Bird poop helps keep coral reefs healthy, but rats are interfering | Science News for Students

Bird poop helps keep coral reefs healthy, but rats are interfering

Nitrogen from bird guano makes for more productive reef ecosystems
Aug 1, 2018 — 6:45 am EST
a red-footed booby chick sitting in a tree

The poop of seabirds, such as this red-footed booby chick, contains nitrogen that leaches into coral ecosystems. That nitrogen helps the reefs stay robust.

N. Graham

When invasive rats eat island seabirds, nearby coral reefs can suffer.

Researchers studied islands with and without the rodents in the Chagos Archipelago. It’s an island group in the middle of the Indian Ocean. On rat-free isles, the team found an average of 1,243 birds per hectare (2.5 acres). On rat-infested islands, they found only about two birds per hectare. What’s more, rat-free areas had healthier coral reef ecosystems.

yellow and blue fusiliers swim in a reef of the Chagos Archipeligo
Coral reefs near islands without invasive rats are healthier than those off of rat-infested isles. This is likely due to more nutrients washing into the sea from bird poop. Here, fish called yellow and blue fusiliers swim in a reef of the Chagos Archipeligo.
N. Graham

The reason: Bird poop.

Feces are naturally rich in nitrogen, a key nutrient. Rain washes the poop’s nitrogen into the ocean. This helps keep reefs productive. The scientists described their findings in the July 12 Nature.

“We’re essentially linking three ecosystems in this study,” explains Nick Graham. Rats affect the seabirds, which affect the reefs, notes this coauthor of the new study. An ecologist, Graham works at Lancaster University in England. Feces are naturally rich in nitrogen, a key nutrient. Rain washes the poop’s nitrogen into the ocean. This helps keep reefs productive. The scientists described their findings in the July 12 Nature.

People introduced rats to the Chagos Archipelago in the late 1700s. Since then, these rodents have taken over ecosystems, devastating populations of native seabirds. These birds have included red-footed boobies and terns.

Rats eat seabird eggs, chicks — even the brains of adult birds, says Holly Jones. She’s a restoration ecologist who was not involved in the study. Rats are a major problem, says Jones, who works at Northern Illinois University in DeKalb. Why? Seabirds are “ecosystem engineers.” They help to keep the ecosystem in balance. When they’re gone, the environment on land and in the water changes dramatically.

Story continues below graph.

nitrogen levels graph
Nitrogen levels were tested in island soil, algae and the muscles of a common reef fish called damselfish. Measurements compared amounts of two forms of nitrogen, heavy and light. Bird guano is a rich source of that type of nitrogen. And there was more heavy nitrogen in areas with no rats.
N. Graham et al/Nature 2018

Looking for signs of poop

Bird poop, or guano, is rich in certain heavy isotopes of nitrogen. (Isotopes are different forms of an element that all have the same amount of protons but different numbers of neutrons.) Heavy nitrogen comes from the birds’ diet. Its presence points to there being bird poop — and thus birds — in an area. Graham and his colleagues tested for these isotopes on 12 islands. Six were infested with rats, the rest had no rats. The researchers also tested nearby coral reefs.

Compared with rat-infested islands, the team found much more heavy nitrogen in the soil of rat-free islands. These were sites where bird populations still thrived. The researchers also found heavy nitrogen in the algae, sponges and fish at reefs surrounding those islands. Rains and lapping waves are known to leach bird guano into the sea. However, that guano’s effects on reefs had been unclear.

Because nitrogen can act as a fertilizer for ocean plants and algae, the researchers now suspect this boosts the health of reefs around rat-free islands. More algae grow. That leads to more fish grazing on the reefs. Those fish also help clear out dead corals. These are all essential processes in a healthy reef. The fish living near reefs with more nitrogen also grew larger and faster, the scientists show.

In addition to these indirect effects on reefs, nitrogen may also directly help the corals, says David Gillikin, who was not involved in the study. He’s a biogeochemist at Union College in Schenectady, N.Y. Between 15 and 50 percent of nitrogen found in corals comes directly from seabird guano, he says.

Eradicating invasive species, such as rats, from the islands will help preserve reefs, Graham says. Rat extermination has been done on 580 islands worldwide. It can be difficult to get rid of every rat on an island. So far, the success rate has been about 85 percent.

Still, many reefs have been in trouble for decades and face other threats, too. These include coral bleaching and ocean acidification. Climate change has been driving both impacts. The UNESCO World Heritage Centre estimates that large coral reefs could be gone by the end of this century. “We’re constantly looking for solutions for the coral reef crisis,” Graham says.

Protecting seabirds to save coral reefs is one solution that doesn’t stink. 

