The ant/bacteria relationship is a good example of a symbiosis—a mutually beneficial relationship. The ants’ food is protected from the mold, and the bacteria get easy access to a source of food. Symbioses are not violations of the law of limited competition since, in this case, neither the ants nor the bacteria are actively seeking out the mold to eliminate it or preventing the mold from surviving altogether.
Symbioses exist throughout nature (in fact, the fungus and ant are symbiotes as well) since the members of the relationship are often better able to survive and reproduce thus out-competing other organisms. Again, they are competing with (albeit more effectively together than apart), but not eliminating the other organisms in the community of life who also need resources to survive.
While not specifically related to symbiotic relationships, question and answer 478 at this website also bears on your inquiry.
A great example of violating the law of limited competition is the way “Takers” operate. We want meat so we graze our cows on grass. The other grazing animals competing with our cows for nutritious grasses are hunted or chased away—we eliminate the cow’s competition. Some species of grass are favored by our cows. We eliminate the grasses that the cows don’t like so that the favored grass species have less competition—we eliminate the grass’ competition.
We also remove any other vegetation that competes for water and soil nutrients. Any disease or insect that might use the favored grass is also eliminated—we eliminate the grass’ and the cow’s predators and diseases. All that remains is our food and our food’s food—all others are eliminated. At least that is the goal of totalitarian agriculture.
NOTE: This question was answered by Dr. Alan Thornhill