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.
Contributed by Andrew Myers
What are we prepared to give up the name of nature and what will we do protect it?
This is the underlying theme of the RADIOLAB podcasts entitled ‘Galapagos’ and ‘For the Birds.’ In ‘For the Birds,’ a group of bird conservationists relocate a flock of whooping cranes, an endangered species, to a marsh in Florida. Once the birds arrive, attracted by birdfeeders, they start to inhabit a woman’s backyard. The conservationists ask the woman to remove her birdfeeders, she refuses. For her, the birds had become an integral part of her relationship with her husband who was suffering from Alzheimer’s disease. For the conservationists, her attitude and actions posed a threat to the future of the birds and what it meant to be wild. By asking her to remove the birdfeeders, they also ask her to give up a part of her relationship with her husband; while simultaneously, her refusal to remove the birdfeeders jeopardizes the very birds she treasures. This dilemma raises problematic questions that involve uncertain facts, opposing values, and may not have any right or wrong answer. They are much political as they are ecological, necessitating comprehensive approaches that integrate the expertise of many different disciplines.
What is a healthy forest?
For some, this involves little to no human activity, for others human activity (e.g., timber management) is critical to maintaining a healthy forest. In this case, the term “health” carries vastly different meanings, formed by different cultural values; “health” is fundamentally political, yet has serious ecological consequences. A healthy forest, in practice, is not defined by objective facts, rather, it is defined by power. For example, the forested landscapes in the U.S. have undergone many shifts, from the American Indians, to European settlers, commercial timber management, to environmental protection. In each of these eras, the health of the forest was defined by those with the most power, resources, and influence to do so, allowing them to implement their vision upon the landscape.
Are we willing to ask a woman to give up part of her relationship for an endangered bird, would we kill thousands of goats to protect turtle habitat, will we prevent timber production to protect bear habitat or will political and economic circumstances open up roadless forests for new roads and logging? While the answers to these questions are no doubt important, the more relevant detail is what the answers to these questions mean for how we live our daily lives. Addressing this point will lead us to a more productive debate about what is best for whom considering the needs and concerns of all living things rather than the perpetual debate about what the ‘correct’ answer is. Doing so requires collaboration across disciplines. Science of course, provides us with some of the necessary information for how our decisions will shape the world, and ultimately, our lives. The challenge, is remembering that we ourselves are political beings.
Bibliography and Further Reading
Berkes, Fikret. 2011. “Implementing Ecosystem-Based Management: Evolution or Revoltuion?”
Fish and Fisheries 13 (4):465-476.
Devlin, Vince. 2014. “Columbia Falls Sawmill Announces Layoffs, Cites Timber Blocked by
Litigation.” Missoulian, August 29, 2014. Accessed,
For the Birds. RADIOLAB audio podcast. Accessed, http://www.radiolab.org/story/birds/.
Galapagos. RADIOLAB audio podcast. Accessed, http://www.radiolab.org/story/galapagos/.
Ludwig, Donald. 2001. “The Era of Management Is Over.” Ecosystems 4 (8):758-764.
Neumann, Roderick, P. 2005. Making Political Ecology. London, England: Hodder Arnold.
Robbins, Paul. 2004. Political Ecology: A Critical Introduction. Malden, MA: Wiley-Blackwell.
Warren, William, A. 2007. “What Is a Healthy Forest?: Definitions, Rationales, and the
Lifeworld.” Society and Natural Resources 20 (2):99-117.