remnant

A remnant is a small piece left over from a larger whole. The end of the roll, or the last of a forest.

Fabric shops sell remnants cheaply, which is the most everyday use, and quilters exist largely because of them. Ecology uses it precisely, since a remnant population or a remnant of ancient woodland is what survives of something once far larger, and those fragments matter enormously for conservation. The word is formal and slightly melancholy, since a remnant always implies loss.