1.2 | Food Chains
Aligns to COR A: The role of higher plants in the living world
Plants occupy a foundational position in almost every terrestrial and many aquatic ecosystems by converting sunlight into chemical energy through the process of photosynthesis. This fixed energy is then transferred through food chains, beginning with primary consumers—herbivores that feed directly on plant tissues. Secondary consumers prey on these herbivores, and tertiary consumers feed on the secondary consumers. At each step, energy is lost as heat through metabolic processes, resulting in progressively lower biomass at higher trophic levels. The efficiency of energy transfer between each link typically ranges between 5% and 20%, shaping the length and complexity of food chains in a given habitat.
In terrestrial grasslands, for example, grasses and forbs harness solar radiation and store energy in leaves and roots. Grasshoppers and small mammals—primary consumers—graze these plants, converting carbohydrate reserves into animal biomass. Predatory spiders, birds, and foxes form subsequent links in the chain. By contrast, in freshwater ponds, phytoplankton and submerged macrophytes serve as primary producers; zooplankton and aquatic insects graze the algae, while small fish and amphibians occupy higher trophic levels. Understanding these pathways sheds light on how changes in plant communities—whether caused by invasive species, land-use changes, or climate shifts—cascade through entire ecosystems.
Human food systems are, in essence, anthropogenic food chains. Crops such as wheat, rice, and maize are cultivated, harvested, and consumed directly or processed into livestock feed. Livestock then occupy higher trophic positions, providing meat, dairy, and eggs. This conversion of plant biomass into animal protein introduces additional inefficiencies: for every kilogram of beef produced, approximately 25 kilograms of grain feed are required. Such insights drive research into alternative protein sources, plant-based meats, and insect farming, which aim to shorten the supply chain and improve energy use efficiency.
Ecologically, long food chains with many trophic levels are inherently unstable; a reduction in primary producer diversity or abundance can ripple upward, leading to declines in consumer populations or trophic imbalances. Keystone species—those with disproportionately large effects on community structure, often occur at middle trophic levels, such as herbivorous sea urchins in kelp forests. Effective conservation and management, therefore, require a clear grasp of the plant-centered energy flow that underpins all higher-level interactions.
Real-World Example
In the San Joaquin Valley’s almond orchards, cover crops (mustard and barley mixes) planted between tree rows serve as primary producers. Beneficial ground beetles and parasitoid wasps feed on the cover-crop insects, which in turn suppress orchard pests such as navel orangeworm. By designing these planted food chains, growers enhance natural biological control and reduce reliance on synthetic insecticides.
Questions to Ponder
- Why do most food chains seldom extend beyond five trophic levels?
- How might the introduction of a novel invasive plant species alter existing food-chain dynamics?
- In what ways do agricultural monocultures simplify or destabilize traditional ecological food chains?
- How can studies of aquatic versus terrestrial food chains inform cross-ecosystem conservation strategies?
Self-Check Exercises
- True or False: Energy transfer between trophic levels is typically more than 50 % efficient.
- Short Answer: Describe one ecological consequence of reduced primary producer diversity.
- Diagram Labeling: Draw a three-link food chain for a California chaparral ecosystem, identifying producers, primary consumers, and secondary consumers.
- Calculation: If a grassland supports 2 000 kg ha⁻¹ of primary producer biomass and energy‐transfer efficiency is 10 % per link, estimate the maximum biomass of tertiary consumers per hectare.
Suggested Images
- Figure 1.2.1: Simplified trophic pyramid showing biomass or energy at each trophic level in a terrestrial ecosystem.
- Figure 1.2.2: Photograph of a cover-crop planting in an orchard with overlay arrows illustrating the food-chain links to beneficial insects.
- Figure 1.2.3: Comparative diagram of terrestrial versus freshwater food chains highlighting differences in energy transfer efficiencies.
