Guest blog post by Daniel Ayllón and Steve Railsback
Early in the morning, Daniel Ayllón and his research mates at the Universidad Complutense de Madrid drive towards the mountains near Madrid. They’re out to survey streams where the endangered Southern Iberian spined-loach and Northern Iberian spined-loach used to coexist. We say “used to,” because once again they fail to find the Northern Iberian spined-loach, probably locally extinct. Such extinctions are not unusual, as freshwater fishes are one of the most threatened groups of animals in the world. There are still many brown trout there, though; the water is still cold enough for them.
Salmonids (trout, salmon and char) are especially challenged by climate change because they need cold, oxygenated and clean water. Trout populations at low altitudes or low latitudes are thus particularly at risk; many in the Iberian Peninsula have been declining for decades as rivers warm and dry. Climate models project a bleak future: such Mediterranean populations will face hotter and drier streams, with more frequent and longer droughts and heat waves, and increasing competition from warm-water fish.
Despite these changes, local extinctions of trout are still rare, because salmonids are among the most adaptable and resilient of freshwater fishes. They are changing their physiology and phenology, growth and reproduction patterns, and life-history strategies to adjust to the new environmental conditions, via evolutionary, plastic and behavioural mechanisms. While evolutionary ecologists typically focus on genetic adaptation to forces such as climate change, behavioural plasticity could be even more important, because it is fast, reversible and often predictable.
In fact, thermoregulatory movements seem a ubiquitous behavioural mechanism in salmonids: individuals move up and down river networks to find less-stressful temperatures and better growth potential. Behavioural plasticity in circadian activity and habitat selection (deciding when and where to feed) also help trout resist short-term environmental changes. However, we don’t know how important changes in circadian activity─or behaviours in general─are to long-term population persistence in the face of climate change. So to shed light on this question, in a recent work published in Individual-based Ecology, we ran two virtual experiments using the inSTREAM individual-based model to represent a trout population in northern Spain.
Steve Railsback and his colleagues at Cal Poly Humboldt University and the US Forest Service’s Pacific Southwest Research Station in Arcata, California, have been developing, testing, and applying inSTREAM for 25 years. The central idea of individual-based models (IBMs) and of individual-based ecology in general is that a biological system can be described through its individual agents, their environment, and the interactions among agents and between agents and environment. The agents of a system (for example, all fish in a population) are modelled as unique and autonomous individuals with their own properties.
Agents also have behaviours: they make decisions, following simple rules or algorithms, independently of other individuals, and seek objectives such as surviving to reproduce in the future. These behaviours are adaptive: agents’ decisions depend on their state and the state of their environment. In this way, population-level results actually emerge from the behaviour of the individuals. In inSTREAM, model trout decide whether to feed vs. hide from predators at different times of day, assumed a trade-off between the need to feed and the predation risk it poses. Temperature has a strong effect on this trade-off because a fish’s metabolic rates, and thus the amount of food it needs, increase sharply with temperature.
What did we learn with our IBM? First, our simulations show what behavioural ecologists know from experiments: that during warm summers trout can meet their metabolic requirements only by feeding at multiple times of day and segregating temporally, so that fish of different size can feed at the same spot but at different times of day. Feeding during daytime is more profitable but riskier, while doing it at night is safer but less efficient, and feeding during twilight provides near-daytime growth and somewhat-reduced risk.
We then analysed how model trout change their circadian foraging behaviour under increasing climate change. As we expected, trout showed great behavioural plasticity: trout of all ages responded to warmer and drier conditions by increasing daytime feeding and overall foraging activity, although there were differences across age classes in the distribution of daily activity. Our second experiment used a great advantage of IBMs as a virtual laboratory: we can run experiments that are impossible in reality. We tested the importance of behavioural plasticity by simply turning the behaviour off. In our simulations, virtual populations of trout capable of flexible circadian feeding were more resistant to climate change─had higher biomass and a more balanced age structure─than were populations of trout that feed only during daytime.
These experiments reinforce that behavioural plasticity can be key for coping with environmental changes, so we shouldn’t minimise its relevance when predicting the persistence of salmonid populations in warming and drying rivers. This conclusion no doubt also applies to other taxa that have powerful adaptive behaviours.
This study epitomises individual-based ecology, the subject of Pensoft’s new journal: we use what we know from empirical research on individual physiology and behaviour, in an individual-based model, to study complex population responses of direct relevance to our changing world.
Research article:
Ayllón D, Railsback SF, Harvey BC, Nicola GG, Elvira B, Almodóvar A (2025) Behavioural plasticity in circadian foraging patterns increases resistance of brown trout populations to environmental change. Individual-based Ecology 1: e139560. https://doi.org/10.3897/ibe.1.e139560
