[Editor’s note: This article was written by Dr. Adam Reddon. Adam is a widely published fish biologist who works at McGill University in Montreal. His research focuses on aggression, learning, predator-prey relationships and neuroscience in fishes. Follow him on Twitter @adamreddon or visit his website at adamreddon.ca ]
Habitat degradation can have major impacts on fish communities. Changing the environment can shift the balance of power between species, allowing some species to flourish while others suffer. Often a small number of species will exploit the new conditions and become highly abundant while the overall diversity of animals found in a degraded area decreases. Altered habitats can reduce the amount of food, shelter and breeding substrate for certain species causing their populations to decline. There may be other, less obvious routes through which habitat degradation impacts fish. For instance, changes in the environment may have impacts on the cognitive or sensory capabilities of some fish, resulting in a reduced ability to avoid predators or find the resources they need to be successful.
McCormick and Lönnstedt (2016) examined how coral reef destruction may impair the ability of certain fish species to learn the identities of new predators and respond correctly to cues of predation risk. Many reef dwelling fish species spend a larval stage floating around the ocean amongst the plankton, before setting down on the reef to spend their adult lives. This transition is a very dangerous time for these fish because they must quickly learn whom among the reef community poses a threat and which animals are benign. Failing to react to a predator can mean certain death, but reacting to non-threatening animals can waste valuable time and energy. Learning all of this information is a challenging task and many of these newly settled reef dwellers do not last long in their new homes. The easiest way to learn if an unfamiliar animal is dangerous is through direct interaction, if a fish chases you trying to eat you and you escape, you now know that species is a threat. However, this is a pretty dangerous way to go about acquiring knowledge, as one false move could spell the end.
Luckily, fish of all kinds have evolved an ingenious system for marking unknown stimuli as dangerous using their sense of smell. Fish normally react very strongly to the smell of damaged individuals of their own species. If another member of your species has been injured or killed, chances are something dangerous is nearby and you should take heed. Fish react to these damage induced odours innately, meaning they don’t have to learn this association, they know from birth that the smell of injured members of their own species means that danger is afoot. What fish do learn is the association between these smells and other odours, sights or sounds in the environment. If a fish smells the scent of a conspecific (a member of the same species) that has been injured along with the odour of some unknown species of animal mixed together, they quickly learn that this other unfamiliar smell is probably from something dangerous and should be treated with caution when it is encountered. Using this powerful ability to form associations, fish can learn about possible dangers without having to engage in risky interactions with unfamiliar potential predators.
Recent evidence has suggested that coral reef bleaching, caused by increasing ocean temperatures and altered water chemistry may impair the ability for reef dwelling fish to learn about new predators using odour cues. McCormick and Lönnstedt (2016) sought to better understand this effect, determine if it affected different species equally and see if the fish had to be living directly on degraded corals or if dead corals nearby were enough to throw the fish off. McCormick and Lönnstedt captured ambon damselfish (Pomacentrus amboinensis) as they were approaching a reef following their larval stage. The authors then trained the fish by exposing them to a combination of the smell of damaged ambon damselfish and water that had been used to hold a novel predator, the dottyback (Pseudochromis fuscus). After this exposure, the authors placed the ambon damselfish onto one of four types of experimental habitats: a healthy piece of live coral surrounded by live corals, a dead coral surrounded by live corals, live coral surrounded by dead coral and dead coral surrounded by dead coral. The scientists then conducted a test where they gave the fish a shot of water containing either the smell of damaged conspecifics, the smell of dottyback predators or no odours at all and then watched the reactions of the ambon damselfish to each stimulus. What the authors found was that ambon damselfish living in the pristine habitat (on a live coral surrounded by live coral) reacted as one would expect. The plain sea water was ignored while the smell of either the damaged ambon damselfish or the smell of the predatory dottyback was treated with fear: the fish stopped moving, stopped eating and quickly retreated to their shelters. However, if the ambon damselfish was exposed to degraded corals, either directly or in the surrounding habitat, their reaction was very different. Instead of hiding in fear in response to the dangerous stimuli, the fish exposed to dead corals did just the opposite, they moved more, went further from their shelters and increased their foraging rate. These are risky things to do if there is a predator around and suggest that the dead corals are interfering with the ability for the ambon damselfish to respond appropriately to cues of danger, something that may get them eaten and thus potentially have a negative impact on their population. Interestingly, its not that the dead coral merely made it impossible to for the ambon damselfish to smell the predators or damaged conspecifics, because the fish exposed to dead coral still reacted to these odours, but something about the dead corals alters the behaviour in response to these cues, potentially with dangerous consequences.
Ambon damselfish are habitat generalists, meaning they will live in a variety of different sorts of places, including both live and degraded reefs. McCormick and Lönnstedt wanted to see if they would find the same sorts of effects of dead coral on the behaviour of another species, the neon damselfish (P. coelestis). Unlike the ambon damselfish, the neon damselfish specialize on living in rubble conditions similar to those found in dead and degraded reefs. When the authors did a similar test on the neon damselfish they found that this species reacted just fine even when living on a dead coral. This suggests that the effects of habitat degradation are species specific and depend on the sorts of habitats the animals normally live on.
Together, the results of McCormick and Lönnstedt’s paper suggest that habitat degradation can affect species through less obvious means than just changes in the resources available to them. Changing habitats can also alter the cognitive abilities and behavioural responses of fish, which may have important effects on how well they adapt to changing conditions. Some species may find ways around these impacts and do well even in degraded habitats, while other species do not. It is important for us to consider the effects of climate change and habitat degradation on the cognition and behaviour of different species, and seek to understand why some species may be impacted while others are not.
Mccormick, M. I., & Lönnstedt, O. M. (2016). Disrupted learning: habitat degradation impairs crucial antipredator responses in naive prey. Proceedings Biological Sciences / the Royal Society, 283(1830), 20160441–8. http://doi.org/10.1098/rspb.2016.0441
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