Kevin is describing three principles in world building to make those breath life and seem more realistic:
1) Creatures are Part of Their Environments
Begin the level design process with your Non Player Characters (NPCs) in mind
2) Creatures are Territorial
Congregate Creatures in Groups Around Resources
3) Creatures Organize Themselves into Innate Hierarchies
Arrange like creatures in a hierarchical manner
These principles are all present in the real world. Just look around, you won't have problems finding a bunch of ants gathering ressources for the nest. This simple example quickly illustrates the three principles: they are part of their environment (they build a nest), they are territorial (they gather ressources) and organized in a hierarchy (they are social creatures organized in different casts).
For more fascinating read on the ecology and the environment, I recommend reading David Suzuki's books "The Sacred Balance" and "Tree: A Life Story".
As for the games' environments, Kevin Carter concludes:
Ultimately, the three guidelines laid out by this article can be seen as the connective tissue that holds a game world together. The tighter and thicker this connective tissue becomes between, the more believable your game world will feel, and the easier it will be to reach what should be the ultimate goal of all game designers, specifically stellar game play that works on multiple levels.