School › Ecosystems › Ages 15–16
Energy flow and interactions in ecosystems
Trophic levels, the 10% rule and the Silver Springs data; why food chains are short; trophic cascades and their uncertainty; and a field investigation linking abiotic factors to plant distribution.
Read
An ecosystem consists of biotic factors (organisms) and abiotic factors such as temperature and humidity, all depending on each other directly or indirectly. Ecosystems range from tide pools to large biomes; the five main types of biome are desert, aquatic, tundra, grassland and forest.[1]
Energy enters almost every ecosystem as sunlight, captured by producers (autotrophs). It then passes along trophic levels: primary consumers eat producers, secondary consumers eat primary consumers, and so on up to apex predators.[2][3]
On average only about 10% of the energy at one trophic level passes to the next. The rest is lost: much escapes as heat during respiration, some food is indigestible and leaves as waste, and some organisms die without being eaten and are broken down by decomposers.[2]
Real ecosystems vary. In the Silver Springs study in Florida, producers captured 20,810 kcal/m²/yr (gross primary productivity), but 13,187 kcal/m²/yr was used in respiration or lost as heat. The transfer efficiency between the first two trophic levels was about 14.5%.[3]
Because energy is lost at each step, after four to six transfers there is not enough energy left to support another trophic level. A pyramid of energy is therefore always upright.[3][2]
Interactions can spread through an ecosystem in a trophic cascade. After wolves returned to Yellowstone, elk became scarce where wolves concentrated and willows, cottonwoods and aspens recovered, helping beavers. But after about ten years bears, cougars and bison made the picture more complicated, and outside the park humans remain the dominant influence.[4]
Try it: Abiotic factors and plant distribution
You need
- Frame quadrats (0.5 m × 0.5 m)
- Two long tape measures
- Random number tables or app
- Light meter or a phone light-meter app
- Thermometer
- Soil moisture meter (or a soil sample and a balance)
- Plant identification key
- Recording sheet
Safety
Teacher checks the site and does a risk assessment. Wear long sleeves and trousers (brambles, nettles, ticks) and sturdy shoes, watch for branches at eye level and uneven ground, never work alone, wash hands after handling soil and plants. Place quadrats; never throw them.
What to do
- Choose two contrasting areas, for example ground under trees and open grass nearby. Write a hypothesis linking an abiotic factor to the plants.
- In each area use two tapes at right angles and random numbers to place quadrats.
- In each quadrat record the plant species present and estimate their abundance (number of individuals or percentage cover).
- At the same spot measure light intensity, air or soil temperature and soil moisture. Take all readings within a short time, so the weather changes as little as possible.
- Take at least 10 quadrats in each area (10–30 is better for statistics).
- Calculate means for each area, draw a chart, and discuss whether the abiotic differences could explain the plant differences.
What happens, and why?
Abiotic factors such as temperature affect which plants can grow in an ecosystem. Sampling two contrasting areas randomly, with enough quadrats in each (often 10–30 for statistical tests), lets you compare them fairly. But many factors differ between the two areas at once (light, moisture, trampling, soil), so a field study shows a correlation, not proof of cause.[1][6]
Play: Where does the energy go?
Does this energy pass on to the next trophic level, or is it lost from the food chain?
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6–7 · Ages 10–11