Chapter 5 Notes: From Ecosystems to Biomes

Ecosystems

Ecosystems are dynamic networks where living communities interact with abiotic factors and energy/nutrient flows.
Key ecosystem processes include photosynthesis, respiration, energy flow, and nutrient cycling.
Humans rely on ecosystems and assign value to ecosystem goods and services (external benefits).
Abiotic factors and biotic interactions set the range of tolerance and limiting factors that determine species distributions.
Ecosystems contain communities of interacting species and their physical environment; predictable patterns of ecosystem distribution exist globally.
Biomes are ecosystems with similar vegetation and climatic conditions; they represent broad geographic patterns rather than strict boundaries.

Trophic levels

In photosynthesis, plants use light energy, CO₂, and H₂O to produce organic molecules (sugar). The general equation for photosynthesis can be written as:
6\,CO{2} + 6\,H{2}O + \text{light energy} \rightarrow C{6}H{12}O{6} + 6\,O{2}
Energy and nutrients flow up through trophic levels (feeding levels) as organisms consume others.
A trophic level is defined by the primary source of energy for that level; energy and materials move upward through the system.

Food chains and food webs

Food chain: describes the linear transfer of energy and nutrients from one organism to another; energy generally moves upward through the chain.
Not all energy/nutrients reach higher trophic levels; much is lost at each step.
Food web: a network of interconnected food chains forming complex feeding relationships, usually more evident in aquatic systems.

Aquatic food webs (illustrative components)

Common components across Arctic and marine systems include:
- Producers: phytoplankton, diatoms, photosynthetic/chemosynthetic autotrophs.
- Primary consumers: herbivorous zooplankton, small fish.
- Secondary/tertiary consumers: arctic cod, ringed seals, bearded seals, bowhead whale, polar bear, arctic fox, beluga whale, etc.
- Carnivorous zooplankton; detritivores and decomposers are also part of the web.
Aquatic webs often feature ice biota (diatoms) and marine/estuarine linkages; these webs illustrate energy flow and trophic interactions in aquatic environments.

Autotrophs and heterotrophs (basics)

Autotrophs: produce organic matter (OM) from inorganic matter using external energy sources.
- Producers: green plants (photosynthetic) and chemotrophic bacteria.
Heterotrophs: consume OM to obtain energy.
- Consumers: eat living prey; include primary (herbivores), secondary, tertiary, and higher-order consumers.
- Decomposers: scavengers, detritus feeders, and chemical decomposers that break down dead OM.

Producers and primary production

Photosynthesis captures light energy to convert CO₂ and H₂O to OM.
Chlorophyll is the green pigment responsible for absorbing light energy.
Chemosynthesis allows bacteria to use energy from inorganic chemicals (e.g., H₂S, NH₃) to synthesize OM.
Primary production (also called primary productivity) is the production of OM through photosynthesis and associated biomass growth.

Global primary productivity (terrestrial)

Net Primary Production (NPP): the classified rate of new plant biomass production (tonnes of dry organic matter per hectare per year) for ecosystems.
Units and scale (terrestrial): often presented as tons/ha/year of total dry matter (above- and below-ground).
Values vary by ecosystem type; general ranges observed include None, Low (~1), and High (~15) for reference scales in teaching materials.

Consumers

Consumers include animals, fungi, and many bacteria; they are categorized by food source:
- Primary consumers (herbivores): feed on producers.
- Secondary consumers (carnivores): feed on primary consumers.
- Tertiary and higher-order consumers.
Carnivores are typically secondary or higher-order meat eaters.
Omnivores feed on both plants and animals.

Grassland food chain (illustrative structure)

Producers → First-level (primary) consumer → Second-level (secondary) consumer → Third-level (tertiary) consumer
This simple chain represents the flow of energy from producers up through consumers; each ascending level typically contains less biomass.

Decomposers and detritus

Decomposers convert detritus (dead plant material, waste, and bodies) back into inorganic forms and energy.
Detritus is high in potential energy and fuels detritivores and chemical decomposers.
Detritus-based pathways are especially important in nutrient cycling.
Key decomposer groups:
- Scavengers (e.g., vultures) break down large pieces of matter.
- Detritus feeders (e.g., earthworms) feed on partly decomposed matter.
- Chemical decomposers (e.g., fungi and bacteria) break down molecule-sized matter.

