Unit 1: The Living World: Ecosystems

Unit 1 covers how ecosystems work. It covers the ways species interact, the world's terrestrial and aquatic biomes, the carbon, nitrogen, phosphorus, and water cycles, primary productivity, and how energy moves through trophic levels and food webs.

1.1 Introduction to Ecosystems

Species in an ecosystem interact in predictable ways, and the exam tests whether you can name the interaction and say who gains and who loses. Symbiosis is a close, long-term interaction between two species. It comes in three types. In mutualism, both species benefit. In commensalism, one benefits and the other is neither helped nor harmed. In parasitism, one benefits at the expense of the other, which is called the host. A predator-prey relationship is not a type of symbiosis. It is a separate interaction in which one organism hunts, kills, and eats another.

Competition is rivalry for limited resources. It happens within a species, called intraspecific competition, or between species, called interspecific competition. Resource partitioning is one way competition eases. Species use resources in different ways, in different places, or at different times, which reduces the negative effect of competition on survival. Warblers feeding at different heights in the same tree are the classic example.

1.2 Terrestrial Biomes

A biome is a large region whose characteristic plant and animal communities result from, and are adapted to, its climate. The major terrestrial biomes to know are taiga, temperate rainforest, temperate seasonal forest, tropical rainforest, shrubland, temperate grassland, savanna, desert, and tundra. The distribution of terrestrial resources like water and lumber trees is not random. It varies with climate, geography, latitude and altitude, nutrient availability, and soil.

Biome boundaries are not fixed. Biome shifts happen because the worldwide distribution of biomes is dynamic. It has changed in the past, and it may shift again as the global climate changes. A warming climate can push the taiga poleward and shrink the tundra, which is the kind of directional shift the exam asks about.

1.3 Aquatic Biomes

Freshwater biomes are streams, rivers, ponds, and lakes. They are a vital resource for drinking water. Marine biomes are the oceans, coral reefs, marshland, and estuaries. Algae in marine systems supply a large portion of Earth's oxygen and also absorb carbon dioxide from the atmosphere, which ties this topic directly to the carbon cycle in topic 1.4.

The distribution of marine resources such as fish varies with salinity, depth, turbidity, nutrient availability, and temperature. Notice how this list differs from the terrestrial list. Salinity, depth, and turbidity replace latitude and soil, because light and dissolved conditions are what structure life in water.

1.4 The Carbon Cycle

The carbon cycle is the movement of carbon atoms and molecules between sources and sinks. A source is a process or location that releases carbon, and a sink, also called a reservoir, stores it. Some reservoirs hold carbon for long periods, like fossil fuels and ocean sediments, while others hold it for short periods, like living organisms and the atmosphere.

The biological engine of the cycle is the pair of photosynthesis and respiration. Photosynthesis removes carbon dioxide from the atmosphere, and cellular respiration releases it back. Over geological time, dead plant and animal material decomposed and stored carbon underground as fossil fuels. Burning those fuels quickly moves millions of years of stored carbon into the atmosphere as carbon dioxide, which is why fossil fuel combustion is a source in this cycle.

1.5 The Nitrogen Cycle

The nitrogen cycle moves nitrogen atoms and molecules between sources and sinks through nitrogen fixation, assimilation, ammonification, nitrification, and denitrification, with soil microorganisms doing much of the work. Nitrogen fixation is the step that makes nitrogen usable. Certain soil bacteria convert atmospheric N2 into ammonia (NH3), which quickly becomes ammonium (NH4+) in soil and is then available for biological uptake.

Two facts about nitrogen show up constantly. First, the atmospheric nitrogen reservoir is the largest nitrogen reservoir on Earth, yet most nitrogen reservoirs hold their compounds for relatively short periods. Second, there is a nitrogen limitation. Nitrogen availability in soil is limited by the rate of nitrogen fixation, and in many ecosystems nitrogen availability limits how much primary production plants and other producers can do.

