how do ecosystems work? - gavilan collegehhh.gavilan.edu/jcrocker/documents/ch29_lecture.pdf ·...

Copyright © 2009 Pearson Education, Inc..

Lectures byGregory Ahearn

University of North Florida

Ammended byJohn Crocker

Chapter 29

How Do Ecosystems Work?

Copyright © 2009 Pearson Education Inc.

29.1 How Do Ecosystems Obtain Energy And Nutrients?

The activities of life are powered by the energy of sunlight.• Solar energy is captured in ecosystems by

photosynthetic organisms transformed and ultimately converted to heat energy that radiates back into space.

• Energy moves through ecosystems in a continuous, one-way flow, and is constantly replenished from outside by the sun.

• Because of this constant flow of energy from an outside source ecosystems can both thrive and obey the laws of thermodynamics



In contrast to energy, nutrients are not replenished.• Nutrients are elements and small molecules

that form the chemical building blocks of life.• Nutrients are transported, redistributed, and

converted to different molecular forms, but do not leave Earth; they are recycled within and among ecosystems.



An ecosystem consists of all of the communities within a defined area, along with the aabiotic environment.• Ecological study of ecosystems focuses on

flows of energy and nutrients.• Ecosystem ecologists follow the pathways of

energy and nutrients to understand the factors that shape the complex interactions within communities, and between communities and the abiotic environment.

Copyright © 2009 Pearson Education Inc.Fig. 29-1

Energy from sunlight

detritus feeders and decomposers

primary consumers

producers

HEAT

higher-level consumers

HEAT

HEAT

NUTRIENTS

heat energy

solar energy

energy stored in chemical bondsnutrients

HEAT


29.2 How Does Energy Flow Through Ecosystems?

Hydrogen molecules in the sun fuse to form helium molecules, releasing tremendous amounts of energy.• A tiny fraction of this energy reaches Earth,

arriving in the form of electromagnetic waves including heat, light, and ultraviolet energy.

• Much of the energy reaching Earth is absorbed or reflected back into space by Earth’s atmosphere, clouds, and surface.

• Only about 1% of the solar energy that reaches Earth makes it to the surface as light that is available to power life.



Energy enters ecosystems mainly through photosynthesis.• During photosynthesis, solar energy powers

reactions that store energy in the chemical bonds of sugar and other high-energy molecules.

• Photosynthetic organisms produce food for themselves, using nonliving nutrients and sunlight.

• Organisms that can produce their own food are called autotrophs, or producers.


Sugar is synthesized and used in plant tissues

plant tissues, growth

Energy is captured from sunlight

Carbon dioxide is absorbed from the air

Oxygen is released

Water is absorbed from soil, used in photosynthesis, and stored in cells

Inorganic mineral nutrients (nitrate, phosphate) are absorbed from soil and used in plant tissues

photosynthesis


Photosynthesis

Fig. 29-2



By manufacturing food for themselves producers directly or indirectly produce food for nearly all other organisms as well.• Organisms that cannot produce their own food

are called heterotrophs, or consumers.• Heterotrophs must acquire energy and many

of their nutrients from other organisms.



Energy captured by producers is available to the ecosystem.• The amount of life that a particular ecosystem

can support is determined by the amount of energy captured by the producers in that ecosystem.

• The energy that producers store and make available to other organisms is called net primary productivity.

• Net primary productivity is measured as the biomass added by producers per unit area over a given time.



Energy captured by producers is available to the ecosystem (continued).• Net primary productivity of an ecosystem is

influenced by many environmental variables• amount of nutrients available • amount of available sunlight• availability of water• temperature

• In deserts, productivity is low due to lack of water; in the open ocean productivity is limited because nutrients are lost to deeper water.



An ecosystem’s overall contribution to the Earth’s productivity depends upon both the ecosystem’s net primary productivity per unit area and the area it covers.• In ecosystems with abundant resources productivity is

high. ex/ tropical rain forests and estuaries.• Rain forests and the open ocean each account for

about a quarter of the planet’s total productivity • Oceans are much less productive per unit area but

cover 65% of the Earth • Tropical rainforests cover only about 4% but are

fantastically productive



Ecosystem productivities compared.

Fig. 29-3

desert (90)

open ocean (125)

continental shelf (360)

tundra (140)

coniferous forest (800)

temperate deciduous forest (1,200)

grassland (600)

tropicalrain forest(2,200)

estuary(1,500)



Energy passes from one trophic level to another.• Energy flows through communities from

producers at the first trophic level, through several levels of consumers.

• Primary consumers (herbivores) feed directly on the producers and form the second trophic level.

