Chapter 5 Notes: Ecosystems to Biomes

Ecosystems

• Ecosystems participate in core processes: photosynthesis, respiration, energy flow, and nutrient cycling.

• Humans rely on ecosystems and assign value to ecosystem goods and services.

• Ecosystems contain communities of interacting species and their abiotic factors.

• Range of tolerance and limiting factors control species distribution.

• Predictable patterns of ecosystem distribution around the world.

Biomes

• Biomes are ecosystems having similar vegetation and climactic conditions.

Trophic levels

• In photosynthesis, plants use light energy, CO₂, and H₂O to produce organic molecules (sugar).

• Compounds and nutrients move up trophic levels (feeding levels) to other organisms.

• Trophic level: feeding level defined by primary source of energy.

• Trophic levels show the movement of energy and materials through the food web.

Food chains and Food webs

• Food chain: describes where energy and nutrients move with respect to one organism to another; energy moves upward along the chain.

• Not all energy and nutrients reach higher trophic levels.

• Food web: interconnection of food chains forming a complex feeding network; usually prominent in aquatic systems.

Aquatic food webs (example components)

• Base/primary producers: phytoplankton, diatoms (ice biota).

• Primary consumers: herbivorous zooplankton, small fish.

• Secondary consumers: carnivorous zooplankton, Arctic cod, carnivorous invertebrates.

• Higher-level consumers: arctic cod, bearded seal, ringed seal, bowhead whale, beluga whale, polar bear, Arctic fox, Arctic terns, loon, sea ducks.

• Key connections: diatoms/phytoplankton → zooplankton → small fish → larger fish/predators; top predators include mammals and seabirds.

Autotrophs, heterotrophs, and functional groups

• Autotrophs: produce organic matter (OM) from inorganic matter using an external energy source.

  • Producers: green plants; photosynthetic and chemotrophic bacteria.

• Heterotrophs: consume OM to obtain energy.

  • Consumers: eat living prey.

  • Decomposers: scavengers, detritus feeders, and chemical decomposers that eat dead OM.

Producers

• Photosynthesis captures light energy to convert CO₂ and H₂O to OM.

• Chlorophyll: green pigment in plant leaves responsible for absorbing light energy.

• Chemosynthesis: bacteria use energy in inorganic chemicals (e.g., H₂S, NH₃) to synthesize OM.

• Primary production: production of OM through photosynthesis and associated growth of organisms/biomass; also termed primary productivity.

Global primary productivity (terrestrial)

• Net Primary Production (NPP): classification of new plant biomass in tons per hectare per year of total dry organic matter (above- and below-ground).

• Categories (approximate): None, Low (1), High (≈15) (illustrative categories used in the slide).

Consumers (by diet)

• Primary consumers (herbivores): eat producers.

• Secondary consumers (carnivores): feed on primary consumers.

• Tertiary consumers and beyond: feed on other consumers.

• Carnivores: secondary or higher-order meat eaters.

• Omnivores: feed on both plants and animals.

Grassland food chain (structure)

• Producers → First-level (primary) consumers → Second-level consumers → Third-level (tertiary) consumers.

Decomposers and detritus

• Decomposers: organisms whose feeding results in decay of OM.

• Detritus: dead plant material, waste, and dead bodies; high in potential energy.

• Energy from detritus is used by:

  • Scavengers (e.g., vultures): break down large pieces of matter.

  • Detritus feeders (earthworms): eat partly decomposed matter.

  • Chemical decomposers (fungi and bacteria): break down molecule-sized matter.

Specialized decomposers

• Some decomposers digest wood (cellulose): termites.

  • Termites maintain mutualistic relationships with gut microorganisms to digest cellulose.

• Cellulose: material in plant cell walls; indigestible by humans; excreted as waste.

• Anaerobic respiration: occurs in absence of oxygen in sediments, lakes, marshes, swamps, and animal guts.

• Fermentation: modified form of cellular respiration used by some decomposers; breaks down glucose without oxygen; produces ethyl alcohol, methane, acetic acid.

Trophic categories (summary)

• Autotrophs: make their own organic matter from inorganic nutrients using environmental energy sources.

• Heterotrophs: must feed on organic matter for energy.

• Producers:

  • Photosynthetic green plants: use chlorophyll to absorb light energy.

