Population Ecology & Food Web

6c. Students know how fluctuations in population size in an ecosystem are determined by the relative rates of birth, immigration, emigration, and death.6d. Students know how water, carbon, and nitrogen cycle between abiotic resources and organic matter in the ecosystem and how oxygen cycles through photosynthesis and respiration.6e.Students know a vital part of an ecosystem is the stability of its producers and decomposers.6f. Students know at each link in a food web some energy is stored in newly made structures but much energy is dissipated into the environment as heat. This dissipation may be represented in an energy pyramid.

A community interacts with abiotic factors, forming an ecosystem

Energy flows from the sun, through plants, animals, and decomposers, and is lost as heat

Chemicals are recycled between air, water, soil, and organisms

Energy flow and chemical cycling are the two fundamental processes in ecosystems

A terrarium ecosystem

Lightenergy

Chemical cycling(C, N, etc.)

Chemicalenergy

Heatenergy

Trophic structure is a key factor in ecosystem dynamics

A food chain is the stepwise flow of energy and nutrients from plants (producers) to herbivores (primary

consumers) to carnivores (secondary and

higher-level consumers)

TROPHIC LEVEL

Quaternaryconsumers

Tertiaryconsumers

Secondaryconsumers

Primaryconsumers

Producers

Carnivore

Carnivore

Carnivore

Carnivore

Carnivore

Carnivore

Herbivore

Plant

Zooplankton

Phytoplankton

A TERRESTRIAL FOOD CHAIN

AN AQUATIC FOOD CHAIN

Decomposition is the breakdown of organic compounds into inorganic compounds• Decomposition is essential for the continuation of life on Earth

• Detritivores decompose waste matter and recycle nutrients

– Examples: animal scavengers, fungi, and prokaryotes

Food chains interconnect,

forming food webs

A food web is a network of interconnecting food chainsIt is a more realistic view of

the trophic structure of an ecosystem than a food chain

Wastes anddead organisms

Detritivores

(Prokaryotes, fungi,certain animals)

Producers

(Plants, algae,phytoplankton)

Primaryconsumers

Secondaryandprimaryconsumers

Tertiaryandsecondaryconsumers

Energy supply limits the length of food chains

Biomass is the amount of living organic material in an ecosystem

Primary production is the rate at which producers convert sunlight to chemical energy The primary production of the entire

biosphere is about 170 billion tons of biomass per year

A pyramid of production reveals the flow of energy from producers to primary consumers and to higher trophic levels

Tertiaryconsumers

Secondaryconsumers

Primaryconsumers

Producers

10 kcal

100 kcal

10,000 kcal

1,000,000 kcal of sunlight

1,000kcal

Only about 10% of the energy in food is stored at each trophic level and available to the next level

– This stepwise energy loss limits most food chains to 3 - 5 levels

– There is simply not enough energy at the very top of an ecological pyramid to support another trophic level

Connection: A production pyramid explains why

meat is a luxury for humans The dynamics of energy flow apply to the

human population as much as to other organismsWhen we eat grain or fruit, we are

primary consumersWhen we eat beef or other meat from

herbivores, we are secondary consumers

When we eat fish like trout or salmon (which eat insects and other small animals), we are tertiary or quaternary consumers

Because the production pyramid tapers so sharply, a field of corn or other plant crops can support many more vegetarians than meat-eaters

TROPHIC LEVEL

Secondaryconsumers

Primaryconsumers

Producers

Humanvegetarians

Corn

Humanmeat-eaters

Cattle

Corn

Chemicals are recycled between organic matter

and abiotic reservoirsEcosystems require daily infusions of

energyThe sun supplies the Earth with energyBut there are no extraterrestrial

sources of water or other chemical nutrients

Nutrients must be recycled between organisms and abiotic reservoirsAbiotic reservoirs are parts of the

ecosystem where a chemical accumulates

There are four main abiotic reservoirs

– Water cycle

– Carbon cycle

– Nitrogen cycle

– Phosphorus cycle

Water moves through the biosphere in a global cycle

Heat from the sun drives the global water cycle PrecipitationEvaporation Transpiration

Water vaporover the sea

Water vaporover the land

Surface waterand groundwater

Precipitationover the land

Flow of waterfrom land to sea

Net movementof water vaporby wind

Precipitationover the sea

Solarheat

Evaporationandtranspiration

Evaporationfrom the sea

Oceans

The carbon cycle depends on photosynthesis and

respiration

Carbon is taken from the atmosphere by photosynthesisIt is used to make organic

moleculesIt is returned to the atmosphere by

cellular respiration

CO2 in atmosphere

Plants,algae,

cyanobacteria

Photosynthesis

Primaryconsumers

Higher-levelconsumers

Wood andfossil fuels

Detritivores(soil microbes

and others)

Decomposition

Detritus

BurningCellular respiration

The nitrogen cycle relies heavily on bacteria

Nitrogen is plentiful in the atmosphere as N2

But plants cannot use N2

Various bacteria in soil (and legume root nodules) convert N2 to nitrogen compounds that plants can useAmmonium (NH4

+) and nitrate (NO3–)

Some bacteria break down organic matter and recycle nitrogen as ammonium or nitrate to plants

• Other bacteria return N2 to the atmosphere

Nitrogen (N2) in atmosphere

Amino acidsand proteins in

plants and animalsAssimilationby plants

Denitrifyingbacteria

Nitrates(NO3

–)

Nitrifyingbacteria

Detritus

Detritivores

Decomposition

Ammonium (NH4+)

Nitrogenfixation

Nitrogen-fixingbacteria in soil

Nitrogen-fixingbacteria in root

nodules of legumes

Nitrogenfixation