Biodiversity is a broad concept, embracing taxa at all levels and different centres of plant origin. These can be used to characterize plant zones, to monitor changes in the status of a flora as a result of human pressure (Protopopova, 1991), and to predict future developments. Human pressure is, on the whole, negative for plants. Humans destroy localities. Air pollution from industry, traffic and other sources influences plants and tends to decrease the natural flora. Paradoxically total numbers of taxa can increase under these conditions because more or less natural colonizer species spread onto waste land. These species, though, tend to be broadly-distributed and cosmopolitan, and induce uniformity: floras of different cities are often similar. The species common to different urban locations (Vasil'yeva-Nemertsalova, 1996) are mostly colonizers, usually weeds and escaped cultivated plants. The flora of Odesa, a big industrial and recreational centre, is typical. In the last hundred years numbers of plant species, genera and families increased, and their proportions changed (the taxonomy used in the older records has been modernized). Common denominator species constitute 14% of the total; half are weeds or escaped cultivated plants; 15 species were previously absent. Under human pressure diversity of natural floras is destroyed, and numbers of common denominator taxa increase.