A lot of wildlife requires specific environmental situation. A lot of species won't survive changes in temperature.
Most poor people will have hard time surviving either increased flooding or increased droughts. Where I live(South MH, India), we are already facing drought every other year, and I am preety sure this is going to get worse. Millions live in this area. Where do you think we will move when we don't have water? (clue: look for places with more water)
So in short, huge parts of world either get submerged in sea, suffer from severe droughts/flooding, from desertification, or from the destruction of crops. Without food, water and land, people migrate in large numbers, and thus exert more pressure on those places.
Some experts believe that a war is likely between India and China over water in next decade.
I haven't touched the issue of lack of antibiotics and resurgence of old diseases at all.