____ are present in the first trophic level in the food chain.

Concept: a trophic level is the position an organism holds in a food chain, counted by the number of feeding transfers that separate it from the original source of energy entering the ecosystem. The counting starts with organisms that obtain no energy by eating anything else — they capture energy from sunlight or from inorganic chemical reactions and build their own organic food. Such self-feeding organisms are called autotrophs, and in ecosystem language they are the producers.

Application — trace a simple grassland chain, grass → grasshopper → frog → snake, and count the feeding transfers from the energy source:

1. Grass builds its own food by photosynthesis from sunlight, carbon dioxide, water and minerals. It consumes no other organism, so zero feeding transfers separate it from the energy source: it stands at trophic level 1.

2. The grasshopper gets its energy by eating grass — one feeding transfer from the source — so it stands at trophic level 2 and is termed a primary consumer (herbivore).

3. The frog eats the grasshopper — two feeding transfers — placing it at trophic level 3 as a secondary consumer.

4. The snake eats the frog — three feeding transfers — placing it at trophic level 4 as a tertiary consumer.

Cross-check — the same counting applied to every level of a food chain:

Contrast with the other groups named in the sentence: primary consumers sit at the second level because one feeding step separates them from the producers, and tertiary consumers sit at the fourth level because three feeding steps do. Only the self-feeding producers need no feeding step at all, so only they can head the chain.

Result: the first trophic level of a food chain is occupied by autotrophs, so the blank is filled by Autotrophs.