1.6 The Phosphorus Cycle

The phosphorus cycle moves phosphorus atoms and molecules between sources and sinks. It has one feature that sets it apart from the other cycles. It lacks a significant atmospheric component. Phosphorus does not move through the air the way carbon and nitrogen do. The major phosphorus reservoirs are rock and ocean sediments containing phosphorus-bearing minerals, and phosphorus-bearing minerals are released as rocks weather.

Because rocks weather slowly, phosphorus is relatively scarce in ecosystems. It is often a limiting nutrient for plants and other producers, particularly in freshwater and some terrestrial ecosystems. This is the contrast the exam loves. Nitrogen limitation is common in terrestrial systems and is tied to fixation rates, while phosphorus limitation is common in freshwater systems and is tied to the slow weathering of rock.

1.7 The Hydrologic (Water) Cycle

The hydrologic (water) cycle is the movement of H2O in its solid, liquid, and gaseous phases between sources and sinks. It is powered by the sun, which drives evaporation and transpiration and keeps the cycle moving. The oceans are the primary reservoir of water at Earth's surface, with ice caps and groundwater acting as much smaller reservoirs.

1.8 Primary Productivity

Primary productivity is the rate at which solar energy is converted into organic compounds via photosynthesis over a unit of time. It is measured in energy per unit area per unit time, for example kcal/m2/yr. The two forms of productivity matter more than the units. Gross primary productivity (GPP) is the total rate of photosynthesis in a given area. Net primary productivity (NPP) is the rate of energy storage by photosynthesizers in that area after subtracting the energy lost to respiration. In symbols, NPP = GPP - respiration. NPP is what is actually available to the next trophic level.

Light limits productivity in water. Most red light is absorbed in the upper 1 m of water, and blue light only penetrates deeper than 100 m in the clearest water. Photosynthesizers in aquatic ecosystems have adapted to the lack of visible light at depth, which is why most aquatic photosynthesis happens near the surface.

1.9 Trophic Levels

All ecosystems depend on a continuous inflow of high-quality energy to maintain their structure and to move matter between the environment and organisms through the biogeochemical cycles. Energy enters from the sun and flows to producers in the lowest trophic levels, then upward to primary consumers, secondary consumers, and tertiary consumers. Matter behaves differently. The conservation of matter means that biogeochemical cycles demonstrate that material is neither created nor destroyed as it cycles. Energy flows through an ecosystem once and is lost as heat. Matter cycles.

1.10 Energy Flow and the 10% Rule

The 10% rule says that energy transfer between trophic levels passes on only about one-tenth of the energy to the next level. If producers capture 10,000 kcal/m2/yr, primary consumers get about 1,000, secondary consumers about 100, and tertiary consumers about 10. The rest is lost as heat and used in metabolism along the way. The thermodynamics behind this is the reason the rule exists. Energy moving from lower to higher trophic levels loses usable energy at each step, as the laws of thermodynamics require.

This is why food chains are short and why eating lower on the trophic pyramid feeds more people. Each added level throws away roughly 90 percent of the energy that entered it.

1.11 Food Chains and Food Webs

A food chain traces the flow of energy and matter from producers (autotrophs) to primary consumers (herbivores) and on to secondary and tertiary consumers (omnivores and carnivores). A food web is the more realistic model. It depicts the interlocking pattern of energy and matter flow in two or more feeding sequences. Real ecosystems are webs, because most organisms eat and are eaten by more than one species.

Detritivores and decomposers close the loop. They return nutrients to the soil and play an essential role in food chains and webs. Without them, the matter in dead organisms would stay locked up instead of cycling back to producers.

Populations in a web push and pull on each other through feedback loops. A change in one species' population feeds back through the feeding links. A negative feedback loop dampens the change, as when predators decline because their prey declined. A positive feedback loop amplifies it. Removing a predator can let prey explode, which is a change that reinforces itself through the web.

