Why are terrestrial food chains usually limited to 4–5 trophic levels?

The correct option is: Energy available decreases at higher trophic levels.

To understand why terrestrial food chains are typically limited to 4–5 trophic levels, we must examine how energy flows through an ecosystem.

Energy enters most ecosystems via photosynthesis, where plants (producers at the first trophic level) convert solar energy into chemical energy stored in organic molecules. When herbivores (primary consumers) eat these plants, and carnivores (secondary and tertiary consumers) eat the herbivores, energy is transferred up the food chain.

However, this energy transfer is highly inefficient. According to the ecological rule of thumb known as the 10% Law, only about 10% of the energy stored as biomass at one trophic level is passed on to the next level. The remaining 90% is lost through several pathways:
1. Respiration and metabolism: Organisms use a large portion of their consumed energy to perform life processes (such as movement, growth, and cellular repair), which is dissipated as heat.
2. Waste: Some organic matter is not fully digested and is excreted.
3. Unconsumed biomass: Not all organisms at a lower trophic level are eaten by those at the higher level.

We can mathematically model the energy remaining at trophic level
$n$
using the formula:
$E_n=E_1\times {\left(0.10\right)}^{n-1}$
where
$E_1$
is the energy captured by the primary producers (the first trophic level).

If the producers capture 10,000 units of energy, the distribution across trophic levels proceeds as follows:
- Trophic Level 1 (Producers): 10,000 units
- Trophic Level 2 (Primary Consumers): 1,000 units
- Trophic Level 3 (Secondary Consumers): 100 units
- Trophic Level 4 (Tertiary Consumers): 10 units
- Trophic Level 5 (Quaternary Consumers): 1 unit

Due to this rapid, exponential decline, the amount of energy available at the 5th level or beyond is usually insufficient to support the metabolic needs of a viable population of top predators. Therefore, terrestrial food chains are naturally constrained to 4–5 trophic levels.