Many hypotheses have been developed to describe the dynamics of invasion, and these explanations variously describe how a barrier to invasion is removed so that an invasive species is successfully transferred to a new location. The explanations are not mutually exclusive.
Undoubtedly an important reason why invasive species do not always invade habitats, the preemption hypothesis suggests that invasives can be prevented from invading because of the presence of other species already on a site. The plant colonization window is closed, because other species are physically occupying the space. Though often overlooked, preemption is logically one of the most important forms of competition in plants. If a species is already occupying a space, it is difficult for an invasive species (or any other) to eject that species from the space.
The 'enemy release hypothesis' is the idea that invasive species are less impacted by enemies (e.g., herbivores) than native species, because in the new geographical location, the invasives species are freed from the parasites that kept their growth in check in their native environment. Therefore, invasive species are thought to compete more successfully with native species in their invasive environment, because the native species have not been released from their traditional enemies. The enemy release hypothesis is the basis for biocontrol programs.
The introduction of the pests of invasive species can have a dramatic impact on the growth and spread of invasive species in biocontrol programs, which is evidence that the enemy release hypothesis is operating. Also, plant species are sometimes (but not always) taller in their introduced ranges, and this is often attributed to the absence of their traditional parasites. Most invasive plant species have an average of 16 parasites in their native range, but only 3 in their introduced range.
Related to the idea that invasive species may be larger in their introduced ranges, is the idea that invasive species may have introgressed genes from hybridization with native species, so that invasive species not be the same type as their progenitors in their region of origin (intro-gression hypothesis; see the section on 'Types of invasive species'). With novel genes introduced, the invasive species with introgressed genes may be larger and with advantages over the original invasive type.
Resource hypothesis, resource-enrichment hypothesis, fluctuating-resources hypothesis
Resource-related hypotheses assume that invasive species are limited by resource availability. According to the various resource hypotheses, invasion is related to the availability of resources including nutrients, light, and water. The resource-enrichment and fluctuating-resources hypotheses assume that communities are more invasible if amounts of unused resources are higher.
The 'diversity-resistance hypothesis' suggests that communities with high levels of diversity are less invasible by exotic species, as based on ideas of competitive exclusion and niche theory. If niches are narrower and already filled, competitive exclusion bars new species from entering very diverse communities. The opposite idea is also claimed, that is, that communities with high levels of diversity are more invasible by exotic species. Studies support both claims. However, studies of the pattern of species invasion at large regional scales suggest that species-rich landscapes are more invaded by invasive species than species-poor landscapes.
After an invasive species has initially established on a new continent, the best predictor of the invasive range is the climatic range of the species on its home continent. Thus, after the invasive species' successful establishment on a new continent, the species is likely to spread throughout the entire climatic range. This spread may take decades or centuries. The 'geographic range hypothesis' is related to niche theory but carried to a landscape scale, because of the idea that species grow best in certain climates and environments. Modelers who attempt to project the eventual spread of an invasive species on a new continent use the ecological niche characteristics of the native range. An interesting wrinkle on this idea is that as world climate changes, the potential ranges of invasive species could either enlarge or shrink depending on their abilities to migrate, and occupy habitats within the new climate.
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