Confusions That Cost Points

Practice Questions

Original questions written for this guide in the style of the AP exam. Answers and explanations are on the next page, so complete the questions before checking them.

Answer Key

1. B. The fish benefits and the anemone is unaffected, which is the definition of commensalism. A is wrong because mutualism requires both species to benefit, and the anemone gains nothing. C is wrong because parasitism requires the second species to be harmed, and the anemone is not. D is wrong because neither organism kills and eats the other, and predator-prey is not a type of symbiosis.

2. C. The 10% rule applies at each transfer. Producers hold 20,000, so primary consumers get about 2,000, and secondary consumers get about 200. B stops after one transfer and reports the primary consumer level. D applies the rule three times, which would be the tertiary consumer level. A ignores the rule entirely.

3. B. Phosphorus reservoirs are rock and ocean sediments, and phosphorus enters ecosystems as rocks weather. It has no significant gaseous phase. A describes denitrification, which releases nitrogen (N2), not phosphorus, into the atmosphere. C confuses phosphorus with carbon, which photosynthesizers do take from the atmosphere as CO2. D describes the carbon cycle's photosynthesis-respiration loop.

4. D. NPP = GPP - respiration, so 12,000 - 7,000 = 5,000 kcal/m²/yr. A adds respiration to GPP instead of subtracting it. B reports GPP rather than NPP. C reports the respiration value alone, which is what is subtracted, not what remains.

One-Page Recall Check

• Define symbiosis and name its three types, stating who benefits and who is harmed in each.
• Explain why a predator-prey relationship is not classified as symbiosis.
• Distinguish intraspecific from interspecific competition and explain how resource partitioning reduces competition.
• List the major terrestrial biomes and the factors that determine the distribution of terrestrial resources.
• Explain what a biome shift is and what can cause one.
• Distinguish freshwater from marine biomes and name the factors that determine marine resource distribution.
• Define a carbon source and a carbon sink, and explain how burning fossil fuels moves carbon between them.
• Describe the steps of the nitrogen cycle and explain why fixation is the bottleneck for plant nitrogen uptake.
• Explain why phosphorus has no significant atmospheric component and where its major reservoirs are.
• State what powers the hydrologic cycle and name the primary water reservoir.
• Define GPP and NPP, give the equation that relates them, and state the units of productivity.
• Explain how light penetration limits aquatic photosynthesis.
• Contrast the direction of energy flow with the cycling of matter.
• Apply the 10% rule across two trophic transfers and explain the thermodynamic reason for the loss.
• Distinguish a food chain from a food web and explain the role of detritivores and decomposers.
• Define negative and positive feedback loops in a food web.

Key terms for this unit

Predator-prey relationship, Symbiosis, Mutualism, Commensalism, Parasitism, Competition, Resource partitioning, Biome, Major terrestrial biomes, Distribution of terrestrial resources, Biome shifts, Freshwater biomes, Marine biomes, Distribution of marine resources, Carbon cycle, Carbon sources and sinks, Photosynthesis and respiration, Fossil fuel carbon release, Nitrogen cycle, Nitrogen fixation, Nitrogen limitation, Atmospheric nitrogen reservoir, Phosphorus cycle, Phosphorus reservoirs, Phosphorus as a limiting nutrient, Hydrologic (water) cycle, Ocean water reservoir, Primary productivity, Gross primary productivity (GPP), Net primary productivity (NPP), Units of productivity, Light and aquatic photosynthesis, Energy inflow requirement, Conservation of matter, Direction of energy flow, 10% rule, Thermodynamics and energy loss, Food chain, Food web, Detritivores and decomposers, Feedback loops in food webs.

About this guide. Written for Rycal and aligned to the College Board AP Environmental Science course framework, Unit 1. All questions and explanations are original Rycal writing. Rycal is independent and is not affiliated with or endorsed by the College Board.