• The third and fourth trophic levels are the secondary and tertiary consumers, and consist of meat-eating predators.



Feeding relationships within ecosystems form chains and webs.• It is common to identify a representative of

each trophic level such that each representative species eats another on the level below it.

• This linear feeding relationship is called a food chain.

• Different ecosystems have radically different food chains.


TERTIARY CONSUMER (4th trophic level)

PRODUCER (1st trophic level)

A simple terrestrial food chain(a)

SECONDARY CONSUMER (3rd trophic level)

PRIMARY CONSUMER (2nd trophic level)


A simple terrestrial food chain

Fig. 29-4a



A simple marine food chain

Fig. 29-4b

SECONDARY CONSUMER (3rd trophic level)

A simple marine food chain(b)

Phytoplankton PRODUCER

(1st trophic level)

Zooplankton PRIMARY CONSUMER

(2nd trophic level)

TERTIARY CONSUMER (4th trophic level)



In natural communities, the actual feeding relationships form a food web of many interconnected food chains.• Some animals act as primary, secondary, and

tertiary consumers (omnivores).• Many carnivores eat both herbivores and

other carnivores, and act as both secondary and tertiary consumers.



Detritus feeders and decomposers release nutrients for reuse.• The detritus feeders are an army of mostly

small and often unnoticed animals and protists that live on the refuse of life—molted exoskeletons, fallen leaves, wastes, and dead bodies.

• The network of detritus feeders is complex, and on land, it may consist of earthworms, mites, protists, centipedes, insects, pillbugs, nematodes, and a few large vertebrates (vultures etc).



Detritus feeders and decomposers release nutrients for reuse (continued).• These organisms consume dead organic

matter, extract some of the energy stored in it, and excrete it in a further decomposed state.

• Their excretory products serve as food for other detritus feeders and for decomposers.



The decomposers are primarily fungi and bacteria.• Decomposers digest food outside their bodies

by secreting digestive enzymes into the environment.

• They then absorb the nutrients they need, and the remaining nutrients are released into the environment.



Detritus feeders and decomposers are absolutely essential to life on Earth.• Their activities reduce the bodies and wastes

of other organisms to simple molecules, such as carbon dioxide, water, minerals, and organic molecules, which return to the atmosphere, soil, and water.

• They perform the vital role of nutrient recycling.



Energy transfer through trophic levels is inefficient.• When a caterpillar (a primary consumer) eats the

leaves of a tomato plant (a producer), only a portion of the solar energy originally trapped by the plant is available to the insect.

• Only a small fraction of the energy captured by the first trophic level is available to organisms in the second trophic level.

• Some of the original energy was used by the plant for growth and maintenance, and some was converted into the chemical bonds of molecules such as cellulose.



Energy transfer through trophic levels is inefficient (continued).• Additional energy is lost in each transfer to a

higher trophic level.• Some of the energy consumed by the

caterpillar is used to power crawling and chewing, and some is given off as heat.

• All of that energy is unavailable to a bird that eats that caterpillar.

• Energy flow through different trophic levels may be illustrated by a deciduous forest ecosystem.



Energy transfer and loss in a forest

Fig. 29-6

HEAT

HEAT

heat

energy stored in chemical bonds

secondary consumerprimary consumer

detritus feeders and decomposers

producer



Energy pyramids illustrate energy transfer between trophic levels.• About 90% of the energy stored at a trophic

level is lost in the transfer to the next level.• Therefore, the energy stored by an

ecosystem’s primary consumers is only about 10% of the energy stored in the bodies of producers.

• This inefficient energy transfer between tropic levels is called the “10% law.”



Energy pyramids illustrate energy transfer between trophic levels (continued).• An energy pyramid, which shows maximum

energy at the base and steadily diminishing amounts at higher levels, illustrates the energy relationships between trophic levels.

• Energy pyramids also show why producers are the most abundant organisms in an ecosystem and carnivores the rarest.


29.3 How Do Nutrients Move Within And Among Ecosystems?

Unlike the energy of sunlight, nutrients are not continuously supplied to Earth in a steady stream from above.• The same pool of nutrients has been

supporting life for more than 3 billion years.• Macronutrients, required by organisms in large

quantities, include carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur, and calcium.

• Micronutrients, including zinc, molybdenum, iron, selenium, and iodine, are only required in trace quantities.



Nutrient cycles (biogeochemical cycles) describe the pathways that substances follow as they move through communities to the nonliving portions of the ecosystem and then back again to communities.• Storage sites for nutrients are called

reservoirs.• For example, carbon’s reservoirs are as

carbon dioxide in the atmosphere, solutes in the ocean, stored fossil fuels, and other organic matter protected from decay.



