Here are some suggested answers. There could be others!

The greater the population, the more natural resources are burnt up and, consequently the more pollution is created.

A city crowded with people leads to roads crawling with cars.

Cities face a number of problems which are due to overcrowding.

The consequence of too dense a population is overcrowding.

In a city light, heat, transport and food must all be supplied artificially as we are living a life removed from nature.

One effect of living on the street is that drug abuse and crime rates rise.

We are part of an unhealthy, consumerist and throwaway society, which creates an environmental crisis.

Owing to being over-crowded, Britain’s main cities all have a number of people living on the street.