Tertiary consumers are animals that feed on other carnivores or omnivores, occupying a position towards the top of an ecosystem’s food chain. They primarily consume secondary consumers, and their lower abundance compared to other organisms is a pattern seen across various habitats. This article explores the ecological reasons behind fewer high-level predators.
Understanding Trophic Levels
Ecosystems are structured by feeding relationships, organized into distinct levels called trophic levels. At the base are primary producers, such as plants and algae, which create their own food through photosynthesis. These organisms convert light energy into chemical energy, forming the foundation of the food web.
Primary consumers are herbivores that feed directly on primary producers. Examples include animals like rabbits, deer, or grasshoppers. Secondary consumers are carnivores or omnivores that prey on primary consumers, such as lions hunting zebras or snakes eating mice.
Tertiary consumers are organisms that primarily eat secondary consumers. These can be carnivores, like a hawk preying on a snake, or omnivores that consume other carnivores. Humans, polar bears, and crocodiles can all function as tertiary consumers depending on their diet.
The Inefficient Transfer of Energy
The primary reason for the decreasing number of organisms at higher trophic levels stems from the inefficient transfer of energy between these levels. This concept is often described by the “10% rule,” which states that, on average, only about 10% of the energy from one trophic level is transferred to the next. The remaining 90% of energy is lost at each step.
Energy loss occurs for several reasons. A significant portion of the energy consumed by an organism is used for its own metabolic processes, such as respiration, movement, and maintaining body temperature, with much of this energy dissipating as heat.
Furthermore, not all the biomass from one trophic level is consumed by the next. Parts of an organism, such as bones, fur, or woody stems, may not be eaten or are indigestible. Additionally, not all consumed food is fully digested and assimilated; some energy is lost through waste excretion. This cumulative loss of energy at each transfer means that progressively less energy is available to support organisms at successively higher trophic levels.
Consequences for High-Level Consumers
The substantial energy loss at each trophic level directly impacts the populations of high-level consumers. Less energy is available to sustain large numbers of tertiary consumers, which explains why food chains rarely extend beyond four or five trophic levels.
The limited energy availability results in fewer individuals at higher trophic levels. This also leads to a smaller total biomass at the tertiary consumer level, forming an ecological pyramid where biomass decreases from the base to the top. Tertiary consumers often require larger territories to find enough prey, as their food source is more dispersed and less abundant.
Due to their position at the top of the food chain, tertiary consumers are more susceptible to environmental disturbances. Any disruption that affects lower trophic levels, such as a decline in primary producers or primary consumers, can have a magnified impact on tertiary consumer populations because their energy base is already so narrow. Their populations are inherently limited by the amount of energy that successfully makes its way up the food web.