Food chains and food webs Knowledge-Based Study

Study notes

1. Energy Flow in Ecosystems

   All biological communities require a continuous input of energy to sustain life. In most ecosystems, this energy enters as sunlight and is converted into chemical energy via photosynthesis by primary producers (autotrophs), such as plants, algae, and cyanobacteria.

   Organisms that cannot synthesize their own organic molecules are called consumers (heterotrophs). They obtain energy and organic nutrients by feeding on other organisms. Feeding positions within an ecosystem are categorized into distinct trophic levels:

   • First trophic level: Primary producers (autotrophs).
   • Second trophic level: Primary consumers (herbivores that feed directly on producers).
   • Third trophic level: Secondary consumers (carnivores or omnivores that feed on primary consumers).
   • Fourth trophic level: Tertiary consumers (apex predators or top carnivores that feed on secondary consumers).

   Decomposers (such as bacteria and fungi) and detritivores (such as earthworms and woodlice) break down dead biomass and organic waste, returning inorganic nutrients to the soil and water for reuse by producers.

   Structure of a Food Chain

   A food chain is a simplified, linear model illustrating the direct transfer of energy and nutrients from one organism to the next. Each link represents a single feeding relationship.

   An example of a terrestrial food chain is:

   Grass → Grasshopper → Frog → Garter Snake → Red-tailed Hawk

   In this model:

   • Grass represents the primary producer.
   • Grasshopper acts as the primary consumer (herbivore).
   • Frog acts as the secondary consumer (carnivore).
   • Garter Snake serves as the tertiary consumer.
   • Red-tailed Hawk functions as the quaternary consumer or apex predator.

   The arrows in a food chain always point from the organism being consumed to the consumer, indicating the direction of energy transfer.

   From Chains to Webs

   In nature, feeding relationships are rarely linear. Most consumers feed on multiple species and, in turn, are preyed upon by several different predators. A food web consists of interconnected food chains within an ecological community.

   Why Food Webs Matter

   • Dietary flexibility: Many animals are omnivores, occupying more than one trophic level simultaneously depending on their food source. For example, a bear eating berries acts as a primary consumer, but when eating salmon, it acts as a secondary or tertiary consumer.
   • Ecosystem resilience: Complex food webs with high biodiversity tend to be more stable. If a disease reduces the population of a single prey species, predators can often switch to alternative prey, buffering the community against systemic collapse.

   The 10% Rule of Energy Transfer

   Energy transfer between trophic levels is inherently inefficient. On average, only about 10% of the chemical energy stored as biomass in one trophic level is converted into new biomass at the next trophic level. The remaining 90% is lost primarily due to:

   • Metabolic heat: Heat dissipated as a byproduct of cellular respiration.
   • Undigested waste: Biomass that is excreted (feces) or not consumed (bones, shells, roots).
   • Life processes: Energy spent on locomotion, thermoregulation, and tissue maintenance.

   Because energy decreases drastically at each step, food chains rarely exceed four or five trophic levels. There is simply insufficient available energy to sustain viable populations of higher-level consumers.