Aug. 6, 2009 Advances in ecology increasingly reveal that conventional agricultural practices have detrimental effects on the landscape ecology, creating problems for long-term sustainability of crops. In a series of sessions at the Ecological Society of America's Annual Meeting, ecologists will present their ideas on how our agricultural practices can take lessons from natural environments.
Perennial plants produce more, require less input than annual croplands
The major crops used globally to feed people and livestock – wheat, rice, maize and soy – are based on an annual system, in which crop plants live one year, are harvested, and are replanted the following year. These systems are notorious, however, for stripping organic nutrients from soils over time.
Perennial systems, on the other hand, contain plants that live longer than one year despite being harvested annually. Many agricultural scientists, including Jerry Glover of The Land Institute, say that perennial crops are the key to creating more sustainable agricultural systems.
"Across agricultural history, we've fundamentally relied on annual grain crops," Glover says. "But at the same time we rely on them, they're degrading the ecosystems they're in, which reduces their productivity."
To compare the long-term sustainability of these two cropping systems, Glover and his colleagues conducted a study on the physical, biological and chemical differences between annual wheat fields and perennial grass fields in Kansas. The fields had each been harvested annually for the past 75 years.
In each test, the researchers found perennial fields to be healthier and more sustainable ecosystems. In the perennial fields, the plants' total root mass was more than seven times that of the annuals, and the roots infiltrated about a foot deeper into the ground. The perennial fields also had higher soil microbe biodiversity and higher levels of dissolved carbon and nitrogen in the soil. All these findings, says Glover, suggest that the perennial field soil is healthy enough to maintain high levels of organic nutrients.
In addition to being more ecologically sustainable, Glover's team found that the perennial fields were more energy-efficient in providing productive harvests. Although only the annual fields received yearly fertilizer inputs, the perennial fields yielded 23 percent more nitrogen harvested over the 75 years, despite requiring only 8 percent of the energy inputs in the field – such as fertilizer and harvesting operations – as the annual systems.
Glover says that these results clearly show the need to move away from annual crops and increase our use and domestication of perennial crops.
"So far, little effort has been made to improve perennial crops," he says. "Some of greatest possibilities for transforming agriculture may well come from overlooked systems such as perennial grasses."
Landscape diversity creates habitat for pest enemies
Farmers spend millions of dollars each year on pesticides to kill crop-eating insects. But these insects have natural enemies, too, and new research is investigating what farmers can do to encourage the proliferation of these pest-eaters. One study, presented as a talk at the ESA meeting, shows that increasing the natural habitat in and around farms can boost populations of pests' natural enemies.
Rebecca Chaplin-Kramer of the University of California Berkeley surveyed the abundance of flies, ladybugs, wasps and other predators of a common agricultural pest, the cabbage aphid, in croplands ranging from 2 percent to about 80 percent natural vegetation. She found that as the proportion of natural area – or complexity – increased, so did the numbers of natural enemies in the croplands.
Chaplin-Kramer shows that increases in predators didn't always result in fewer aphids in the croplands, but she points out that agents of control are only half of the equation and sources of the pests themselves must also be considered. In the absence of predators, pest levels would likely rise even more dramatically.
"By having complexity, you're supplying a community of insects to that farm that will be waiting when –and if – more pests show up," Chaplin-Kramer says.
Fostering larger predator communities is time-consuming and can take years to show results, Chaplin-Kramer says, which is why many farmers are skeptical of the idea. But, she says, there's no doubt that a strong predator base is more sustainable than simply using pesticides.
"Pesticides are a short-term solution, because pests can build up resistance, and new pesticides are constantly being developed," she says. "Building up predator communities takes time, but the systems are more stable and will provide more ecosystem services in the long term."
Reduced tilling improves soil microbe biodiversity
The idea of using biological instead of chemical methods to create healthy croplands doesn't include just above-ground approaches. Soil bacteria can affect the growth and success of crop plants by fixing nitrogen, aiding in the uptake of nutrients and decomposing dead organic matter. Some current farming practices, however, may disrupt the soil ecosystem and decrease the effectiveness of the microbe community.
In his poster, Shashi Kumar of Texas Tech University will explore the relationship between conventional tilling and low-tilling practices on farms in semi-arid areas of west Texas. In areas where soil tilling was kept at a minimum, Kumar and his colleagues found a higher diversity of soil bacteria; conventional tilling produced lower bacterial diversity.
Kumar says that conventional tillage systems disrupt soil particles and decrease soil pore size, which can lead to decreased water and soil access for microbes. Although he recognizes that tillage is necessary, he thinks that farmers can reduce their tillage, even in semi-arid regions, to promote soil bacterial biodiversity.
"We are currently using so many different crop management systems, like pesticides, insecticides and fungicides, which are damaging to our soil system," Kumar says. "Why shouldn't we focus on biological methods, since the bacteria are already there?"
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