Ecology and the Environment: Feeding Relationships
Hello Biologists! Welcome to one of the most fundamental and fascinating topics in ecology: Feeding Relationships. Don’t worry if the term ‘ecology’ sounds complicated—it’s just the study of how living things interact with each other and their environment.
Understanding who eats whom, and how energy moves through an ecosystem, is essential. It explains why we have certain numbers of different animals in the wild and why certain ecosystems are stable. Let’s dive in!
I. The Roles We Play: Producers and Consumers
In any ecosystem (like a forest, pond, or desert), every living organism has a specific job related to how it gets its energy. These jobs define the trophic level (feeding level) of the organism.
1. Producers (The Food Makers)
Producers are the starting point of almost every feeding relationship. They are the organisms that make their own food, usually through a process called photosynthesis.
- They capture light energy from the Sun and convert it into chemical energy (like glucose).
- Key Examples: All green plants, algae, and some bacteria.
- Analogy: Producers are like the self-sufficient chefs who run their own solar-powered kitchens.
2. Consumers (The Food Eaters)
Consumers cannot make their own food, so they must eat other organisms (producers or other consumers) to get their energy.
Consumers are categorised based on what they eat:
- Primary Consumers (Herbivores): They eat only producers (plants).
Example: Rabbits, cows, deer, caterpillars. - Secondary Consumers (Carnivores or Omnivores): They eat primary consumers.
Example: Foxes eating rabbits, small birds eating caterpillars. - Tertiary Consumers (Carnivores or Omnivores): They eat secondary consumers.
Example: Eagles eating foxes, large snakes eating small birds. - Quaternary or Apex Consumers: These are often at the top of the food chain and have very few, or no, natural predators.
Example: Polar bears, Killer whales, humans (sometimes).
Quick Review: Every food chain starts with a Producer.
3. Decomposers (The Clean-up Crew)
Decomposers are vital, though they often aren't included directly in the main food chain diagram. They break down dead organisms (producers and consumers) and waste materials (faeces).
- Role: They recycle essential nutrients (like nitrates and phosphates) back into the soil, which producers then use to grow.
- Key Examples: Bacteria and Fungi.
- Did you know? Without decomposers, all the nutrients would be locked up in dead matter, and new plants couldn’t grow!
II. Following the Energy: Food Chains and Food Webs
A food chain is the simplest way to show how energy is transferred from one organism to the next.
1. Food Chains: The Simple Path
A food chain shows a single path of energy flow. We use arrows to show this movement.
The most important rule for drawing food chains:
The arrow points in the direction of energy flow (from the organism being eaten to the organism that does the eating).
Producer → Primary Consumer → Secondary Consumer → Tertiary Consumer
Example:
Grass → Grasshopper → Frog → Snake
- The grasshopper gets energy from the grass.
- The frog gets energy from the grasshopper.
- The snake gets energy from the frog.
Common Mistake to Avoid: Drawing the arrow pointing backwards! Remember: The arrow points to where the energy is going (like pointing the food into the mouth of the consumer).
2. Food Webs: Connecting the Paths
In the real world, organisms rarely eat just one type of food. A fox might eat rabbits, mice, or berries. This interconnectedness is shown by a food web.
- A food web is a network of interconnected food chains.
- Food webs show much more realistically how organisms rely on each other.
- Stability: Ecosystems with complex food webs are generally more stable than those with simple chains. If one food source disappears, a consumer can switch to another source.
Think of it: If a mouse only ate one type of plant, and that plant died out, the mouse would starve. If the mouse is part of a complex food web, it can switch to eating seeds or insects.
Key Takeaway: Food chains show single paths; food webs show multiple, interconnected paths of energy flow.
III. Energy Transfer Efficiency: Why Ecosystems Need So Much Grass
When an organism eats another, it doesn’t gain 100% of the energy stored in that food. Most of the energy is lost at every stage. This is why food chains usually only have 4 or 5 steps—there isn't enough energy left to support more levels!
1. The 10% Rule
Only about 10% of the energy available at one trophic level is transferred and incorporated into the biomass of the next trophic level. The rest (about 90%) is lost to the environment.
2. How Energy is Lost (The 90% Loss)
The energy that is lost is used up by the organism before it gets eaten, or it is released as unusable heat.
Energy is lost in the following ways:
- Respiration: Energy is needed for basic life processes like breathing, muscle movement, pumping blood, and growing. This energy is ultimately lost as heat to the environment.
- Waste (Faeces/Excretion): Some parts of the food (like cellulose in plants) are undigested and passed out as waste. This energy remains unavailable to the consumer.
- Uneaten Parts: Not all of the organism is consumed (e.g., bones, roots, fur).
Analogy: Imagine you earn \$100 (100% of energy). Before you can save it (pass it to the next level), you have to spend \$90 on rent, food, and bills (respiration and heat loss). You can only pass on the remaining \$10 (the 10%).
This massive energy loss explains why the base of a food web (the producers) must be so much larger than the top (the apex consumers).
IV. Visualising Relationships: Pyramids of Numbers and Biomass
Pyramids are used to visualise the structure of an ecosystem, usually by showing the size of each trophic level relative to the others.
1. Pyramids of Numbers
These pyramids show the total number of individual organisms at each trophic level. They are drawn to scale, with the producer level forming the largest base.
- The number of organisms generally decreases at each level as energy is lost.
The Problem with Numbers: Pyramids of numbers can sometimes look strange, or even be inverted (upside down), if the producer is very large.
Example of an Inverted Pyramid of Numbers:
One large Oak Tree (Producer) ← hundreds of Caterpillars (Primary Consumers) ← dozens of Birds (Secondary Consumers).
Because one Oak Tree supports many caterpillars, the base of the pyramid (the Tree) is smaller than the level above it. This makes the pyramid look irregular, which is why scientists prefer pyramids of biomass.
2. Pyramids of Biomass
Biomass is the total mass of living material (usually measured as dry mass) at each trophic level.
- Pyramids of biomass provide a much clearer picture of the feeding relationships because they show the actual amount of biological matter available.
- Because energy is always lost at each step (the 10% rule), the total biomass at each successive level must get smaller.
- Rule: Pyramids of biomass are almost always pyramid-shaped (broad base, narrow top).
Summary Comparison:
Pyramid of Numbers: Counts individuals; can be inverted.
Pyramid of Biomass: Measures total mass; nearly always a regular pyramid shape because energy loss is measured.
Final Encouragement: You've covered the core concepts of energy flow! Remember the 10% rule and the direction of the arrow, and you'll be set for any exam question on food chains and webs!