A food chain illustrates a linear path of feeding relationships, showing which organism consumes another for sustenance. A food web is a more complex, interconnected network of multiple food chains, providing a comprehensive picture of energy and material transfer in an ecosystem. These feeding relationships are the pathways through which various components move throughout the environment. The movement of these components—specifically energy, physical matter, and unintended pollutants—is fundamental to understanding the structure and function of biological communities.
The One-Way Movement of Energy
Energy represents the foundational currency that moves through every food chain and web, beginning with an external source. For nearly all ecosystems, this source is the sun, with producers converting solar radiation into chemical energy through photosynthesis. This stored chemical energy then begins its one-way journey as organisms consume one another across different feeding layers, known as trophic levels.
The transfer of energy between trophic levels is inefficient, described by the “10% rule.” When a consumer eats an organism, it captures only about 10% of the stored energy; the remaining 90% is lost as heat or used for metabolic processes.
This significant energy loss explains why food chains are typically short, rarely extending beyond four or five trophic levels. Since energy cannot be recycled once dissipated, a continuous input from the sun is required to sustain life.
The Cycling of Essential Matter
While energy flows in a single direction, physical matter, composed of elements and nutrients, is constantly recycled. Atoms like carbon, nitrogen, and phosphorus are repeatedly transferred between organisms and the non-living environment. Organisms acquire these elements from food, incorporating them into tissues for growth and maintenance.
When organisms die or produce waste, this matter becomes available to decomposers, such as fungi and bacteria. Decomposers break down dead organic material and complex waste products. They secrete digestive enzymes that convert complex organic compounds into simpler, inorganic substances.
This process returns essential elements to the soil, water, or atmosphere in a form that producers can absorb and reuse. This continuous cycle ensures that the limited supply of physical elements is constantly available to support new life.
The Accumulation of Harmful Substances
Food webs also act as pathways for unintended substances, particularly synthetic chemicals and heavy metals. Harmful compounds, such as the pesticide DDT or mercury, are not easily broken down or excreted by organisms. These substances enter the food web and build up in an individual organism’s tissues over its lifetime, a process called bioaccumulation.
This localized buildup leads to a broader problem known as biomagnification. As a consumer at a higher trophic level eats many contaminated organisms, the toxin concentration in its body increases substantially. Because of inefficient energy transfer, a top predator must consume a large mass of prey, concentrating the pollutant with each meal.
This increasing concentration means that organisms at the top of the food web, such as raptors and large predatory fish, carry the highest toxic loads. Mercury, for example, exhibits significant biomagnification, resulting in the highest concentrations in species like swordfish and tuna.