Many pests have only become a problem because of the direct actions of humans. Modifying these actions can often substantially reduce the pest problem. Living organisms evolve and increase their resistance to biological chemical, physical or any other form of control. Unless the target population is completely exterminated or is rendered incapable of reproduction, the surviving population will inevitable acquire a tolerance of whatever pressures are brought to bear – these results in an evolutionary arms race. House flies tend to accumulate wherever there is human activity and are virtually a global phenomenon, especially where food or food waste is exposed. Similarly, seagulls have become pests at many seaside resorts. Tourists would often feed the birds with scraps of fish and chips and before long; the birds would become dependent on this food source and act aggressively towards humans.