BIOLOGY 103
      Chapter 51:  ECOSYSTEMS

 

The most important concept 

Critical materials are recycled within food webs, but energy cannot be recycled; 9/10 of potential energy is converted to entropy at each typical "link" in the food web.


The length of the food web and population sizes are limited by energy produced by photosynthesis. The "leaks" of critical materials out of the system are frequently the limiting factor for photosynthesis and thus the entire food web and species diversity.

for Wednesday, pp. 981b-5,989-995a
Key textbook points  
diversity review
food web review
quiz

 

for Friday, pp. 995b-1001
Key textbook points 
Sample Quiz
 eco-cycle review


Scientific Method in this Chapter

food WEB LINKS

back to Bio103 homepage

Key textbook points for Wednesday

  • Species Diversity in our class will be defined as the number of different types of critters.  
    This definition is technically known as species richness. If you are working on Lab 10, you may need to consider Box 50.1 for other definitions.
  • Species Diversity logically:  
    • depends on the probability that each species there can persist despite competition, predation, lethal parasitism, stochastic effects on its population growth, etc.
    • also depends on how likely it is that different species have the dispersal ability and numbers of recruitment units (immigrants, seeds, etc.) to get there
    • and richness also depends on how long the habitat and its conditions have existed; if it's relatively young and rare, maybe only a few species have had a chance to evolve adaptations suitable for it
  • These logical points are based on MacArthur & Wilson's Island Biogeography Theory (1967, updated) :
    • Island Biogeography Theory

      • generalizes (lawlike) that succession eventually leads to the same number of species in a particular habitat, and that big and nearby habitats have more species than smaller and more isolated habitats;

      • explains the causes (theories explain): according to the original theory (1967), the two main forces which determine species numbers are immigration and interspecific competition.  A third force, speciation, was documented in 2000.
  • TEXTBOOK chapter 50 pp. 981-2 describes some hypotheses for causes of higher species diversity; connect the logical points from Island Biogeography Theory above to these hypotheses about diversity:
    • latitude:  species numbers usually increase toward the equator
    • environmental heterogeneity
    • time/stability
    • intermediate disturbance
    • (from lab 4) area  
    • (also consider what you know from previous chapters and labs, like 
      • the Law of the Minimum 
      • and the Law of Competitive Exclusion 
      • and keystone predators
      • environmental stochasticity
      • demographic stochasticity
      • genetic stochasticity
      • other factors contributing to extinction, like habitat destruction)
  • Textbook pp. 983-5 Why is higher diversity "good" for 
    • productivity? (fig. 50.15)
    • resistance to disturbance? (fig. 50.16a)
    • resilience (recovery) after disturbance (fig. 50.16b)
  •  
  • 989- ECOSYSTEM PROCESSES.  
  • click to enlarge:  foodChain1.jpg (151897 bytes)foodChain2.jpg (74396 bytes)

    The diagrams shown above are another way to illustrate the concepts shown on textboook figures 51.2 and 51.4 and 51.6b and the paragraphs on pp. 990-995. 

    All the diagrams show how energy and matter flow through the biosphere or a part of it--one ecosystem. These figures are key to understanding this chapter. 