  • Photosynthetic bacteria: use light energy.

  • Chemosynthetic bacteria: use high-energy inorganic chemicals (e.g., H₂S).

• Consumers:

  • Primary consumers/herbivores: feed exclusively on plants.

  • Omnivores: feed on both plants and animals.

  • Secondary consumers/carnivores: feed on primary consumers.

  • Higher-order carnivores: feed on other carnivores.

• Parasites: feed on another plant or animal over an extended period.

• Decomposers, scavengers, detritus feeders, chemical decomposers (fungi/bacteria).

Limits on trophic levels

• Terrestrial ecosystems typically have 3–4 trophic levels; marine systems can have up to 5.

• Biomass = total combined (net dry) weight of organisms.

• Rule of thumb: each higher trophic level has about 90% less biomass than the level below it.

  • Example (grassland):

  • Grass biomass (producers) ≈ 100 (units)

  • Herbivores (first-level consumers) ≈ 10

  • Primary carnivores (second-level consumers) ≈ 1

  • Higher carnivores (third-level consumers) ≈ 0.1

• Biomass pyramid: segments show relative biomass at each trophic level.

Biomass pyramid (illustration from slide)

• Producers: 100

• First-level consumers: 10

• Second-level consumers: 1

• Third-level consumers: 0.1

• Note: The pyramid demonstrates the dramatic drop in biomass at higher trophic levels.

Flow of energy in terrestrial ecosystems

• Standing-crop biomass: biomass of primary producers in an ecosystem at a given time (above-ground biomass is often highlighted).

• Biomass and primary production vary greatly by ecosystem type and climate:

  • Forests have large biomass.

  • Grasslands have high primary production.

• Sunlight is the primary energy source for most ecosystems.

• Primary production uses only about 2\% of available solar energy, yielding approximately 120\ \text{gigatons of OM/year}.

Fate of food energy

• About 60-90\% of ingested energy is oxidized for energy needs.

• About 10-40\% is converted to body tissues for growth, repair, and maintenance.

• Undigested food is excreted as waste.

• Excretions include carbon dioxide, nitrogen, phosphorus, and water (via urine).

Energy flow inefficiency and secondary production

• Energy flow between trophic levels is inefficient; only a small percentage is transferred to the next level.

  • Much biomass is not consumed by herbivores.

  • Some energy is used for cellular metabolism.

  • Some is not digested and excreted as waste.

• Secondary production: rate of growth of consumers over time; incorporation of matter and energy from a lower trophic level into a consumer's body (tissue formation/growth).

Trophic inefficiency and bioaccumulation

• Higher trophic levels require more energy for the same amount of tissue.

• More energy, time, water, and resources are needed to produce a consumer than a producer.

• Some materials (heavy metals, pesticides) are hard to degrade and can be excreted slowly; they bioaccumulate within individuals and biomagnify up the food chain.

Aquatic systems: key differences from terrestrial systems

• Aquatic systems follow similar energy flow with two major differences:
  1) More energy is available at each level; energy transfer is more efficient; cold-blooded animals require less energy.
  2) Aquatic systems have a reversed biomass pyramid: smaller biomass of algae and greater biomass of larger fish at higher trophic levels.

• Implications: food chains can be longer; bottom-dwelling organisms (algae/phytoplankton) have shorter lifespans with rapid turnover, while upper-level organisms live longer and may have more biomass per individual.

From ecosystems to biomes

• Ecosystem patterns show predictable populations of organisms under particular conditions.

• Distinct biotic communities characterize different regions.

• Biomes are large geographical terrestrial biotic communities controlled by climate.

• Biomes are often named after dominant vegetation and do not have strict boundaries.

Climate and its effects on biomes

• Climate = average temperature and precipitation (weather) of a region; varies widely.

• Equatorial regions: warm, high rainfall, little seasonality.

• Temperate zones: seasonal temperatures.

• Polar regions: longer and colder winters.

• Average temperature varies with latitude and altitude.

Latitude, altitude, and microclimates

• Latitude increases away from the equator; altitude increases away from sea level.

• Microclimate: environmental conditions in a localized area can differ from regional climate.

• Microclimates create variations of ecosystems within a biome.

Precipitation and species distribution

• Precipitation varies from nearly 0 to over 100 inches per year.

