Barriers are various natural or artificial borders that fragment the landscape. They reduce the habitat, block pathways and separate the populations into smaller segments. A natural barrier may be a sierra, a valley, a river, a sea, the distance between optimal habitats etc. Human-related barriers are the roads, fences and cities. The following hypothesis was tested: the barrier-effect's strength depends on the barrier itself as well as on the size, migratory behavior and other properties of

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... affected species. This review was written with the focus on the species size, the barrier type and the effect's strength in the case of multiple species and barriers. The results are mostly in agreement with this hypothesis, but it is revealed that the evaluation is not fully standardized yet. Wright's Fst value is an exact number, but researchers have handled it differently. In some cases they were more permissive and ascribed less impact (mostly in the case of low Fst values). On the other hand, there are authors who valorized the low Fst values because of the possible cascade effect caused by genetic division and changing behaviour. This review describes the possible effects of barriers, but every species and habitat is unique, therefore the method described should not be regarded as 100% accurate. With this comparing technique the effects may be predicted more precisely prior to carrying out the impact studies, thus the additional costs and the impact on the actual species will be easier to estimate.