define enemy release hypothesis

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Do invasive species escape their enemies?

One of the invasive plants found in the experiment, Dianthus armeria

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Invasive species, like zebra mussels and garlic mustard, are species that have been introduced by humans to a new area. Where they invade they cause harm. For example, invasive species outcompete native species and reduce diversity, damage habitats, and interfere with human interests. Damage from invasive species costs the United States over $100 billion per year.

Scientists want to know, what makes an invasive species become such a problem once it is introduced? Is there something that is different for an invasive species compared to native species that have not been moved to a new area? Many things change for an invasive species when it is introduced somewhere new. For example, a plant that is moved across oceans may not bring enemies (like disease, predators, and herbivores) along for the ride. Now that the plant is in a new area with no enemies, it may do very well and become invasive.

Scientists at Michigan State University wanted to test whether invasive species are successful because they have escaped their enemies. They predicted invasive species would get less damage from enemies, compared to native species that still live near to their enemies. If native plants have tons of insects that can eat them, while an invasive plant has few or none, this would support enemy escape explaining invasiveness. However, if researchers find that native and invasive species have the same levels of herbivory, this would no support enemy escape. To test this hypothesis, a lab collected data on invasive and native plant species in Kalamazoo County. They measured how many insects were found on each species of plant, and the percent of leaves that had been damaged by insect herbivores. The data they collected is found below and can be used to test whether invasive plants are successful because they get less damage from insects compared to native plants.

Featured scientist:  Elizabeth Schultheis  from Michigan State University

Flesch–Kincaid Reading Grade Level  = 11.3

• For a lesson plan on the Enemy Release Hypothesis,  click here .
• The Denver Museum of Nature and Science has a short video giving background on invasive species, here . 

Do insects prefer local or foreign foods?

One of the invasive plants found in the experiment, Centaurea stoebe.

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• Student Activity, Graph Type B, Level 2
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• STATISTICS EXTENSION – Data Worksheet – Student Activity
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Insects that feed on plants, called herbivores , can have big effects on how plants grow. Herbivory can change the size and shape of plants, the number of flowers and seeds, and even which plant species can survive in a habitat. A plant with leaves eaten by insect herbivores will likely do worse than a plant that is not eaten.

Plants that naturally grow in an area without human interference are called native plants. When a plant is moved by humans to a new area and lives and grows outside of its natural range, it is called an exotic plant. Sometimes exotic plants become invasive , meaning they grow large and fast, take over habitats, and push out native species. What determines if an exotic species will become invasive? Scientists are very interested in this question. Understanding what makes a species become invasive could help control invasions already underway and prevent new ones in the future.

Because herbivory affects how big and fast a plant can grow, local herbivores may determine if an exotic plant thrives in its new habitat and becomes invasive. Elizabeth, a plant biologist, is fascinated by invasive species and wanted to know why they are able to grow bigger and faster than native and other exotic species. One possibility, she thought, is that invasive species are not recognized by the local insect herbivores as good food sources and thus get less damage from the insects. Escaping herbivory could allow invasive species to grow more and may explain how they become invasive.

To test this hypothesis, Elizabeth planted 25 native, 25 exotic, and 11 invasive species in a field in Michigan. This field was already full of many plants and had many insect herbivores. The experimental plants grew from 2011 to 2013. Each year, Elizabeth measured herbivory on 10 individuals of each of the 61 species, for a total of 610 plants. To measure herbivory, she looked at the leaves on each plant and determined how much of each leaf was eaten by herbivores. She then compared the area that was eaten to the total area of the leaf and calculated the proportion leaf area eaten by herbivores. Elizabeth predicted that invasive species would have a lower proportion of leaf area eaten compared to native and noninvasive exotic plants.

Featured scientist: Elizabeth Schultheis from Michigan State University

Flesch–Kincaid Reading Grade Level  = 10.9

There is one scientific paper associated with the data in this Data Nugget. The citation and PDF of the paper is below, as well as a link to access the full dataset from the study:

• Schultheis, E.H., A.E. Berardi, and J.A. Lau (2015). No release for the wicked: enemy release is dynamic and not associated with invasiveness . Ecology 96(9) 2446-2457.
• Data available from the Dryad Digital Repository

For two lesson plans covering the Enemy Release Hypothesis,  click here  and here . 

Aerial view of the experiments discussed in this activity:

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• Contents Contents
• Article Info Article Info
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• Cited Cited
• Nanopubs Nanopubs
• Reviews 3 Reviews
• Article metadata
• Introduction
• Hypothesis name
• Identifiers
• Domains that make use of this hypothesis
• Reviews and meta-analyses
• Related hypotheses
• Hypothesis definitions
• Formalized representation of hypothesis variants
• Acknowledgments
• Nanopublications

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That actually explain what's on your next test, enemy release hypothesis, from class:, intro to botany.

The enemy release hypothesis is a concept in ecology that suggests non-native plant species can thrive and become invasive in new environments because they escape their natural predators, pathogens, or herbivores found in their native habitats. This lack of biotic resistance allows these species to grow unchecked, potentially leading to significant ecological impacts and alterations in local ecosystems.

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5 Must Know Facts For Your Next Test

• The enemy release hypothesis highlights that invasive plants often experience rapid growth and reproduction when introduced to new areas due to the absence of their native enemies.
• This hypothesis is often used to explain why some non-native species become invasive while others do not; successful invaders typically have few or no predators in their new habitats.
• Research has shown that invasive plant species can alter nutrient cycling and water availability in ecosystems, leading to further ecological changes.
• Invasive plants can outcompete native flora for resources, causing declines in biodiversity and disruption of existing ecological relationships.
• The enemy release hypothesis supports the idea that managing invasive species may require understanding their interactions with local predators and competitors to restore balance.

Review Questions

• The enemy release hypothesis explains that successful invasive plant species often thrive in new environments because they escape their natural enemies like herbivores and pathogens. This lack of predation allows them to grow rapidly and reproduce more effectively than they would in their native habitats. In contrast, non-invasive species tend to face similar pressures from local competitors and predators, which limits their ability to establish and spread.
• The implications of the enemy release hypothesis suggest that invasive plants can significantly reduce biodiversity by outcompeting native species without the pressure from their natural enemies. This alteration of community structure can lead to monocultures and disrupt established ecological relationships. Therefore, effective ecosystem management must consider the dynamics of invasive species and potentially involve introducing or enhancing local predator populations to help control invaders.
• The enemy release hypothesis can guide future research by encouraging scientists to investigate the specific interactions between invasive plants and their potential predators or pathogens in introduced ranges. Understanding these dynamics can help identify effective biological control agents or methods for managing invasive species. Furthermore, insights gained from this research can inform ecological restoration efforts aimed at re-establishing native plant communities, ensuring they are resilient against invasive species encroachment.

Related terms

Biotic resistance : The ability of native species and ecological interactions to limit the establishment and spread of invasive species through predation, competition, and disease.

Invasive species : Non-native organisms that spread widely in a new environment, often causing harm to the local ecosystem, economy, or human health.

Ecological impact : The effects that organisms have on their environment, including changes in biodiversity, ecosystem structure, and function due to species interactions.

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