Lönnestedt, O.M., M.I. McCormick, D.P. Chivers, and M.C.O. Ferrari. 2014. Habitat degradation is threatening reef replenishment by making fish fearless. Journal of Animal Ecology 83: 1178-1185. DOI: 10.1111/1365-2656.12209
Have you ever watched a horror flick in which the protagonists are in a rundown building, the power goes out, so one goes to the basement to find the fuse-box? And then he/she is met with an abundance of warnings while returning to the main floor—blood on the floor, knife missing from the block, etc.? But he/she keeps exploring the house, calling friends’ names? And then of course, he/she gets murdered! And you sit there, thinking “why didn’t you hide?” Well, reef fish on degraded reef are somewhat like our misguided slasher flick protagonist.
Habitat destruction causes many conservation problems, including loss of biodiversity. Coral reefs worldwide are degrading and the biodiversity they support is diminishing. Reefs are declining due to ocean acidification, increasing temperature, pollution, overfishing, and other sources of damage. As corals die, rubble (broken, worn-down pieces of dead coral) and algae become the dominant features of the reef. Algae can out-compete, overgrow, and essentially smother live coral. As live coral declines so too does the biodiversity supported by the reef.
Predator-prey relationships are responsible for energy transfer throughout an ecosystem, meaning that a balanced dynamic is incredibly important for a healthy system. Prey organisms are constantly faced with a trade-off: avoid perceived threats (stay alive by hiding out) or ensure fitness by foraging and mating (but risk being eaten). The habitat characteristics (openness, presence of shelters, chemical cues of environmental health) will change prey’s perception of risk. Researchers set out to determine how prey respond to habitat degradation.
The Study & Major Findings
Lönnestedt et al. conducted their research on the Great Barrier Reef in Australia, focusing on a species of damselfish (Pomacentrus amboinensis) in transition from the larval life-phase to juvenile phase. These phases take place in two different environments: the larval phase is pelagic (open ocean) and the juvenile phase is benthic (bottom-dwelling, specifically reef-dwelling). As they transition, they rely on chemicals released from other injured damselfish to indicate predation risk so they can learn which predators to avoid.
The team used wild-caught fish and released them onto artificial reef patches that were in one of two categories: 1) live coral or 2) dead coral (overgrown by algae and invertebrates). The researchers constructed the reef patches, keeping the patch size, spaces for shelter, and structure consistent. They built the patches using one species of hard coral (Pocillopora damicornis). Colonies that were alive and healthy comprised live coral patches and those that were dead and covered with algae and/or sessile invertebrates made up the dead coral patches.
Researchers then sought to answer three questions:
A) Does coral status affect the behavior of the damselfish?
To answer question A, the behavior of each individual was observed and recorded for 3 minutes. The researcher noted time spent at 0, 2, 5 or 10 cm. from the reef, height above the bottom of the reef, and time spent in shelter.
Result: Fish behavior was impacted by habitat health. Fish on the live coral were more cautious—remaining closer to the base of the coral, closer to shelter, and taking shelter for longer time periods—than the fish on dead corals (Fig. 1).
B) Does it affect their ability to respond to risk cues?
In this part of the experiment, one of three chemicals was released into the water and behavior was observed again. The same fish used in A were used for B. The three chemicals were: (1) injury cues from conspecifics (the same species); (2) injury cues from heterospecifics (distantly related species); and, (3) saltwater as a control. The heterospecific cues were from cardinalfish (Apogon doederleini). Damselfish should respond most strongly to injury cues from their own species.
Result: When exposed to injury cues from their own species, damselfish on live coral increased their use of shelters and minimized their movement around the coral patch more than when exposed to another species’ injury cues. The fish on dead coral patches did not change their behaviors when exposed to any of the cues (Fig. 2).
C) Does the survival of the damselfish differ at the two types of reefs after 48 hours?
To determine survival (C), a different set of fish were released on the live and dead patch reefs. Researchers counted the fish three times a day for 2 days. The migration of this species of damselfish between patches was assumed to be negligible based on other studies. In a pilot study in which mesh cages protected the fish from predators, all damselfish survived for 48 hours regardless of whether the coral was alive or dead. Therefore, if a damselfish was missing from the patch reef, researchers assumed it had been consumed by a predator.
Result: 79% of damselfish on dead coral patches became victims of predators compared to 45% of those on live coral patches. This equates to a 75% increase in survival of fish on live coral reef habitat (Fig. 3).
Habitat degradation disrupts the normal behaviors of these fish, turning them into risk-takers and thereby reducing their chances of survival. Damselfish on dead coral can no longer effectively protect themselves from predation, even when exposed to predator cues. These transition stage recruits to the reef are important to population maintenance. The damselfish may not repopulate and therefore will not continue to support their predators. Therefore, if they don’t survive the transition, the stability of the food web itself is lost.
It is important to properly understand and explain why degradation takes such a toll on diversity. It is more than the immediate loss of live coral or an increase in algal cover. Ultimately, the habitat degradation changes the way resident organisms behave and respond to their environment and to each other.