Power Words

acidification     A process that lowers the pH of a solution. When carbon dioxide dissolves in water, it triggers chemical reactions that create carbonic acid.

algae     Single-celled organisms, once considered plants (they aren’t). As aquatic organisms, they grow in water. Like green plants, they depend on sunlight to make their food.

archipelago     A group of islands, many times forming in an arc across a broad expanse of the oceans. Examples include the Hawaiian islands, the Aleutian islands and the more than 300 islands in the Republic of Fiji.

average     (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.

biogeochemistry     A term that covers processes that cycle (or eventually deposit) pure elements or chemical compounds (including minerals) between living species and nonliving parts (such as rock or soil or water) within an ecosystem. A scientist who works in this field is a biogeochemist.

birds     Warm-blooded animals with wings that first showed up during the time of the dinosaurs. Birds are jacketed in feathers and produce young from the eggs they deposit in some sort of nest. Most birds fly, but throughout history there have been the occasional species that don’t.

climate     The weather conditions that typically exist in one area, in general, or over a long period.

climate change     Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.

coauthor     One of a group (two or more people) who together had prepared a written work, such as a book, report or research paper. Not all coauthors may have contributed equally.

colleague     Someone who works with another; a co-worker or team member.

coral     Marine animals that often produce a hard and stony exoskeleton and tend to live on reefs (the exoskeletons of dead ancestor corals).

diet     The foods and liquids ingested by an animal to provide the nutrition it needs to grow and maintain health. (verb) To adopt a specific food-intake plan for the purpose of controlling body weight.

ecology      A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.

ecosystem     A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra. The term can also be applied to elements that make up some an artificial environment, such as a company, classroom or the internet.

element     A building block of some larger structure. (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.(in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.

engineer     A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.

environment     The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of components in some electronics system or product).

feces     A body's solid waste, made up of undigested food, bacteria and water. The feces of larger animals are sometimes also called dung.

fertilizer     Nitrogen, phosphorus and other plant nutrients added to soil, water or foliage to boost crop growth or to replenish nutrients that were lost earlier as they were used by plant roots or leaves.

invasive species     A species that is found living, and often thriving, in an ecosystem other than the one in which it evolved. Some invasive species were deliberately introduced to an environment, such as a prized flower, tree or shrub. Some entered an environment unintentionally, such as a fungus whose spores traveled between continents on the winds. Still others may have escaped from a controlled environment, such as an aquarium or laboratory, and begun growing in the wild. What all of these so-called invasives have in common is that their populations are becoming established in a new environment, often in the absence of natural factors that would control their spread. Invasive species can be plants, animals or disease-causing pathogens. Many have the potential to cause harm to wildlife, people or to a region’s economy.

isotopes     Different forms of an element that vary somewhat in mass (and potentially in lifetime). All have the same number of protons in their nucleus but different numbers of neutrons.

leach     (in geology and chemistry) The process by which water (often in the form of rain) removes soluble minerals or other chemicals from a solid, such as rock, or from sand, soil, bones, trash or ash.

native     Associated with a particular location; native plants and animals have been found in a particular location since recorded history began. These species also tend to have developed within a region, occurring there naturally (not because they were planted or moved there by people). Most are particularly well adapted to their environment.

neutron     A subatomic particle carrying no electric charge that is one of the basic pieces of matter. Neutrons belong to the family of particles known as hadrons.

nitrogen     A colorless, odorless and nonreactive gaseous element that forms about 78 percent of Earth's atmosphere. Its scientific symbol is N. Nitrogen is released in the form of nitrogen oxides as fossil fuels burn.

nutrient     A vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organism requires as part of its food in order to survive.

population     (in biology) A group of individuals from the same species that lives in the same area.

proton     A subatomic particle that is one of the basic building blocks of the atoms that make up matter. Protons belong to the family of particles known as hadrons.

reef     A ridge of rock, coral or sand. It rises up from the seafloor and may come to just above or just under the water’s surface.

rodent     A mammal of the order Rodentia, a group that includes mice, rats, squirrels, guinea pigs, hamsters and porcupines.

sea     An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.

sponge     A primitive aquatic animal with a soft, porous body.

UNESCO    Short for United Nations Educational, Scientific and Cultural Organization. This agency, based in Paris, France, is part of the United Nations. It was created in November 1945, several months after the close of World War II. Among its major accomplishments, it created the intergovernmental conference to set up an international system for issuing copyrights. In 1960, it launched a 20-year program to move 22 monuments and architectural complexes in Egypt that were about to be flooded by the Nile. In 1972 it created a world heritage program, inducting the first sites onto its list for protection in 1978. That same year, it adopted a declaration that support for racism had no scientific basis. In 2000 it committed governments across the world to providing basic education for all residents.

wave     A disturbance or variation that travels through space and matter in a regular, oscillating fashion.


Journal:​ N. Graham et al. Seabirds enhance coral reef productivity and functioning in the absence of invasive rats. Nature. Vol. 559, July 12, 2018, p. 250. doi:10.1038/s41586-018-0202-3.