Enemy release hypothesis

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The enemy release hypothesis is among the most widely proposed explanations for the dominance of exotic invasive species. In its native range, a species has co-evolved with pathogens, parasites and predators that limit its population. When it arrives in a new territory, it leaves these old enemies behind, while those in its introduced range are less effective at constraining them. The result is sometimes rampant growth that threatens native species and ecosystems.

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Explanations for invasive species success

Ecologists have identified many potential reasons for the success of invasive species, including higher growth rates or seed production than native species, more aggressive dispersal, tolerance of environmental heterogeneity, more efficient use of resources, and phenological advantages such as an earlier or longer flowering season. [1] [2] [3] Invasive species may have greater phenotypic plasticity in important traits than their native competitors, allowing them to tolerate more environmental variation, [4] or exhibit the ability to evolve rapidly to adapt to their new conditions. [5] In addition, some habitats, due to disturbances or other factors, may be more vulnerable to invasion than others. [6] Most exotic species do not become invasive, [7] and some authors suggest that those that do represent repeated and larger introductions that generate propagule pressure. [8] Among the many explanations for invasive success, however, the enemy release hypothesis has had the most support. [9]

Enemy release hypothesis

The enemy release hypothesis (ERH) is most often applied to invasive plants, but there is evidence for its usefulness in other systems, including fish, [10] amphibians, [11] insects, [12] and crustaceans. [13] The ERH assumes that: (1) herbivores, pathogens and parasites suppress plant population growth, (2) these enemies plague native plants more than immigrating non-native species, and (3) non-native plants are able to leverage this advantage into more rapid population growth. [14]

An early study of the flowering plant Silene latifolia found that about 60% of its invasive populations in North America were free from herbivory, while 84% of those in its native Europe exhibited damage from at least one herbivore. [15] A study of almost 500 exotic plant species in the United States found that they were infected by 84% fewer fungi and 24% fewer virus species than in their native ranges. [16] And a meta-analysis covering 15 exotic plant studies found the number of insect herbivores on average to be greater in their native than in their introduced range, with overall damage greater on native plants than on the introduced species. [17]

Support for the theory, however, is not universal. [18] In some cases, native pathogens, parasites and herbivores present significant biotic resistance to potential invasive species, [19] [20] as do non-native enemies that may have arrived prior to the exotic plant. [21] Enemy release may be weaker, too, when an exotic species is more closely related to native species in their introduced ranges, making them more likely to share herbivores or pathogens. [22] In a meta-analysis of 19 research studies involving 72 pairs of native and invasive plants, invasive exotic species did not incur less damage than their native counterparts and, in fact, exhibited lower relative growth rates. [23] In other cases, invasive success was due not to release from herbivory but greater tolerance of it. [24]

The ERH is closely related to two other important theories for invasive species success: the evolution of increased competitive ability (EICA) and novel weapons hypotheses (NWH). EICA asserts that because exotic plants are released from the burden of defending themselves against herbivores in their native range, they evolve to reallocate those resources to traits, such as growth and seed production, that make them more formidable competitors in their introduced range. [25] ERH is an ecological mechanism, while EICA rests on evolutionary adaptation. [26] The experimental support for EICA is mixed. [27] For example, Solidago altissima plants artificially released from herbivory became more competitive against other plant species. [28] However, a meta-analysis of 30 studies that found evidence of evolutionary shifts in introduced species, showed no indication of a trade-off between herbivore defenses and growth. [26]

The novel weapons hypothesis (NWH) is another perspective on the enemy release hypothesis. Some plants evolve chemical defenses to compete in their original range. In their introduced range, the native species are highly vulnerable to these chemicals because they have no prior experience with them, giving the exotic species a competitive advantage. [29] [30] [31]

Practical applications

A final argument for the ERH lies in the success of biological control of some invasive species, in which herbivores or other enemies from their native environment are introduced to suppress population growth in their adopted range. [32] For example, when conservationists sought to control the invasive St.-John's-wort ( Hypericum perforatum ) in North America, they imported a leaf herbivore ( Chrysolina quadrigemina ) from its native range in Europe. [33]

Related Research Articles

<span class="mw-page-title-main">Herbivore</span> Organism that eats mostly or exclusively plant material

A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthparts adapted to rasping or grinding. Horses and other herbivores have wide flat teeth that are adapted to grinding grass, tree bark, and other tough plant material.

