Darlene Panvini
Biology Department, Vanderbilt University


This case examines the biological, ecological, social, political, and economic factors surrounding exotic species as well as the role of resource managers in shaping public policy on environmental issues.

Variations of the case have been used at Vanderbilt University for several years in a freshman seminar biology course, "Conservation Ecology: Problems and Potential Solutions." Students in the course are first-semester freshmen, only a few of whom intend to major in biology or science. They generally have a disparate knowledge of biological or conservation-related concepts. All first-year students in the College of Arts and Science are required to take a freshman seminar and this course is just one of many from which they may choose.

The course focuses on biodiversity: the different ways it is defined, historical and current losses, and efforts to protect it. The session on exotic species is part of a larger unit focusing on factors that contribute to the loss of biodiversity. Discussion of this case generally precedes a service learning experience at a local park in which students spend four hours removing amur honeysuckle, the exotic plant in the case. One of the objectives for the course requires that students be able to analyze the role that economic, political, and social factors play in biodiversity conservation so that they can assess current conservation issues they encounter in the media and become informed spenders of their money and votes. The case therefore incorporates news articles and radio segments that students might realistically encounter.

In addition to conservation ecology courses, this case would be appropriate for a non-majors science course, a bioethics course, or a majors biology course such as ecology. More or less emphasis can be placed on the science involved in ecosystem stability and the physiological characteristics of exotics that contribute to their success. Broader application to exotic non-plant species can be made, although there are references to animals and fungi in the case.


Exotic species are believed by some conservationists to be one of the worse threats to biodiversity, second to loss of habitat. Alternatively, some scientists recently have claimed that the spread of exotics is part of globalization and that exotics can (do) play an important role in urban landscapes. Certainly, exotics are the most frequently used plants of many landscapers. In working through the case, students are asked to:


Globalization of the world has had an impact on natural ecological systems. While species have always expanded their ranges and spread into new areas, humans have accelerated this process by facilitating the distribution of species outside the species’ native range and into other habitats, especially disturbed habitats. Non-indigenous species are introduced into ecological systems through a variety of mechanisms: purposefully (e.g., for hunting), by escaping cultivation (e.g., landscaping plants and pets), and accidentally (in ship ballast or overseas packages) (Ruesink, et al., 1995). Often referred to as "exotics" or "biological pollution," these non-indigenous species can be found at all taxon levels. Eurasian species account for as much as 80 percent of the invading species worldwide (Hutchinson and Vankat, 1997). Reichard and Hamilton (1997) report that 85 percent of the 235 woody plant species that naturalized in North America were introduced for ornamental and functional landscaping and 14 percent were introduced for agriculture or production forestry.

Certain physiological traits are commonly found among successful invaders. These include high reproductive output, plastic responses to the environment, high net primary productivity (for plant species), and lack of coevolved biological control agents (Hutchinson and Vankat, 1997).

Communities themselves have certain characteristics that make them more or less susceptible to invasion. Disturbance, either naturally occurring or human-caused, has been positively correlated to invasibility. Late-successional systems are generally more resistant to invasion than younger systems (Hutchinson and Vankat, 1997).

Non-indigenous species can impact the integrity of natural systems by altering nutrient regimes, forming monocultures, and causing the extinction of native species (Ruesink, et al., 1995). The impact of exotics on natural systems varies from system to system, but studies have shown an inverse relation between the invasion of exotic species and the regeneration of native woody species, herb abundance, and diversity. Over time, this has the potential of altering the composition and structure of forests and other late successional systems as early-successional exotic species can dominate and prevent the establishment of late successional species (Hutchinson and Vankat, 1997).

While this case focuses only on the invasion of non-indigenous species into natural ecological systems, a discussion of exotics could include the many beneficial aspects of exotics in agriculture and horticulture. One could also examine the problems posed in agricultural systems by exotic pests introduced unintentionally. In all situations, we are posed with the problem of determining the risks associated with non-indigenous species and balancing those with the values placed on these species by various interest groups (Ruesink, et al., 1995).

Resource managers struggle with establishing policies that account for the dynamic nature of ecological systems and with defining the term "biodiversity." Older policies assumed that systems were balanced and stable assemblages of species that should reflect pre-Colombian conditions. Newer policies, in recognizing that systems are not static, acknowledge that the historical absence of a species is not sufficient to warrant its present removal and that species should be evaluated on the basis of their functional roles. Some exotic species may play important ecological roles in disturbed urban landscapes, providing nutrient retention, carbon storage, and animal habitat (Luken and Thieret, 1996).

