|Concepts of Biology (BIOL116) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; email@example.com; http://www.employees.csbsju.edu/ssaupe/|
Ecology: Ecosystems & Nutrient Cycles
Recall that organisms exchange materials with the environment. Essentially, living things must obtain two major materials from the environment:
A. Energy (click here for notes)
B. Proper elemental building blocks
elements are found in a typical organism.
O, N make up 99% of the total in any individual.
The other 24 or so elements are also important (especially P and
S), but required in lesser amounts.
As an aside, the growth and survival of an individual is dependent upon obtaining adequate materials from the environment. The Law of Limiting Factors states that the element in least amount relative to needs to individual will limit its growth. A good analogy is a barrel - just as you can only fill a barrel as full as its longest stave, an organisms growth will be dictated by its most limiting factor.
Elements are recycled (unlike energy!).
The cycling or exchange of elements through the ecosystem is called a biogeochemical cycle. The elements ["chemical"] cycle between biotic (living organisms, "bio-") and abiotic (rock, air, water; "geo-") reservoirs.
A. General model for a biogeochemical cycle (diagram provided in class)
organisms comprise the biotic reservoir
An element can exist in various forms in the biotic reservoir.
Elements move in the biotic reservoir via food chains/webs
Element movement through the biotic reservoir parallels energy flow.
Nutrient elements return to the abiotic reservoir via death, excretion, wastes.
There are various types of abiotic reservoirs (air - atmospheric cycle such as for carbon and nitrogen; water - hydrological cycle especially for hydrogen and oxygen; rock - sedimentary cycle as for phosphorus).
Elements exist in various forms in the abiotic reservoir.
Geological activity moves materials through the abiotic reservoir.
Movement through the abiotic reservoir is typically much slower than through the biotic reservoir.
Points to consider.
As you study each biogeochemical cycle, consider the following questions:
In what form does the element exist in the abiotic and biotic reservoirs?
In what form does the element return to the abiotic reservoir (and by what process/es)?
How did the element enter the biotic reservoir and in what form?
How does the element move in the biotic and abiotic reservoirs?
Does the element move fast or slow?
Is it an atmospheric, sedimentary or hydrological cycle?
Hydrological cycle - the
cycling of water (hydrogen, oxygen).
See diagram in text. We won't discuss this one directly so check it out in the text. Some important points to remember: evaporation, runoff, precipitation, ground water, transpiration, watershed, aquifer.
IV. Carbon cycle - an atmospheric cycle.
See diagram in text and provided in class. Points to consider:
reservoir = carbon dioxide in air (0.033%) and dissolved in water
(carbonates), rock/fossils = petroleum products, limestone, sediments.
reservoir - carbon is the basic structural building block for virtually all
molecules (such as proteins, carbohydrates, lipids or fats, and nucleic
enters the biotic reservoir via photosynthesis by plants
Carbon moves in the biotic reservoir via food
to the abiotic reservoir via respiration, death, excretion
combustion, uplifting, etc., return carbon trapped in rock/petroleum to
return to the atmosphere.
Effect - increased global
temperature, like inside an automobile on a hot day, which is the result of
increased levels of carbon dioxide and other greenhouse gases like CFC's
(industrial byproduct, 15% of total), methane (microbial fermentations,
especially in rice fields, cow guts and termite hills; 15%), and nitrous oxide
(N2O, 10%). They act like a transparent blanket that
absorbs heat, but allows light to penetrate. Carbon dioxide levels
have increased dramatically in recent history because: (a) increased combustion (fossil fuels, slash/burn
agriculture in tropics) and (b) deforestation (decreased number of plants
that can remove carbon dioxide).
There is a little controversy about the relative importance of organisms in this cycle. Some believe that the role of organisms is minor compared to cycling that occurs in the abiotic reservoir.
Nitrogen Cycle - another atmospheric cycle.
