What led to the writing of the constitution?

It was primarily the failure of the Articles of Confederation to deal with the issues arising between the states that led to the writing of the Constitution, since it was clear there needed to be a "stronger" central government. 

Yes, studying how mayfly population affects the number of trout in a stream is ecology.
Ecology seeks to unveil how different organism relate with one another as well as their environment.
Ecology is basically the study of distribution,abundance and relation of organisms and their interaction with their environment.