• Distribution can be even or seasonal.

• Climate supports species within their range of tolerance; highest densities occur where conditions are optimal; species are excluded where any condition exceeds their tolerance.

Temperature delineated biomes

• Tropical rainforest: cannot tolerate freezing.

• Deciduous forest: trees drop leaves and go dormant during freezing temperatures.

• Coniferous forests: tolerate harsh winters and short summers (northern regions).

• Permafrost: permanently frozen subsoil, roots cannot penetrate; limits tree growth.

• Tundra: ecosystem of organisms that can live where permafrost persists.

Distribution of terrestrial biomes

• Global map shows latitudinal and altitudinal distribution of biomes (examples include tundra, boreal forests, temperate forests, savannas, deserts, tropical forests).

Aquatic systems: depth, salinity, and permanence

• Aquatic and wetland ecosystems are determined by depth, salinity, and permanence of water.

• Freshwater systems include lakes, marshes, streams, and rivers.

• Mixed gradients exist (estuarine environments) between freshwater and marine systems.

• Marine systems include oceans and coastal regions.

Productivity across biomes

• Biomes differ in primary productivity; tropical rainforests are among the most productive.

• Open oceans contribute a large share of global productivity but individual site productivity is often limited by nutrient availability.

• Estuaries and upwelling zones are particularly productive due to nutrient inputs.

Productivity: examples and rankings (illustrative)

• Open Ocean: relatively high region-wide productivity but low per-area rates in many zones; productivity often limited by nutrients.

• Upwelling zones: among the highest productive marine areas.

• Estuaries: extremely productive due to mixing and nutrient influx.

• Freshwater lakes/streams: substantial productivity, supporting diverse communities.

• Terrestrial productivity varies: dense productivity in tropical rainforests; moderate in temperate forests and grasslands; lower in deserts.

• Notable terrestrial productivity examples (order roughly from high to lower): Estuaries, Upwelling zones, Temperate forests, Tropical rainforests, Grasslands, Deserts, Tundra.

Productivity data (selected examples from the slide)

• Average annual net primary productivity (g m^{-2} yr^{-1}) and % of global surface area (for context):

  • Estuary: approximately $1300$ g m^{-2} yr^{-1}$ (high productivity) and substantial local area.

  • Open Ocean: approximately $125$ g m^{-2} yr^{-1}$ (lower per-area productivity compared with estuaries but vast area).

  • Upwelling zones: around $500$ g m^{-2} yr^{-1}$ (high productivity).

  • Continental shelf: around $360$ g m^{-2} yr^{-1}$.

  • Lake and stream: around $250$ g m^{-2} yr^{-1}$.

  • Boreal forest / temperate forests: several g m^{-2} yr^{-1} (typical values lower than large open-water zones per unit area but extensive).

  • Tropical rainforest: among the higher rainforest biomes in total productivity due to year-round warmth and moisture.

  • Desert and semidesert scrub: around $90$ g m^{-2} yr^{-1}$ (low per-area productivity).

  • Tundra: around $140$ g m^{-2} yr^{-1}$ (low but not negligible in nutrient-poor conditions).

• Note: values vary by source; the slide presents a comparative table with multiple biomes and involves both productivity and area metrics to illustrate global patterns.

Ecosystem disturbance and resilience

• Disturbance: natural or human-induced event that interrupts ecological succession and creates new conditions on-site (e.g., volcanoes, fires, human activities).

• Different ecosystems have different capacities to respond to disturbances (resilience).

• Disturbances can kill or displace many community members and alter energy flow and nutrient cycling.

Resilience

• Resilience: ability of a system to absorb, adapt to, or recover from disturbances while maintaining ecosystem services, structure, and biodiversity.

• Mechanisms include:
  1) Resistance: ability to withstand stress or disturbance without significant change (e.g., mature temperate forests with high biodiversity and complex structure).
  2) Recovery: speed and extent to which an ecosystem returns to its pre-disturbance state and re-establishes equilibrium.

Succession

• Ecological succession: transition from one biotic community to another after a disturbance.

• Pioneer species: first colonizers of a newly opened area.

• Facilitation: early species modify conditions to aid subsequent species, driving succession forward.

