Principles of Biology - Biology 102
Spring Quarter
Lake Tahoe Community College
Instructor: Sue Kloss
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Ch. 55 - Ecosystems
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I. Ecosystem Structure and Dynamics
A. Ecosystems and physical laws
1. Laws of thermodynamics
a. energy can be neither created nor destroyed, only transformed
b. in any transformation, there is always some inefficiency - energy lost as heat
B. Ecosystem – biotic community and abiotic environment (Fig. 1)
1. Energy flow is linear – eventually all energy is released as heat (less organized)
2. Chemical cycling is circular –chemical elements are cycled between living and nonliving components
a. plants acquire them in inorganic form from atmosphere/soil and fix them into organic molecules
b. microbes break down dead organism and return the components in inorganic form to air/soil
C. Trophic structure is key factor in ecosystem dynamics
1. trophic structure – transfer of food from level to level is food chain –(Fig. 2)
2. this doesn’t ever really happen like this – its much more complex = food web
3. arrows move in the direction of energy flow
a. Producers are autotrophs – Ps or Chemosynthesis
b. Primary consumers eat autotrophs. E.g grasshopper
c. Secondary consumers eat primary consumers
d. Tertiary consumers eat consumers from the levels below
e. Quaternary consumers – eat other 3 levels of consumers
f. Detritivores – dead material produced by all levels – animal wastes, plant litter, carrion
1. rodents, earthworms, crayfish, crabs, vultures, as well as fungi and prokaryotes
2. decomposition – turning organic matter into inorganic matter – we all do it, but prokaryotes and fungi
decompose less usable material and make it available to all of life
3. food chains are an over simplification ; interrelated food chains are called food webs
II Physical and chemical factors limit primary production in ecosystems
A. Ecosystem energy budgets
1. Of all the visible light that strikes earth, only about 1% is converted to chemical energy in Ps
2. Gross primary production (GPP) = the amt of light energy that is converted to chemical energy by Ps
a. Net Primary production (NPP) = the amt of GPP - Respiration - R- of primary producers
b. for example, forests may have NPP only 25% of GPP; can be expressed in g/m2/yr (dry wt)
c. different ecosystems have Different NPP (Fig. 4)
d. satellite images can give us info about global NPP (Fig. 5)
3. Limiting factors are factors that must be added for productivity to increase; in L. Tahoe, it’s P limited
a. eutrophication - increased nutrients so that there’s more available food for organisms
1. can be useful - upwelling
2. can turn oligotrophic lakes into algal filled lakes - (Fig. 7)
III. Energy transfers btn trophic levels are usually less than 10% efficient
A. Energy Supply limits length of food chain
1. every day, earth receives about 100 million atomic bombs worth of solar energy
a. absorbed
b. reflected
c. refracted
2. only 1% converted to chemical energy (Ps)
a. in one yr, this is enough to produce 170 billion tons of biomass each year
b. primary production – turning solar energy into chemical energy
c. 1/6 energy eaten in leaf incorporated into caterpillar biomass- what happens to rest? (Fig. 10)
3. different ecosystems produce wildly varying amts of biomass
a. open ocean has very low production but large;
b. tropical rain forests have only 5% of ocean’s area, but produce almost as much
d. pyramid of energy (Fig. 11)
e. 10% avg. of energy is transferred up, differs among ecosystems – from 5 – 20%
f. explains why top level consumers need much bigger home range- it takes a lot of vegetation to support
so many trophic levels
so limited biomass at top of pyramid is concentrated in relatively small number of large individuals who
need large areas
h. no wonder that many face extinction
i. not enough energy at top of pyramid to support another level
B. This is why meat is a luxury for humans and why ecologically it is better to eat lower on the food chain (Fig. 14)
C. Pyramid of numbers - most predators are larger than prey (not always, think of insects in trees) (Fig. 13)
D. Green world hypothesis
1. with so many consumers, how can many terrestrial ecosystems support large, standing crops?
