Contributed by Jacob Lucero
If you’ve ever walked through a field and emerged with your socks full of pokey grass seeds or if you’ve ever popped a bike tire on a sharp thorn, you may have had a close encounter with an invasive plant. Many a hiker has paused to remove the seeds of cheatgrass (Bromus tectorum) from their socks, and the sharp seeds of puncture vine (Tribulus terrestris) have been the bane of many a bike ride. Although both cheatgrass and puncture vine are common in the western United States, neither is native to our region. Both plants hail from Eurasia; both have managed to hitch a ride to a new home (North America) and both thrive in their new environment. In addition to annoying hikers and bikers, exotic species can also perpetrate drastic ecological disruption, threatening the vitality of ecosystems and even jeopardizing human health. Accordingly, researchers, conservationists, and governments around the world expend considerable resources in fighting, preventing, and managing biological invasions. For example, David Pimentel and colleagues (2005) estimated that invasive species cost the United States over $120 billion in losses each year. Yet robust solutions to this worldwide threat remain elusive. So why, then, are invasive species so successful?
One of the most oft-cited explanations for the success of invasive species in novel environments is the enemy release hypothesis. This hypothesis suggests that natural enemies like competitors, predators, and diseases check the population growth of an organism in its native habitat. However, translocation across an ocean or a continent may allow that organism to escape the effects of its natural enemies. Without natural enemies to regulate its population growth in its new home, an introduced species could proliferate to become a noxious invader.
Although the enemy release hypothesis is touted in textbooks, classrooms, and scientific literature, it has rarely been tested empirically! To rigorously test the enemy release hypothesis, a researcher should protect a focal invader from natural enemies in both the native and non-native ranges. If the enemy release hypothesis is true, protection from natural enemies should only improve the invader’s population growth at home because natural enemies are assumed to be important only in the native range. Thus, parallel experiments must be executed in both the native and non-native ranges to test this hypothesis.
With the help of international collaborators, I am currently carrying out such parallel experiments in North America, Turkey, Iran, and Uzbekistan to test the enemy release hypothesis with respect to one of the most virulent invasive plants in the world, cheatgrass (Bromus tectorum). We predict that seed predators (important natural enemies) such as mice and ants limit the establishment of cheatgrass more strongly in Eurasia (the native range of cheatgrass) than North America (the non-native range of cheatgrass). Understanding the effects of natural enemies in both the native and non-native ranges of this invader could help us better understand the invasion process and illuminate the mechanisms that allow invasive species to become so successful in novel environments.