Climate variation and population dynamics in tropical seabirds

Impacts from variation in climate associated with global warming are often difficult to discern in natural ecosystems. The effects of climate fluctuations appear to manifest differently both temporally and spatially depending on the prevailing environmental conditions to which species are exposed throughout the course of their reproductive and/or life cycles. This may be particularly prevalent in tropical systems where food availability is temporally and spatially more variable compared to higher latitudes. To examine current and potential future impacts of environmental variability on tropical seabirds, I examined the population and reproductive dynamics of multiple sympatric species using different foraging guilds and breeding at northern and southern breeding sites on the Great Barrier Reef, Australia.\ud \ud Sooty terns (Onychoprion fuscata), Common noddies (Anous stolidus) and Black noddies (A. minutus) share the life-history traits of pelagic/offshore foraging regimes, suspected breeding-site fidelity, clutch size of one and protracted post-natal growth. Crested terns (Sterna bergii) breed at similar locations and adhere to an inshore foraging regime, a brood size of one, variation in colony sites between and within breeding seasons and rapid postnatal growth. \ud \ud The population dynamics of Sooty terns, Common noddies and Crested terns breeding at Michaelmas Cay, northern Great Barrier Reef were examined over 18 years relative to a broad range of environmental and anthropogenic covariates. I also used several seasons of fieldwork to compare foraging success and chick growth of Sooty terns at Michaelmas Cay and Black noddies breeding at Heron Island, southern Great Barrier Reef, to fine-scale variation in sea-surface temperature. Next, the potential for Black noddies to respond to predicted future changes in climate through behavioural and developmental flexibility was assessed using variation in natural and artificial feeding conditions over two breeding seasons. Trophic mechanisms underlying these relationships were also examined, thereby identifying the potential for these same processes to both operate within the Great Barrier Reef and affect taxa at other trophic levels. Finally, while I primarily focussed on determining the relative role of large-scale oceanographic variation on seabird biology, I could not ignore potential confounding influences not related to major oceanography, such as catastrophic events (cyclones), direct (tourism) or indirect (fisheries) human pressure.\ud \ud Results from the long-term monitoring component of this research suggest that breeding Sooty terns and Common noddies are sensitive to changes in both surface chlorophyll concentration and thermocline depth that occur well in advance of El Niño generated seasurface temperature anomalies. The most likely explanation for this finding is that these relationships are mediated through a strict reliance by these tropical pelagic piscivorous seabird species on underwater predators, such as tuna and mackerel, to push prey to the surface where it becomes available to these aerial predators. In contrast, Crested tern breeding is not correlated with any environmental/biological parameters investigated. Inshore foraging species such as Crested terns depend on more localised and a larger variety of prey types and do not rely on underwater predators to capture food. These findings demonstrate that El Niño related phenomena do not affect seabird prey dynamics solely via productivity shifts at seasonal scales, nor in similar ways across different foraging guilds. The findings also suggest that population declines observed at multiple colonies of a variety of pelagic/offshore foraging species in western tropical Pacific may be directly related to the frequency and intensity of El Niño anomalies over the study period.\ud \ud Direct analyses of the influence of fine-scale variation in sea-surface temperature on foraging\ud success and chick growth of Sooty terns at Michaelmas Cay and Black noddies at Heron Island confirmed within-season variation in sea-surface temperature as another important determinant of foraging success at a reef-wide scale for these two similar tropical seabird species. Consistent patterns were observed on the northern reef with significant negative relationships between day-to-day variation in sea-surface temperature and both the amount of food fed to Sooty tern chicks and the feeding frequency in both years. For Black noddies on the southern reef, sea-surface temperature was negatively related to meal sizes and feeding frequency in a season of extremely high sea-surface temperatures but not when sea-surface temperatures were average. Considering forecasts for increases in extreme temperature events associated with global warming, as well as similar recent findings from the southern reef on a separate species, these results suggest that sea-surface temperature variation commonly does and will continue to influence the foraging success of multiple tropical seabird species across the Great Barrier Reef and implies a significant negative effect of forecasted climatic changes on seabirds breeding in the region.\ud \ud Next, my assessment of Black noddy flexibility in provisioning behaviour and development of offspring suggests limited plasticity in both life-history characteristics and that the potential capacity of this species to resist future environmental variation associated with climate change is low. Results garnered from the four food treatment groups (two natural and two artificially supplemented) demonstrated that during periods of low prey abundance, parents brought back smaller meals but at the same rate as chicks exposed to normal conditions. Super-abundance of food through food supplementation, also resulted in delivery of smaller meals by adults, but again did not influence feeding frequency. Evidence from this study suggests that smaller meals fed to artificially supplemented chicks resulted from a decrease in chick demands associated with satiation, rather than adult behavioural responses to chick condition. Chicks maintained structural growth while sacrificing body condition during periods of low prey abundance, and chicks were unable to take advantage of periods of high prey abundance by increasing growth rates, suggesting chick development is driven by an obligate rather than facultative mechanism. Combined these results suggest that future changes in sea-surface temperature and other environmental variation may require evolutionary responses to natural selection in these birds. The long generation time and low fecundity of this taxon make such responses over short periods unlikely. If so, this, and possibly similar, species may be at high risk from climate change. \ud \ud The potential complication of other anthropogenic stressors in the system influencing the main findings of this thesis is effectively negated. The 18 year dataset on population trends for Sooty terns, Common noddies and Crested terns breeding at Michaelmas Cay was assessed relative to two potential threatening processes: anthropogenic disturbance (as either tourism of commercial fishing) and cyclone activity. Longer-term trends in population sizes of all species remained unaffected by direct human pressure through tourism. Long-term population trends were also not closely related to indirect pressure through competition for resources via commercial fishing. Nor were long-term population trends in any species threatened by localised disturbance from cyclones or associated changes in cay size and/or nesting habitat. Field studies into the immediate influence of tourism on foraging success and chick growth of Sooty terns at Michaelmas Cay support these results, with only minor, short term impacts on breeding success observed as a result of the proximity of nesting sites to tourists.\ud \ud Combined, the results from this study have defined climatic variation at multiple temporal and spatial scales as the primary driver of population dynamics in pelagic/offshore foraging tropical seabird species across the Great Barrier Reef. The findings significantly enhance our\ud mechanistic understanding of how El Niño-related phenomena impact biological processes in tropical marine systems by substantially increasing knowledge of the potential effects of climate change on trophic dynamics and by elucidating how these impacts vary by taxa. Affected species have life-history characteristics in common which made them particularly sensitive to ENSO-associated fluctuations in food availability, while the single unaffected species has life-history characteristics which naturally buffered it to climate variability. These patterns are similar to comparative findings from elsewhere. The information compiled in this thesis can be used to guide enlightened management decisions, on a species by species and/or regional basis, that protect Australia's marine biodiversity and allow the long-term sustainable use of the Great Barrier Reef and other tropical ecosystems