Ecosystems

CORE IDEA An ecosystem is a functional system formed by organisms and their physical environment, linked by energy flow, food webs, decomposition, and nutrient cycling. Scale is flexible: a rotting log, pond, forest, watershed, or ocean basin may all be studied as ecosystems.

1. The ecosystem framework

Biotic components: producers, consumers, decomposers, detritivores, pathogens, and their interactions.

Abiotic components: light, temperature, water, atmosphere, salinity, soil or sediment, nutrients, topography, and disturbance.

Energy flows one way: sunlight (or chemical energy) enters, passes through trophic levels, and is ultimately dissipated as heat.

Matter cycles: water and elements such as carbon, nitrogen, and phosphorus are repeatedly transformed and reused.

Controls and feedbacks: climate and resources constrain organisms; organisms also reshape soils, water, fire regimes, and local climate.

Do not confuse these terms

Term	Meaning	Useful distinction
Habitat	The place and conditions used by a species or population.	Species-centered: a forest contains many habitats.
Community	All interacting populations in an area.	Living component only; it omits the physical environment.
Ecosystem	Community plus abiotic environment and their processes.	Functional and scale-flexible.
Biome	Very large climatic region characterized mainly by vegetation and climate.	A biome contains many ecosystems; boundaries and classifications vary.
Biosphere	All ecosystems and all regions of Earth where life exists.	The global ecological system.

2. Major terrestrial systems

System	Climate / water	Dominant producers	Productivity	Biodiversity	Signature process	Major pressures
Tropical forest	Warm; generally wet	Trees; layered canopy	Very high	Very high	Rapid cycling; nutrients often held in biomass	Deforestation, drought, fire
Temperate forest	Seasonal; moderate rain	Deciduous and/or conifer trees	Moderate-high	High	Pronounced seasons; fertile to acidic soils	Logging, fragmentation, pests, altered fire
Boreal forest (taiga)	Long cold winter	Cold-tolerant conifers	Low-moderate	Moderate	Large soil/peat carbon stores; slow decay	Warming, fire, insects, logging
Grassland / savanna	Seasonal rain; dry periods	Grasses, forbs; scattered trees in savanna	Moderate-high	Moderate-high	Fire and grazing maintain openness; deep roots build soil	Conversion, overgrazing, woody encroachment
Shrubland / Mediterranean	Mild wet winter; hot dry summer	Sclerophyll shrubs and small trees	Moderate	Moderate-high	Frequent drought; many fire adaptations	Urbanization, invasive plants, altered fire
Desert	Very low or unpredictable rain	Sparse drought-adapted plants	Low	Often moderate, specialized	Water pulses drive activity; biological soil crusts important	Water withdrawal, off-road disturbance, warming
Tundra / alpine	Cold; short growing season	Low shrubs, sedges, mosses, lichens	Low	Low-moderate	Permafrost common in Arctic; slow decomposition	Rapid warming, thaw, development

What chiefly separates terrestrial systems?

Water availability and temperature set the broad limits. Seasonality, soils, fire, herbivory, elevation, and land use then determine which vegetation structure persists. Boundaries are usually transition zones (ecotones), not sharp lines.

