How Climate Change Amplifies Damage from Invasive Species
Economic and environmental damage to U.S. agriculture from the introduction of plant and animal species as well as insect pests and diseases from foreign countries has been an increasing concern to American farmers and ranchers in recent decades, and it seems likely to only get worse in the future as climate change expands the climatic zones vulnerable to these so-called invasive alien species (or IAS). The most comprehensive estimate of the economic impact of these IAS came in 2005, when several Cornell University scientists estimated that nearly $120 billion in damages to the U.S. economy from invasive plants, pathogens, diseases, and animals, with about half of that damage accruing to U.S. agriculture. Other economic sectors that are impacted by IAS include maritime and river shipping, fishing, forestry, food processing, and recreation. Public health is also impacted, as some of the IAS are zoonotic pathogens which also affect humans, such as the Zika virus, parrot fever, West Nile fever, and influenza.
Many of these IAS, especially the plant varieties, were introduced deliberately into the United States during the 19th or 20th centuries, many for ornamental purposes, and then later got out of control and spread throughout the environment. One notorious example of this vector is kudzu, a viny plant species that originated in Asia and was first brought into this country as an exhibit in the 1876 World Fair held in Philadelphia, PA. It did not become widely adopted until the 1930’s, when it was thought that it could be helpful in anchoring soils in regions where fields otherwise might be prone to wind erosion such as occurred during an eight-year period in the southern Plains states known as the Dust Bowl.. Farmers were encouraged to plant kudzu by USDA’s Soil Conservation Service, even provided a modest financial incentive to do so. Although probably not as commonplace as the widely cited 7-9 million acre figure for the Southern United States, kudzu remains a problem in part because it can serve as a host for other IAS such as Asian soybean rust.
While climate change is described mainly as representing a long-term increase in average temperature, how it manifests in on-the-ground weather is actually quite complex. From a policymaker’s viewpoint, the goal is to reduce emissions of greenhouse gasses so as to avoid having the average global temperature increase by 2 degrees Centigrade above pre-industrial levels recorded in the mid-19th century. However, the other dimensions of climate change beyond average temperature increases are already having an adverse impact on global agriculture, such as greater variability in precipitation, which shows up as both longer and more severe droughts as well as more severe precipitation events (both rain and snow) when they do occur. A recent study by researchers at Penn State University found that the prime growing zone for mainstay U.S. row crops such as corn and soybeans has already begun to shift northward out of Iowa and Illinois into northern tier states such as Minnesota and the Dakotas. The shift has been and is likely to continue to be a gradual one, but will require additional research efforts to develop new seed varieties to help farmers adapt to the changes.
These shifts in growing zones will also facilitate the spread of the current array of weeds, pests, and diseases to new regions as well, many of them qualifying as IAS as described earlier. As a Senate Ag Committee staffer, I visited Brazil in the spring of 2004, to learn more about the impact on Asian soybean rust (Phakopsora. pachyrhizi) on that country’s soybean crop, and what they were doing to combat it. The disease had arrived in that country in 2001, and the spores that caused it had spread widely across the country and were generating serious damage to Brazil’s soybean crop, causing losses of up to 90 percent in affected fields. The spores were first found in the United States about six months later, in the fall of 2004, but fortunately to date have not caused much damage to the U.S. soybean crop, in large part because the dominant growing region for the crop is in the Midwest, which generally has a much cooler climate than the soybean growing regions in Brazil. As a result, the spores do not survive over winter in those states, when the primary reservoir during the off-season for the Asian soybean rust spores has been kudzu vines in the southern states. Kudzu is a legume species and is closely enough related to soybeans that the spores can survive on those plants during winter months.
To this point, soybean rust has not spread from south to north during the growing season quickly enough to cause serious damage to the Midwest states that dominate U.S. soybean production, such as Illinois and Iowa. However, as of 2019, kudzu plants had been found in several Midwest states, including Illinois, Missouri, and Nebraska. As winter weather in the Midwest continues to get warmer on average, the risk of soybean rust getting established earlier in the growing season will increase. Scientists have been working to develop soybean rust resistant seed varieties for the last few decades, but no varieties with such traits are yet commercially available, so the main mechanism currently used to control its presence is fungicide application.
Cattle ranchers in the south are dealing with a couple of invasive plant species that are degrading the quality of their pastures and forcing them to increase their application of herbicides to compensate. In particular, Renee Strickland, a Florida cattle rancher who serves as Farmer Ambassador from that state for the Farm Journal Foundation has experienced increases in tropical soda apple and smut grass in her fields in recent years. These are both invasive species that arrived in the state a few decades ago. The foliage for both weed species are unpalatable for livestock, so its presence reduces weight gain for animals grazing in those pastures. Scientists at the University of Florida have identified a ladybug like species as a biological control for tropical spot apple, but they consume the weed too slowly to keep up.