Carbon cycles through the atmosphere, oceans, and communities.• Carbon enters the living part of the ecosystem

when producers capture CO2 during photosynthesis and incorporate its carbon atoms in organic molecules.

• Primary consumers, such as rabbits, shrimp, and grasshoppers, eat the producers and acquire the carbon stored in their tissues.

• Producers and consumers die, and the carbon in their bodies is returned to the reservoirs by decomposers.


consumers producers

decomposition of wastes and dead bodies fossil

fuels limestone

soil bacteria and detritus

feeders

CO2 in the atmosphere

CO2dissolved in the ocean

respirationburning of fossil fuels fire

photosynthesis

reservoirsprocessestrophic levels



The major reservoir for nitrogen is the atmosphere.• The atmosphere contains 79% nitrogen gas

(N2 ), and is thus the major reservoir for this nutrient.

• Nitrogen enters the food web mainly through certain bacteria in soil and water that engage in nitrogen fixation, a process in which nitrogen and hydrogen are combined to form ammonia (NH3 ).

• The ammonia is converted into nitrate (NO3 ).



The major reservoir for nitrogen is the atmosphere (continued).• Plant roots can absorb ammonia and nitrate, and

plants incorporate the nitrogen from these molecules into amino acids, proteins, nucleic acids, and some vitamins.

• As nitrogen passes through the food web, some is returned to soil or water when decomposer bacteria convert wastes and dead bodies back to nitrate and ammonia.

• Atmospheric nitrogen is replenished by denitrifying bacteria, which break down nitrate, releasing nitrogen gas back into the atmosphere.


nitrogen in the

atmosphere

electrical storms

produce nitrateburning fossil fuels produces nitrogen

oxides

reservoirsprocessestrophic levels/ organisms

decomposition of wastes and dead bodies

soil bacteria and detritus

feeders

ammonia and nitrate in soil

and water

nitrogen-fixing bacteria in

legume roots and soil

producers

denitrifying bacteria

uptake by

plants

consumersapplication of manufactured

fertilizer



The major reservoir for phosphorus is rock.• The phosphorus cycle does not include the

atmosphere.• The reservoir for phosphorus in ecosystems is

rock, where phosphorus is bound to oxygen to form phosphate.

• Dissolved phosphate is readily absorbed through the roots of plants and is passed on to other organisms through the food web.

• Phosphorus is important in ATP and NADP, nucleic acids, and the phospholipids of membranes.


phosphate in

rock

geological uplift

reservoirsprocessestrophic levels

consumers

detritus feeders

producers

phosphate in

soilphosphate

in sediment

runoff from rivers

runoff from fertilized

fields

phosphate in

water



Phosphorus in the living part of ecosystems is eventually returned to the nonliving parts.• Organisms excrete excess phosphate into

their surroundings, and decomposers return phosphate from dead bodies back to the soil and water.

• Some of the phosphate that dissolves in fresh water is carried to the oceans, and may be absorbed by marine producers.

• From marine producers, the phosphate may travel along a marine food web.



Water remains unchanged during the water cycle.• The hydrologic cycle differs from most other

biogeochemical cycles in that most water is not incorporated into other molecules during the cycle.

• The major reservoir of water is the ocean.• The hydrologic cycle is driven by solar energy

evaporating water and by gravity drawing the water back to Earth as precipitation.

• A small amount of this water is incorporated into the living parts of the ecosystem.


water vapor in the atmosphere

evaporation from land and

transpiration from plants

evaporation from the ocean

precipitation over

ocean

reservoirsprocesses

precipitation over land

runoff from land surface

groundwater

water in the ocean

lakes and streams


29.4 What Happens When Humans Disrupt Nutrient Cycles?

Many of the environmental problems that plague modern society are caused by human disruption of biogeochemical cycles.• Some disruptions are direct, as when

industrial processes transfer toxic substances such as lead, arsenic, mercury, uranium, and oil from their normal reservoirs into the environment.

• Other human-caused cycle disruptions, such as addition of herbicides and pesticides to lawns and shrubs, have more indirect effects.



Overloading the phosphorus and nitrogen cycles damages aquatic ecosystems. • In farm fields, gardens, and suburban lawns,

ammonia, nitrate, and phosphate are supplied by chemical fertilizers.

• This phosphorus is removed from its reservoirs in rocks, converted to phosphate fertilizer, and applied to cultivated land.

• Similarly, nitrogen is applied to these areas in the form of ammonia and nitrate fertilizers.



Overloading the phosphorus and nitrogen cycles damages aquatic ecosystems (continued).• The soil that erodes from fertilized fields

carries large quantities of phosphorus and nitrogen into lakes, streams, and the ocean.

