The number and kinds of organisms is never constant. Populations are constantly affected by various factors. These factors include:

1. Season: Some organisms, plants and animals alike, are present in the ecosystem only during certain time. For example Mayfly, Mayflies spend a year awaiting their birth, and then most die after living just one day.

2. Extinction: Some species might be wiped out due to human or environmental activities.

3. Human activities: Deforestation causes a loss of a great number of organisms, hunting reduces the number of organisms.