Effects of removing predators

This article, by George Foulsham in Futurity.com, could be read in connection with Communities & ecosystems for Food chains & webs as well as with Populations for factors that influence population growth; Option E Stimulus & response for how animal responses can be affected by natural selection; and  Option G Population Ecology for the conservation of fish stocks.  Elizabeth Madin begins her abstract with "Fishing has clear direct effects on harvested species, but its cascading, indirect effects are less well understood."

“In more pristine areas, predators abound, and small prey fish are cautious and move only short distances from their hiding places,” says Robert Warner, professor of ecology, evolution, & marine biology at the University of California, Santa Barbara. “On heavily populated islands, where fishing has dramatically reduced numbers of predators, small fishes are much bolder. What we’ve seen is that these behavioral responses can lead to cascading effects through the ecosystem.” 

This is a great reference for the discussion about animal responses and natural selection in Option E Stimulus & response. Where grazer fish are not subject to predation, the behaviour to move further is selected while those under the predation pressure the behaviour to remain close to the nest or protective cover is selected.

Investigating the coral reefs of the central Pacific Ocean’s Line Islands, scientists collected data from waters where predator fish species have been  severely fished near some islands and not  near others. The team observed how fishing had impacted populations of sharks and other predators.  The conservation of top predators is essential for the wellbeing of the entire ecosystem so is relevant to the conservation of fish stocks in Option G Population Ecology.

"What would happen to the reef if the small, seaweed-eating fish had nothing to fear,” says Elizabeth Madin, the lead author of both studies. “What if they could graze more like cows in a pasture than wildebeest on a lion-infested African plain?”

Distinct changes in the patterns of seaweed growth were seen when predators were removed by fishing which then caused differences in the grazing behavior of small fish. 

Students should be able to see that changes in food chains and webs when the predators are removed changes the dynamics of the producer seaweed populations which in tern causes increase food supply for the grazer fish species which is relevant to topic 5  Communities & ecosystems in Ecology.

Coral is a sessile organism is easily  impacted by seaweed growing over the colony.  Grazing fish can crop down the seaweed so that corals can grow and thus provide safe hiding areas for the fish. Thus fishing away the predator fish will be a factor in how the reef grows.

Competing for space on the reef with seaweed, corals spawn need to find bare patches of reef that they can settle on, so the places where new coral can grow could be more limited. This is another reference to factors that effect coral growth as well as how the seaweed population is not eliminated by the grazer fish species but is maintained, thus inhibiting the coral embryos from becoming established.

“What our results show is that fishing can have surprising, but very clear, effects throughout coral reef ecosystems,” Madin says. “Hopefully, these results will help conservation practitioners and resource managers move toward true ecosystem-based management, where the full suite of ecological interactions and human impacts guide policy decisions.”"

This could be an excellent source  for use in the discussion about international measures that would promote the conservation of fish as the protective measures require many nations to accept for successful conservation. ^[1]