Overview
This lecture explains the difference between food webs and food chains, how energy and matter flow in ecosystems, and how changes in one population can affect others through trophic cascades.
Food Chains and Food Webs
- Food chains show a simple, linear flow of energy and matter from one organism to another.
- Food webs consist of at least two connected food chains that share a common organism.
- Arrows in food chains and webs indicate the direction of energy and matter transfer (pointing to the eater).
- Even herbivores eating plants are considered predators in this context.
Trophic Levels
- Trophic levels classify organisms by their position in the energy flow (e.g., producers, primary consumers, secondary consumers).
- One organism in a food web may occupy multiple trophic levels depending on what it eats.
- Example: An owl is a secondary consumer when it eats a rabbit, and a tertiary consumer when it eats a robin.
Population Changes and Trophic Cascades
- An increase in a predator's population (e.g., python) reduces prey populations (e.g., rat, frog).
- Fewer prey can lead to more of their food sources (e.g., more grasshoppers if rats and frogs decline), impacting plants negatively.
- Decreased populations at one level can affect predators and other organisms up and down the web.
- Trophic cascade: A significant change in one population (often a top predator) creates ripple effects throughout the ecosystem.
- Example: Losing wolves increases deer, which overconsume plants, leading to habitat and vegetation loss.
Practice/Skill Application
- Practice describing the effects of population changes in a food web using visual models.
Key Terms & Definitions
- Food Chain — A linear sequence showing energy flow from one organism to another.
- Food Web — A network of interconnected food chains sharing common organisms.
- Trophic Level — A classification of an organism's position in a food chain/web (producer, consumer, etc.).
- Trophic Cascade — Ecosystem-wide effects caused by changes in the population of one species, especially predators.
Action Items / Next Steps
- Practice: Describe the direct effect of a frog population decline on another organism in a food web.
- Identify an organism that acts as both a secondary and tertiary consumer in a given food web.