The number and kinds of organism is not constant.How do you explain this statement?

Reproduction is one of the characteristics of living beings. Because of reproduction, the number of organisms present at a place keeps on changing i,e, the number of organism is not constant.
The kinds of organism found at a place depend upon various environmental factors like temperature, availability of food, water and other resources. As the environmental factors also keep on changing with time, therefore kinds of organism j is not constant at a place.