Unit Review - Ecosystems: Interactions and Energy
Grade 8 Science Unit Review
Learning Objectives
Describe how water is distributed on Earth and explain why freshwater is a limited resource
Explain the processes of the water cycle and the concept of residence time
Describe how surface and deep ocean currents form and affect climate, including the Gulf Stream, El Nino, and upwelling
Explain how groundwater forms and moves, describe watersheds, and analyze human impacts on water quality
Model energy flow through food webs, apply the 10% rule, and explain why there are fewer apex predators
Introduction
~1 minutesUnit Review: Ecosystems - Interactions and Energy
This review covers everything we learned about Earth's water systems and ecosystem energy flow. Use it to prepare for your unit assessment.
Review: Water Distribution
~4 minutesTopic 1: Water Distribution
Earth is 71% water-covered, but most is NOT usable:
• Oceans (saltwater): 96.5%, too salty
• Ice caps & glaciers: 1.74%, frozen
• Groundwater: ~1%, underground
• Surface freshwater (lakes, rivers): <0.01%
Only about 0.5-1% of Earth's total water is readily accessible freshwater.
Analogy: If all Earth's water were 100 liters (a bathtub), only about 2 tablespoons would be the rivers and lakes we depend on.
Review: The Water Cycle
~5 minutesTopic 2: The Water Cycle
The hydrologic cycle is powered by the Sun. Six major processes:
• Evaporation: Liquid → gas (absorbs energy)
• Transpiration: Plants release water vapor (a large tree: 400+ liters/day)
• Condensation: Vapor → liquid droplets (releases energy, forms clouds, powers storms)
• Precipitation: Rain, snow, sleet, or hail
• Infiltration: Water soaks into ground
• Runoff: Water flows over land to streams/rivers
Residence Time: How long water stays in a reservoir. Atmosphere = 9 days. Rivers = 2-6 months. Groundwater = 100-10,000+ years.
Why groundwater pollution is critical: It persists for centuries. Prevention >> cleanup.
Review: Ocean Currents
~5 minutesTopic 3: Ocean Currents
Surface currents are driven by: wind, the Coriolis effect (curves right in N. Hemisphere, left in S.), and continental boundaries. They form gyres (circular patterns).
The Gulf Stream: Moves 100x the Amazon's flow. Keeps Western Europe 5-10°C warmer than expected.
Deep currents (thermohaline circulation): Driven by density differences (cold + salty = dense = sinks). The global conveyor belt takes ~1,000 years for one circuit.
Climate effects: Warm currents → warmer/wetter coasts. Cold currents → cooler/drier coasts. Upwelling brings nutrient-rich deep water up → great fishing.
El Niño: Trade winds weaken, warm water spreads east. La Niña: Trade winds strengthen, cold water dominates.
Review: Groundwater & Watersheds
~4 minutesTopic 4: Groundwater & Watersheds
Porosity = empty space. Permeability = how easily water flows through. Clay: HIGH porosity but LOW permeability (tiny pores, poorly connected).
Underground structures: Zone of Aeration → Zone of Saturation → Water Table (boundary) → Aquifer (holds usable groundwater)
Watersheds: All land draining into the same river. Bounded by drainage divides. Everything upstream affects everything downstream.
Human impacts: Impervious surfaces increase runoff. Agriculture adds fertilizer/pesticide runoff. Two types:
• Point source: Specific location (factory pipe), easier to regulate
• Non-point source: Diffuse (runoff), harder to control
Review: Food Webs & Energy Flow
~4 minutesTopic 5: Food Webs & Energy Flow
Energy enters most ecosystems as sunlight. Producers capture it via photosynthesis: 6CO2 + 6H2O + light → C₆H₁2O₆ + 6O2
Roles: Producer → Primary Consumer → Secondary Consumer → Tertiary Consumer; Decomposers recycle nutrients.
Two rules: Energy flows ONE way (sunlight → heat). Matter CYCLES (recycled).
The 10% Rule: Only ~10% of energy passes to the next trophic level. The rest is lost as heat.
Energy pyramid: 10,000 J → 1,000 J → 100 J → 10 J
This is why there are fewer apex predators, less energy available at the top.
Food web arrows point FROM what's eaten TO what eats it (direction of energy flow).
Unit Assessment
~22 minutesWhat percentage of Earth's total water is salty ocean water?
Where is MOST of Earth's freshwater located?
The vast majority of Earth's water is NOT usable for drinking, agriculture, or industry.
What provides the energy that powers the water cycle?
When water vapor condenses to form clouds, it:
Match each water cycle process with its description:
Why is groundwater pollution considered more serious than air pollution?
Put these water reservoirs in order from SHORTEST residence time to LONGEST:
[Unsupported question type: ordering]
What is the PRIMARY force that drives surface ocean currents?
The Coriolis effect causes ocean currents in the Northern Hemisphere to:
What drives deep ocean currents (thermohaline circulation)?
The Gulf Stream keeps Western Europe warmer than expected because:
Upwelling is beneficial for fisheries because:
Put these steps of the global conveyor belt in correct order:
[Unsupported question type: ordering]
What is the difference between porosity and permeability?
Clay has high porosity but low permeability. Why?
What defines the boundaries of a watershed?
Which type of pollution is HARDER to control and regulate?
How do impervious surfaces (pavement, buildings) affect watersheds?
Which are functions of a HEALTHY watershed? (Select ALL that apply)
Select all that apply.
In most ecosystems, what is the original source of the energy stored in food?
In a food web, an arrow from grass to rabbit shows:
Photosynthesis converts ___1___ energy into ___2___ energy stored in food.
Match each role with its function in energy flow:
If a producer level contains 5,000 Joules of energy, how much energy is available to the secondary consumers?
Why do food webs usually have fewer apex predators than herbivores?
Place these organisms in the correct order of energy flow:
[Unsupported question type: ordering]
Decomposers return nutrients to the environment that producers can use again.
If wolves are removed from an ecosystem, what is the most likely result?
Which best defines a biome?