DOES THE EARTHWORM SUSTAIN HUMANKIND as the tiller of fertile topsoil or subvert it as spoiler of the atmosphere? Is its role as decomposer beneficial to soil cultivation or detrimental to the environment? Is its presence a blessing or a curse?
The answers to these questions used to be clear-cut: earthworms indicated fertile land. But changes in earthworm populations and the discovery that nonnative earthworms have significantly increased the release of carbon dioxide into the atmosphere make the answers much more uncertain. There's hardly a creature we deem more insignificant than the shy and lowly earthworm, yet they profoundly affect our environment.
Changes in land-use, especially in the tropics, alter the role earthworms play in the balance of their ecosystems. And exotic earthworms, species not native to the habitat, appear with land-use change. Cutting down forests for agriculture or development affects the abundance and species diversity of earthworms. As forests vanish, native earthworms disappear and exotic earthworms overpopulate, reducing biodiversity and releasing more carbon dioxide, the same pollutant emitted by the burning of coal, oil and natural gas, into the atmosphere. Deforestation also reduces photosynthesis. Photosynthesis removes 120 billions metric tons of carbon dioxide globally from the air each year.
When CO2 increases drastically, it escalates the chances of global warming. While most of the increases in atmospheric CO2 have come from combustion of fossil fuels, changes in land use–deforestation and the accompanying overpopulation of earthworms–could be a cause of global warming.
Xiaoming Zou, Ph.D., a biologist at the University of Puerto Rico, Río Piedras's Institute for Tropical Ecosystem Studies, focuses his research on how soil biota regulates soil processes and fertility, sustainable use of soil resources, and the health of the ecosystem in general. He is one of the world's foremost experts on earthworm ecology. It was he and Zhigang Liu, a postdoctoral scientist at ITES, who discovered that CO2 released from soil increased with earthworm activity in the tropics.
"I chose earthworms to study because they are among the most abundant of soil creatures in moist tropical soil. Earthworms play a critical role because they're tillers. Without them, the whole ecosystem and human society would be different," Zou says.
Earthworms accelerate decomposition, which means that they make more nutrients available to plants and improve their growth. But faster decomposition also means that more CO2 is released into the atmosphere. Each year soil releases about 60 billion metric tons of carbon into the atmosphere. Fossil fuel combustion releases only 6 billion tons. In other words, soil releases 10 times more CO2 into the atmosphere than the burning of fossil fuels. And if soil respiration increased by just 10 percent, it would be equivalent to doubling the amount of carbon dioxide released into the atmosphere by fossil fuel consumption.
"Earthworms can modify the soil environment by mixing organic and mineral particles and changing the water infiltration and aeration regimes. Tillering by soil fauna directly alters soil's physical, chemical, and biological properties," says Grizelle González, Ph.D., a research ecologist at the Forest Service's International Institute of Tropical Forestry in Puerto Rico and an expert on the ecology of Puerto Rican earthworms.
"Earthworms can modify the soil environment by mixing organic and mineral particles and changing the water infiltration and aeration regimes. Tillering by soil fauna directly alters soil's physical, chemical, and biological properties," says Grizelle González, Ph.D., a research ecologist at the Forest Service's International Institute of Tropical Forestry in Puerto Rico and an expert on the ecology of Puerto Rican earthworms.
"Given that approximately 50 percent of world forests are located in the tropics and that this area has the highest rate of deforestation and land conversion in the world, deforested tropical land is considered to be a major source of CO2 to the atmosphere. We as a society haven't paid enough attention to the ecological consequences of tropical land use and the invasion of exotic earthworms," says Zou.
In the tropics, one specific species, Pontoscolex corethrurus, is causing the problems. "It's the culprit," says Zou.
Exotic earthworm expansion
No one knows when the first exotic earthworms of the Pontoscolex corethrurus species emigrated from South America to other parts of the tropics. Zou estimates that they came to Puerto Rico sometime between 1800 and 1845, when trees were being felled for agriculture and a variety of plants were being introduced to the island. But they may have arrived along with indigenous people coming to Puerto Rico from South America more than 2,000 years ago. The first were probably stowaways in the soil of transplanted potted plants. Hitchhiker worms could also have arrived in mud on tires of imported vehicles and on the bottoms of boats in soil that survived the voyage. In addition some people, thinking that they would increase the fertility of the land, might have deliberately imported and released earthworms not native to the tropics. However, points out González, the deliberate introduction of earthworms occurs more often in the States, as people became aware of the benefits of vermicomposting, and the introductions are related to the fish bait industry.
