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The Washington Post

Health & Science

Decline of predators such as wolves throws food chains out of whack, report says

By Darryl Fears

July 14, 2011

The decline of large predators such as big cats, wolves, sharks and giant whales may be “humankind’s most pervasive influence on the natural world,” causing prey animals to swell in population and throw food chains out of balance, a new report says.

Humans have touched off the world’s latest mass extinction, according to the report, published Thursday in the journal Science, and the consequences are being felt on land and in water systems as large predators vanish.

“Recent research suggests that the disappearance of these animals reverberates further than previously anticipated,” says the report, “Trophic Downgrading of Planet Earth.” In addition to creating an overabundance of prey, the dwindling number of predators contributes to the spread of disease, wildfires and invasive species.

The decline of wolves in Yellowstone Park is cited as an example of what can happen. Elk and deer in the park once flourished on willow trees and saplings, threatening a crucial part of the forest on which other creatures rely.

The report also mentions the slaughter of lions and leopards by hunters and herders in parts of sub-Saharan Africa. As a result of the killings, disease-carrying olive baboons have thrived without their top predators and inched closer to food crops and people.

The decimation of sharks along the U.S. Atlantic Coast has allowed their main prey, the cow-nosed ray, to proliferate and dine heavily on the threatened Chesapeake Bay oyster.

A reduction of big herbivores such as buffalo and wildebeest in East Africa through hunting is also a problem, the report says. Their demise has led to increases in plants that fuel giant wildfires in the dry season.

Americans don’t have to visit federal parks or sub-Saharan Africa or plunge into seas to see the consequences, said Ellen K. Pikitch, a co-author of the report and a professor at Stony Brook University in New York. Many experience the problem every day in their own back yards.

“People who live in North America know it’s hard to grow a garden because deer will eat it,” said Pikitch, a marine biologist. “The lack of wolf populations throughout North America has led to an expansion of the deer

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population.

“You may hate wolves. You might think they’re dangerous. But without them, the land changes,” Pikitch said. “Deer carry ticks. We humans become more susceptible to diseases such as Lyme disease.”

Wildlife advocates say efforts to protect one species of predator in the United States were set back when the Obama administration signed a bill in April that removed 1,300 wolves from the endangered species list in northern Rocky Mountain states. It was the first time Congress had taken a species off the endangered list. The law allows limited hunting of the animals to begin this summer.

Other studies have examined the collateral damage caused by the near-extinction of large predators and herbivores. But the report in Science is the first to tie together the impact on land animals as well as salt and freshwater marine life, Pikitch said. It was conducted by an international team of 24 scientists and funded primarily by the Institute for Ocean Conservation Science at Stony Brook.

Much of the science in this area of study has focused on the threat to life at the bottom of the food chain, theorizing that small animals and plants are important because so many creatures rely on their survival.

Although “bottom-up” research is fundamental and important, the report says, “top-down” research deserves wider consideration “if there is to be any real hope for understanding and managing the workings of nature.”

The report acknowledges that top-down research of the food chain is difficult to conduct, noting that it can take decades to measure the effects of the disappearance of large predators.

“The irony . . . is that we often cannot unequivocally see the effect of large apex consumers until after they have been lost” and the ability to restore the species has also been lost, the report says.

Large predators, or apex species, include animals that people adore, such as otters, and others not so popular, such as vultures.

On the Pacific Coast, from Alaska to the southern tip of California, sea otters were hunted in the 1900s to near-extinction for their pelts. Their absence started a chain of events that nearly eliminated the kelp forests that nurture all manner of marine life on the coast.

Sea otters feed on sea urchins, which dine on kelp. Without otters, the sea urchin population exploded. The kelp forest started to disappear. When sea otter populations elsewhere were re-introduced to a few areas along the coast, the kelp started to rebound.

A telling consequences of the absence of large predators can be found on the Scottish island of Rum, where wolves have been gone for more than 250 years and red deer thrive, the report says. The once forested island is now treeless.

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Darryl Fears Darryl Fears is a reporter on the national staff who covers the Interior Department, issues affecting wildlife and the Chesapeake Bay watershed. Follow 

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Climate change: changes in the Earth’s weather that scientists predict will happen over many years.

Industrial Revolution: a time of development from the late 1700s to the mid-1800s that led to increased use of machines, factories, and energy, and that resulted in increased pollution….more (https://en.wikipedia.org/wiki/Industrial_Revolution)

Ocean acidification: a reaction that occurs when there is more carbon dioxide present in ocean waters than normal due to an increase in carbon dioxide in the air. Carbon dioxide can split into smaller ions, releasing hydrogen ions into the water. This causes the water to become acidic….more (https://en.wikipedia.org/wiki/Ocean_acidification)

Productive: getting a lot done or providing a favorable environment with food and shelter for many organisms to grow.

