TEACHING ALL VOLUMES SUBMIT WORK SEARCH TIEE
VOLUME 6: Table of Contents TEACHING ISSUES AND EXPERIMENTS IN ECOLOGY
ISSUES: FIGURE SETS

Figure Set 1: What are the effects of herbivory on individual plant survival and growth?

Purpose: To interpret graphical results and to examine how herbivory affects individual plant survival and growth.
Teaching Approach: Think-pair-share
Cognitive Skills: (see Bloom's Taxonomy) — knowledge, comprehension
Student Assessment: One minute paper

BACKGROUND

Invasive species are species that have been introduced from their native range into an area where they do not have an evolutionary history. Further, invasive species are categorized as highly problematic species, causing both economic and ecological harm (Pimentel et al 2005). There are numerous invasive plant species that have been introduced to the United States that are problematic and need to be managed, including bush honeysuckle (Lonicera maackii), garlic mustard (Alliaria petiolata), and spotted knapweed (Centaurea maculosa). Typical management strategies often include hand-pulling, mowing, chemical spray, or a combination thereof. For some species, these management strategies can be an effective means to reduce the abundance of invasive species, ultimately reducing the negative effects they have on native species and communities. In other cases, typical management strategies are not enough to reduce invasive species' abundance or are too costly to employ. When such traditional management techniques prove inadequate (based on cost or effectiveness), biological control is often considered a feasible alternative. In plants, for instance, biological control consists of introducing enemies (herbivores), often referred to as biological control agents, from the plant's native range. The herbivores are meant to "damage" the invasive plant species by consuming plant tissue, reducing plant resources, and therefore curbing its population growth.

One invasive species that is considered highly problematic is Lespedeza cuneata (common name: sericea lespedeza or Chinese lespedeza). Lespedeza cuneata is a perennial legume native to eastern Asia. It was introduced in to the United States in the 1930s to stabilize areas that had been strip mined. It was also recommended the Department of Transportation in many states to use for quickly stabilizing roadsides. While the plant grows quickly in poor soil and requires little maintenance, it is those same traits that also make it an invasive threat. Lespedeza cuneata does not stay put. From the initial plantings, L. cuneata has spread by the movement of animals, hay, and equipment used to cut hay, and through the blowing wind. It can now be found throughout the eastern and Midwestern United States. It encroaches on our native prairies, savannas, glades, woodlands and forests. Normal grassland management practices such as grazing and burning do not adequately control L. cuneata and can actually increase its spread.

Lespedeza cuneata produces prolific amounts of seed, and some of that seed can remain dormant in the soil and germinate at a later time, making it very difficult to eradicate the species once it establishes. Further, L. cuneata makes two different types of seeds: cleistogamousseeds are produced from flowers that never open and are completely self fertilized and chasmogamous seeds are produced from flowers that are open; these seeds may be outcrossed. Cleistogamous seeds are cheaper for the plant to produce, and L. cuneata produces a higher proportion of cleistogamous seeds when growing in stressful environments (Schutzenhofer 2007). This flexibility in its mating system might allows L. cuneata to produce numerous seeds in many different types of environments.

Lespedeza cuneata's high tannin content makes it nearly unpalatable for cattle. It displaces more desirable native forages and generally degrades the quality of our landscapes wherever it is present. There are native options that are better for livestock and for wildlife, and native plants can also provide stable, low maintenance roadsides. Lespedeza cuneata's initial advantages, that it was cheap and easy to establish, have already cost the United States plenty in management costs. For example, in the state of Missouri alone, hundreds of thousands of dollars are spent by state, federal and private agencies to control the spread of L. cuneata. Management of L. cuneata typically entails herbicide spraying (Wehtje et al. 1999) and combinations of herbicide, mowing, native seed planting, and burning (Price and Weltzin 2003). However, these techniques are costly and in many cases do not effectively control L. cuneata (Brandon et al. 2004). It is for these reasons, among others, that biocontrol may be considered an option for controlling L. cuneata. To learn more about L. cuneata, see www.moprairie.org/documents/PrJnl_Vol28No2_07.pdf, page 34.

Much time and effort goes into researching biological control strategies, including finding an appropriate biological control agent to introduce. An appropriate agent must negatively impact the fitness and population growth rate of the target species, and not negatively affect other species in the community. For example, if a leaf-chewing insect is found to consume L. cuneata in its native range in Asia, but the same insect is not found in the United States, we might consider introducing that insect to the United States. One question we might ask is: How many leaves does the insect need to damage or consume to control to curb the population growth rate of L. cuneata in its invasive range? Answering this question can be achieved before the insect is introduced, by experimentally removing leaves (with scissors) at different levels (e.g., 20% removal, 40% removal.) and measuring plant fitness and plant population growth rate.  

References

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