A worker fire ant is steadily traversing the landscape looking for a good meal to take back to the nest. Suddenly, it finds a lizard basking in the sun lying directly in its path. Undaunted, the ant goes back to the nearest foraging trail and recruits colony members to the site of the lizard. Armed with a venomous sting, the ants prepare to launch their attack.
Red imported fire ants have been marching across the southeastern United States since they arrived from South America on a cargo ship in Mobile, Alabama in the 1930s. They have spread across 14 states and 343 million acres , disrupting agriculture, infrastructure, and recreation along the way. Red imported fire ants are just one example of an invasive species. Invasive species are plants, animals, and microbes that have been introduced by humans to locations where they would not be found naturally. The term “invasive” is usually reserved for species that are so successful at colonizing new environments that they disrupt or alter the original ecosystem. The U.S. government also includes non-native species that cause economic harm or harm to human health in its definition of an invasive species . Non-native species that persist without becoming overabundant or causing significant harm are usually termed exotic species.
An Overview of Invasive Species
The United States is home to approximately 50,000 exotic species, including ~25,000 of plants, ~20,000 microbes, nearly 4,600 invertebrates, and some 300 vertebrates . However, only a fraction of these species drive large-scale disturbances associated with invasive species. Invasive species are introduced through a number of different pathways. Most commonly, invasive species arrive as accidental hitchhikers in transported goods or shipping materials, such as wooden pallets or ballast water used by cargo ships. An increasingly global economy has accelerated the rate at which these introductions occur. Gardens are another common source of invasive species. Exotic species are often planted for ornamental use in landscaping, but can escape from yards and nurseries and establish wild populations. In some cases, species become invasive after being intentionally released as a way to control the population of an established pest — which is often an invasive species itself! This is an example of biological control, which, despite good intentions, often leads to the establishment of a new invasive species without having any impact on the intended pest population. This is the case with the notorious cane toad in Australia, which was introduced in order to control the cane beetle population on sugarcane plantations. However, the poisonous toads spread rapidly throughout the country without having any effect on the cane beetles.
Red imported fire ants are voracious predators, pictured here devouring a large insect. (Photo credit: T. Langkilde)
Invasive species are usually good competitors, which helps them gain a foothold in new environments. Once established, invasives often outcompete native species. Many invasive plant species quickly come to dominate their new communities, sometimes to the exclusion of nearly all other species. The success of “weedy” plants often results from high reproductive rates, ease of dispersal, and the ability to tolerate a wide range of environmental conditions. Loss of plant diversity simplifies the structural complexity of habitat and reduces the variety of potential food sources available to native wildlife. Cogon grass, for example, is an invasive grass in the southeast US, and it grows so densely that ground-nesting animals are forced out along with the vast majority of native plants. Its leaves are studded with silica crystals, making it an unpalatable food source. It even alters the fire regime of the ecosystem, burning at a higher temperature than native grasses, thereby increasing the mortality of other plants during fire events . Invasive plants have also been shown to alter the flux of carbon and nutrients such as nitrogen through an ecosystem.
Other invasive species succeed for different reasons. Enemy release occurs when the new environment lacks a natural predator or herbivore of the invader. Some invaders have novel weaponry, such as a toxin, that makes them efficient predators. Invasive animals are best known for their devastating effects on native prey populations. The arrival of the brown tree snake to Guam in the 1940s led to the loss of 9 of 11 species of native, forest-dwelling birds on the island .
Although invasive species tend to disrupt ecosystems, in rare cases they serve positive ecological functions. One study found that the abundance of two native bird species was strongly positively correlated with the abundance of fruit-producing Asian honeysuckle, suggesting that the invasive honeysuckle is a valuable food source for these birds .
Human and Economic Impacts
Invasive species pose significant costs for human society. In an agricultural setting, crop yields suffer from competition with weeds and experience direct attacks from invasive insects, mites, and plant pathogens. Livestock can be poisoned by toxic weeds and attacked by pests. Invasive species are also a serious public health concern because they can carry and transmit diseases as well as directly injure humans with bites or stings. Invasive species can interfere with infrastructure such as electric utilities and roadways, reduce the productivity of industries dependent on particular ecosystems, and lower the recreational value of natural areas. The total cost of invasive species in the United States is estimated to be $120 billion per year. This figure results both from costs spent on control of invasives (e.g., pesticide application) and losses attributed to these species (e.g., smaller crop yields, reduced recreational use of lakes) . The majority of the cost can be ascribed to direct losses. The total figure would be even greater if losses in biodiversity and ecosystem services were monetized as well.
Evolution in Action
So what does the future hold for the native species that stand in the way of these invaders? In some cases, the outlook is grim: we will probably never recover pristine landscapes from the grip of cogon grass, and invasive species like the brown tree snake are responsible for driving many threatened species closer to extinction . However, not all native species are unequally matched in the fight against invasion. Recall the lizard lying in the path of the red imported fire ants: the ants crawl on the lizard, insert their stingers underneath its scales, and begin injecting powerful neurotoxic venom. This might sound like a death sentence, and for lizards that have never encountered these ants before, it is. However, a recent study by Dr. Tracy Langkilde has found that lizards that have been invaded by these ants for many years have developed a way of coping: when attacked by fire ants, the experienced lizards twitch their bodies, flick their legs, and flee to safety! Lizards from invaded populations also have relatively longer hind limbs that help them remove the ants from their bodies and run away faster. All of these changes have occurred within the last 70 years — that’s about 35 lizard generations — and this suggests that the lizards are rapidly adapting to the presence of the fire ants . Although invasive species are accelerating the decline of some native species, others are able to stand their ground in the face of invasion.
Native fence lizards are developing new behaviors to deal with the invasion of red imported fire ants. (Photo credit: K. Boronow)
Katie Boronow is a PhD student in the Department of Organismic and Evolutionary Biology at Harvard University.
 United States Department of Agriculture, Animal and Plant Health Inspection Service. 2011. Imported Fire Ants. http://www.aphis.usda.gov/plant_health/plant_pest_info/fireants/
 National Invasive Species Information Center. 2011. What is an invasive species? http://www.invasivespeciesinfo.gov/whatis.shtml
 Pimentel, D, R Zuniga, and D Morrison. 2005. Update on the environmental and economic costs associated with alien-invasive species in the United States. Ecological Economics 52:273-288.
 Plant Conservation Alliance, Alien Plant Working Group. 2009. Cogon Grass. http://www.nps.gov/plants/alien/fact/imcy1.htm
 Fritts, TH, and D Leasman-Tanner. 2001. The Brown Treesnake on Guam. http://www.fort.usgs.gov/resources/education/bts/bts_home.asp
 Gleditsch, JM, and TA Carlo. 2011. Fruit quantity of invasive shrubs predicts the abundance of common native avian frugivores in central Pennsylvania. Diversity and Distributions 17:244-253.
 Clavero, M, and E García-Berthou. 2005. Invasive species are a leading cause of animal extinctions. TRENDS in Ecology and Evolution 20:110.
 Langkilde, T. 2009. Invasive fire ants alter behavior and morphology of native lizards. Ecology 90:208–21.