Year-round movement and foraging ecology of frigatebirds, Fregata spp.
thesisposted on 2017-01-10, 00:49 authored by Rowan William Mott
Biodiversity is increasingly threatened as a result of intensifying anthropogenic activities. To counter this decline there is an increasing recognition of the importance of sound conservation management decision-making. The central tenet of this process relies on scientific knowledge of the ecology and biology of the systems in order to implement conservation measures with the highest efficacy. In Section 1 of my thesis I undertook a structured literature review to identify spatial patterns in existing research effort into the at-sea distribution of seabirds. Of concern, this review highlighted many areas that had received little research attention despite high seabird species richness or exposure to high cumulative human impacts. Mismatches between the distribution of research effort and areas of conservation importance will need to be addressed and increasing international collaboration and facilitating uptake of platforms of opportunity for vessel-based survey seem the most likely to overcome current impediments. In marine systems, the large spatial extent, dynamic nature, and financial and logistical constrains make collection of ecological data difficult. Consequently, marine biologists often use indicator species to provide information on marine condition and ecological processes. Accordingly, Section 2 of my thesis examined the movements and foraging ecology of two congeneric seabirds in a region identified in Section 1 as underrepresented in the seabird spatial ecology literature. These species, Great Frigatebird Fregata minor and Lesser Frigatebird F. ariel, possess adaptations unique among seabirds that heighten their sensitivity to change in the marine environment. In Section 2, resource partitioning between Great Frigatebirds and Lesser Frigatebirds breeding at the same location primarily involved differences in the type of prey consumed rather than spatial partitioning of foraging grounds. Conversely, two populations of Lesser Frigatebirds breeding on separate, but nearby, islands had stable isotope δ 13C values indicating differences in location where prey was sourced. Prey availability differed markedly between inshore locations favoured by one colony and offshore locations used by the other colony. This disparity likely contributed to the large difference between the reproductive outputs of the two colonies. For both inter- and intraspecific partitioning cases, central place foraging constraints may have influenced the observed patterns. Feather samples, which reflect non-breeding resource acquisition, had a larger trophic niche width than samples reflecting breeding season diet (red blood cells and plasma) suggesting increased variation in the foraging strategy when not breeding. Data from GPS tracking devices identified marine areas in south-east Asia as important locations for Great and Lesser Frigatebirds that undertook a post-breeding dispersal. Habitat suitability modelling using MaxEnt identified proximity to small (<1000 ha) islands as the most important environmental predictor of habitat suitability for non-breeding frigatebirds, with sea surface temperature also influencing predicted habitat suitability to a lesser extent. Proximity to small islands is a spatially static variable and, consequently, additions to the marine protected area network informed by this habitat feature are likely to have more predictable long-term benefits relative to those informed by dynamic features, such as chlorophyll-α concentration. Together, these results demonstrate the complex mechanisms used by tropical seabirds to partition foraging resources and exploit their marine environment most effectively across time. They also provide ecological insight that will be useful for conservation planning in the eastern Indian Ocean and marine waters of southeast Asia.