The introduction of the Brown Treesnake (BTS; Boiga irregularis) to Guam in the mid-20th century caused the extinction of the vast majority of its bird species. Of the remaining bird species, the native Såli (Aplonis opaca) is now limited to breeding and roosting in certain developed areas on the island. Previous research has shown that although Såli adults appear to have high survival, their fledglings (young birds that recently left the nest) are highly vulnerable to predators. My research focused on refining our understanding of how BTS are impacting Såli life cycles, and how BTS management can benefit Såli conservation. In my first chapter, I conducted the first field study to investigate the influence of both prey and predator sizes on the capacity for snakes to swallow their prey. I found that nearly half of attempts by BTS to swallow fledgling Såli were unsuccessful, and that BTS routinely pushed the upper limits of their capacity to swallow prey. These results suggest that relatively small BTS pose a greater predation risk to birds than previously recognized, and broaden the size range of BTS that should be targeted in management efforts for avian conservation. In my second chapter, I investigated a potential life-history trade-off for Såli between how close their nest sites are to foraging areas and the risk of fledglings being killed by BTS. I found that Såli nesting closer to the forest make more nesting attempts per year, but that fledgling survival was higher the farther their nest site was from the forest. Såli therefore balance the potential number of young they can raise with the predation risk to those same young in their choice of nest sites. In my third chapter, I examined whether the suppression of BTS led to a change in the survival of fledgling Såli. I found that snake control did not lead to a change in fledgling survival from the period before the treatment or in comparison to areas where snakes were not controlled, and that there was no shift from snake predation to other causes of death. Changes to the strategies or tools employed will therefore be required in order for future BTS control efforts within similar habitats to effectively improve avian survival. This body of work demonstrates that BTS are affecting Såli in unexpected ways, and offers new guidance on how to manage damaging invasive species for native species conservation. |
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