Tropical or temperate?

Warming waters and ocean acidification could have opposing effects on the movements of tropical fish, according to a study published  in the journal Nature Climate Change – which would be good news for temperate fish in cooler waters.

The study also highlights the importance of considering the complexities of climate change impacts on ecosystems.

Tropical fish in north-eastern Australia have been moving south each year when their habitat gets uncomfortably warm. But as the oceans absorb carbon dioxide (CO₂) and become more acidic, the cooler waters might become less appealing for the migrant fish.

It’s a complex scenario. As temperate waters in the southeast have warmed up, black spined sea urchins (Centrostephanus rodgersii) have thrived and decimated kelp forests. This creates “urchin barrens”, which tropical coral reef fish prefer to kelp habitats.

But when CO₂ levels increase, the study found that numbers of sea urchins – which need calcium carbonate minerals to build their skeletons – drop by 87%. This transforms the barrens into algal turfs, in turn becoming less attractive for tropical fish species.

Whilst these competing forces could lower the northern fishes’ choices for escaping warm waters, it might help preserve temperate habitats for species whose options are dwindling.

“Slowing down of tropical invasions into temperate Australia is not a bad thing because these tropical fishes can start competing with native temperate fishes in kelp-dominated systems,” says senior author Ivan Nagelkerken, from the University of Adelaide.

“The native temperate species already have difficulties dealing with warmer waters at their trailing edges, and competition with invader species will only decrease their survival and health.”

While the impact of global warming on poleward migration of species from warm climates has been well studied, virtually nothing is known about the role that ocean acidification might play, note first author Ericka Coni and colleagues in their paper.

To explore this, they used real-life ecosystems: warming waters on Australia’s south-east coast (“tropicalisation hotspots”) and volcanic CO₂ vents off New Zealand’s coast that cause acidification levels emulating those predicted by the end of the century.

“This enabled us to see how whole species, communities and ecosystems were changing due to climate stressors,” explains Nagelkerken.

“We found that kelp habitat harboured the least amount of tropical fishes, whilst harbouring a high abundance of temperate species. Hence, by protecting kelp habitat … we might slow down tropical invasions and provide temperate species with opportunities to adjust to climate change.”

More broadly, the study highlights the importance of exploring a range of elements for a more complete picture.

“When we combine multiple stressors and look at the effects at ecosystem level – which includes all of the complexity of species interactions, habitat changes and so on – we often see different, or more nuanced effects than expected based on more than simple single-species or single stressor aquarium experiments,” says Nagelkerken.

“Whilst the latter are still important to understand isolated effects, natural laboratories are important to reveal whole ecosystem effects.”