Diversity is a main component of a healthy urban forest. Different species are susceptible to different pests and different kinds of damage, so the more different kinds of trees are planted in a community, the less vulnerable the overall community forest will be to invasive species, climate change, or severe weather.
One of the major reasons that Dutch elm disease and emerald ash borer have been so devastating to communities is that many had planted nothing but elms or a very high proportion of ashes.
The more closely related tree species are, the more likely they are to be vulnerable to the same pests and damage. So the goal is to follow the 5:10:15 rule: In any community, no more than 5 percent of trees should be of the same species; no more than 10 percent should be from the same genus; and no more than 15 percent should be from the same family. When choosing a tree to plant, make sure it is suitable for the growing conditions of the site and try something new. Look around your street and neighboring yards to see what they have planted, and choose something different.
Searchable tree database at the Virginia Tech Department of Forest Resources and Environmental Conservation.
Vascular Plant Name Search at the University of Wisconsin Robert W. Freckmann Herbarium.