Biodiversity can be measured in different ways: species richness (number of species), number of populations, or amount of genetic variation within a population or species. Often, the popular press and resource managers only employ the species richness definition. When biodiversity is measured this way, the presence of exotic species can be said to increase species richness, accounting for greater diversity in an ecological system. Many scientists agree that this is a short-term measure of biodiversity because eventually the composition and structure of the system changes as the exotics form a monoculture, resulting in a reduction in biodiversity. Clearly, more research is needed that examines the function of non-indigenous species in urban landscapes and explores methods of assessing biodiversity. Inherent to understanding these questions about exotics is the need to be able to predict the invasibility of an introduced species.

Predicting the invasibility of a species has important policy implications. Reichard and Hamilton (1997) note that the U.S. Federal Noxious Weed Act (1974) prohibits the import of 94 species listed as "weeds." Yet these are known problem species, especially in agriculture. The Act does not stipulate the evaluation of new introduced species to determine their potentiality for invasiveness. The authors outline four possible policy strategies: (1) admit all species unless proven to be invasive and specifically prohibited; (2) prohibit all species unless proven to be non-invasive; (3) test each species for invasiveness before making a decision regarding entry; and (4) make an informed estimate about the invasive potential of a species based on available information about other invasive species. The authors then discuss a model for evaluating the invasive potential of new woody plant introductions and then divides species into three categories: admit, deny admission, and delay admission for further analyses and/or monitor intensively.

On February 3, 1999, President Bill Clinton signed Executive Order 13112 to coordinate a federal strategy among the Commerce, Interior, and Agriculture Departments to address the environmental and economic threats posed by exotics. The Order created a National Invasive Species Council charged with developing an invasive species management plan within 18 months. Federal agencies were mandated to prevent the introduction of invasive species and to restore native species on federal lands and in federally-related projects. To fund these initiatives, as well as research on habitat restoration and integrated pest management tactics, $28.8 million in funding was proposed as part of the fiscal year 2000 budget (White House Press Release on Alien Species; see The National Invasive Species Council sought public input in October 2000 on its Draft National Invasive Species Management Plan. This plan included recommendations to improve and support coordination, control efforts, research, education programs, prevention, and exchange of information regarding invasive species (Southeast Exotic Pest Plant Council; see

Questions and Answers
  1. List the characteristics of exotics (ecological and biological) that enable them to be effective invaders?
  2. High reproductive output, plastic responses to the environment, high net primary productivity (for plant species), and lack of coevolved biological control agents.

  3. List the reasons (social, political, and economic) that people use exotics.
  4. Students can discuss the various uses of exotics: for ornamental landscaping, erosion control, wildlife habitat. Political factors include: few laws regulating the distribution of exotics, the U.S. government importing exotics and encouraging their use, lobbying power of nursery associations. Economic reasons include: exotics are often cheaper, exotics can grow more quickly so they are more profitable for nurseries to propogate.

  5. Is there a difference between "invasive exotic" and "exotic"? Think about the tulips commonly seen in yards in the spring orr potatoes and rice grown in the U.S.
  6. The purpose of this question is to get students to realize that not all exotics are "bad" and that exotics are commonly used in gardening and agriculture. In fact, we rely heavily on exotic species used in agriculture. The problem is that we often don’t know which species are going to be invasive of natural areas and we don’t know to what extent they will alter natural systems.

  7. Describe the characteristics of communities that increase their invasibility.
  8. Disturbance, either naturally occurring or human-caused, has been positively correlated to invasibility. Early successional systems are more susceptible to invasion.

  9. Describe the effect of exotics on community structure.
  10. While we are not sure of the process – whether it is due to competition or by altering nutrient regimes and other environmental factors – we do know that exotics can form monocultures and result in the extinction of native species. As a result, the structure and composition of natural areas change.

  11. Defend or dispute the statement "Exotics increase the biodiversity of an area."
  12. Depending on how biodiversity is measured (number of species, genetic differences, etc.) and at what point in time, exotics can increase the biodiversity of a natural area. If the exotic alters the system so that native species are displaced, then biodiversity decreases. In many cases, exotics form monocultures so that biodiversity of that area is decreased.

  13. Defend or dispute the statement "Exotics play an important function in urban landscapes."
  14. Defense: In heavily disturbed areas, exotics may be some of the few species that survive. They can then play important roles in nutrient retention, carbon storage, and providing animal habitat.

    Dispute: Native species can still play important ecological roles in urban landscapes and pose less of a risk to natural areas.

  15. Should legislation be enacted to regulate the import, sale, and distribution of exotics? If so, should this legislation be at the local, state, or federal level? If not, then why not?
  16. This question is certainly open to interpretation and students often have strong reactions to it. Those who favor legislation generally feel that it should be at the federal level since we have spent a lot of time in previous classes talking about how species do not recognize political boundaries and that the strongest levels of protection will consider landscape approaches to managing biodiversity. These same students also acknowledge the difficulties of getting federal legislation to trickle down to the local level where laws can realistically be implemented. Students opposing legislation usually favor an "it will work itself out" philosophy. Some of these students also voice concern about "discrimination" of a species, private property rights, and excessive governmental control.

