Ecosystems
From Chapter 28.
Major Concepts:
- Chemicals cycle within ecosystems
- Ecosystems are structured by trophic levels
- Energy fows through ecosystems
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:
- The Atmosphere (C, N, O)
- The Water
- 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
Groundwater
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)
Madagascar
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
- ferredoxin
- nitrogen reductase
- 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
To report problems, email webmaster.