• Succession is not infinite; a climax ecosystem is the final stage but can change with new species introductions or removals.

• Adjoining ecosystems in the same environment can be at different successional stages.

Primary and secondary succession

• Primary succession: occurs in areas lacking plants and soil (e.g., retreating glacier).

• Secondary succession: occurs after a disturbance in areas with pre-existing soil (e.g., fire, floods, human clearing).

• Process: recolonization by plants and animals from surrounding areas; starts with soil.

Aquatic succession

• Lakes and ponds undergo aquatic succession: soil from land erodes and settles in water; lake gradually fills in; terrestrial species move in as the lake fills; aquatic species move toward the center and eventually the lake disappears.

Disturbance and resilience in ecosystems

• All stages of succession are present in landscapes; disturbances create gaps or patches and biodiversity is enhanced by disturbance.

• Natural succession can be blocked or modified if key species are eliminated.

Fire and succession

• Fire is a major disturbance in many ecosystems.

• Tolerance to fire varies by species:

  • Grasses and pines tolerate fire.

  • Broad-leaved trees are more damaged by fire.

• Fires release nutrients from dead matter and some plants require fire to germinate (fire-adapted ecosystems).

• Fire-climax ecosystems: ecosystems that depend on periodic fire to maintain their existence (e.g., certain grasslands and pine forests).

Resilience mechanisms and limits

• Resilience mechanisms help restore ecosystem function after disturbance (energy flow and nutrient cycling).

• Limits exist: a severely degraded ecosystem may not recover its original functions, leading to a new, less productive ecosystem.

Ecosystem capital and human welfare

• Ecosystems provide valuable services to humans and other species (flood control, soil maintenance, CO₂ absorption, nutrient cycling).

• Goods and services are not fully captured by markets; ecosystem capital underpins many economic and social systems.

Economic valuation of ecosystem services

• Incremental value of services: economic value of how changes in quantity/quality of services affect human welfare.

• Example: converting forest to palm plantations may yield short-term revenue ($1,000–$2,000 per hectare per year) but incur larger losses in ecosystem services (~$5,000–$10,000 per hectare per year).

• Estimates suggest roughly $44 trillion per year in ecosystem goods and services across major services, contributing to more than half of global GDP when aggregated across sectors.

Ecosystem restoration

• There is capacity to restore ecosystems, though some sites require intensive restoration work.

• Global demand for ecosystem goods and services (e.g., agriculture, infrastructure) creates pressure to convert ecosystems.

• Alternatives can be pursued when:

  • Society recognizes how essential ecosystems are.

  • Ecosystem sustainability is promoted.

  • Alternatives are economically viable.

Managing ecosystems

• Effective ecosystem management relies on understanding:

  • How ecosystems function.

  • How they respond to disturbances.

  • What goods and services they provide.

• U.S. agencies involved in ecosystem management include:

  • Forest Service

  • Department of Wildlife and Fisheries

  • National Park Service

  • Environmental Protection Agency (EPA)

  • National Oceanic and Atmospheric Administration (NOAA)

Equations and key quantitative references (LaTeX)

• Primary production uses only a small fraction of available solar energy:
  \text{Primary production} \approx 0.02 \times (\text{total solar energy input})

• Energy yield from primary production (example):
  120\ \text{gigatons of organic matter per year}

• Biomass ratio across trophic levels (typical approximate rule of thumb):
  \text{Producer biomass} : \text{First-level consumer biomass} : \text{Second-level consumer biomass} : \text{Third-level consumer biomass} \approx 100 : 10 : 1 : 0.1

• Energy fate: energy costs and tissue formation ranges:

  • Energy used for metabolism, growth, and maintenance typically: 60\% \le E_{metabolism} \le 90\% of ingested energy.

  • Energy converted to tissue: 10\% \le E_{tissue} \le 40\% of ingested energy.

• Energy transfer efficiency between trophic levels is low; a common conceptual statement is that only a small percentage is transferred to the next level (no single universal percentage, but the 1/10 rule is often cited in teaching).

• Ecosystem services valuation: a large-scale estimate cited is approximately 44\times 10^{12}\$ (\$44 trillion)\per\year in ecosystem goods and services contributing to human welfare; this reflects global significance beyond market transactions.

// End of notes on Chapter 5: From Ecosystems to Biomes