The number and kinds of organisms are not constant. How do you explain this statement?

(N/A) The population of any organism cannot remain constant over time. In some generations,the population may increase due to favorable environmental conditions. Conversely,in other generations,the population may decrease due to unfavorable conditions or resource limitations.
Furthermore,the kinds of organisms cannot remain constant because evolution is a continuous,never-ending process. Throughout geological time,new species originate through speciation,while older species may become extinct due to their inability to adapt to changing environments.
For example,the population of mosquitoes is very high during warm and humid weather,whereas they are rarely seen during extreme summer or winter conditions.