    Make sure you understand how these figures connect to the other diagrams and concepts throughout chapters 48 and 49 and 51.  This is a very good time to review  these relevant concepts:  biodiversity, law of the minimum, predator, competitor, prey.   
  •  
  • 990-1:  Primary productivity (= photosynthesis) provides all the energy that goes into the food web for most ecosystems.  Which kinds of ecosystems have unusually high productivity?  Which ecosystems are very low in photosynthesis?  (See figure 51.3  as another way to see the paragraph-type explanation)  Photosynthetic rates depend most often on light, water (but ice doesn't count, since plants can't use it), and nutrients (especially nitrogen, phosphorus, potassium, sometimes calcium or iron).  Which factors in particular help explain the differences on the figures on figure 51.3?  How can agricultural practices change the productivity of agricultural ecosystems?  (You can probably find the answer in your own brain, but you could review limiting factors.)
  • 990-4:  9/10 of potential energy (biomass) is converted to entropy at each typical "link" in the food web.  Use this fact to explain the pyramids of energy on fig 51.6b..   
  • 994-5:  Food Webs and species diversity.  Re-think the hypotheses about species diversity in light of these ideas.
  • CHECKLIST herbivores, carnivores, detritivores, disturbance, species diversity, latitudinal gradient, resistance, resilience, stability, community, ecosystem, energy flow, primary producers, autotrophs, consumers, herbivores, carnivores, decomposers, abiotic environment, biotic environment, biomass, photosynthesis, chemosynthesis, gross photosynthesis, gross photosynthetic efficiency, net primary production, grazing food web, secondary production, decomposer food web, detritus, trophic structure, trophic level, food chains, food webs
  • WEDNESDAY'S QUIZ & EXAM PREVIEW
    • http://wps.prenhall.com/esm_freeman_biosci_1/0,6452,501340-,00.html
      • summary review #17, 18, 19
      • Applying #3 (think RibbonWalk) and #5
    • http://wps.prenhall.com/esm_freeman_biosci_1/0,6452,501340-,00.html
      • summary review #4, 5, 6, 7, 8, 9
      • p. 1001 Content #1, 2, 3, 4; Conceptual #1, 2, 4; Applying #2
    • and these:
      1. Biologists call critters which eat plants
        [a] autotrophs.   [b] carnivores.    [c] decomposers. [d] herbivores.

      2. An ecological pyramid of energy suggests that
        [a] most of the energy available in a habitat is lost to the system at each trophic level.
        [b] most energy is recycled in a stable community.
        [c] the critters toward the top have much more food energy available for consumption.

      3. Productivity at the surface of the open ocean  is limited most by the availability of
        [a] light.   [b] phosphorus.    [c] water.   [d] other physical factors.

      4. Productivity in the arctic is limited most by the availability of
        [a] light.   [b] phosphorus.    [c] water.   [d] other physical factors.          click here for answers

Critical materials are recycled within food webs, but energy cannot be recycled; 9/10 of potential energy is converted to entropy at each typical "link" in the food web.    food web links

FRIDAY

  • 995b-1000:  9/10 of organic matter (biomass) is converted to inorganic nutrients at each typical "link" in the food web.  But, unlike entropy, these molecules can be recycled.  By Friday be prepared to draw and explain the biogeochemical cycles in general (fig. 51.8) and specifically  carbon (Activity 51.1 The Global Carbon Cycle and fig. 51.11) and nitrogen. (fig. 51.13a)
    • The book doesn't say much about the impact of living things and their biomass on the water cycle.  Can you?  Remember RibbonWalk?  optional:  water cycle movie http://www.brainpop.com/science/ecology/watercycle/index.weml?&tried_cookie=true   
    • What is the connection of the carbon cycle to global temperature change?  What are fossil fuels http://www.nytimes.com/library/national/science/051600sci-environ-carbon.1.jpg.html What are some possible solutions to global temperature change?
    • What is "fixed nitrogen" and why does it matter?  The book says a lot, but not enough, about the human impact on the nitrogen cycle.  What can you add?  Remember RibbonWalk?
    • How do these cycles help explain low productivities on Figure 51.3?  Why do upwellings and turnovers and eutrophication cause changes in productivity?  Are these changes always "good" from a human being's viewpoint?  Be prepared to draw or explain the biogeochemical cycle connection to the Law of the Minimum and to these issues:   global warming, fossil fuel use, world hunger.
    • more FIGURES of global cycles:  phosphorus, sulfur, water
  • don't forget Activity 51.1 The Global Carbon Cycle
  • CHECKLIST:  biogeochemical cycles, nutrient flow, nutrient export, soil organic matter, humus, vegetated, devegetated, global carbon cycle, greenhouse gas, global nitrogen cycle, "fixed" nitrogen
  • Sample Quiz for Friday
    • The textbook authors emphasize the importance of human activity in altering the limiting resources of habitats. You should expect questions where you can apply the most important concept to specific situations involving food web processing of energy and matter, like draining wetlands or paving parking lots or farming tropical forests or eutrophication of aquatic habitats or the greenhouse effect or being a carnivore or finding solutions to world hunger.

