Widespread seabird deaths during La Niña

There have been a few reports this year of Black Noddies washing up dead along Australia’s coastline. While we can’t rule out climate change effects, widespread seabird deaths during La Niña are not uncommon. There is a practical explanation. Nonetheless, whenever there is substantial mortality of adult seabirds, it may also be cause for concern.

In this post, I’ll explain a little bit about seabird population ecology and how it connects to ocean productivityThe power of an ecosystem to process energy. The most productive ecosystems have reached a steady stable-state with maximum entropy production. That’s to say, the number of species has reached an optimum and the functions they fulfil, have translated free surplus energy into nutrients that is either stored inside plants and animals, or is entrained within the biological cycles that More and climate cycles.

La Niña and ocean productivity

During cooler La Niña conditions in the Pacific there is more surface productivity than during La Niño. You would think this means more food but as usual, ecosystemsHow ecosystems function An ecosystem is a community of lifeforms that interact in such an optimal way that how ecosystems function best, is when all components (including humans and other animals) can persist and live alongside each other for the longest time possible. Ecosystems are fuelled by the energy created by plants (primary producers) that convert the Sun's heat energy More work differently to how we might imagine. Warm water floats and this usually forms a blanket over the sea, which stops many creatures from reaching the rich food beneath. You see, at the surface, sunlight, photosynthesisMeaning how plants extract energy by absorbing water and using radiation from the Sun to combine it with carbon dioxide to create sugars. More and food chains tend to use up all the nutrientsEnergy and nutrients are the same thing. Plants capture energy from the Sun and store it in chemicals, via the process of photosynthesis. The excess greenery and waste that plants create, contain chemicals that animals can eat, in order to build their own bodies and reproduce. When a chemical is used this way, we call it a nutrient. As we More quite quickly. When you see ‘blue water’ in the ocean, it’s a sign that the surface conditions are pretty bleak.

During La Niña years, the surface is cooler and this layer is spread thin. This means there is mixing of deeper layers and this promotes more nutrients reaching the surface and the water turns a bit green. There is a surplus of algae and zooplankton. For microscopic animals that live inside this layer, it’s rich pickings. They can have a bumper year. But for marine predators that feed on them, all the way up to sharks and seabirds, the situation is reversed.

Marine vertebrates need the strongest energy gradients

If you’re a predator searching for food, you need access to patches that are rich in prey such as flying fish and other filter-feeders. The problem is, when there is zooplankton everywhere, your prey is thinly spread out across large areas of ocean. You have to travel much further to find food and there comes a point when it’s no longer viable. La Niña years are not the greatest for breeding seabirds in particular. Black Noddies at Heron Island for example, have to return daily to their nests to incubate eggs and raise chicks.

The best breeding years are during El Niño, when the ocean is more barren. This forces fish to congregate in far higher densities around islands and upwelling zones. This is when the birds can reliably forage nearby and find ample food for their chicks. Also, the activity of the birds transferring nutrientA substance that contains the raw materials for life. At a chemical level, these are contained inside compounds that are absorbed into the body and essential energy-containing molecules are extracted, so that energy can be transformed into other chemical processes that use the energy for living. More to and from land, amplifies nutrient concentrations, effectively prolonging the season. The whole food chainA single thread in a food web illustrating the chain of animals that eat each other. At the base of the food chain are small high-energy (fast metabolism) animals and at the other end large low metabolism animals. An example would be whales eating krill that eat plankton that eat algae. Or lions that eat gazelles that eat grass. More flourishes during these conditions.

Living on the edge

One thing we always have to remember, is that wildlife lives on the very edge of survival. Seabirds congregate and feed in some of the lowest nutrient areas of the planet. Ironically, the ocean has some of the highest biomassThe weight of living organisms. Biomass can be measured in relation to the amount of carbon, the dry weight (with all moisture removed) or living weight. In general it can be used to describe the volume of energy that is contained inside systems, as the size of animals relates to their metabolism and therefore, how much energy they contain and More. Not really ironic though, when you realise that is how and why it functions … it’s the absorption of energy into biological food chains that keeps ecosystems intact. That’s what animals do. But it also means that the more extreme changes in climatic conditions are going to cause significant mortality.

Boom bust cycles

Ecosystems need time to breathe. The boom bust cycles between La Niña and El Niño years give populations chance to recover. Fish disperse out into the ocean and mature, before congregating closer inshore to breed, then relaxing their distribution again. If climate change is having an effect, it’s in the duration and intensity of the peaks and troughs between these events. If they become too frequent, or not frequent enough, that is when it could start to impact whole populations of marine animals like seabirds.

Adult seabird deaths

Adult seabird mortality is a cause for concern because these are the birds that are most mature and connected to existing ecosystem processes. Seabird populations are split between adult breeders and adult non-breeders. Normally, there is a slow turnover of breeding adults as some die of natural causes and new adults are recruited to fill in the gaps. This provides a level of natural resilienceReferring to an ecosystem’s ability to maintain a steady stable-state. The need to build resilience is entirely anthropocentric and symptomatic of ecosystems that are damaged or declining, leading to loss of ecosystem services on which humans depend. More against periodic environmental changes. What can also happen is that if too many adults die, the overall fitness of new recruits can drop below a viable level. When that occurs, it can lead to sudden population collapse – but due to the elasticity of seabird populations, these symptoms can take many years to reveal themselves.

This year there is a La Niña alert so what we’re seeing could be a sign of those conditions taking hold. At the same time, the amount of adult mortality is something to keep an eye on.