About 80% of air is made up of N2 gas. Points to consider:
reservoir - N2 (air), ammonia (NH4), nitrate (NO3),
& nitrite (NO2). The latter three are mostly dissolved
reservoir - nitrogen used in many molecules especially in proteins and
nucleic acids (DNA)
fixation - converts nitrogen gas into ammonia.
This is the result of the action of:
(a) aquatic microbes like Anabaena and Nostoc, which
are types of blue-green algae that are better called cyanobacteria); (b)
electrical discharges ("the grass is always greener after a thunder
storm"), (c) free-living soil microbes (Azotobacter); and (d)
symbiotic nitrogen-fixing bacteria (Rhizobium) in nodules (bubbles)
on the roots of legumes (called poor-man's persons, rich in protein cause
they fix nitrogen) and a few other plants.
absorb ammonia (conifers, grasses) or nitrate (most others).
- conversion of ammonia to
nitrate by soil microbes (Nitrosomas converts ammonia to nitrite; Nitrobacter
converts nitrite to nitrate). Favored
by warm temperature and neutral pH.
and decayed organic materials decomposed by another set of bacteria into
ammonia - called ammonification. Various
microbes are responsible, favored by cool temperatures, all pH's.
- closes the cycle. Returns
nitrogen to the atmosphere. Conversion
of nitrate, nitrite, and/or ammonia back to nitrogen gas by other microbes.
Note the heavy reliance on microbes for the
function of this cycle.
plants can usually out compete others in nutrient poor soil
fixation is "expensive" - it requires a lot of energy (high
Legumes are excellent green manure
Phosphorus Cycle - a sedimentary cycle.
Much slower than other cycles.
is the main form of phosphorus in the abiotic reservoir that is available to
plants. Also occurs as guano,
is used in nucleic acids, membranes, energy metabolism
bacteria return phosphate back to the abiotic reservoir when they
decompose organic wastes and decaying materials
is relatively insoluble in water - it precipitates out of solution easily.
Ultimately, phosphate forms deposits in the oceans.
returns (moved) via weathering, volcanic activity, etc.
Eutrophication often related to phosphorus availability - which is typically a limiting element. Do you recall our lab last semester when we studied the water chemistry of East Gemini Lake?
Homeostasis exists in nutrient cycles
In a stable ecosystem, biogeochemical cycles show homeostasis. In other words, the cycles are intact and efficient. Or more simply stated: Input = Output.
Hubbard Brook Forest
in the White Mountains (NH) is a massive project by Gene Likens, G. Bormann and
colleagues that demonstrated the importance of an intact ecosystem for nutrient
cycling. Hubbard Brook is comprised
of a series of forested valleys each with watershed that is drained by a single
stream. Thus, it was an ideal
"contained" system. Scientists could
measure inputs and outputs before and after clear cutting the forest (Table 1).
Table 1: Selected results from studies on nutrient cycling at Hubbard Brook Forest (arbitrary units)
Undisturbed (before clear cutting)
Disturbed (after clear cutting)
Output (Net change)
Results: Runoff, nutrient loss (see Table) and erosion all increased dramatically after clear cutting.
Conclusion: Destruction of vegetation alters nutrient cycles. (were you really surprised?)
This idea was first proposed by James Lovelock and later championed by Lynn Margulis and others. Lovelock has a couple of books out, check them out. His ideas are considered somewhat skeptically by many ecologists - mostly because they are difficult to test scientifically.
Named for Greek Goddess of the Earth.
According to Lovelock - the Earth (biosphere) is a self-regulating system. The evolution of life and the planet are intimately connected. They each influence on
Essentially, the Earth is like a giant organism.
environment (i.e., temperature, moisture, radiation.)
influences the environment. For example: oceanic producers
use CO2 →
less greenhouse effect →
ice age →
oceanic life dies →
CO2 increases →
oceanic producers →
requires active biogeochemical cycles - Plate tectonics.
Persistence of life was a puzzle to early geologists who predicted
that nutrients should all wash into ocean sediments (Harold Morowitz)
Maintenance of nutrient cycles (e.g., Hubbard Brook).
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Last updated: April 22, 2004 � Copyright by SG Saupe