Final answer:
The herbivore is sometimes a 1st level and sometimes a secondary level consumer in the given food web.
Step-by-step explanation:
In the given food web, the organism that is sometimes a 1st level and sometimes a secondary level consumer is the herbivore.
The primary producers like plants and phytoplankton are at the 1st trophic level. The herbivores, which consume the primary producers, are at the 2nd trophic level. However, some herbivores can also be preyed upon by carnivores, placing them at the 3rd trophic level as secondary consumers.
For example, if a deer eats grass (primary producer), it is a 2nd trophic level consumer. But if that deer is subsequently eaten by a lion, it becomes a part of the 3rd trophic level as a secondary consumer.