Iowa State University

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Where Will Our Next Big Weed Come From?
by Bob Hartzler

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 December 22, 2005 - Paper prepared for Iowa Integrated Crop Managment Conference, November 30, 2005


 Weed communities within agronomic fields are defined by the number of different species present and the relative abundance of the individual species. At any given time, three or four species within a field typically dominate the weed community, with the remaining species present at relatively low densities. A weed shift represents a change in the weed community within a field. The majority of shifts are caused by differential selection pressure placed on the weeds present in a field. Species or biotypes adapted to current weed management tactics increase, whereas weeds susceptible to those tactics decrease. The rate of change varies with the characteristics of the weed species and the diversity of the control tactics used in a field. Occasionally, weed shifts occur due to the introduction of a new species to a region. This paper will provide an overview of characteristics that allow certain plants to become invasive and expand their range by invading new regions.

Exotic– Naturalized - Invasive

An exotic plant is any species that occurs artificially outside of their known historical range. In this context, artificial refers to humans having been involved in the movement of the plant from its native range. Since the European migration to North America began, hundreds of exotic plants have been introduced to the continent. Some were brought intentionally, such as musk thistle as an ornamental flowering plant and velvetleaf for use as a fiber source. Others were accidental introductions, brought over in ship ballast or as contaminants in food, feed or other imports.

Exotic plants introduced to a new region may or may not be adapted to the new environment they encounter (Anon. 2005). Exotic plants that become a permanent component of a region’s flora are referred to as naturalized. A small percentage of naturalized exotic plants earn the term of invasive plants. Invasive plants typically spread rapidly and replace native vegetation in their new habitats. Giant foxtail, multiflora rose, velvetleaf, and garlic mustard are examples of exotics that have earned invasive status.

The ‘Tens Rules’ (Williamson and Fitter, 1996) was proposed to quantify the risk of exotic introductions. The authors state that approximately 10% of the exotic plants that are brought to a new region will actually escape into the wild. Of these escaped plants, only 10% will become naturalized, and 10% of the naturalized plants will become invasive. Thus, although the likelihood of a exotic plant becoming invasive is relatively small (1 in 1000), the increasing rate that exotics are brought to North America due to the global economy has resulted in a large effort to better understand characteristics that lead to invasiveness.

Why do certain plants become invasive?

 Weeds of agricultural lands share many common traits that allow them to persist in spite of our efforts to control them. Most of these traits are associated with reproduction, and include: 1) prolific seed production under good growing conditions, 2) ability to produce seed under adverse growing conditions, 3) lack of specific requirements for seed germination, 4) persistent seed, 5) ability to reproduce via self and cross pollination, 6) short juvenile period (woody species) and 7) dispersal mechanisms for short and long distance transport.

Many invasive species behave differently in the regions they colonize compared to their native land. Thus, an introduced species that is aggressive and displaces native species in new regions may be much less competitive in its native range. These plants act as ‘good neighbors’ in their native range and to not dominate plant communities. The traditional explanation for this newfound aggressiveness is that in new territories the plant is released from the suppressive effects of natural enemies (insects, diseases). The foundation of classical biological control of weeds is to identify these natural enemies in the weed’s native range and introduce the natural enemies to the plant’s new territory. Numerous successful biological control programs have greatly reduced the economic impact of invasive weeds, the most recent being leafy spurge in states west of Iowa.

Recently it has been proposed that factors other than release from natural enemies may play a role in the aggressive behavior of invasive plants (Callaway and Ridenour, 2004). One hypothesis to explain invasiveness is the ‘evolution of increased competitive ability’ (EICA). This idea is based on the fact that plants have developed defense mechanisms (chemicals) that reduce the damage inflicted by natural enemies. When removed from the natural enemies, the resources previously used to protect the plant are gradually shifted towards growth. The greater resources available to support growth enhance the competitiveness of the plant and contribute to the invasiveness of the plant. Exotic plants often have a lag period (several decades) between introduction and becoming invasive. The EICA hypothesis helps explain this lag period since this shift in resource allocation would not occur instantly upon removal from natural enemies.

A new hypothesis has been proposed that is based on allelopathy. Allelopathy is the production and release of chemicals by a plant that suppress growth of neighboring plants. It has been proposed that in a plant’s native region other members of the plant community would develop resistance to the allelochemicals over time (Callaway and Ridenour, 2004). When a plant is introduced to a new region, the native species may have not encountered the allelochemical and thus are highly susceptible to its toxic effects. This allows the exotic species to be much more competitive in its new range than it was in its native land.

The next big weed?

The focus of much of the research on invasive weeds has targeted species that are adapted to less disturbed habitats than agricultural fields. This would include prairies, rangeland, woodlands and wetlands. Invasive plants reduce the value of these areas in many ways, including: 1) replacing native species and reducing diversity (garlic mustard), 2) interfering with recreation (multiflora rose), and 3) eliminating food and nesting areas for wildlife (purple loosestrife). These natural areas are prone to invasion because of past disturbances, fragmentation, loss of natural control measures such as burning, and other disruptions caused by human activity.

Exotic plants also pose a threat to agricultural production, as documented by the success of velvetleaf, foxtail spp., woolly cupgrass, common cocklebur and Canada thistle. The most recent exotic to pose problems in agricultural fields is Asiatic dayflower – the magnitude of the problem that will be caused by dayflower is yet to be determined. In addition to exotics from other continents, it is likely we will see weeds traditionally viewed as adapted to southern climates move into Iowa. In the past 20 years there has been a significant increase in infestations of morningglory and prickly sida found in Iowa fields. It is impossible to predict which exotic plants possess the combination of traits that will allow them to join the communities of plants that currently infest Iowa corn and soybean fields. However, the success of past exotics and increased global travel suggest that new introductions will pose challenges for future weed management.

Anonymous. 2005. Weeds gone wild. Alien Plant Working Group webpage. Accessed Nov. 7, 2005.

Callaway, R.M. and W.M. Ridenour. 2004. Novel weapons: invasive success and the evolution of increased competitive ability

Williamson, M., and A. Fitter. 1996. The varying success of invaders. Ecology 77:1661-1666.

Prepared by Bob Hartzler , extension weed management specialist, Department of Agronomy, Iowa State University