Rat poison is not always an effective rodenticide and will reduce in effectiveness with age. Explain why

Any type of poison is not guaranteed to eradicate rats entirely. This is due to genetic mutations allowing an individual to be immune to certain types of toxins. Whilst the population may be eradicated, if enough individuals can survive they can breed and their offspring will also be immune to the rodenticide thus making it redundant. This is due to vertical gene transmission. The population is then likely to return to its optimum level quickly as there is reduced competition for resources.