Excellent fun-time game, nice and easy!
Each map starts with a ready-made watering hole and a few plants. You will add more as you play.
The key is to place the animals where they belong, which starts near watering holes. Place enough animals and you receive a bonus build which allows you to place Grass, Shrubs, Tress (all needed by herbivores to eat) and water (needed by most, but not all, animals to drink). Then, you will gingerly place carnivorous animals into your ecological domain to, well, feast on the plant-eaters. The main thing to remember is that the plant-eating animals will eat all the plants in a given area and may die anyway, either to lack of food or they went to far from water. So, the meat-eaters are actually a good thing ... just like in reality, since they keep the populations of the plant-eaters down. In the end, the entire Eco-System reaches a balance.
Several scenarios exist, and they get tougher as you progress. Sometimes you have to place certain animals on the map, even though you know they won't last long, just to get more special builds. Placing a storm with a special build will fill up the rare empty watering hole, allowing you to start a whole new 'family' of animals.
The animals are also realistic in their habits. Placing Elephants, for example, near a watering hole with a lot of Birds will most likely kill off many of the flyers since they live in the water and, when the elephants 'make like an elephant' and rush into the water (they occasionally do this), they will force many of the birds out. Likewise, Elephants will kill some smaller animals, including Lions, and even knock off that voracious Crocodile.
There is a little voice acting in the game in the way of mission briefings. These are short but kinda cute.
Also, as you progress through the main mission tree, you will unlock larger, special missions. These allow you to build huge Eco-Systems of animals and plants on the maps used in the main mission tree. These scenarios have no ending, and will last as long as you wish to continue and watch your animals thrive.
All in all, this is a Fun game. It is not a serious, die-hard RTS, but that makes the game all that much more interesting to play. A great leisure time game to play and just relax.