Estimating the population size and ecological implications of a Sula sula colony on Mona Island

Abstract

En el capítulo 1, estimé la población de S. sula en la isla de Mona usando el método de distancia y comparé los resultados obtenidos de los realizados durante el día y la noche para determinar el momento más adecuado para realizar su estimación. Probé la efectividad del uso de drones para el conteo de nidos activos y la comparé con el número obtenido a partir de conteos de a pie. Encontré que la isla Mona alberga la segunda colonia más grande de S. sula del Caribe, con aproximadamente 5.500 aves. Descubrí que el uso de drones puede proporcionar una alternativa a estos estudios y que detectan mejor los nidos activos. En el capítulo 2, exploré algunas consecuencias y efectos ecológicos de la colonia de S. sula en el ecosistema terrestre, así como áreas sin tales influencias en la isla. También logré generar un mapa que reconstruyera la extensión original de las colonias de aves marinas en la isla mediante el análisis de trazas de isótopos estables de nitrógeno (¹⁵ N). Encontré que los insumos alóctonos tienen efectos positivos en el sistema, aumentando la productividad de las plantas, y por tanto aumentando así la densidad y la actividad de los animales. Además, parecen desencadenar algunos efectos en los herbívoros y depredadores introducidos que afectan a plantas y animales locales. Finalmente, hallé evidencia de un área comparable en tamaño a la colonial actual que probablemente haya sido ocupado por una colonia, pero que desapareció —y con ella— los subsidios positivos a las poblaciones locales de plantas y animales endémicos.

Terrestrial ecosystems of small oceanic islands are often nutrient poor, and marine-derived inputs from mobile species such as seabirds can have key direct and indirect effects on their terrestrial food webs. Nutrients in the form of guano and carcasses generated by seabird colonies can readily be used by plants, detritivores and coprophagous arthropods. Increases in plant biomass and populations of smalls arthropods can in turn indirectly affect populations of animal consumers at higher trophic levels. Unfortunately, seabird colonies are declining worldwide due to harmful human impacts on islands and oceans. Thus, seabird declines can be expected to negatively impact the terrestrial ecosystems through nutrient reductions and subsequent losses of biodiversity. This thesis is focused on estimating the population size of the Sula sula L. colony in Mona Island and the indirect and direct effects on the terrestrial ecosystem. In chapter 1, we provided an estimate for the population of S. sula in Mona and compared results from surveys conducted at different times in order to determine the most adequate survey time. We also tested the effectiveness of drone photography to count active nests and compared it to data collected from ground-based surveys. We found that Mona island supports the second largest colony of S. sula in the Caribbean region, with approximately 5,500 birds. We also found that drone photography surveys can provide a cost-effective alternative to the more labor-intensive point-count or transect surveys given that more active nests are detected from aerial photographs than from ground surveys. In chapter 2, we explored some ecological consequences and effects of the S. sula colony on the terrestrial ecosystem, as well as areas without such influences in Mona Island. We also attempted to generate a map reconstructing the original extent of seabird colonies in Mona island by studying and comparing the remaining traces of stable isotopes of nitrogen (15N). We found that the allochthonous inputs have positive bottom-up effects in the system by increasing the productivity of plants, thus increasing the density and activity of animals. Also, they appear to unleash some top-down effects from introduced herbivores and predators that affect plants and animals. Furthermore, we discovered that an area comparable in size to the current colony probably existed but disappeared – and with it – the positive subsidies to the local populations of endemic plants and animals. The result from this thesis provided a reliable method to estimate seabird populations and highlight their importance in small oceanic islands.