This blog post is provided by Ericka O. C. Coni, David J. Booth, Camilo M. Ferreira and Ivan Nagelkerken and tells the #StoryBehindThePaper for the paper “Behavioural generalism could facilitate co-existence of tropical and temperate fishes under climate change”, which was recently published in the Journal of Animal Ecology.

No sailor wants to find themself entering uncharted waters, yet it has happened many times in the past when people were exploring unmapped parts of the world. When this happens, like in all instinctive creatures, survival mode is activated, and strategies are quickly deployed or created to guarantee their well-being. With the ongoing intensification of climate change, many species are shifting their distribution to seek more tolerable conditions and, as a consequence, species invasion rates have intensified. Feeling the heat, for example, many marine tropical fish species are entering uncharted waters by moving towards cooler environments where they face a new landscape, novel prey, predators, and competitors, and suboptimal temperatures. For at least two decades, hundreds of tropical fish species have ventured into Australia’s temperate reefs due to the strengthening of the East Australia Current, but successful colonisation depends on how they respond to and cope with unfamiliar environmental conditions, especially during the winter, when the water temperatures are below their thermal tolerance. In contrast, native-temperate fishes are forced to interact, and share habitats and resources with these tropical range-extending fish, potentially triggering modifications in the structure and function of temperate communities. So, how do tropical fishes manage to cope with such unfamiliar conditions found in cooler environments? And, from the point of view of native-temperate fishes, how are they handling the invasion of these tropical fish?

The key to these two questions may depend on the behavioural flexibility of both tropical and native temperate fish. In this case, their ability to modify their behaviour, is a potential mechanism adopted to adjust to new environments and novel species interactions under climate change. To better understand the importance of behavioural modification in the future ocean, we would like to introduce you to the species Abudefduf vaigiensis (popularly known as the Indo-Pacific Sergeant). The Indo-Pacific Sergeant is an ordinary tropical coral reef fish that has been observed year after year venturing into Australians temperate reefs during warmer seasons (Jan-May).

Over the last two decades when summer time arrives and the ocean starts to warm up, millions of baby fishes are born at the southern Great Barrier Reef. This is where the likely journey of an Indo-Pacific Sergeant starts. An Indo-Pacific Sergeant larva will have to spend some time navigating in the open ocean alongside many other fishes until growing large enough and finding a suitable habitat to settle. During this pelagic period, fish can be transported for long distances by currents. During this time in the water column, the dispersal of the Indo-pacific Sergeant into uncharted water is mediated by climate change. First, their larvae are caught by one of the strongest ocean currents along the Australian coast, the East Australian Current (EAC). This current is constantly increasing the flow of warmer water from the Great Barrier Reef towards the southern cold-temperate waters. Larvae then navigate until they reach and settle on temperate reefs, located faraway and being much colder than coral reefs. After surviving this long journey, larvae grow into small fish (recruits). In these novel environments, these larvae probably experience many unfamiliar biotic and abiotic conditions. Yet, the Indo-pacific Sergeant has an instinctive goal: survive, grow and reproduce in this novel environment.

Finding a suitable place to settle in such a far away and cold environment is only the first challenge in the Indo-Pacific Sergeant’s life. They will still need to deal with other adversities such as lower seawater temperatures, and novel predators and competitors, especially those that use similar niches. To try to overcome all these adversities, a common first response is to quickly change their behaviour. Especially species or individuals with a broad behavioural repertoire have increased chances of survival in such novel environments compared to those with a narrower repertoire. Luckily, the Indo-Pacific Sergeant has a broad behavioural repertoire, allowing them to exploit a wider variety of resources within a broad spectrum of ecological conditions and also avoiding conflict with local species.

The story of the Indo-Pacific Sergeant shows what many other tropical fish species might be dealing with to guarantee their survivorship when range extending into temperate ecosystems. By using an established approach from stable isotope studies (ellipse-based metric within a Bayesian framework) to calculate the niche breadth in a multivariate space as a proxy of behavioural modification, we revealed that behavioural niche shifts, expansions and segregations are potential mechanisms adopted by the tropical vagrant and native-temperate fishes to adjust to changing community structures under climate change. Whilst tropical fishes changed and become more generalists in their behavioural repertoire in their temperate ranges, native-temperate fishes use this same strategy along their warmer ranges.

Foraging position (bottom vs. water column) and social interaction were the most important behaviours responsible for the behavioural niche alteration and segregation across the range positions for tropical and temperate species. Prey species probably differed between warmer to cooler latitudes, which might explain the changes in the tropical and temperate species’ foraging behaviour. Furthermore, the Indo-Pacific Sergeant and the other tropical species, as well as the native-temperate fishes, are very flexible in their shoaling behaviour. On temperate reefs, tropical vagrant fishes shoal with local species, suggesting that they are flexible in employing social interactions with unknown species. The tropical fishes appear ‘smart’ in adopting this strategy, as by shoaling with native-temperate species they could learn how to recognise unfamiliar prey and predators.

Our study highlights some of the potential mechanisms allowing tropical range-extending fishes to persist in temperate ecosystems and for temperate fishes to adjust to this tropical invasion in the near future. Behavioural modification (ability to change their behavioural niche breadth) and behavioural generalism (ability to expand their behavioural repertoire) displayed by both tropical and temperate species can be adaptive and play a vital role in increasing their success as the environment changes under ocean warming. The ability of tropical range-shift species to be flexible in their behavioural niche and behavioural segregation (low degree of behaviour overlap) with local species at temperate latitudes might allow tropical fishes to exploit novel habitats and avoid or reduce competition with temperate species, increasing the likelihood of these species to establish and maintain a viable population as they enter temperate communities. So, it is expected that the ultimate fate of the Indo-Pacific Sergeant and other behavioural generalist species in temperate environments is to successfully establish.