Croak: what's killing these frogs? (Life/Earth science: water pollution/ecosystems).
In the last decade, about 20 known frog species around the world have gone extinct (died off), with an estimated 200 more species in fast decline. Why? Frog skin is porous, or riddled with holes; frogs breathe and "drink" water through their skin. Like sponges, they soak up their external environment. And their health is a telltale report card on the well-being of planet Earth: that's why frogs are known as an indicator species. As fewer frogs are found, and more malformed ones appear in remaining populations, alarmed scientists can't help but ask: What's in the water?
FOOD FRIGHT CYCLE
Tipped off by an avalanche of reports of odd-legged frog sightings, Pieter Johnson, a University of Wisconsin ecologist (scientist who studies the interactions of organisms and their environment), sloshed through 101 sites in California, Oregon, Washington, Idaho, and Montana. Says Johnson: "Diseases are natural. But when some factor changes so drastically, like an outbreak in wildlife, it's a symptom of something gone wrong." Malformed flogs were found breeding not only in small ponds, but also in lakes, irrigation canals, ephemeral (temporary) pools, and reservoirs. "I didn't expect this to occur in such a huge diversity of systems."
Johnson discovered that bodies of water with higher frequencies of abnormal frogs also teem with a trematode--or wormlike parasite (organism that lives on or inside another organism)--called Ribeiroia ondatrae. This parasite has a multiple-host life-cycle (see diagram, above). "First, we tried to understand what caused the parasite to be there," says Johnson. "Then we considered the ultimate factors of why the parasites are more abundant at these sites." One disturbing conclusion: Nutrient-rich agricultural fertilizers and cattle waste that seep into the water may contribute to deforming frogs.
More nutrients in water elevate algae (plantlike aquatic organisms) growth. In turn, more algae attract more Planorbella, an aquatic snail, to feed on the algae. The snail is a common host to trematodes, which navigate their way through water to burrow into the skin of a new host: tadpoles (developing flogs). "They don't just burrow anywhere," says Johnson. Specifically, they worm their way to the basal tissue (growing tissue) of the tadpole's hind limbs, forming cysts. "Imagine someone jamming a golf ball right into the middle of the tissue where your leg's trying to grow," he says. The budding bone distorts, splitting into two, or more. "Or it stops limb development all together."
The parasitic journey doesn't end with monster frogs. Malformed frogs are weak and easy targets for predators like birds or mammals. Inside the new host, parasites mature and spawn eggs, which are released into the environment via the host's feces. The eggs hatch in the water and attack snail tissues, then frogs. "If the frog doesn't get eaten by a predator, the parasite can't survive," says Johnson. On the other hand, nutrient runoff is suspected to increase the presence of snails, trematodes--and malformed frogs.
In a separate study, atrazine, the most widely used agricultural herbicide (weed killer) in the U.S., has been found to turn male frogs into hermaphrodites (having both male and female reproductive organs). "I was really surprised,'" says biologist Tyrone Hayes at the University of California, Berkeley. "At the time, atrazine wasn't known as an endocrine disruptor [mechanism that interferes with the natural secretion of hormones, or chemicals that control body functions]. And it wasn't expected to have such dramatic effects--especially in such low doses."
More than 30,000 tons (60 million pounds) of atrazine blanket U.S. farmlands each year, says the U.S. Department of Agriculture. Atrazine is so abundant in the environment it's even detected in rain and snow. The EPA allows 3 parts per billion (ppb) in drinking water (3 parts atrazine per billion parts drinking water). But Hayes found that frogs were impacted by doses as low as 0.1 ppb. "Let's say you have 5 grains of salt," Hayes says. "Imagine 5,000 times less than that in a four-liter aquarium. That low level still makes a difference to a tadpole."
Atrazine causes developing male frogs to produce aromatase, an enzyme (protein that speeds up chemical reactions) in vertebrates. The enzyme converts testosterone (male hormone) into estrogen (female hormone). "In males, this enzyme isn't normally active," says Hayes. But atrazine turns it on in the testes (male reproductive organs). Since all the testosterone is converted into estrogen, part of the frog's testes turn into an ovary (female reproductive organ). Also, the demasculinized frog's larynx (voice box) is smaller, producing a more feminine sound. Female frogs normally make estrogen--Hayes thinks they might regulate its levels. "So, we haven't seen any super female frogs."
"Some people may say `It's just frogs, so what?'" says Hayes. But he describes frogs' vital role as one piece of a complex, delicate puzzle: In a world without frogs, what would happen to mosquitoes and other insects that frogs normally eat? What about the fish, which share the same water as the frogs, and can land on your dinner plate? How about animals that prey on the frogs and fish? And then, there's the water.
