What controls population growth? How does density affect population growth?
Population growth takes place when mortality is less than natality. Environmental effects on population are many which control the population size.
Population growth without regulation may lead to exponential, often a disastrous increase in population size.
Several factors directly or indirectly control the population growth.
Population is made up of individuals. Proper growth and development of an organism will lead to reproductive success and then the population size will increase.
- Every individual faces competition for food and space. Thus the ecosystem in which the organism resides, may counter an abnormal growth in number of individuals.
- Then there are predators and diseases which also affect population size.
- Seasonal change in temperature and precipitation are other factors which control population size.
All the above factors may act simultaneously but in varying degree.
- Natural calamities like volcanic eruption or flood, drought, etc. may also randomly alter population size.
Now the second part of your answer.
Some factors controlling population size of an organism may actually be density dependent.
- These are resources I first mentioned i.e. resources in limited supply, such as space, food and water. Individual organisms may suffer higher mortality rate due to fierce intra-specific (and often inter-specific) competition.
- Rapid spread of infectious disease is also common when population density is high. Disease epidemic leads to a drastic derease in population size.