As the environment changes, some species adapt in a way that them survive and reproduce, while other species do not adapt well and die. this process is called:

The answer is natural selection. Charles Darwin popularised the term "natural selection". The change in the heritable traits characteristic of a population over generations. Natural selection is reproduction of individuals due to differences in phenotype and the differential survival It is a key mechanism of evolution. 

because they are eaten by the frog and the frog getseaten by the snake and the snake by the eagle

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.