From Chapter 28.

Major Concepts:

Your textbook defines an Ecosystem as "A major interacting system that involves both organisms and their nonlivng environment" and states that it is the most complex level of biological organization. We can think of an ecosystem as being defined by the transfer of energy and mass, governed by the concepts we have been discussing in the last 4 lectures. The controlled cycling of nutrients is a key defining property of ecosystems, and collectively, the organisms that comprise and ecosystem regulate the capture and expenditure of energy and the cycling of chemicals. All organisms within an ecosystem depend on the phsyiological and ecological activities of the other organisms within the ecosystem to stay alive.

I. Chemicals Cycle Within Ecosystems

Biogeochemical cycling is the movement of mass, energy and nutrients through both biological and geological processess.

Pools are the stores or reservoirs of substances that are involved in biogeochemical cycles

Globaly the major inorganic poos are:

  1. The Atmosphere (C, N, O)
  2. The Water
  3. Rocks (P, K, S, Mg, Ca,Na, Fe, Co)

Fluxes are the movements of substances among the pools.

Rocks -> Water -> Organisms ->2° Organism -> -> -> rocks/water

The Water Cycle

Figure 25.2 from your textbook

Most living things are composed primarily of water. As the ultimate source of H+, leads to the formation of ATP through chemiosmosis.

The path of free water

Oceans cover 3/4 of the surface of the Earth

Evaportationis powered by the Sun.

90% of the Atmospheric moisture comes from Evapotranspiration

~2% of the water on the earth is captured in any form - frozen, held in soil or incorporated into living things

Free Water is the the other 98% (oceans, rivers, streams, lakes, seas)

The importance of water to organisms

The availability of water can determine which organisims live and which die.

crops require about 1000kg of water to 1 kg of food!

Free water can determine the regional abundance of living organisms present


Aquifers - permeable, saturated, underground layers of rock, sand and gravel

Can be the most important reservoir of water available to living organisms

96% of all fresh water in US

25% of all water used by US

50% of the drinking water used by the US

Water Table - upper unconfined portion of the groundwater

flows into streams

partly accessible to plants

recharged by soil percolation - slowly

Goundwater flow is very slow, mm to m day-1

Can be consumed faster than it is recharged (Ogallala Aquifer in the Great Plains)

Chemical pollution is very serious problem

2% of the US groundwater is polluted

pesticides, herbicides, fertilizers

slow turnover, location size of groundwater sources make removing pollutantsvirtually impossible.

Breaking the water cycle

90% of water in a tropical forest passess through the plalnts - making them the primary source of local rain fall

Deforestation can break the water cycle

Von Humbolt in Columbia (late 1700's)


Can be directional change that is very difficult if not impossible to reverse

The Carbon Cycle

CO2 ~ 0.036% of the atmosphere

10% of Atmospheric pool is fixed (removed from the atmosphere) via photosynthesis each year

Ultimately Respiration returns this carbon to the atomsphere or water

Carbon can accumulate in peat, which can then be incorporated into fossil fuels such as coal or oil

The oceans can absorb a large amount of carbon

Human activities are altering the global carbon cycle

Figure 25.4 from your textbook

The Nitrogen Cycle

Nitrogen fixation (the coversion of atmosphereic N2 gas to NH3) is the ultimate source of N in living things

All living things require N (proteins, nucleic acids and other N containing compounds)

Atmosphere is 78% N2 (no mineral sources of N)

very strong triple bond, difficult to break

Biologically fixed N is only 0.03% of the atmospheric pool

N2 fixation

N2 + 3 H2 --> 2 NH3

transforms N2 gas into a biologically useful form of N

bacteria are the only organisms that can complete the process

  • free living soil bacteria
  • symbotic with legume roots and some other plants

over time N fixation has led to the accumulation of a significant pool of available N for plant growth

requires 3 proteins

  1. ferredoxin
  2. nitrogen reductase
  3. nitrogenase

requires ATP and a reducing agent

Bacteria and fungi breakdown nitrogen-containing compouds from decaying organic matter release excess ammonium ions (NH4+) via Ammonification

Ammonium ions can be converted to soil nitrites and nitrates

NH4+, NO3- and some amino acids can be absorbed by plants.

Some fixed N is returned to the atmosphere as N2 and N20 (nitrous oxide) gas by Denitrification

Figure 25.5 from your textbook

The Phosphorus Cycle

representative of mineral cycles.