Specialized decomposers and microbial pathways

Some decomposers digest wood (cellulose) via mutualisms with gut microorganisms (e.g., termites).
Cellulose: plant cell-wall material; humans cannot digest it.
Anaerobic respiration occurs in the absence of oxygen (in sediments, guts, etc.).
Fermentation is a form of anaerobic respiration used by some decomposers; it breaks down glucose in the absence of oxygen and produces byproducts like ethanol, methane, and acetic acid.

Trophic categories (summary)

Autotrophs: make their own organic matter from inorganic nutrients using energy sources.
Heterotrophs: must feed on organic matter for energy.
Producers:
- Photosynthetic green plants (chlorophyll)
- Photosynthetic bacteria (light-absorbing pigments)
- Chemosynthetic bacteria (oxidation of inorganic chemicals like
  hydrogen sulfide)
Consumers:
- Primary consumers/herbivores: feed on plants
- Omnivores: feed on both plants and animals
- Secondary consumers/carnivores: feed on primary consumers
- Higher-order consumers: feed on other carnivores
Parasites: feed on another organism over time
Decomposers, scavengers, detritus feeders, chemical decomposers (as defined above)

Limits on trophic levels and biomass pyramids

Terrestrial ecosystems typically have 3–4 trophic levels; marine systems can have up to 5.
Biomass pyramid concept: the total biomass at higher trophic levels is smaller than that at lower levels, with about a 90% decrease per step.
Example relation:
ext{Biomass}{n+1} \approx 0.1 \times \text{Biomass}n
or equivalently a ~90% reduction moving up a trophic level.
This creates a pyramid shape when biomass is plotted by trophic level.

Biomass pyramid (illustrative description)

Producers form the base with the largest biomass.
1st-level consumers (herbivores) have about one-tenth the biomass of producers.
2nd-level consumers (primary carnivores) have about one-tenth the biomass of 1st-level consumers.
The biomass at higher levels decreases progressively, forming a pyramid.

Flow of energy and standing-crop biomass (terrestrial)

Standing-crop biomass: the biomass of primary producers in an ecosystem at a given time (above-ground focus is common).
Biomass and primary production vary greatly by ecosystem type and climate.
- Forests tend to have large biomass.
- Grasslands tend to have high primary production.
Sunlight is the primary energy source for most ecosystems; primary production uses only about 2\% of available solar energy, yielding roughly 120\ \text{gigatons of OM/year}
Note: These numbers illustrate the general scale and efficiency, not exact values for every ecosystem.

Fate of consumed energy and efficiency

Of food energy consumed:
- About 60\%\text{–}90\% is oxidized for energy (maintenance, activities).
- About 10\%\text{–}40\% is converted to body tissues for growth and repair.
- Undigested energy leaves as waste.
Excretion also removes carbon dioxide, nitrogen, phosphorus, and water.

Energy flow is inefficient and production terminology

Only a small fraction of energy passes to the next trophic level; much is not consumed, used in metabolism, or excreted.
Secondary production: the rate of growth of consumers over time; energy and matter transfer from lower to higher trophic levels to form new tissue.

Trophic inefficiency and biomagnification

Higher trophic levels require more energy to produce the same amount of tissue (greater energy demand).
Some materials (e.g., heavy metals, pesticides) are hard to degrade and are excreted slowly; they bioaccumulate within individuals and magnify up the food chain (bioaccumulation and biomagnification).

Aquatic vs. terrestrial systems: key contrasts

Aquatic systems follow similar energy flow patterns but with two major differences:
1) More energy is available at each level; energy transfer is more efficient; cold-blooded (ectothermic) animals require less energy for body mass.
2) Aquatic systems tend to have a reversed biomass pyramid: smaller biomass of algae/phytoplankton at the base but greater biomass of larger fish higher up, in some contexts.
Bottom-up dynamics in aquatic systems can lead to longer chains and different turnover rates; algae and phytoplankton often have rapid growth and turnover, while upper-level organisms live longer and may accumulate biomass.

From ecosystems to biomes: climate and geography

Ecosystem patterns have predictable populations of organisms under particular conditions.
Regions host distinct biotic communities and know patterns of energy flow and food webs/trophic levels.
A biome is a large geographical terrestrial biotic community controlled by climate; often named after dominant vegetation; boundaries are not sharply defined and can vary with time.

