Climate Change
With approximately 70% of the Earth’s surface covered with seas and oceans, we can expect the world’s seabirds to be greatly impacted by predicted changes to the Earth system as the planet warms. Just 3% of the world’s c.10,000 species of birds are seabirds, clearly underlining just how hard it is for a bird to live off and survive in the marine environment.
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Projected sea-level rises will encroach upon many coastal areas currently used for breeding and roosting by seabirds – such as penguins, gulls and terns. Species breeding on rocky habitats such as those found on oceanic islands have perhaps less to fear from sea-level rises.
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In association with increased levels of CO2 in the Earth’s atmosphere, there has been a parallel but lower increase in CO2 in the world’s oceans, increasing their acidity. This increased ocean acidity is of concern as it will adversely affect the capacity of many crustaceans and molluscs to construct their shells and exoskeletons. Coral reefs, some of the most productive marine ecosystems, are particularly at risk of impact from increased ocean acidity.
Another consequence of warming oceans is that less oxygen can be dissolved in warmer waters. This is already evident in the overall productivity of tropical seas (low diversities and low abundances of seabirds) compared to high-latitude polar waters (high diversities and high abundances of seabirds). The combination of more acidic and warmer, less productive oceans will threaten the prey stocks of all seabirds around the world.
The warmer atmosphere will also result in a greater frequency, duration and intensity of extreme weather events, which have been shown to adversely affect seabird populations by large-scale breeding failure or mass mortality events (wrecks).
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Marine heatwaves (MHWs) are periods of anomalously warm water associated with changes in ocean structure, based on the horizontal advection of water masses and atmospheric exchange of heat. The longest MHWs persist for many months and dramatic effects on marine life have been reported from around the world. As top-order predators, seabirds are particularly sensitive to MHWs, with high levels of mortality associated with some events, but not with others. Thus, prediction of impacts is not straightforward, as mortality is not linearly related to simple measures of MHW intensity, persistence, and areal coverage. MHWs represent a window into future environmental changes associated with global warming.
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These anomalous high-temperature events occurring as part of a long-term warming trend are spikes that foreshadow projected conditions at some time in the future. The higher the spike, the greater the MHW anomaly and the farther into the future the MHW allows us to predict how seabirds may respond to projected environmental conditions. Seabirds’ responses, based on demographic parameters and the geographic proximity and phenological timing of MHWs will be complex (with some responses likely to be unpredictable) and extend over broad spatial and temporal scales.
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The spatial proximity of anomalous marine conditions to breeding colonies, their overlap with foraging areas, and the degree to which MHWs coincide with pre-breeding and breeding seasons presently generate the greatest pressures on seabird populations. It is likely that area-restricted seabird species, in terms of movement and breeding strategies, are at greater risk from MHWs, but that non-linear effects complicate prediction.
Responses to climate change will differ among species. While many species’ populations are expected to decrease, some species, such as King Penguins, are increasing as increased habitat becomes available. Distant foraging species such as many albatrosses, petrels and shearwaters may be less impacted by changing conditions compared to species with more restricted ranges such as penguins and terns. Generalist species such as gulls are likely to benefit, at least in the near-term.