The sum of the partial pressures of the gases that make up a gaseous mixture is equal to the total pressure of said mixture, according to Dalton's law:
P total = P N2 + P 02 + PC02
PC02 = P total - P N2 - P 02= 12, 5 atm -3, 7 atm -4,8 atm= <em>4 atm</em>
Migratory birds would not be able to utilize any other habitat in their place and a lot of animals wouldn't either so those are some negative impacts hope this helps
