Adeer is considered a heterotroph because it

it obtains it energy by feeding on other organisms

Explanation:animals and plants needs energy to carry out their normal functions such as growth.some are autotrophs ,others are heterotrophs.

Autotrophs can make their own food through photosynthesis.that is they use energy from the environment and inorganic nutrients to synthesis their food.autotrophs are majorly green plants,algae and cyanobacteria.

Chemoautotrophs are autotrophs which get their energy from chemical reactions in order to produce their food.example is bacteria

Heterotrophs get their energy by feeding on other organisms and digesting them .they depend on autotrophs directly or indirectly for their source of energy.for example,a deer feeds on green plants ,which are autotrophs.in this case,the deer is said to be a herbivore