Theories for invasive species and why they occur have been hypothesized in recent ecological research. These hypotheses cover a variety of thoughts and multiple may be true concurrently: the enemy escape hypothesis, EICA hypothesis, EDCA hypothesis, Novel Weapons hypothesis, Missed Mutualism hypothesis, Invasional Meltdown hypothesis, Biotic Resistance hypothesis, unusual refuge hypothesis, global competition hypothesis, introduction pressure hypothesis, unintentional screening hypothesis, intentional screening hypothesis and the related idea of biological control corollary. The enemy escape hypothesis states that species are less subject to specialized predators and pathogens where they are introduced than where they are native. This is due to the lack of selection for a specialized predator where the organism was not existing. The EICA (Evolution of Increased Competetive Ability) hypothesis is where plants evolve to grow faster than other plants. EDCA (Evolution of Decreased Competitve Ability) hypothesis is where plants evolve to reproduce faster than other plants, but have no competetive ability. Novel Weapons Hypothesis explains how native species have not evolved to be selected for defenses against introduced species. This means that the introduced species defense mechanisms are more likely to target and harm the native species. Missed Mutualism Hypothesis states that the invading species will be less likely to take over it if is introduced without its native mutualists because it no longer benefits from the relationship with them. The opposite is true of the Invasional Meltdown Hypothesis which discusses the idea that if the mutualists of an introduced species are brought with it, the species will be highly invasive and almost always win out. The Biotic Resistance Hypothesis states that introduced species are less likely to be invasive because the presence of native predators, pathogens, and competitors will limit their spread. The idea that specific location yield species that are highly specialized to live there and only there comes into play in the Unusual Refuge Hypothesis, where it is thought that lack of adaption to local stresses limits invasive-ness, then locations with unusual stresses are less invasible. Global Competition Hypothesis simply states that it is more likely that some species elsewhere is a better competitor in a single niche than the local species because of the opportunity for better plants to be located and adapted to any other location on the planet. The introduction pressure hypothesis believes that the greater the number of individuals of a species are introduced and the more times that species arrives, the more likely it is to take hold in the introduced environment. That is because increasing the number of individuals reduces the limitations on the genetic pool of the migrated population and decreases the affects of genetic drift, and interbreeding between previously separate species populations increases fitness. Intentional screening causes certain traits that are more likely to yield an invasive species to be artifically selected for when people pick plants to bring. The unintentional invasion hypothesis deals with the traits that a plant species is likely to have to have survived travel and dispersal to become invasive. Plants that cling to boats and crops and survive travel are likely to be more drought tolerant, more fecund, parasitic to plants that were intentionally brought, and from places disturbed by people and therefore disturbance tolerant. The biological control corollary is the idea that a specialized predator from the native location of an invasive plant can be brought to reduce the invasive-ness of that plant.
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