I was just reading a study written by a scientist who was talking about a particular biological issue and he said that the dogma is that diversity is good, but in his study it turns out that what you really need are certain types of species that keep out bad species. And that is all the diversity you need.

And it made me think, duh, how many times have we heard about an invasive non-native species being introduced through human folly and it ruins the balance and processes of the productive native plants or animals that are critical to the local environment.

So certain examples of diversity are a real problem in nature or in the environment if they upset the optimal natural functions of native species and also we as humans can see that diversity is not always and everywhere a good thing.

Plants that cause problems when they are not in their native environment are what we call weeds. Animals that become infected with a sickness may be different from animals that are healthy, but that diversity is not a strength.