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It is the same as creating more pests or setting up dominant species which upset the ecological food pyramid leading to decimation of trophic levels or predators and energy cycling. Fishes feed on microscopic algae and animals the primary base of the food chain in water, if they are not checked they will choke the rivers and oceans causing toxic waters. Fish feed to contain them, in the process they grow and increase in numbers but their population is checked by bigger fishes and so on till it reaches a top predator which is usually a large animal.
In determining species, it is best to differentiate in terms of the population that is being adapted in which, a certain niche. So, if it has a small gene pool, and the different species will be interbreeding each other with a much genetic variability in gene pool, it would most likely cause a less adaptation. The niche will be having a less adaptation if too much genetic variability will occur.
