cannot have null values in several fields on the Ecology tab

[ljwalker]

What module(s) and tab(s) does this bug affect?
Taxonomy module: Ecology tab

Describe the problem
Project, Abundance, Depth Zone, and Substrate do not allow default = null/blank and we are forced to use "-" as a placeholder value for blanks.

Describe the solution you'd like
Allow the fields above to be blank.