Why do land animals need more air?

It's not more "air", but more *oxygen* that land animals need. Cal King nicely explains why. Being on land produces a higher environmental temperature than being in water, and the higher the core temperature, the faster the metabolism (breakdown of food to release energy) ... but faster metabolism requires more oxygen. Luckily, lungs are more efficient at deriving oxygen than gills.

But if I may expand on that answer, what is true for ectothermic (cold-blooded) land animals like amphibians and reptiles, is true on steroids with the evolution of endothermic (warm-blooded) animals like the dinosaurs and early proto-mammals, as well as their later descendants, the birds and mammals respectively. Endothermy requires constant expenditure of energy to raise the internal core body temperature, and thus even greater oxygen requirements. (This is why the evolution of external insulators -- feathers, fur, and later blubber in aquatic mammals -- to reduce the loss of body heat, we're important parts of dinosaur, bird, and mammal evolution.)

Finally, I will mention two other developments in land animals that increased oxygen requirements even further ... the evolution of flight (e.g. In birds and bats) ... and the evolution of big brains (in mammals, particularly in the primates). Big brains require voracious amounts of oxygen, which is why the evolution of human-like intelligence could not have occurred in cold-blooded, gill-breathing aquatic animals. (The octopus is a valiant attempt.)

As body temperature increases, the metabolic rate of ectothermic animals also increase. To fuel the increased metabolism, more oxygen is needed. An ectothermic land animal can maintain a body temperature higher than the surrounding air by basking or coming into contact with warm objects like rocks and road pavements (ever seen a snake pressed tightly against the road surface to get warm?). However, fish are unable to do so because water is a huge heat sink. It takes a lot of energy to raise the body temperature of a fish above that of the surrounding water. Therefore most fish simply maintain a body temperature that is the same as the water they live in. Tuna fish is a well known fish that can raise its CORE temperature to swim faster, since muscles are more efficient when they are warm. Nevertheless, it minimizes the amount of energy needed by maintaining a body surface temperature that is much cooler than its core temperature (BTW, some seals do the same to conserve energy).

Since it takes a lot of energy to raise the temperature of water just a single degree C, most bodies of water are cooler than the surrounding air. Therefore, a lizard can often maintain a body temperature as much as 90-95 degrees F, while fish in the nearest body of water swim in temperature that is only about 70 deg. F or even lower. Therefore the often higher body temperature of the land animals are responsible for their greater need for oxygen. If you take a lizard, a frog and a fish, and they all have the same body temperature, their metabolic rates, and their need for oxygen, should be similar.

I'm not sure that land animals DO need more air than gilled animals. Fish and things that have gills just have a different respiratory system adapted for their enviroment (underwater). Their gills filter the oxygen out of the water as they swim. It may seem like they need less oxygen, but it probably all depends on the species. I know that fish tanks usually have a filter that causes bubbles in the water, which aerates it, and that some people with small fishing pond will pump that water out and back in through a hose and sprinkler to do the same and keep the water oxygen rich.