<i>Erysimum</i> Genus of flowering plants

Erysimum, or wallflower, is a genus of flowering plants in the cabbage family, Brassicaceae. It includes more than 150 species, both popular garden plants and many wild forms. The genus Cheiranthus is sometimes included here in whole or in part. Erysimum has since the early 21st century been ascribed to a monogeneric cruciferous tribe, Erysimeae, characterised by sessile, stellate (star-shaped) and/or malpighiaceous (two-sided) trichomes, yellow to orange flowers and multiseeded siliques.

<span class="mw-page-title-main">Invasive species</span> Non-native organism causing damage to an established environment

An invasive or alien species is an introduced species to an environment that becomes overpopulated and harms its new environment. Invasive species adversely affect habitats and bioregions, causing ecological, environmental, and/or economic damage. The term can also be used for native species that become harmful to their native environment after human alterations to its food web – for example, the purple sea urchin which has decimated kelp forests along the northern California coast due to overharvesting of its natural predator, the California sea otter. Since the 20th century, invasive species have become a serious economic, social, and environmental threat worldwide.

<span class="mw-page-title-main">Introduced species</span> Species introduced by human activity

An introduced species, alien species, exotic species, adventive species, immigrant species, foreign species, non-indigenous species, or non-native species is a species living outside its native distributional range, but which has arrived there by human activity, directly or indirectly, and either deliberately or accidentally. Non-native species can have various effects on the local ecosystem. Introduced species that become established and spread beyond the place of introduction are considered naturalized. The process of human-caused introduction is distinguished from biological colonization, in which species spread to new areas through "natural" (non-human) means such as storms and rafting. The Latin expression neobiota captures the characteristic that these species are new biota to their environment in terms of established biological network relationships. Neobiota can further be divided into neozoa and neophyta (plants).

<i>Centaurea diffusa</i> Species of flowering plant

Centaurea diffusa, also known as diffuse knapweed, white knapweed or tumble knapweed, is a member of the genus Centaurea in the family Asteraceae. This species is common throughout western North America but is not actually native to the North American continent, but to the eastern Mediterranean.

<span class="mw-page-title-main">Plant defense against herbivory</span> Plants defenses against being eaten

Plant defense against herbivory or host-plant resistance (HPR) is a range of adaptations evolved by plants which improve their survival and reproduction by reducing the impact of herbivores. Plants can sense being touched, and they can use several strategies to defend against damage caused by herbivores. Many plants produce secondary metabolites, known as allelochemicals, that influence the behavior, growth, or survival of herbivores. These chemical defenses can act as repellents or toxins to herbivores or reduce plant digestibility. Another defensive strategy of plants is changing their attractiveness. To prevent overconsumption by large herbivores, plants alter their appearance by changing their size or quality, reducing the rate at which they are consumed.

Trophic cascades are powerful indirect interactions that can control entire ecosystems, occurring when a trophic level in a food web is suppressed. For example, a top-down cascade will occur if predators are effective enough in predation to reduce the abundance, or alter the behavior of their prey, thereby releasing the next lower trophic level from predation.

<span class="mw-page-title-main">Community (ecology)</span> Associated populations of species in a given area

In ecology, a community is a group or association of populations of two or more different species occupying the same geographical area at the same time, also known as a biocoenosis, biotic community, biological community, ecological community, or life assemblage. The term community has a variety of uses. In its simplest form it refers to groups of organisms in a specific place or time, for example, "the fish community of Lake Ontario before industrialization".

The evolution of increased competitive ability (EICA) hypothesis was first proposed by Bernd Blossey and Rolf Nötzold in 1995 as a way to explain the success of invasive, non-indigenous species. Observing that:

  1. there is usually a lag period between the time of introduction of an invasive species and the point at which it is considered invasive, and
  2. invasive plants seem to be more virulent in habitats into which they have been introduced, the scientists presumed a sort of naturalization through modification for non-indigenous plants.

