Lesson 3 of 6 · Biosphere & ecosystems
Life within a changing environment
7 minute lesson · 3 checks along the way
Your goal for this lessonDistinguish material cycling from energy flow and choose evidence for a proposed change in an ecosystem.
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Life belongs inside the system
The includes life across Earth, from organisms in soils to microscopic life in seawater. An brings a community of organisms together with its physical and chemical environment. Water availability, temperature, light, and nutrients influence what can live in a place. Organisms also change their surroundings through processes such as growth, respiration, decomposition, and the movement of soil and water.
These interactions do not require a planet with intentions or a community planning for balance. Organisms respond, compete, cooperate, reproduce, and die under particular conditions. A population can grow, shrink, or move; an ecosystem can change substantially. Humans belong within these relationships. Agriculture, fishing, habitat alteration, and pollution affect other organisms and the environmental processes on which people themselves depend.
Follow matter and energy separately
Photosynthetic organisms use light energy to build organic molecules from carbon dioxide and water. Plants and many microscopic aquatic organisms support food webs this way. Consumers obtain material and usable chemical energy by feeding. Decomposers transform dead material and wastes, returning nutrients to the surroundings. Plants respire too: photosynthesis and respiration are different processes, not separate labels for plants and animals.
Atoms can be reused as matter moves among organisms, water, air, and soil. Energy does not make the same reusable circuit. Energy entering an ecosystem is transformed and transferred, with increasing amounts becoming dispersed heat; continued biological activity needs continuing supplies. Sunlight supports most familiar food webs, while some organisms use chemical energy. A food-web arrow needs a stated meaning so a diagram does not confuse matter transfer with a claim of perfect energy recycling.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
Work through a decision
Investigate a green lake
A satellite image shows a green patch in a lake, and its source identifies an algal bloom. A team proposes that nutrient-rich runoff contributed to the growth. Choose evidence to test that explanation: nutrient measurements entering the lake, rainfall and river-flow records, repeated observations of the bloom, and samples identifying the organisms. Color alone cannot determine which nutrient sources supplied the water or whether a particular toxin is present.
Compare the proposed sequence with the timing. Did increased nutrient delivery precede the bloom, or did the bloom begin earlier? Were light, temperature, and mixing also changing? An investigation should allow several influences instead of treating one correlation as a complete explanation. A photograph helps locate the visible pattern, while measurements of water and organisms test the processes that might have produced it.
Lake Erie’s algal bloom in a true-color Suomi NPP satellite image from August 16, 2015. The NOAA observation shows the bloom’s visible pattern, not a photograph of the satellite itself. Its color alone does not identify every organism, toxin, nutrient source, or oxygen concentration.
Look closely
Compare the green western water with the darker part of the lake. What would you measure next to connect this visible difference with the conditions experienced by fish?
See what to notice
Measure dissolved oxygen at relevant locations and depths, and identify organisms and water conditions. The image helps locate a pattern but does not supply all of those measurements. Repeated observations and samples can test how growth, decomposition, circulation, and oxygen are connected.
Check 2 of 3
10 pointsUse the reading above. You can try again without losing points.
More growth is not always better
When a large amount of organic material dies and is decomposed, organisms carrying out respiration can consume dissolved oxygen. If oxygen is used faster than it is resupplied, aquatic animals may lose suitable habitat. This connects life to water circulation and chemistry. Not every bloom produces toxins, and a bright green photograph does not by itself measure oxygen concentrations below the surface.
Before deciding that an ecosystem is healthy or damaged, identify the question and relevant observations. Measures might include oxygen, species composition, growth, or the persistence of suitable habitat. No single color stands for all of them. Explain how an atom in an organism could return to its surroundings, then explain why the energy supporting that organism does not simply circulate forever in the same usable form.
Check 3 of 3
10 pointsUse the reading above. You can try again without losing points.
Put it together
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