P can limit plant growth

Phospates relatively unavailable in most soils

relatively insoluble

accumulate in sdiments

Guano is rich in Phosphorous

Phosphate rich rocks (apatite)

Ocean is largest pool - deep seabed mining has been considered

Figure 25.6 from your textbook

Hubbard Brook Ecosystem Experiment

Experimental Forest in New Hampshire, studied since 1963

Watershed - an area defined by topography that drain into a common river system

Temperate decidous forest

V-notch weirs measure water flow in 6 tributaries of Hubbard Brook, the central stream

Stream chemistry, flow and precipitation rates measured in each of the 6 watersheds

Undisterbed forest are very efficient at retaining nutrients

Nutrients in (rain and snow) = Nutrients out (stream water)

very small % of total nutrients cycled in the biology of the watershed

very small loss of Ca, small gain of N and K

1965 & 1966, one watershed was clear cut and treated with herbicides

Stream flow increased by 40%

Ca loss increased by 10 times

N loss of 120 kg ha-1 yr-1

Figure 25.7 from your textbook

[NO3-] in the stream was elevated above EPA approved levels

alagal blooms

P did not change

Rapid decrease in fertility

II. Ecosystems Are Structured By Who Eats Whom, i.e. Trophic Levels.

Autotrophs, or Primary Producers - plants, algaeand some bacteriathat are able to capture light energy and manufacture their own food. Capture about 1% of the energy that reaches the leaves.

Heterotrophs, or Consumers - animals, fungi, most protists and bacteria and nongreen plants - obtain organic molecules sysnthesized by autotrophs.

Primary consumers - feed directly on autotrophs (herbivores)

Secondary consumers - feed on primary consumers (carnivors and perasites)

Decomposers - break down the organicmatter accumulated inthe bodies of other organisms.

Detritivores - live on refuse of an ecosystes, includes decomposers and the large scavengers such as crabs, vultures and jackals.

An Ecosystem contains all of these levels of organization and can be broken down into trophic levels

Organisms in each trophic level feeds on on another in a Food Chain

Figure 25.8 from your textbook

Food chain length and complexity are variable

Food Webs are branching food chains - a series of orgainisms that feed on each other

As energy is transfred among the trophic levels some is always lost as heat (why?)

> 40% of energy ingested goes towards growth and reproduction

Invertebrates ~ 25% towards growth (10% of total ingested)

Carnivors ~ 12% towards growth (5% of total ingested)

Herbivors ~ 50% towards growth (20% of total ingested)

10% of the energy reaches the next trophic level.

Figure 25.9 from your textbook

III. Energy Flows Through Ecosystems

Primary Productivity

1 to 5% of solar energy falling on a plant is converted to organic matter

Amount of orgainc matter produced from solar energy in a given area during a given periodof time - Primary Production

Gross Primary Production (GPP) - includes organic matter respired by autotrophs

Net Primary Production (NPP) - amount of production available to heterotrophs

NPP = GPP - autotrophic respiration

Biomass - the weight of all the organisms living in an ecosystem, increases as a result of NPP

Productive Biological Communities

Ecosystems differ in the rate of NPP

wetland and rainforests have high levels of NPP, 1500 to 3000 g m-2 yr-1

Ecosystems differ in their total biomass

rainforest > wetlands

Table 25.1 from your textbook

Secondary Productivity

rate of production by heterotrophs - Secondary Productivity

approximately an order of magnitude lower than NPP

some biomass not consumed (feeds decomposers)

some energy passes through the heterotrophs (feces)

some lost as heat (our old friend the second law of thermodynamics)

Figure 25.10 from your textbook

The Energy in Food Chains

Food chaims generally have three or four steps, after that too little energy remains to support other organisms

Community Energy Budgets

Cayuga Lake

150 of each 1000 cal fixed by algae and cyanobacteria are transferred to small heterotrophs

30 cal to secondary consumers (smelt)

6 cal to humans eating smelt

If trout eat smelt and a human eats the trout - human captures about 1.2 cal of the orginal 1000

The lower you eat on the food chain, the more energy is conserved

Figure 25.11 from your textbook

Factors Limiting Community Productivity

In theory, communities with higher NPP can support longer food chains

Ultimate constraint is the amount of sunlight received

NPP increases as growing season lengthens

NPP is higher in warm vs. cold climates

more N available

Ecological Pyramids

Plants capture about 1% of suns energy

Each successive member of the food chain gets about 10% of what was available to the last level

Generally more individuals at the lower levels

Generally more biomass at the lower levels

large animals are characteristically members of the hiher trophic levels.

Inverted Pyramids

some aquatic ecosystems

requires a rapid turnover at the lower level (autotrophs - high reproduction rate)

Top Carnivores

Energy loss between trophic levels limits the number of top carnivors that a community can support

1/1000 of the energy captured by photosynthesis can pass to a tertiary consumer such as a snake or a hawk

Why aren't there any predators living primarily on lions or eagles?

Figure 25.12 from your textbook

Useful Links

Lecture by Professor Kevin Griffin.

Updated April 20, 2005
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