In ecology, a priority effect refers to the impact that a particular species can have on community development as a result of its prior arrival at a site. There are two basic types of priority effects: inhibitory and facilitative. An inhibitory priority effect occurs when a species that arrives first at a site negatively affects a species that arrives later by reducing the availability of space or resources. In contrast, a facilitative priority effect occurs when a species that arrives first at a site alters abiotic or biotic conditions in ways that positively affect a species that arrives later. Inhibitory priority effects have been documented more frequently than facilitative priority effects. Studies indicate that both abiotic and biotic factors can affect the strength of priority effects. Priority effects are a central and pervasive element of ecological community development that have significant implications for natural systems and ecological restoration efforts.

The Janzen–Connell hypothesis is a well-known hypothesis for the maintenance of high species biodiversity in the tropics. It was published independently in the early 1970s by Daniel Janzen, who focused on tropical trees, and Joseph Connell who discussed trees and marine invertebrates. According to their hypothesis, host-specific herbivores, pathogens, or other natural enemies make the areas near a parent tree inhospitable for the survival of seeds or seedlings. These natural enemies are referred to as 'distance-responsive predators' if they kill seeds or seedlings near the parent tree, or 'density-dependent predators' if they kill seeds or seedlings where they are most abundant. Such predators can prevent any one species from dominating the landscape, because if that species is too common, there will be few safe places for its seedlings to survive. Both Janzen and Connell originally proposed that for natural enemies to increase local diversity, they must be host-specific and relatively immobile, such that they disproportionately reduce the density of the more locally common tree species. This prevents any one species from becoming dominant and excluding other species through competition, allowing more species to coexist in small areas. This can be classified as a stabilizing mechanism.

<span class="mw-page-title-main">Ecological fitting</span> Biological process

Ecological fitting is "the process whereby organisms colonize and persist in novel environments, use novel resources or form novel associations with other species as a result of the suites of traits that they carry at the time they encounter the novel condition". It can be understood as a situation in which a species' interactions with its biotic and abiotic environment seem to indicate a history of coevolution, when in actuality the relevant traits evolved in response to a different set of biotic and abiotic conditions.

<span class="mw-page-title-main">Plant use of endophytic fungi in defense</span>

Plant use of endophytic fungi in defense occurs when endophytic fungi, which live symbiotically with the majority of plants by entering their cells, are utilized as an indirect defense against herbivores. In exchange for carbohydrate energy resources, the fungus provides benefits to the plant which can include increased water or nutrient uptake and protection from phytophagous insects, birds or mammals. Once associated, the fungi alter nutrient content of the plant and enhance or begin production of secondary metabolites. The change in chemical composition acts to deter herbivory by insects, grazing by ungulates and/or oviposition by adult insects. Endophyte-mediated defense can also be effective against pathogens and non-herbivory damage.

<span class="mw-page-title-main">Escape and radiate coevolution</span>

Escape and radiate coevolution is a hypothesis proposing that a coevolutionary 'arms-race' between primary producers and their consumers contributes to the diversification of species by accelerating speciation rates. The hypothesized process involves the evolution of novel defenses in the host, allowing it to "escape" and then "radiate" into differing species.

<span class="mw-page-title-main">Tritrophic interactions in plant defense</span> Ecological interactions

Tritrophic interactions in plant defense against herbivory describe the ecological impacts of three trophic levels on each other: the plant, the herbivore, and its natural enemies. They may also be called multitrophic interactions when further trophic levels, such as soil microbes, endophytes, or hyperparasitoids are considered. Tritrophic interactions join pollination and seed dispersal as vital biological functions which plants perform via cooperation with animals.

Local adaptation is a mechanism in evolutionary biology whereby a population of organisms evolves to be more well-suited to its local environment than other members of the same species that live elsewhere. Local adaptation requires that different populations of the same species experience different natural selection. For example, if a species lives across a wide range of temperatures, populations from warm areas may have better heat tolerance than populations of the same species that live in the cold part of its geographic range.

