Alien species, or species that are not native, invade habitats and alter ecosystems around the world. Invasive species are only considered invasive if they are able to survive and sustain themselves in their new environment. A habitat and the environment around it has natural flaws that make them vulnerable to invasive species. The level of vulnerability of a habitat to invasions from outside species is defined as its invasibility. One must be careful not to get this confused with invasiveness, which relates to the species itself and its ability to invade an ecosystem. There are many factors, abiotic and biotic, that can impact habitat invasibility, such as stress, disturbance, nutrient levels, climate, and pre-existing native species. Typically invasive species favor areas that are nutrient-rich, have few environmental stresses, and high levels of disturbances. This explains why areas in the United States such as Hawaii, Florida, and California are infested with invasive species. These invasions are one of the biggest and most consistent threats to biodiversity across the globe. Antarctica is the only natural reserve on Earth that is without invasive species, due to its environmental factors.
Ecological factors
Abiotic factors Abiotic factors serve as the first filter of invasive species within a respective habitat. Invasive species are typically limited to habitats that are ecologically similar to their native habitats. The relative efficiency of invasibility is dependent on the abiotic niche being conserved over time or if the invader is able to adapt rapidly to their newly invaded abiotic niche. For example, the Asian shore crab (Hemigrapsus sanguineus) has almost identical abiotic niches in its native and invaded habitat but the European Green Crab (Carcinus maenas) has the ability to adapt its abiotic niche as it can survive in a wide range of water temperatures and salinities. Some invasions are only dependent on abiotic factors and not biotic factors. For example, the invasion of cordgrass (Spartina anglica) in salt marshes was highly dependent on salinity and sediment type and no biotic factors. Propagule pressure is a composite measure of the number of individuals of a species released into a region to which they are not native. It has been found that species with weak dispersal agents create increased invasibility, especially near stream sides. The absence of habitat fragmentation allows for greater dispersal and high invasibility at edges of habitat boundaries. Propagule pressure is the main reason why the density of some invasive species is higher near the site of introduction. Disturbance is another abiotic factor that can affect invasibility. Disturbance is defined as a punctuated event that kills organisms or removes part of their biomass. Both increasing and suppressing disturbances can increase invisibility. For example, in the North American grasslands fire was found to decrease invasion but increased grazing increased invasion. Fires increase the invasibility of some species of pine trees but decreases the invasibility of other pine species. It has been proposed that disturbances increase invasibility to the degree that they cause the natural habitat to deviate from its original state. Stresses such as nutrient availability, water availability, light availability and extreme conditions affect invasibility. Low stress environments increase invasibility because the invaders are more likely to be able to take advantage of the increased resources. Addition of nutrients has been found to enhance the invasibility of an area. For example, when nutrients were added to an aquatic plankton environment it resulted in increased bacteria growth which facilitated the invasion of ciliates since the competition for resources was decreased. Also, nutrient addition to grasslands can have a variety of effects as it can shift species composition towards or away from a smaller number of relatively fast growing species, woody species such as annual herbs, perennial herbs, grasses or shrubs. Nutrient addition to a nutrient poor environment can not only promote invasions but can prompt the eventual dominance of non-native species within the habitat. Large scale nutrient addition can have a significant positive impact on invasibility. For example, large scale nutrient run-off in the New England area has caused the invasion of Phragmites australis an exotic vascular plant. Limiting nutrient resources has a negative impact on invasibility as resources will be limited increasing the competition between invasive and native species. For example, there is decreased water flow inside eelgrass habitats thus eelgrass limits the growth and survival of the invasive mussel (Musculista senhousia) This reduces the extent of its invasion compared to areas where eelgrass meadows are fragmented or absent. Stresses that limit metabolism or resource acquisition - such as extreme temperature and toxins - has the ability to both increase or decrease invasibility; this is dependent on the invader's characteristics. If the invader can live in a wider temperature range than the native species it will have a greater success rate. For example, an invasive C4 plant would have a better chance of surviving than a native C3 plant during a drought.
Biotic factors
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