Road diversion: a special route for use by traffic when a road is temporarily closed or has restrictions.

Trophic flow: the transfer of energy between trophic levels in a food web….more (https://en.wikipedia.org/wiki/Energy_flow_(ecology))

Trophic level: a position in a food web, based on relationships with other organisms in the food web….more (https://en.wikipedia.org/wiki/Trophic_level)

Vibrant: bright, or brightly colored.

Countdown to Disaster in Marine Food Webs?

Written by: Tin Hang (Henry) Hung

show/hide words to know

What's in the Story?

It’s noon and your stomach starts rumbling—you’re hungry. You want to get some Japanese ramen for your empty stomach. Stepping out of your house, you are hit by a wave of heat.

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What would happen if the fish we rely on for food was no longer available? Image by Douglas Perkins.

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Global carbon emissions, in the 1800s and 1900s. Click for more detail.

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“Maybe I should get a cold lunch like sushi instead,” you think. “Hot ramen may not be good on this hot day.” Other people had the same idea, and the restaurant has already sold all of its sushi for the day. You go off to find something else that is cold for lunch.

Weather often affects your food choice. You might drink more hot chocolate in the winter, and eat more ice cream in the summer. Something similar can happen in the sea. When temperatures warm up, some organisms may be found in greater numbers. Changes like these impact what is being eaten in the ocean. But what would happen if an organism was “sold out” in the sea?

Researchers discuss how the changing climate can alter the feeding relationships in the sea in the PLOS Biology article “Climate Change Could Drive Marine Food Web Collapse Through Altered Trophic Flows and Cyanobacterial Proliferation (https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.2003446).” They may have some sad news – the marine food web could collapse. And the clock is ticking.

Hot and Acidic Oceans?

Standing in a corn field, you see a machine drive by, picking corn off the plants. A lot of work that used to be done by hand is now done using machinery. This trend started in the late 1700s and increased heavily by the 1900s. The increase in machines and power usage was called the Industrial Revolution. All of this production put off a lot of air pollution.

After the Industrial Revolution, the amount of carbon dioxide (CO ) in air rose drastically. This increase in CO caused many problems in today’s world, including two big ones: climate change and ocean acidification. Because of these changes, our oceans are getting warmer and more acidic. How will this affect relationships in the sea?

A Taste of Trophic Levels

Species are highly connected to one another. The changing climate can alter their relationships. To understand how these relationships change, let’s look at how we divide organisms in a food web. Different levels of the food web, or food chain, are called trophic levels. “Trophic” just means related to eating. So let’s look at the different eating levels of a web.

Primary producers (like plants) are at trophic level one. They use sunlight to make sugars to feed themselves. The next level has herbivores, or plant eaters. These include large animals like deer, and tiny animals, like zooplankton. The third level is filled by animals that eat other animals. For example, small fish eat zooplankton. Some food webs have a higher levels, with predators that eat animals from the level below.

In these feeding relationships, the energy stored in prey flows to predators, up through trophic levels. This is known as a trophic flow. And because each organism may have multiple food choices, a food web is created, instead of just specific food chains.

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A food web showing the feeding relationships among organisms in the Canadian arctic. Note that some species have more than one food choice (e.g. narwals eat fish (cod) and benthic, or ground dwelling, invertebrates). Also note that some species are a common food choice to more than one predator (e.g. cod are eaten by seabirds, seals, and some whales). Image from "Current State and Trends in Canadian Arctic Marine Ecosystems II," by Darnis et al., in Climatic Change. Click to enlarge.

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Mesocosms can be built on land to hold water, or they can be created in the marine environment. Click for more detail.

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How did the treatments affect energy flow between levels of the food web? Click for more detail.

Humans are part of marine food webs, as marine organisms, like fish, are part of our diet. Billions of people depend on fish for nearly one-fifth of the animal proteins they eat. With such a dependence on marine food webs, we need to understand how the

changing climate may affect the oceans.

Studying Mini Marine Worlds

Researchers built 12 mesocosms to study how changing climate affects marine food webs. The word “mesocosm” comes from two Greek words: meso– means “medium”, and –cosm means “world”. They are “medium worlds” that researchers build, with habitats and organisms inside. Researchers can then experiment within these man-made worlds. For this study, the mesocosms were about the size of a hot tub, which holds around 1,800 liters of water (almost 500 gallons).