Today Pontoscolex corethrurus has spread across the tropics–throughout the Hawaiian Islands, the Caribbean, Mexico, Africa, Australia, and even the tropics of Asia. Zou spent a year in Taiwan and China following the trail of this invasive species and was the first to find it in South Taiwan and in Yunnan, a Chinese province bordered by Vietnam and Laos. "It seems that Pontoscolex corethrurus is in every part of the moist tropics where humans exist."
In their research in Puerto Rico, Zou and Lui found that the Pontoscolex corethrurusspecies increased CO2 soil respiration between 20 and 30 percent. An earlier study by Zou and González showed that the exotic earthworm population had exploded with land-use changes. The researchers found that the Pontoscolex corethrurus population increased from 89 earthworms per meter in a tropical wet forest to 831 earthworms per meter after the forest was converted to a pasture.
Because CO2 is the most important greenhouse gas, most of the work of ITES scientists studying earthworm populations has centered on CO2 increases. But another important effect of tropical land-use changes is soil emission of the greenhouse gas nitrous oxide and the tropospheric ozone precursor nitric oxide. Nitrous oxide has a warming effect because it absorbs infrared radiation, just as CO2 does. Consequences of earthworm activity on these two gases in the tropics are still unknown and need further study.
Reforestation in Puerto Rico
Puerto Rico, says Zou, is an exceptional place to study land-use changes because while other tropical areas have continued the trend of deforestation, in the last 50-60 years this island has experienced significant reforestation. As Puerto Rico changed from an agriculture-based economy to an industrial economy, abandoned pasturelands reverted to forests. ITES scientists have a unique opportunity to study how native earthworm populations recover in secondary forests.
Their findings so far have been encouraging.
As forests recover, earthworm diversity increases and the native earthworm comes back. We could control exotic earthworm populations with reforestation," says Zou. "Particularly, if we consider forest management practices that can positively affect abundance and community structures of native earthworms," adds González.
"Most other places in the tropics are still cutting forests. We are setting an example for the rest of the world. The shift from an agriculture-based economy to an industrial economy is the real cause of reforestation," says Zou.
But as John Thomlinson, Ph.D., a biologist at ITES specializing in landscape ecology and global information systems, points out, the process of reforestation in Puerto Rico isn't a clear-cut victory. "We are seeing a very dynamic situation in Puerto Rico. The forest regeneration has been remarkable, and it demonstrates the resilience of these ecosystems. What we have found is that small remnant patches, left over from before the clearing took place, greatly accelerate regeneration, presumably by serving as sources for new trees. This is an important finding for countries that are still clearing land, in that it can be used as a management tool.
"It isn't clear, however, that the trend of increasing forest cover in Puerto Rico will continue. In fact, it already seems to have reversed as land is cleared for development. This is the two-edged sword of industrialization–with greater affluence comes more development, especially in the form of urban sprawl. It will be interesting to see how these earthworms respond to urban environments."
Plants, animals, soil, and the atmosphere interact to make up the carbon cycle, in which every carbon atom in existence is recycled. For example, the same carbon in our bodies has been used in numerous other molecules in the past. The coal burned a few decades ago that produced carbon dioxide could become part of a plant through photosynthesis. When we eat the plant, that same carbon from the coal becomes part of us. In turn, our waste products contain carbon, which is released into the atmosphere as CO2. The earth's ecosystems take in carbon from the atmosphere through photosynthesis and add it back through respiration and decay. The ocean also stores a significant amount of CO2 that plankton and algae, through photosynthesis, change into organic materials.
When the rates of photosynthesis and respiration are equal, the earth's ecosystems are in balance. But when more CO2 is released into the atmosphere than is absorbed through photosynthesis, carbon either accumulates on land or is released into the atmosphere. Forest destruction, therefore, upsets ecosystems because much of the carbon stored in trees is released into the atmosphere in the form of CO2 when forests are cut, and fewer trees mean a decrease in the amount of CO2converted to organic materials through photosynthesis.
Carbon dioxide is the major greenhouse gas released into the atmosphere. Greenhouse gases can raise the overall global temperature, change climates, and raise the sea level. Scientists at the Tyndall Centre for Climate Change Research and School of Environmental Sciences in the United Kingdom have observed a two mm per year rise in sea level from 1993-2000, lengthened growing seasons (by two to three weeks in the past 15-20 years), and changes in precipitation intensities. Never before have climatic changes been so rapid. Scientists have also noted a 1 degree Fahrenheit rise in the earth's mean temperature over the last century and they expect it to continue increasing by 2 to 6 degrees over the next century. Elevated temperatures have led some scientists to predict the extinction of ten percent of animal species in the next 100 years.
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