3. Major freshwater, wetland, and marine systems

System	Water setting	Primary controls	Production base	Characteristic services / roles
Streams and rivers	Fresh; flowing	Current, oxygen, channel form, watershed inputs	Algae, aquatic plants; large terrestrial subsidies	Transport, migration corridors, water supply
Lakes and ponds	Fresh; standing	Depth, light, temperature stratification, nutrients	Phytoplankton and rooted plants	Water storage, fisheries, recreation
Freshwater wetlands	Saturated or flooded soils	Hydroperiod and low-oxygen soils	Emergent plants, shrubs, trees, algae	Flood storage, water-quality improvement, habitat
Estuaries and salt marshes	Brackish; tidal	Salinity gradient, tides, sediment, nutrient inputs	Marsh plants, phytoplankton, algae	Nursery habitat, filtration, shoreline protection
Mangroves and seagrass beds	Coastal; saline; shallow	Light, wave energy, sediment, temperature	Mangrove trees; submerged flowering plants	Blue-carbon storage, nurseries, sediment stabilization
Coral reefs	Marine; shallow, clear water	Light, temperature, water chemistry, low nutrients	Coral-algal symbioses and algae	Exceptional habitat complexity, fisheries, wave buffering
Kelp forests	Marine; cool, nutrient-rich coast	Light, nutrients, waves, grazing	Large brown algae	High production, habitat, coastal food webs
Open ocean (pelagic)	Marine; water column	Light, nutrients, currents, depth	Microscopic phytoplankton	Global climate regulation, oxygen production, fisheries
Deep sea / benthic	Marine; dark, cold, high pressure	Organic fall, sediment, chemistry; vents locally	Mostly detritus-based; chemosynthesis at vents	Nutrient regeneration, carbon burial, unique biodiversity

Aquatic contrast in one line

Terrestrial ecosystems are commonly classified by climate and vegetation; aquatic ecosystems are more often separated by salinity, flow, depth, light, temperature, nutrients, oxygen, substrate, and connection to land.

4. Compare and contrast: patterns that matter

Comparison	Typical examples	Why / caution
Highest biodiversity	Often tropical forests and coral reefs	Warmth and structural complexity create many niches, but diversity depends on scale and taxonomic group.
Highest productivity per unit area	Often wetlands, estuaries, reefs, and some forests	Abundant light, water, nutrients, or tight recycling support rapid production.
Lowest productivity	Deserts, tundra, and much of the open ocean	Water, temperature, or nutrients limit primary production.
Largest spatial extent	Open ocean	Large area does not mean high productivity per square metre.
Strongest soil-building role	Grasslands and long-lived wetlands/peatlands	Dense roots or slow decomposition can accumulate organic matter and carbon.
Most disturbance-shaped	Grasslands, savannas, shrublands, floodplains, deltas	Fire, grazing, floods, storms, and sediment movement can maintain system identity.
Most land-water integration	Rivers, wetlands, estuaries, and coasts	They receive and transform materials from entire watersheds.
Most directly engineered by people	Croplands, cities, reservoirs, plantations	Inputs and built structures redirect energy, water, nutrients, species, and disturbance.

5. Shared processes; different rates

Primary production: Producers convert light or chemical energy into biomass. Limited by water/temperature on land and often light/nutrients in water.

Food webs: Energy moves through many overlapping feeding pathways; detritus is a major foundation in forests, wetlands, estuaries, and the deep sea.

Decomposition: Microbes and detritivores release nutrients. Warm, moist conditions usually accelerate it; cold, drought, acidity, or oxygen-poor soils slow it.

Succession: Communities change after disturbance. The trajectory depends on legacies, colonists, soils/sediments, repeated disturbance, and climate.

Resilience: A system may absorb disturbance, recover, reorganize, or cross a threshold into a different persistent state.

Connectivity: Organisms, water, sediment, nutrients, pollutants, and heat move among ecosystems; upstream actions often produce downstream effects.

6. Human-modified ecosystems (anthromes)

Cities, croplands, pastures, reservoirs, plantations, and managed forests remain ecosystems—not ecological blanks. They contain food webs, soils, microbes, hydrology, and nutrient cycles, but human decisions dominate many inputs, removals, disturbances, and species filters.

Common strengths: high intended production, concentrated services, engineered water control, and opportunities for restoration and green infrastructure.

Common weaknesses: simplified food webs, fragmented habitat, invasive species, excess nutrients, contaminants, heat, altered flow, and dependence on continued inputs.

Best management lens: treat the site as part of a larger landscape and watershed; protect connectivity, heterogeneity, soils, water, native biota, and natural disturbance where feasible.