Climate and its role in biomes

Climate describes average temperature and precipitation (weather) patterns of a region and varies widely.
Latitudinal and altitudinal gradients drive climate differences:
- Equatorial regions: warm, high rainfall, minimal seasons.
- Temperate regions: seasonal variation with distinct seasons.
- Polar regions: long, cold winters.
The average temperature in a region varies with latitude and altitude.

Latitude, altitude, and microclimates

Effects of latitude and altitude shape which biomes occur where:
- Needleleaf forests, temperate deciduous forests, tropical rain forests, tundra, ice and snow, etc.
Microclimates create localized variations within a biome, leading to finer-scale ecosystem differences.

Precipitation and climate suitability

Precipitation varies from near zero to >100 inches/year and can be evenly or seasonally distributed.
Species persist where temperature and precipitation fall within their tolerance ranges; highest densities occur where conditions are optimal; species are excluded when conditions exceed tolerance limits.

Temperature-delineated biomes

Tropical rain forests: broad-leaved evergreens; cannot tolerate freezing.
Deciduous forests: trees drop leaves and go dormant in freezing temperatures.
Coniferous forests: tolerate harsh winters and short summers in northern regions.
Permafrost: permanently frozen subsoil; roots cannot penetrate; tundra biome persists where permafrost is present.

Terrestrial biomes map (conceptual overview)

Biome distribution is often depicted across latitude/longitude with zones for tundra, boreal forests, temperate forests, deserts, tropical forests, etc., reflecting patterns of temperature and precipitation.

Aquatic systems: freshwater and marine

Aquatic systems are determined by depth, salinity, and permanence of water.
Freshwater systems include lakes, marshes, streams, and rivers.
Mixed ecosystems (estuaries, bays) occur where fresh and saltwater mix.
Marine systems include oceans and coastal regions.

Productivity across biomes

Biomes differ in primary productivity; tropical rain forests are highly productive due to warm temperatures and abundant rainfall.
Open oceans account for high global productivity due to vast area, but local productivity rates can be low because nutrients are often scarce.
Overall, tropical and coastal systems tend to have high productivity; open oceans contribute significantly to global productivity despite lower rate per unit area.
Productivity is commonly measured as net primary production (NPP) in units such as \text{g m}^{-2}\text{ yr}^{-1}.

Productivity by ecosystem (conceptual overview)

A range of ecosystems show varying NPP values; some examples include algal beds/reefs, tropical rain forests, estuaries, temperate forests, savannas, boreal forests, wetlands, cultivated land, lakes/streams, deserts, and open ocean.
Relative rankings typically place tropical rain forests and estuaries/high-nutrient systems among the highest, while deserts and open ocean surface waters can be lower on a per-area basis but contribute substantially to global productivity due to large area.
A representative way to compare is by plotting Average annual NPP (g/m²/yr) against the biome’s share of global surface area and its contribution to global NPP.
If you need exact numeric values for each biome, refer to the table that lists NPP by biome (values in \text{g m}^{-2}\text{ yr}^{-1}) and corresponding surface-area shares; the table shows tropical forests, algal beds and reefs, estuaries, open ocean, lakes, deserts, tundra, etc., with a wide range across biomes.

Ecosystem disturbance and resilience

An ecosystem experiences disturbances (natural or human-caused) that interrupt ecological succession and create new conditions on site (e.g., volcanoes, fires, logging).
Different ecosystems have varying capacities to respond to disturbances; resilience is the ability to absorb, adapt to, or recover from disturbances while maintaining ecosystem services, structure, and biodiversity.
Components of resilience include:
- Resistance: the ability to withstand stress without significant change (e.g., mature forests resisting fire due to biodiversity and structure).
- Recovery: the speed and extent to which an ecosystem returns to its pre-disturbance state.

Succession and its stages

Ecological succession is the transition from one biotic community to another following a disturbance.
Pioneer species colonize first and facilitate subsequent stages (facilitation).
Succession does not proceed indefinitely; it can be punctuated or redirected by disturbances.
Climax ecosystem: final, relatively stable stage of succession, though it can change if new disturbances or species alter conditions.
Adjacent ecosystems in the same environment can be at different successional stages.

Primary and secondary succession

Primary succession: occurs in areas lacking soil and vegetation (e.g., retreating glaciers); begins with bare rock and proceeds to soil formation and vegetation.
Secondary succession: occurs after a disturbance clears an area but soil remains; surrounding plants/animals reinvade and re-establish the community (e.g., after fire or floods).
Typical sequence (example) includes pioneer grasses, tall grasses/herbaceous plants, pines, and eventually hardwood forests (hardwoods) leading to a climax forest over decades.

