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1

Dynamics of Ecosystems

Chapter 57

Biogeochemical Cycles

• Ecosystem

– Includes all the organisms that live in a particular place, plus the abiotic environment in which they live and interact

• Biogeochemical cycles

– Chemicals moving through ecosystems

– Biotic and abiotic processes

• Biogeochemical cycles usually cross the

boundaries of ecosystem

– One ecosystem might import or export

chemicals to another2


• Carbon cycle

– Carbon is a major constituent of the bodies

of organisms

– Carbon fixation: metabolic reactions that

make nongaseous compounds from

gaseous ones

– Aerobic cellular respiration releases CO2

– Methanogens: produce methane (CH4) by

anaerobic cellular respiration

3

4

Carbon cycle

Photosynthesis

Photosynthesis

Respiration

Combustion of fuels in

industry, homes, and cars

Plant

Respiration

Food

chainsAnimal

Respiration

Conversion

by geological

processes

Microbial

Respiration

Food

chains

Exchange

between

water and

atmosphere

Oxidation

of methane

Release

of methane

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

CO2 in atmosphere

Dissolved CO2 and

HCO3–

Carbon in algae

and plantsCarbon in

animals

Carbon in

fossil fuels

(coal, petroleum)

Carbon in dead

organic matter

Carbon in

animals

Carbon in

plants


• Basic water cycle

– Liquid water from the Earth’s surface evaporates into the atmosphere

– Occurs directly from the surfaces of oceans, lakes, and rivers

– Terrestrial ecosystems: 90% of evaporation is through plants

– Water in the atmosphere is a gas

– Cools and falls to the surface as precipitation

5

Water cycleCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Percolation through soil

EvaporationPrecipitation

Condensation

Precipitation

Precipitation

Water in

the oceans

Transpiration

Flow of rivers

to the sea

Evaporation

Gaseous water

(water vapor)

in the

atmosphere

Water in lakes

and rivers

Groundwater

Droplet

water

6


• Nitrogen Cycle

– Nitrogen is a component of all proteins and

nucleic acids

– Usually the element in shortest supply

– Atmosphere is 78% nitrogen

– Availability

• Most plants and animals cannot use N2

(gas)

• Use instead NH3, and NO3–

7


• Nitrogen fixation: synthesis of nitrogen containing compounds from N2

– Nitrification: N2 → NH3 → NO3–

– Denitrification: NO3– → N2

– Both processes are carried out by microbes: free or living on plant roots

– Nitrogenous wastes and fertilizer use radically alter the global nitrogen cycle

– Humans have doubled the rate of transfer of N2 in usable forms into soils and water

8

Nitrogen Cycle

Urea

Excretion

DecompositionDecomposition

Microbial metabolism

Animal excretion

Growth

Food chains

of NH3

N2 in

atmosphere

Nitrogen in

tissues of algae

and plants

Nitrogen in

animal tissues

Nitrogen fixation

by aquatic

cyanobacteriaActivity of

denitrifying microbes

Dissolved NH3 and NO3–

Nitrogen fixation by

soil microbes

Nitrogen in

animal tissues

Nitrogen in

plant tissues

Uptake

by roots

Release

of ammonia

by soil

Soil NH3 and NO3–


Food

chains

9


• Phosphorus cycle

– Phosphorus is required by all organisms

• Occurs in nucleic acids, membranes, ATP

– No significant gas form

– Exists as PO43– in ecosystems

– Plants and algae use free inorganic phosphorus; animals eat plants to obtain their phosphorus

10

11

Phosphorus cycle

Excretion

Decomposition

Loss in drainage

Precipitation Uptake

by roots

Food

chains

Excretion and

decomposition

Food

chains

Uptake

by roots


Phosphates in sediment

Phosphates in

plant tissuesPhosphates in

animal tissues

Phosphates in

solution

Soluble

phosphates

in soil

Phosphates in

plant tissues

Phosphates in

animal tissues

Phosphates in

rocks and minerals

Weathering

Flow of Energy in Ecosystems

• Energy is never recycled

• Energy exists as:

