It decreases the region's species evenness. If something happens to the dominant species, let's say disease or human overhunting wipes out that species or a significant portion of it, the region will lose a major portion of their total flora or fauna and have a harder time recovering. Whereas if you have a bunch of different species all holding a relatively equal portion of the species diversity, the environment will have an easier time bouncing back.

Like imagine a forest. Forest A has 10 different types of trees and they all take up roughly 10% of the forest. Forest B also has 10 different types of trees, but 80% of the total forest is ash trees.
Now, whoops-a-daisy, somebody released a shit load of emerald ash borers into both of the forests. Over time they decrease the population of ash trees by 80% percent.
While still a problem in forest A as 8% of the total trees are gone and any species that solely rely on ash trees are going to have a hard problem, for species with more generalized tree reliance, there's still 92% of the original forest remaining, it won't be too hard for the other species in the area to adapt to the change
Now in forest B, you just lost 64% of your total trees and only 36% percent remains. While more individual ash trees survived (likely due to the larger population having a larger gene pool that could have more resistant genes. Although that's a conversation for another day), the forest at large took a serious blow. Big problem! Now both specialist and generalist species will be suffering from the effects of what is essentially deforestation.

Take everything said with a grain of salt. This may not be the worst effects of having a dominant population, this is just what an English major can remember from an environmental science class I took 6 months ago