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The correct word to fill the gap would be indirect.

In other words, an indirect survey makes use of evidence, such as the footprints left behind by animals, to follow animals that are difficult to track.

Ecological surveys are of 2 types:

  • Direct
  • Indirect

Direct ecological surveys have to do with studying phenomena directly and recording whatever that is observed.

Indirect surveys involve an indirect way of studying phenomena. The signs left by phenomena are observed and logically used to make deductions about such phenomena.

More on ecological surveys can be found here: https://brainly.com/question/1182023?referrer=searchResults