Harder Wildlife is an alteration to the Minecraft world at large. Animals and trees automatically repopulate and die off leaving behind rotting wood, seasons change, snow falls, accumulates, and melts. Crop growth is slowed and takes into account biome attributes: warm (but not hot) and wet (but not soggy) biomes will grow crops the fastest (especially during the spring and summer). Saplings and other plantable items (mushrooms, reeds, etc.) will automatically get planted if left on the ground if possible.
In addition it takes more wheat (seeds, etc) to make animals go into "love mode" but they drop more meat, etc. Leather is no longer painfully scarce, as both pigs and cows drop it and in larger quantities. Killing a sheep gives more wool on average than sheering (but shearing leaves the sheep alive) making it easier to get started without having to murderize a whole flock just to make a bed (but with long-term usefulness of shears).
Goats and Monitor Lizards are added to the mesa biomes, to provide additional access to leather, wool, and food for biomes where it is otherwise scarce, particularly if these biomes are Large or surrounded by other vacant biomes, such as oceans.