Overview
This lecture covers the structure and function of food webs and food chains, explaining how energy and matter flow through ecosystems and how changes in one population impact others.
Food Chains and Food Webs
- A food chain is a single pathway showing energy and matter flow from one organism to another.
- A food web is at least two interconnected food chains sharing a common organism.
- Arrows in food chains and webs point in the direction of energy and matter flow (toward the eater).
Energy and Matter Flow
- The energy and nutrients move from the organism being eaten (prey) to the organism eating (predator).
- Herbivores eating plants are still considered a form of predation.
- Example: Corn → Grasshopper (energy and matter go to the grasshopper).
Trophic Levels
- Trophic levels classify organisms by their position in the food chain or web.
- An organism can occupy multiple trophic levels in different food chains within a food web.
- Example: An owl is a secondary consumer when eating a rabbit and a tertiary consumer when eating a robin.
Population Changes and Trophic Cascades
- Increases or decreases in one organism's population affect other species up and down the food web.
- A trophic cascade is a ripple effect through trophic levels resulting from a major change like the addition or loss of a top predator.
- Example: Removing wolves increases deer populations, reducing plant biomass and potentially harming the ecosystem.
Visual Models and Environmental Concepts
- Food webs visually represent energy/matter flow and relationships between organisms.
- They help describe direct effects of population changes (e.g., decline in frogs increases grasshopper population).
- Practice involves describing these effects and identifying organisms at multiple trophic levels.
Key Terms & Definitions
- Food Chain — a linear sequence of organisms through which energy and matter flow.
- Food Web — a network of interconnected food chains.
- Trophic Level — a position an organism occupies in a food chain or web.
- Trophic Cascade — ecosystem effects caused by changes in the population of a key species.
Action Items / Next Steps
- Practice: Describe a direct effect of a frog population decline in a food web.
- Identify an organism that is both a secondary and tertiary consumer in a given food web.