dropping vs leaf litter

dropping

noun
  • A piece of animal excrement; dung. 

  • The act of something that drops or falls. 

  • Something dropped; a droplet. 

leaf litter

noun
  • Dead plant material, such as fallen leaves, bark, and twigs, that has fallen to the ground and may be used by animals as habitat or for nest-building, eventually decomposing and releasing nutrients into the soil. 

How often have the words dropping and leaf litter occurred in a corpus of books? (source: Google Ngram Viewer )