Chapter 54

Ecosystems

Ecosystems• An ecosystem consists of all the

organisms living in a community as well as all the abiotic factors with which they interact

• Ecosystems can range from a microcosm to lakes and forests

• Most ecosystems are powered by energy from sunlight, which is transformed to chemical energy by autotrophs and passed to heterotrophs

• Most energy is continually dissipated in the form of heat

Ecosystem Dynamics

Decomposition• Decomposition connects all trophic levels• Detritivores that feed on plant remains

often form a link between producers and consumers

• Fungi and prokaryotes (bacteria) are the most important decomposers; they convert organic material from all trophic levels to inorganic compounds that can be recycled by autotrophs

Laws of Physics and Chemistry• The first law of thermodynamics

states that energy cannot be created or destroyed, only transformed– Sunlight → chemical energy → heat

• The second law of thermodynamics states that in each energy conversion, some energy is lost as heat

• Ecologists trace the energy flow in ecosystems and the efficiency of ecological energy conversion

Primary Production• Primary production is a measure of

the amount of light energy converted to chemical energy – output by autotrophs

• Only about 1-2% of the solar radiation that strikes the earth is converted to chemical energy – varies with latitude

• The worldwide photosynthetic production is about 170 billion tons of organic material per year

Primary Production• Net primary production is the gross

primary production minus the energy used by plants in their own cellular respiration (R)– NPP = GPP - R

• Primary production can be expressed as energy per unit area per unit time (J/m2/yr) or as biomass measured in terms of dry weight of organic material added (g/m2/yr)

• Standing crop is the total biomass of plants in an ecosystem

Primary Production in Different Ecosystems

Regional Annual NetPrimary Production

Primary Production in Marine Ecosystems

• In aquatic ecosystems, light and nutrients limit primary production

• Light only reaches the photic zone – more than half is absorbed in the first meter

• Nutrients limit marine production more than light

• Nitrogen and phosphorous, as well as iron, levels limit the growth of phytoplankton – known as limiting nutrients

Primary Production in Freshwater Ecosystems

• In freshwater ecosystems, light intensity, temperature, and availability of minerals affect production

• Eutrophication, the shift in composition of phytoplankton from green algae and diatoms to blooms of cyanobacteria, has been linked to phosphorous pollution from sewage and fertilizer runoff

Primary Production in Terrestrial Ecosystems

• Both water supply and temperature vary more in terrestrial ecosystems – production is related to precipitation and temperature

• Nitrogen or phosphorous is the limiting nutrient in many ecosystems

Secondary Production• Secondary production – is the rate at

which consumers produce new biomass from their food

• The efficiency of energy transfer between trophic levels is usually less than 20%

• Herbivores consume only a fraction of the plant material produced; they cannot digest all they eat; and much of the energy they do absorb is used for cellular respiration

Trophic Efficiency• Trophic efficiency is the percentage of

the energy of one trophic level that makes it to the next level, usually ranging from 5-20%

• The pyramid of production shows the multiplicative loss of energy via heat, respiration, and feces

The Biomass Pyramid• A biomass pyramid illustrates the

standing crop biomass of organisms at each trophic level.

Pyramid of Numbers• The pyramid of numbers illustrates that

higher trophic levels contain small numbers of individuals, resulting from the larger size of these animals and the greatly decreased energy availability illustrated by the pyramid of production

Cycling of Chemical Elements

• Biochemical cycles – chemical elements are passed between abiotic and biotic components of an ecosystem

• After chemicals are passed through the food chain, they are returned to the atmosphere, water, or soil through respiration and the action of decomposers

Cycling of Nutrients

The Water Cycle

The Carbon Cycle

The Nitrogen Cycle

The Phosphorous Cycle

Cycling of Nutrients• Decomposition rates largely determine

the rates of nutrient cycling• Nutrient cycling is strongly regulated by

vegetation

Human Impact on Ecosystems• Human activity often intrudes in nutrient

cycles by removing nutrients from one part of the biosphere and adding them to another

• Agricultural effects – harvesting of crops removes nutrients that would otherwise recycle into the soil – requires the addition of fertilizers

• Accelerated eutrophication – leads to oxygen shortages and fish kills

• Combustion of fossil fuels is the main cause of acid precipitation

Distribution of Acid Precipitation in the US

Human Impact on Ecosystems• Toxins can become

concentrated in successive trophic levels of food webs

• Biological magnification of DDT in a food chain

Human Impact on Ecosystems• Rising atmospheric CO2 – combustion of

fossil fuels and burning of wood• The greenhouse effect – CO2 and water

vapor in the atmosphere absorb infrared radiation reflected from Earth and re-reflect it back to Earth, causing an increase in temp.

• Global warming – caused by an increase in CO2 levels

– Climatologists predict a temp. rise of 2°C in the next 100 years if CO2 levels continue to rise

The Greenhouse Effect

Increase in Atmospheric CO2 and Average Temp. from 1958-2000

Depletion of the Ozone Layer

• The layer of ozone molecules (O3) in the lower stratosphere absorb damaging ultraviolet radiation

• This layer has been gradually thinning since 1975, largely as a result of the accumulation of breakdown products of chlorofluorocarbons in the atmosphere

• Loss of the ozone layer leads to more intense UV radiation and an increase in skin cancer risk.

Depletion of the Ozone Layer