Energy flows through Earth’s diverse ecosystems, primarily entering from the sun, captured by organisms like plants and algae. This captured energy then transfers between organisms as they consume each other. However, this transfer is not entirely efficient; a significant portion is used or dissipated at each step, making energy movement a complex process.
The Rule of Energy Transfer
Organisms within an ecosystem are organized into distinct feeding levels, known as trophic levels. The base of this structure consists of producers, typically plants or algae, which create their own food using sunlight. Organisms that consume producers are called primary consumers, often herbivores. Secondary consumers feed on primary consumers, while tertiary consumers prey on secondary consumers.
As energy moves from one trophic level to the next, a substantial amount is lost. This phenomenon is often described by the “10% rule,” which posits that, on average, only about 10% of the energy from one trophic level is incorporated into the biomass of the next level. The remaining 90% is used by the organisms for their own life processes or dissipated into the environment. For instance, if producers capture 1,000 units of energy, only about 100 units become available to primary consumers. Subsequently, secondary consumers would only acquire roughly 10 units from those primary consumers. This consistent reduction explains why ecosystems can support fewer organisms at successively higher trophic levels.
Mechanisms of Energy Loss
The substantial loss of energy between trophic levels occurs due to several biological and physical factors. A considerable portion of the energy consumed by an organism is utilized for its own metabolic processes, such as respiration, movement, and reproduction. These essential life activities convert energy into forms that are not transferable to the next trophic level, with much of it ultimately dissipating as heat. This heat loss aligns with the second law of thermodynamics, which states that energy transformations always result in some energy becoming less organized or usable.
Another reason for energy inefficiency is incomplete consumption. Not all biomass from one trophic level is eaten, or if eaten, it may be indigestible. For example, parts of plants like roots or woody stems, or animal parts such as bones, fur, or feathers, may not be consumed. Energy is also lost through waste products like feces, which contain unassimilated food.
Consequences for Ecosystems
The significant energy loss at each trophic transfer has profound implications for the structure and function of ecosystems. This energy reduction is visually represented by energy pyramids, which depict a broad base of producers and progressively smaller layers for each successive consumer level.
Similarly, biomass pyramids generally show a decrease in the total mass of living organisms at higher trophic levels. While most terrestrial ecosystems exhibit an upright biomass pyramid, some aquatic ecosystems can have inverted biomass pyramids where the biomass of primary consumers temporarily exceeds that of rapidly reproducing, but short-lived, producers like phytoplankton. The limited energy availability also restricts the length of food chains, which typically consist of only three to five trophic levels, as there simply isn’t enough energy to support many more. This constraint on food chain length influences the overall biodiversity and the carrying capacity of an ecosystem.