die off

Short definition from Wiktionary

Verb

1. 1
   To suffer a series of deaths; to die one by one; to progressively become extinct. (If a group of plants, animals, or people dies off, the entire group dies over a period of time.)
2. 2
   To become defunct or nonexistent.
3. 3
   Of music, noise, etc., to reduce in volume until no longer audible.

Noun

1. 1
   An incident or episode in which a particular species or organism dies off, especially in large numbers.