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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