But they're not competing. They're not trying to do something similar in a more efficient way. They're parasites - genuinely, even literally. They're taking away the energy sentry needs to run themselves but, if they succeed too well, they will destroy their own capacity to run.

Two people can compete. Tomatoes and people are complementary - it's nice to have both. Tapeworms and people are playing a game in which, if the people win, the tapeworms lose, and if the people lose, the tapeworms lose to.