• In the water, the nutrients stimulate growth of photosynthetic algae, and when they die, their bodies are consumed by decomposer bacteria that use up most of the oxygen in the water, killing other organisms.



Overloading with nitrogen and sulfur cycles causes acid deposition.• The combustion of fossil fuels in our cars,

power plants, industrial boilers, smelters, and refineries releases sulfur dioxide and nitrogen oxides into the atmosphere.

• The amounts present can far exceed what natural systems can absorb and recycle.

• Because of this, the acids fall as acid rain, or more accurately, as acid deposition.



Reactions in the atmosphere form acid deposition.• When combined with water vapor in the atmosphere,

nitrogen oxides and sulfur dioxide are converted to nitric acid and sulfuric acid.

• The acids fall in rain or snow, eating away at statues and buildings and can render lakes lifeless.

• Acid rain interferes with the growth and yield of many farm crops by leaching out essential nutrients, such as calcium and potassium, killing decomposer microorganisms, and direct physical damage.



Acid deposition can destroy forests.

Fig. 29-13



Reactions in the atmosphere form acid deposition (continued).• Acid deposition also increases the exposure of

organisms to toxic metals, including aluminum, lead, mercury, and cadmium, which are more soluble in acidified water than in neutral pH water.



Overloading the carbon cycle contributes to global climate change.• Much of Earth’s carbon is stored in reservoirs such as

fossil fuels formed from the buried remains of plants and animals.

• Over millions of years, the carbon in the organic molecules of these organisms is transformed by high temperature and pressure into coal, oil, or natural gas.

• Burning fossil fuels transfers the carbon locked in these reservoirs into atmospheric CO2 .



Overloading the carbon cycle contributes to global warming (continued).• Since the Industrial Revolution, humans have

increasingly relied on the energy stored in fossil fuels.

• When we burn them for power CO2 is released into the atmosphere.

• The amount of CO2 in the atmosphere has increased by more than 36% since 1850, from ~280 parts per million (ppm) to ~380 ppm.



Overloading the carbon cycle contributes to global warming (continued).• Deforestation adds CO2 to the atmosphere by

removing a CO2 sink.• When forests are cut and burned or allowed to

decompose the carbon stored in the bodies of the trees returns to the atmosphere.

• Altogether, humans release almost 7 billion tons of CO2 into the atmosphere each year.


atmosphere

sun

sunlight

CO2 methane

nitrous oxide

agricultural activities

heat trapped in the atmosphere

heat radiated into space

power plants and factories

homes and buildings

forest fires

vehicle emissions

deforestation



Greenhouse gases trap heat in the atmosphere.• Atmospheric CO2 acts like glass in a

greenhouse allowing solar energy to pass through it to reach Earth’s surface then trapping it preventing or slowing its release back into space, resulting in increasing heat content of the atmosphere.

• Most climate scientists have concluded that this greenhouse effect has been intensified by human activities that produce greenhouse gases such as CO2 and methane.



The average global temperature has increased since 1860, parallel to the increase in CO2 .

Fig. 29-15

CO2

temperature

1860 1880 1900 1920 1940year

1960 1980 2000

380

400

360

340

320

300

280

14.6

14.4

14.2

13.8

14.0

13.6

13.4

58.3

57.9

57.6

56.8

57.2

56.5

56.1

°C°F

CO

2co

ncen

tratio

n(p

arts

per

mill

ion

by v

olum

e)

aver

age

wor

ld te

mpe

ratu

re



Global warming will have consequences some of which may be severe.• Scientists have documented effects of

warming on glaciers and polar ice caps, on weather patterns, and on ecosystems.

• The speed and magnitude of these changes are expected to increase as warming continues.



A global meltdown is underway (continued).• As glaciers and polar caps melt, global sea

levels rise threatening coastal cities and island nations.

• Permafrost in northern lands are also melting releasing more organic matter to be decayed adding more carbon and methane.

• Thawing permafrost also impacts adjacent ecosystems as previously firm ground is now subject to shifting and sliding.



Glaciers are melting.

Fig. 29-16



Weather is growing more extreme.• Scientists predict that global warming will

increase the severity of extreme weather events.

• As the world warms further, droughts in certain areas will likely last longer and be more severe, disrupting agriculture.



Ecosystems are affected.• On land, plant distributions will change as rainfall and

temperature change.• Sugar maples may disappear from northeastern U.S.

forests while southeastern forests could be partially replaced by grasslands.

• Coral reefs are likely to suffer damage from warmer waters and increased acidity from dissolved CO2 .

• Accumulated data from scientific studies around the world provide strong evidence that warming-related biological changes are well underway.

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