  17. Assuming that legislation of exotic species should be enacted, what should it include and what information should be used to determine the laws governing exotic species?
  18. With this question, I try to get students to talk about the realities of controlling the import of exotics: of having ships' ballast purified, airplanes fumigated, and imported crops regulated. I also want them to talk about factors used to determine if and when a species can be imported and how we can evaluate the invasive potential of introduced species (see the Reichard and Hamilton (1997) models mentioned earlier).

  19. Should Brian say anything to the sales clerk or manager about the honeysuckle? What role should biologists play in forming public policy?
  20. There is no right or wrong answer to this question. Students should explore various reasons for why biologists should/should not get involved in public policy decisions.


This case has been used in varying formats in the freshman seminar described above, which meets for two 75-minute periods a week. Students were divided into four teams at the beginning of the semester and often worked in and out of class on group tasks. For this lesson, students were asked to read the case before coming to class and to answer the accompanying discussion questions individually. Because of the links embedded within the case and to Internet sites, students need to access the case via a website or online classroom management tool (e.g., Prometheus).

Th class began with a 30-minute discussion asking students to respond to the 10 questions they answered for homework. The main goal of the discussion was to ensure that students understood the basic concepts involved in the case, not to necessarily cover every question. During the discussion, key terms and concepts were written on the board. Students then assembled in their teams, were assigned one of the following exploration questions, and given 15 minutes to prepare a response to present to the class. Each team then had five minutes to act out their dialogues or read the e-mail response they created. [I find it helpful to put the questions on a handout and display them on a transparency during class.]

The following Saturday, the class spent four hours at a local park removing amur honeysuckle. After this service learning experience, students were asked to complete a short in-class free-write exercise in which they described their reactions to exotics before and after the service learning experience.

Exploration Questions

Group 1: Write a dialogue of Eleni Galatos interviewing Brian Farley. Focus on what Brian might say about the Presidential Executive Order and its impact on Franklin Park. Your dialogue should incorporate the key points highlighted during the discussion on exotics. Be prepared to role play your dialogue to the class.

Group 2: Write a dialogue of the phone conversation that Brian Farley might have with Chris Patrick in which he talks about the role that the Franklin Park staff can have in lobbying for legislation on the sale and distribution of exotic plants. Your dialogue should incorporate the key points highlighted during the discussion on exotics. Be prepared to role play your dialogue to the class.

Group 3: Write a dialogue of the conversation between Mark Taylor and Brian Farley in which they talk about plans for a spring workshop on anthracnose and how Mark should follow-up with the landscapers at the condominium. Your dialogue should incorporate the key points highlighted during the discussion on exotics. Be prepared to role play your dialogue to the class.

Group 4: Write an e-mail responding to Anagha Dahr’s question about Tennessee Department of Transportation (T--DOT). How would Brian Farley recommend she reply to Ralph Gorlin? Your e-mail should incorporate the key points highlighted during the discussion on exotics. Be prepared to read your e-mail to the class.

Variations for Teaching the Case

  1. The entire class period, or a longer portion of it, could be spent discussing the 10 discussion questions that students get with the case and covering the key features of the case. The exploration questions could then be omitted, assigned as individual or group homework, or done during the next class period.
  2. The instructor could choose to focus on only one of the exploration questions and ask students to respond to it as an individual free-write activity during the last 10 minutes of class and turn in their response before leaving. Or alternatively, each group could develop a response and turn it in before leaving. Or individuals or groups could develop a response to one exploration question for homework.
  3. At the time the case is distributed, each group could be assigned one to two of the discussion questions and asked to prepare a response to be given at the beginning of class.
  4. The instructor could require that all students hand in a response to the discussion questions and/or the exploration questions.
  5. Students could role play a conversation between Brian Farley and the sales clerk at the nursery in which Brian decides to confront the issue and ask the store to stop selling amur honeysuckle.
  6. Debates could be set up in which students defend or dispute the following statements. Each group could debate from a different stakeholder’s perspective.
  7. "Exotics increase the biodiversity of an area."

    "Exotics play an important function in urban landscapes."

    "Legislation should be enacted to regulate the sale and distribution of exotics."

  8. Other topics that could be explored in discussion or projects include:

Regardless of how the case is used, it will undoubtedly have more impact if students can relate it to exotics commonly known in their own geographical areas. Users of the case are encouraged to seek out local samples of exotics to show students – either in a demonstration or a field trip. The Tenenbaum (1996) article provides a good summary of exotic plant species found throughout the United States.


Go Back to the Case

Acknowledgements: This case study was developed with support from The Pew Charitable Trusts and the National Science Foundation as part of the Case Studies in Science Workshop held at the University at Buffalo, State University of New York, on May 22-26, 2000.

SEND Comments HOME READ Comments click left to send comments, middle to go home, right to read comments