The smallest unit of matter that has the characteristic properties of it’s element is called an “atom”.
Individual atoms are called “elements” for that reason.
There are smaller divisions of matter, however those no longer have the characteristics of the element that they may be a part of. A hydrogen electron is indistinguishable from an argon electron, or any other.

Last option: Preserve an ecosystem

• The establishment of a bird reservation is a way to preserve an ecosystem because reservation provides suitable environment to organism. A healthy ecosystem means large number of biodiversity.
A characteristic one has. It could be appearance or personality