"Take nutrient runoff for example," says Pieter Johnson. "It's currently considered the number-one source of contamination facing freshwater ecosystems around the world." Water contamination could have a huge impact on how much water is available for humans to drink. "There are so many different types of assaults that are going on, All of them are affecting humans in ways that humans haven't even figured out yet," says Johnson.
"Ribbit"-ing Experiment Frogs don't drink water--instead, they depend on their porous skin to absorb it. The process is called osmosis, or movement of molecules from a higher to lower concentration through a semi-permeable membrane. Check out how osmosis works. YOU NEED: 2 raw eggs * 2 large glasses or beakers * 1 measuring cup * red food coloring * water * metal spoon TO DO: 1 Pour a cup of water and 10 drops of food coloring into 2 glasses labeled "A" and "B." 2 Gently tap one egg with the spoon. Without breaking the egg membrane, carefully pick off approximately 1 cm x 1 cm of the shell pieces. (Note: If you break the membrane, start over!) 3 Carefully place the egg into glass A and an uncracked egg into glass B. 4 Let stand undisturbed for two days. CONCLUSIONS: Compare the results. How are the eggs different? Why? How is the red food coloring like water pollutants? Why? And how does it affect the eggs? DON'T STOP NOW! Research and report: What do some frogs do with their old skin?
HOW A PARASITIC WORM DEFORMS FROGS
Agricultural fertilizers and cattle waste seep into water.
More nutrients in water create more algae (plantlike, rootless, aquatic organisms). And more algae attract more Planorbella, an aquatic snail, to eat the algae.
Aquatic snails are hosts to a trematode, or a wormlike parasite.
Parasites crawl into and embed themselves in the base and tail of developing frogs, forming cysts beneath the skin.
Parasites block the development of normal frog limbs, causing malformed features.
Malformed frogs are more vulnerable to prey like birds or mammals. Parasites mature in the new host and release eggs via the host's feces. Eggs hatch in water and invade snail tissues.
Did You Know?
* In addition to frogs, the parasite Ribeeirovia ondatrae has also caused deformities in amphibian cousins such as toads and salamanders.
* Atrazine has been used to kill weeds for more than 40 years. Today it's used in more than 80 countries, but European nations such as France, Germany, and Italy recently banned the herbicide because of human health risks.
* Rising global temperatures and exposure to ultraviolet radiation have also been linked to declining frog populations.
Language Arts: Research the life cycle and habitats Of frogs. Then write an essay on "a day in the life" in the first person--or frog's--point of view.
Critical Thinking: If all the flogs in the world were killed by water pollutants, how would it impact you? Describe the scenario as a sequence of environmental events.
Directions: Answer the following questions in complete sentences.
1. Why are frogs known as indicator species?
2. Describe the trematode life cycle and how it malforms frogs. Be sure to mention how human activity may contribute to the cycle.
3. What is atrazine? How does it affect male frogs?
4. If all frog species died off, how could that impact you? Give three examples.
Answers will vary but should include the following points.
1. Frogs are known as indicator species because they have porous skin, which they use to breathe and to "drink" water. Like sponges, frogs soak up their external environment. And their health is a telltale report card on the well-being of planet Earth.
2. Nutrient-rich agricultural fertilizers and cattle waste seep into water. And more nutrients in water elevate algae growth. More algae attract more Planorbella, an aquatic snail, to feed on. The snail is a common host to a trematode called Ribeiroia Ondatrae. The parasite burrows into the basal tissue of a new host, tadpoles, and forms cysts over where hind limbs form. The budding bone distorts, splitting into two, or more. Malformed frogs are weak and make easy targets for predators like birds or mammals. Inside the new host, parasites mature and spawn eggs, which are released into the environment via the host's feces. The eggs hatch in the water and attack snail tissues. The cycle repeats.
3. Atrazine is the most widely used agricultural herbicide in the U.S. Scientists have found that even in low doses it causes developing male frogs to produce the enzyme aromatase. The enzyme converts testosterone into estrogen. Since testosterone is converted into estrogen, part of the frog's testes turn into an ovary. Also, the demasculinized frog's larynx is smaller, producing a more feminine sound.
4. If all frog species died, mosquitoes and other insects that frogs normally eat could become more abundant, leading to an increase of diseases (such as malaria or West Nile virus). Fish that share the same contaminated waters as frogs could be harmful for human consumption. And contaminated water could affect the quantity available for humans to drink.
* Pieter Johnson's study appeared in the May 2002 issue of the Ecological Society of America's journal, Ecological Monographs. For more information: www.esa.org * Tyrone Hayes's study was published in Proceedings of the National Academy of Sciences: www.pnas.org/cgi/content/full/99/8/5476 * help scientists: Report malformed frogs to the North American Reporting Center For Amphibian Malformations: www.npwrc.usgs.gov/narcam
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|Date:||Sep 13, 2002|
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