The researchers split the mesocosms into four groups, or treatments. In the first, they matched current conditions in the ocean. In the second, they increased the CO level to 900 ppm (“OA” treatment). In the third, they increased the temperature by 2.8°C (“T” treatment). And in the last, they combined those CO and temperature increases (“OAT”). They also called OAT the “business-as-usual” condition, as this is where our environment will end up in the year 2100 if we keep polluting as much as we do now.

Altered Trophic Flows: Road Diversions

Researchers treated the mesocosms for over four months, and measured the effects on the amounts of plants, invertebrates, and fish. They weighed the organisms at different trophic levels. Then they determined the energy flow by comparing the weights among levels.

Together, warming and higher acidity decreased energy flow from the producers (trophic level one) to the herbivores (level two). Warming also reduced energy flow to higher trophic levels. There were clearly some changes or “barriers” that slowed energy flow between trophic levels.

Researchers also found that the combination of warming and acidity increased the energy stored (known as biomass) in producers. More plants? It sounds like good news, right? However, this combination could decrease biomass at higher trophic levels. Researchers found that although there was more energy in producers, organisms in higher trophic levels could not use the extra energy. This is why energy flow among the trophic levels was reduced.

Energy can neither be created nor destroyed. Energy in producers must end up somewhere else, if it is not available to higher trophic levels. These altered energy paths are like road diversions in your city. The carriers cannot efficiently deliver the goods to the receivers, and

the energy ends up on a different route.

But what makes this happen? What are the “diversions” that keep this energy from moving through the food web as it would usually?

Cyanobacteria Takes Over

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How did ocean acidification and warming affect bacteria and algae? Click for more detail.

Cyanobacteria (blue-green algae) are bacteria that capture sunlight and make food. They are producers, in the first trophic level. However, to avoid being eaten, some cyanobacteria release toxins that can kill. Because of this, most herbivores prefer another food choice, turf algae.

In this study, researchers found that cyanobacteria grew better in warmer waters. But herbivores did not eat these cyanobacteria. This means more food was produced at trophic level one, but the energy did not flow to trophic level two or beyond.

Meanwhile, carnivores at higher trophic levels needed to eat more herbivores, because they had higher energy demands in warmer water. Herbivores were sandwiched; they ate less, but were eaten more. The death of herbivores eventually could no longer support the food web. The energy in uneaten cyanobacteria built up at the base of the food web, and organisms at higher levels didn’t have enough food to survive.

No More Business as Usual

In this way, future warming could very likely cause marine food webs to collapse. Considering how much we depend on fish, that’s a scary thought. If we don’t change our ways, and reduce our carbon emissions, our oceans may soon be less productive and vibrant than they are now.

Additional images via Wikimedia Commons. Coral reef by US Fish and Wildlife Service – Pacific Region. Coral reef at lighthouse by Holobionics.

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Bibliographic details: Article: Countdown to Disaster in Marine Food Webs?

Author(s): Tin Hang (Henry) Hung

Publisher: Arizona State University School of Life Sciences Ask A Biologist

Site name: ASU – Ask A Biologist

Date published: January 4, 2019

Date accessed: January 6, 2021

Link: https://askabiologist.asu.edu/plosable/marine-food-web-collapse

APA Style

Tin Hang (Henry) Hung. (2019, January 04). Countdown to Disaster in Marine Food Webs?. ASU – Ask A Biologist. Retrieved January 6, 2021 from https://askabiologist.asu.edu/plosable/marine-food-web-collapse

American Psychological Association. For more info, see http://owl.english.purdue.edu/owl/resource/560/10/ (http://owl.english.purdue.edu/owl/resource/560/10/)

Chicago Manual of Style

Tin Hang (Henry) Hung. "Countdown to Disaster in Marine Food Webs?". ASU – Ask A Biologist. 04 January, 2019. https://askabiologist.asu.edu/plosable/marine-food-web-collapse

For more info, see http://owl.english.purdue.edu/owl/resource/717/04/ (http://owl.english.purdue.edu/owl/resource/717/04/)

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MLA 2017 Style

Tin Hang (Henry) Hung. "Countdown to Disaster in Marine Food Webs?". ASU – Ask A Biologist. 04 Jan 2019. ASU – Ask A Biologist, Web. 6 Jan 2021. https://askabiologist.asu.edu/plosable/marine-food-web-collapse

Modern Language Association, 7th Ed. For more info, see http://owl.english.purdue.edu/owl/resource/747/08/ (http://owl.english.purdue.edu/owl/resource/747/08/)

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