– Light

– Chemical-bond energy

– Motion

– Heat

• First Law of Thermodynamics: energy is

neither created nor destroyed; it

changes forms12


• Second Law of Thermodynamics: whenever organisms use chemical-bond or light energy some is converted to heat (entropy)

• Earth functions as an open system for energy

• Sun is our major source of energy

13


• Earth’s incoming and outgoing flows of

radiant energy must be equal for global

temperatures to stay constant

• Human activities are changing the

composition of the atmosphere

• Greenhouse effect: heat accumulating

on Earth, causing global warming

14


• Trophic levels: which level an organism

“feeds” at

• Autotrophs: “self-feeders” synthesize the

organic compounds of their bodies from

inorganic precursors

– Photoautotrophs: light as energy source

– Chemoautotrophs: energy from inorganic

oxidation reactions (prokaryotic)

• Heterotrophs: cannot synthesize organic

compounds from inorganic precursors

– Animals that eat plants and other animals15


• Trophic levels

– Primary producers: autotrophs

– Consumers: heterotrophs

• Herbivores: first consumer level

• Primary carnivores: eat herbivores

• Secondary carnivores: eat primary carnivores or herbivores

• Detritivores: eat decaying matter

–Decomposers: microbes that break up dead matter

16

17

Trophic levels within an ecosystem


Primary producers

Herbivores

Primary carnivores

Secondary carnivores

Detritivores

Trophic Level 1

Trophic Level 2

Trophic Level 3

Trophic Level 4

Sun


• Productivity: the rate at which the

organisms in the trophic level

collectively synthesize new organic

matter

– Primary productivity: productivity of the

primary producers

– Respiration: rate at which primary

producers break down organic compounds

18


• Gross primary productivity (GPP): raw

rate at which primary producers

synthesize new organic matter

• Net primary productivity (NPP): is the

GPP less the respiration of the primary

producers

• Secondary productivity: productivity of a

heterotroph trophic level

19

Ecologists figure as a rule of thumb that the amount of

chemical-bond energy available to a trophic level over

time is about 10% of that available to the preceding

level over the same period of time20


Heat

1%

Feces and death

Detritivores

Chemical-bond

energy in dead

bodies, feces,

and other non-

living organic

products useful

only to

detritivores.

Solar energy

Herbivores

Primary

producers

Primary

carnivores

Respiration

Secondary

carnivores

Trophic-level interactions

• Trophic cascade: process by which effects exerted at an upper level flow down to influence two or more lower levels

– Top-down effects: when effects flow down

– Bottom-up effects: when effect flows up through a trophic chain

21

22

Top-down effectsEnclosures with brown trout had fewer herbivorous

invertebrates and more algae than ones without trout


0

5000

No fish Trout No fish Trout

2.0

1.5

1.0

0.5

4000

3000

2000

1000

0

Inve

rte

bra

tes (

nu

mb

er/

m2)

Alg

ae

(u

g c

glo

rop

hyll

a/c

m2)

23

Top-down effectsStream enclosures with large carnivorous fish have

fewer primary carnivores, more herbivorous insects, and

a lower level of algae


Carnivorous

damselfly

nymphs

No Fish

High

Herbivorous

insects

Low

Algae Carnivorous

damselfly

nymphs

Low

Herbivorous

insectsAlgae

Fish

High

Po

pu

lati

on

Siz

e

Po

pu

lati

on

Siz

e

Biodiversity and Stability

• Species richness is influenced by

ecosystem characteristics

– Primary productivity

– Habitat heterogeneity

• Accommodate more species

– Climatic factors

• More species might be expected to

coexist in seasonal environment

24

Biodiversity and Stability

• Tropical regions have the highest diversity

– Species diversity cline: biogeographic gradient in number of species correlated with latitude

• Reported for plants and animals

– Evolutionary age of tropical regions

– Increased productivity

– Stability/constancy of conditions

– Predation

– Spatial heterogeneity25

26

Latitudinal cline in species richness

Biodiversity and StabilityCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Number

of species

650–700

600–650

550–600

500–550

0–50

50–100

100–150150–200

200–250

250–300

300–350350–400

400–450

450–500

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