Ecosystems, food webs & energy flow

Year 7 answers

Fluency

Habitat: where an organism lives. Population: all individuals of one species in an area. Community: all populations living together. Ecosystem: community plus abiotic environment.
Biotic (pond): fish, algae, frogs, insects. Abiotic: water, temperature, light, dissolved oxygen, pH.
A producer makes its own food from sunlight or chemicals. Examples: grass, algae, gum tree.
Decomposers break down dead organic matter and return nutrients to the soil. Examples: fungi, bacteria, earthworms.
Producer: grass. Primary consumer: rabbit. Secondary consumer: fox.
The arrow shows the direction energy flows — from the food to the feeder (prey to predator).
About 10% of the energy at one trophic level is transferred to the next; 90% is lost mostly as heat.
Energy decreases by roughly 90% each link, so after 4 or 5 steps there is not enough to support another level.
Biotic: trees, birds, insects. Abiotic: sunlight, rainfall, temperature, soil type.
Examples: cane toads, rabbits, foxes, European carp, lantana.

Reasoning

Grasshoppers: $500$ kg. Lizards: $50$ kg. Eagles: $5$ kg.
Because only about 10% of energy transfers up each level, the total biomass shrinks by 90% per step. Top predators have very little energy to share.
Birds lose insect food → population drops. Pesticide may also accumulate in birds’ bodies through the insects they do eat, causing further harm (bioaccumulation).
Arrows represent energy flow from prey to predator. Reversed arrows would say the predator feeds the prey, which is biologically false and would mislead anyone trying to predict effects of change.
Short droughts stress organisms but do not remove species. A keystone species — one whose role disproportionately affects the web — removes many connections at once, so recovery may be impossible without reintroduction.
They recycle nutrients (carbon, nitrogen, phosphorus) locked in dead matter back into the soil for producers. Without them, dead material would pile up, nutrients would be trapped, and producers would starve.

Reasoning

Accept any web with at least 6 organisms and arrows pointing prey → predator. Example: grass → cricket → magpie → cat; grass → snail → magpie; leaves → possum.
Abiotic: warmer water holds less dissolved oxygen; evaporation increases. Biotic: cold-water fish die or migrate; algae grow faster, possibly causing blooms.
Short term: rabbit population surges. Mid term: grass overgrazed, soil erosion, eagles switch to other prey. Long term: grass dies back, rabbits crash from starvation, whole system destabilised.
90% of energy is lost at each trophic step. Feeding grain to cattle first loses that 90% before it reaches humans, so only a small fraction of the original grain’s energy becomes beef.

Reasoning

Fertiliser runoff → algal bloom → algae die and are decomposed → bacteria using the dead algae consume dissolved oxygen → oxygen level in water crashes → fish suffocate. Extra food at the bottom does not help if decomposition strips the oxygen fish need.
IV: presence/absence of introduced fish. DV: native fish population (count per week). Controlled: pond size, water temperature, food added, predators excluded, duration. Use replicate ponds with and without the introduced species; compare native numbers over time.
Each link transfers only about 10% of the energy (producing the pyramid), but toxins like mercury are not broken down — they are stored in body tissue. A predator eats many prey, so it collects all their mercury, multiplying the concentration even as the energy thins.
After 1 year: dead plant and animal matter accumulates, nutrient cycling stops, soil becomes impoverished. After 10 years: producers decline from lack of nutrients, herbivores starve, chains collapse upward. After 100 years: the forest is gone, replaced at best by pioneer species that can tolerate nutrient-poor soil; long term, some decomposition may restart from airborne microbes.