Aquatic succession

Succession also occurs in lakes and ponds:
- Soil and sediments erode into the water body, gradually filling it.
- Terrestrial species move into the lake; aquatic species retreat, and over time the lake may disappear.

Fire, disturbance, and resilience in succession

Fire is a major disturbance shaping many ecosystems.
Species exhibit varying fire tolerances:
- Grasses and pines tolerate fire; broad-leaved trees are more susceptible.
Fire recycles nutrients from dead organic matter and in some ecosystems is essential for germination and maintenance (e.g., fire-climax ecosystems such as certain grasslands and pine forests).
Resilience mechanisms include nutrient renewal, regrowth from seeds and roots, and recolonization; but resilience has limits—severe disturbances can lead to a degraded ecosystem with altered functioning.

Ecosystem capital and services

Ecosystems provide valuable services to humans and other species, including flood control, soil maintenance, CO₂ storage, and nutrient cycling.
These goods and services are not always captured by market prices; non-market values are substantial and essential for well-being.
An ecosystem’s services contribute to human welfare; estimates suggest a broad range of services have a multitrillion-dollar impact.
A notable figure cited: 17 major ecosystem goods and services contribute about $44 trillion to human welfare each year, which is over half of global GDP.
The concept of ecosystem capital emphasizes the economic value of the relationships between ecosystem changes and human welfare.
Example illustrating ecosystem service valuation and losses:
- Converting 1 hectare of forest to an oil palm plantation can yield $1,000 to $2,000 in annual revenue but incur ecosystem service losses of $5,000 to $10,000 per hectare per year (losses include carbon storage, biodiversity, water regulation, etc.).
- This demonstrates that the gains from exploitation often do not outweigh the losses in ecosystem services.

Ecosystem restoration and sustainable management

Ecosystem restoration is possible and may require intensive effort from restoration scientists.
There are growing pressures on Earth to provide goods and services (e.g., land for agriculture, irrigation, deforestation for agriculture; land and energy for infrastructure).
Alternatives can be pursued when:
- Society recognizes the essential role of ecosystems.
- Ecosystem sustainability is promoted.
- Alternatives are economically viable.

Managing ecosystems and institutions

Understanding ecosystem function, disturbance responses, and the goods/services provided is essential for management.
In the United States, agencies that engage in ecosystem management include:
- U.S. Forest Service
- Department of Wildlife and Fisheries
- National Park Service
- Environmental Protection Agency (EPA)
- NOAA (National Oceanic and Atmospheric Administration)

Practical and ethical implications

Valuing ecosystem services helps justify conservation and restoration investments beyond market prices.
Trade-offs between economic development and ecosystem health must be evaluated, including potential long-term losses from ecosystem degradation.
Restoration aims to recover ecosystem structure and function, but it may not fully restore original conditions or biodiversity, and may require ongoing management.
Policy and planning should emphasize precaution, resilience, and sustainability to maintain ecosystem services for future generations.

Quick reference formulas and numbers (key points)

Photosynthesis equation (summary):
6\,CO{2} + 6\,H{2}O + \text{light energy} \rightarrow C{6}H{12}O{6} + 6\,O{2}
Energy transfer efficiency between trophic levels (typical):
\text{Biomass}{n+1} \approx 0.1 \times \text{Biomass}n
(about a 90% decrease per trophic step)
Primary production efficiency (solar energy capture):
\eta \approx 2\%
Global primary production scale (terrestrial estimate):
\text{Total OM produced per year} \approx 120\ \text{gigatons (Gt) of OM/year}
Fate of energy (typical ranges):
- Energy used for metabolism/maintenance: 60\%\text{–}90\%
- Energy converted to tissue: 10\%\text{–}40\%
Waste and excretion contribute to nutrient cycling (CO₂, N, P, H₂O).
Economic scale (ecosystem services):
- 17 major ecosystem services: approx. 44\times 10^{12} USD/year (about half of global GDP).
- Replacement or conversion costs can exceed gains: example of forest-to-palm conversion shows net ecosystem service losses of 5{,}000\$\text{–}10{,}000\$\text{ per hectare per year} alongside lower immediate revenue ($$1{,}000\$\text{–}2{,}000\$ per hectare/year).