2. green world hypothesis - terrestrial herbivores held in check by predators, parasites, disease (Fig. 15)
a. how green is the world? approx. 83 x 1010 metric tons of C are stored in plant biomass
b. production is about 5 x 1010 metric tons/yr in terrestrial ecosystems
c. herbivores consume annually less than 17% of primary productivity annually
3. green world hypothesis proposes factors that keep herbivores in check a. Plants have defenses vs. herbivores
b. nutrients, not energy supply, typically limits herbivores
c. abiotic factors limit herbivores (unfavorable temp/moisture for herbvrs, etc., keeps them far below K)
d. intraspecific competition limits herbivores
e. interspecific competition limits herbivores
IV. Biological and geochemical processes move nutrients between organic and inorganic parts of ecosystems
A. Chemicals are recycled between organic matter and abiotic reservoirs
1. no extraterrestrial sources of food or water, so life depends on recycling of chemicals
2. We will look at 4: water, carbon, nitrogen and phosphorous
3. Chemicals pass back and forth btn organic matter and abiotic components of ecosystems (Fig. 16)
4. If chemical is stored in large amts outside of living organisms, this is called abiotic reservoir or a sink
B. Water moves thru biosphere in global cycle – see fig. 17
1. water from pacific ocean may end up in animal’s body far inland in N. America
2. human activity affects water cycle
a. destruction of rain forest
b. pumping up of ground water
c. irrigation
C. Carbon Cycle depends on Ps and Respiration
1. increased burning of wood and fossil fuels is putting more CO2 into air
a. global warming
D. Nitrogen cycle relies on bacteria
1. atmosphere is huge N sink – N2
2. nitrate (NO3) and ammonium (NH4) needed by plants, produce by bacteria from N2
3. plants called nitrogen fixers have root nodules for bacterial symbiosis
4. human activities –
a. sewage plants increase N in water bodies
b. N fertilizers end up in water
c. Algae grow in water bodies, kill fish
d. Nitrates in groundwater convert to nitrites, toxic to humans – Salinas area
E. Phosphorus cycle depends on weathering of rock 1. main sink is in rocks
2. same is true of K and Ca
3. slow weathering of rocks adds Phosphates to soil, absorbed by plants and added to biomass for food chain
4. human activities –
a. fertilizers w/P increase algal growth
b. sewage tment increases P wastes
F. Decomposition may limit nutrient cycling rates (Fig. 18)
G. Ecosystem alteration can upset chemical cycling – Hubbard Brook
1. clear-cut and sprayed to prevent plant growth –
a. water runoff increased 30-40% - no plants to cycle it
b. net losses of nutrients- huge Fig. 19
c. nitrate drained out of system; water levels had unsafe amts for humans
d. no Ca ions- leached out- no plant growth as of 1996
V. Human Population is disrupting chemical cycles throughout the biosphere
A. Nutrient enrichment - Agriculture and nitrogen cycling
1. different types of systems hold nutrients for different lengths of time
a. temperate prairie (North America - many years) vs. tropical forests (2 yrs or so)
b. humans have huge imputs of especially N (approximately doubled earth’s version of fixed N)
1. NO x contribute to atmospheric warming, depletion of Ozone, acid rain)
2. Contamination of aquatic ecosystems
g.
B.
C.
D.
E.
a. plants can absorb some amt. of nutrient in ecosystem without damage
b. after that, they leach into ground water and runoff into aquatic systems
1. contaminate water supply
2. choking water ways
3. kill aquatic life
Acid Precipitation - burning wood/petroleum - rel. SO x and NO x
1. Acid precip has pH < 5.6 (Fig. 21, 22)
a. some storms have pH < 3
b. damages plants, alters chemical stores and cycling in soils and aquatic systems, esp. freshwater
toxins in the Environment
1. Biological magnification
a. endocrine disrupters - PCBs. (Fig. 23)
1. in gull eggs, 5000 x greater than phytoplankton
b. DDT, etc.
Carbon Dioxide in atmosphere (Fig. 24)
1. global warming and greenhouse effect - CO2 and water vapor- transparent to light, absorb IR as heat
2. migration of plants and animals may not occur = speed of global warming
depletion of Ozone (Fig. 26, 27)
Ch. 55 Objectives
1. Describe the fundamental relationship between autotrophs and heterotrophs in an ecosystem.
2. Explain how the first and second laws of thermodynamics apply to ecosystems.
3. Explain how decomposition connects all trophic levels in an ecosystem.
4. Explain why the amount of energy used in photosynthesis is so much less than the amount of solar energy
that reaches Earth.
5. Define and compare gross primary production and net primary production.
6. Define and compare net primary production and standing crop.
7. Compare primary productivity in specific marine, freshwater, and terrestrial ecosystems.
8. Explain why energy is said to flow rather than cycle within ecosystems.
9. Explain what factors may limit production in aquatic ecosystems.
10. Explain why areas of upwelling in the ocean have exceptionally high levels of primary production.
11. Distinguish among pyramids of net production, pyramids of biomass, and pyramids of numbers.
12. Explain why aquatic ecosystems may have inverted biomass pyramids.
13. Explain why worldwide agriculture could feed more people if all humans consumed only plant material.
14. Explain the green-world hypothesis. Describe five factors that may act to keep herbivores in check
15. Describe the four nutrient reservoirs and the processes that transfer the elements between reservoirs.
16. Name the main processes driving the water cycle.
17. Name the major reservoirs of carbon.
18. Describe the nitrogen cycle and explain the importance of nitrogen fixation to all living organisms. Name
three other key bacterial processes in the nitrogen cycle.
19. Describe the phosphorus cycle and explain how phosphorus is recycled locally in most ecosystems.
20. Explain how decomposition affects the rate of nutrient cycling in ecosystems.
21. Describe how net primary production and the rate of decomposition vary with actual evapotranspiration.
22. Describe the experiments at Hubbard Brook that revealed the key role that plants play in regulating nutrient
cycles.
23. Describe how agricultural practices can interfere with nitrogen cycling.