Types of Ecosystems

Why This Matters

Understanding ecosystem types isn't just about memorizing a list of environments—it's about recognizing the underlying principles that determine how energy flows, how nutrients cycle, and how organisms interact with their physical surroundings. On the AP exam, you'll be tested on concepts like primary productivity, biogeochemical cycles, biodiversity patterns, and human impacts, and ecosystems are the stage where all of these processes play out. When you understand why a tropical rainforest stores carbon differently than a tundra, you're demonstrating the kind of systems thinking that earns high scores.

Each ecosystem type represents a unique combination of abiotic factors (climate, water availability, soil type) and biotic communities that have evolved together. The exam loves to ask you to compare ecosystems, explain why certain adaptations exist, or predict how environmental changes will affect ecosystem services. So don't just memorize that wetlands filter water—know why they do it and how that connects to nutrient cycling. Master the mechanisms, and you'll be ready for any question they throw at you.

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Terrestrial Ecosystems: Climate Determines Community

Land-based ecosystems are shaped primarily by temperature and precipitation patterns, which determine what vegetation can survive and, in turn, what animals can thrive. The interaction between climate and geography creates distinct biomes, each with characteristic productivity levels and species adaptations.

Forest Ecosystems

• Highest terrestrial biodiversity and productivity—dense vegetation creates complex food webs and multiple habitat niches from forest floor to canopy
• Major carbon sinks through photosynthesis and biomass storage; tropical forests alone store approximately 25% of terrestrial carbon
• Three main types: tropical (high rainfall, year-round warmth), temperate (seasonal variation, deciduous or mixed), and boreal/taiga (cold, coniferous)—each with distinct nutrient cycling rates

Grassland Ecosystems

• Dominated by grasses due to insufficient rainfall for trees—typically 25-75 cm annual precipitation creates fire-adapted communities
• Critical for agriculture as fertile soils develop from deep grass root systems and organic matter accumulation
• Types include savannas (tropical, scattered trees), prairies (temperate, tallgrass), and steppes (semi-arid, shortgrass)—all support large grazing herbivores

Desert Ecosystems

• Defined by aridity (less than 25 cm annual precipitation), not temperature—includes both hot deserts (Sahara, Sonoran) and cold deserts (Gobi, Atacama)
• Specialized adaptations like succulence, deep root systems, nocturnal behavior, and water-conserving metabolisms (CAM photosynthesis in plants)
• Low net primary productivity but high resilience; organisms demonstrate extreme examples of evolutionary adaptation to limiting factors

Tundra Ecosystems

• Permafrost defines this biome—permanently frozen soil layer restricts root depth and decomposition rates
• Short growing season (50-60 days) limits plant growth to low shrubs, mosses, lichens, and grasses; trees cannot establish
• Massive carbon storage in frozen soils; thawing permafrost releases methane and $CO_2$, creating a dangerous positive feedback loop for climate change

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Aquatic Ecosystems: Water as the Medium for Life

Aquatic ecosystems cover approximately 75% of Earth's surface and are distinguished by water chemistry, depth, flow rate, and salinity. The availability of light and nutrients determines productivity zones, while water's high heat capacity makes these systems crucial for global climate regulation.

Freshwater Ecosystems

• Include lentic (still) and lotic (flowing) systems—lakes, ponds, rivers, and streams each have distinct oxygen levels, nutrient dynamics, and community structures
• Critical for human water supply and support disproportionate biodiversity relative to their small global coverage (less than 1% of Earth's surface)
• Highly vulnerable to pollution because they concentrate runoff from surrounding watersheds; eutrophication from excess nitrogen and phosphorus is a major threat

Marine Ecosystems

• Cover 71% of Earth's surface and produce approximately 50% of global oxygen through phytoplankton photosynthesis
• Stratified by light penetration: photic zone (sunlit, photosynthesis occurs), aphotic zone (dark, chemosynthesis or detritus-based food webs)
• Ocean currents regulate global climate by distributing heat; also serve as the largest carbon sink, absorbing roughly 30% of anthropogenic $CO_2$

Wetland Ecosystems

• Transitional zones where terrestrial and aquatic systems meet—defined by hydric soils saturated with water for significant periods
• Ecosystem services powerhouses: filter pollutants, buffer floods, recharge groundwater, and provide critical breeding habitat for fish and waterfowl
• Include marshes (herbaceous plants), swamps (woody plants), and bogs (acidic, peat-forming)—each with distinct hydrology and species composition

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Transitional and Human-Modified Ecosystems

Some ecosystems don't fit neatly into terrestrial or aquatic categories, or they've been fundamentally altered by human activity. These systems reveal how ecological principles operate under stress and how humans can either degrade or enhance ecosystem services.

Coral Reef Ecosystems

• Built by coral polyps secreting calcium carbonate skeletons; require warm, clear, shallow water with specific salinity and light conditions
• "Rainforests of the sea" supporting approximately 25% of marine species despite covering less than 1% of ocean floor—extreme biodiversity in nutrient-poor waters
• Highly threatened by ocean acidification ($CO_2$ absorption lowers pH, dissolving carbonate structures), warming (coral bleaching), and sedimentation from coastal development

Estuarine Ecosystems

• Where rivers meet the sea—brackish water with fluctuating salinity creates challenging but productive conditions
• Nursery habitats for commercially important fish and shellfish; high nutrient input from rivers supports exceptional productivity
• Vulnerable to upstream pollution, sea level rise, and coastal development; serve as natural buffers against storm surge

Urban Ecosystems

• Human-dominated systems where built infrastructure interacts with remnant natural elements like parks, street trees, and urban wildlife
• Heat island effect demonstrates altered energy balance; impervious surfaces change hydrology and increase runoff pollution
• Opportunities for sustainability: green infrastructure (green roofs, rain gardens) can restore ecosystem services like stormwater management and habitat connectivity

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Quick Reference Table

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Self-Check Questions

1. Which two ecosystem types store the most carbon, and how do their storage mechanisms differ (biomass vs. soil vs. water)?

2. Compare coral reef and tropical forest ecosystems: what do they share in terms of biodiversity patterns, and why are both considered highly vulnerable to climate change?

3. A river carries agricultural runoff into a coastal wetland and eventually an estuary. Trace how excess nitrogen would affect each ecosystem along this path—which ecosystem services might mitigate the damage?

4. If an FRQ asked you to explain how climate determines biome distribution, which three terrestrial ecosystems would best illustrate the relationship between precipitation, temperature, and vegetation type?

5. Urban ecosystems and wetlands seem very different, but both demonstrate how ecosystems can provide services to human communities. Compare one service each provides and explain the underlying ecological mechanism.