<i>Scaptomyza flava</i> Species of fly

Scaptomyza flava is an herbivorous leaf mining fly species in the family Drosophilidae. In Latin, flava means golden or yellow. The fly is amber to dark brown in color and approximately 2.5 mm in length. In Europe and New Zealand the larvae are pests of plants in the order Brassicales, including arugula, brassicas, broccoli, Brussels sprouts, bok choy, cabbage, canola, cauliflower, horseradish, kale, kohlrabi, napa cabbage, nasturtium, radish, rapini, rutabaga, turnip, wasabi and watercress. In New Zealand, its range has expanded to include host species that are intercropped with salad brassicas, including gypsophila, otherwise known as baby's breath, which is in the pink family (Caryophyllaceae) and the pea in the Fabaceae. More typically, S. flava is oligophagous within the Brassicales. Scaptomyza are unusual within the Drospophilidae because the group includes species that are truly herbivorous. Other herbivorous drosophilids include D. suzukii, which attacks fruit very early during ripening and species within the genus Lordiphosa, from Africa and Asia, which also include leaf miners. Most drosophilids feed on microbes associated with decaying vegetation and sap fluxes.

Naomi Cappuccino is an associate professor of biology at Carleton University in Ottawa, Canada. Her research primarily focuses on population ecology and biological control of invasive species.

Invasion genetics is the area of study within biology that examines evolutionary processes in the context of biological invasions. Invasion genetics considers how genetic and demographic factors affect the success of a species introduced outside of its native range, and how the mechanisms of evolution, such as natural selection, mutation, and genetic drift, operate in these populations. Researchers exploring these questions draw upon theory and approaches from a range of biological disciplines, including population genetics, evolutionary ecology, population biology, and phylogeography.

Cryptic mimicry is observed in animals as well as plants. In animals, this may involve nocturnality, camouflage, subterranean lifestyle, and mimicry. Generally, plant herbivores are visually oriented. So a mimicking plant should strongly resemble its host; this can be done through visual and/or textural change. Previous criteria for mimicry include similarity of leaf dimensions, leaf presentation, and intermodal distances between the host and mimicking plant.

References

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  18. ↑ Jeschke, Jonathan; Gómez Aparicio, Lorena; Haider, Sylvia; Heger, Tina; Lortie, Christopher; Pyšek, Petr; Strayer, David (2012-08-22). "Support for major hypotheses in invasion biology is uneven and declining". NeoBiota. 14: 1–20. doi: 10.3897/neobiota.14.3435 . ISSN   1314-2488.
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  23. ↑ Chun, Young Jin; Van Kleunen, Mark; Dawson, Wayne (2010-06-10). "The role of enemy release, tolerance and resistance in plant invasions: linking damage to performance: Invasive plants and enemy release". Ecology Letters. 13 (8): 937–46. doi:10.1111/j.1461-0248.2010.01498.x. PMID   20545733.
  24. ↑ Ashton, Isabel W.; Lerdau, Manuel T. (2007-10-04). "Tolerance to herbivory, and not resistance, may explain differential success of invasive, naturalized, and native North American temperate vines: Resistance and tolerance of invasive vines". Diversity and Distributions. 14 (2): 169–178. doi:10.1111/j.1472-4642.2007.00425.x. S2CID   83429134.
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  26. 1 2 Felker-Quinn, Emmi; Schweitzer, Jennifer A.; Bailey, Joseph K. (2013). "Meta-analysis reveals evolution in invasive plant species but little support for Evolution of Increased Competitive Ability (EICA)". Ecology and Evolution. 3 (3): 739–751. doi:10.1002/ece3.488. PMC   3605860 . PMID   23531703.
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  33. ↑ DeLoach, C.J. (1997). "Biological control of weeds in the United States and Canada". In Luken, James O.; Thieret, John W. (eds.). Assessment and Management of Plant Invasions. New York: Springer New York. pp. 172–194. ISBN   978-1-4612-1926-2. OCLC   840278263.
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