     

The biology of this chapter rests on the laws of thermodynamics from physics:

THERMODYNAMICS

First Law: Matter and energy cannot be created or destroyed in ordinary chemical reactions. (Law of conservation of matter and energy)

Second Law: Whenever energy is converted from one form to another (like from water pressure to electricity, from electricity to light), some of the energy is converted to entropy and can no longer be used to do work. (Law of entropy)

WHY THE LAWS OF THERMODYNAMICS ARE IMPORTANT IN THIS COURSE:

  1. Biologists assume that living critters and their ecological communities "obey" the laws of nature, including both laws of thermodynamics. So far, scientists have demonstrated no "vital force" beyond the natural processes of physics and chemistry.

  2. Applied to food webs, the First Law tells us that critters do not make their fur and fronds from nothing,. Instead, all critters re-arrange pre-existing molecules (=matter) into new molecules, using energy released when some large molecules are converted into smaller molecules. The energy-releasing molecules are rebuilt from smaller molecules with energy transferred from solar energy in photosynthesis.

  3. According to the Second Law of Thermodynamics, every time a photosynthetic leaf channels rays of sunlight energy into its molecular reaction centers, some of the solar energy ends up in the calories of new sugar molecules, but some energy is also released from the leaf as heat energy, or entropy. When the plant converts the sugar molecules to molecules of protein or fat, again some of the calories of energy are released as entropy. When a bug eats the leaf, the bug's cells rearrange the plant's sugar and fat and protein molecules into new buggy versions, and more of what was once solar energy is released as entropy. A bird eats the bug and rearranges bug molecules into egg and feather molecules; again more calories are "burned." Every time a new molecule is assembled or moved to a new location, some entropy is produced and some other molecules are broken down to release calorie energy for assembling and moving. As a general rule, only 10% of the calories digested remain in the consumers' bodies, available to provide energy for the predators in the next trophic level of the food web. In other words, 9/10 of the energy eaten by the average critter becomes entropy. At the same time 9/10 of the big energy-containing molecules they eat are converted to smaller, low-energy molecular "wastes" like carbon dioxide and water and urea.

  4. So is there a food web theory? Ecologists have spent a century or so documenting food webs, comparing the sizes and numbers of trophic levels, measuring the flow of matter and energy through them. Ecologists unraveled the "rules" for food webs gradually; so nobody wrote a formal famous food web theory. However, you don't have to be a genius to recognize the informal theory which could summarize Chapter 25: The quantities of matter and energy contained in living tissue decrease by about 90% at each trophic level because of the entropy generated in conversions. The entropy leaves the ecosystem, but matter can be recovered by the producers and cycled through food webs again.

  5. Connections among food webs, mineral cycles, productivity (=the rate of food production or photosynthesis by the primary producers) and biodiversity are complex. 

    • A few weeks back, you studied the LAW of LIMITING FACTORS, which implied that The species richness (=number of different kinds of critters) in a habitat will be limited to the species which can find the resources they need.  

      • Obviously, mineral deficiencies will limit not only plant species richness but also their photosynthetic rate and therefore their productivity, the  amount of energy at the bottom of the food chain.

      • Energy (= food!) is obviously a limiting factor for all the consumers in the other trophic levels of the food web.

    • Biodiversity of the plant producers seems to increase productivity (see http://www.sciencemag.org/cgi/content/full/286/5442/1123 for some of the most recent research).  Increases in productivity logically might be expected to cause increases in biodiversity at consumer levels of the food chain; since there's more food to divide you might expect the "pie to be sliced" into more species.  Species richness patterns do often follow this trend, but there are many exceptions. 

    • For the exceptions, just think about how higher species richness of at the bottom of the food web creates more types of food and therefore more potential niches at the second level and therefore....

    • But on the other hand, sudden increases in productivity typically give a temporary advantage to one consumer species, which then competitively excludes other species at the same trophic level, and thus wipes out their predators higher on the food chain.  The web collapses; a good example is eutrophication, too much of a good thing (a mineral nutrient, in this case).

  6. The First Law of Thermodynamics is an example of a law which changed. Can you figure out which part of the First Law changed? When? Why?

  7. Critics of biology sometimes claim (incorrectly!) that evolution cannot be true because the Second Law of Thermodynamics requires that entropy always increases. Thus, the critics mistakenly conclude, life cannot change from simple to more complex without supernatural miracles. The error these critics make is in forgetting that as living processes convert small molecules to larger ones, they release other small molecules and entropy into the environment; they don't increase the entropy within their own bodies. All living things accumulate energy and matter by increasing entropy outside their own bodies. An obvious example is the "miracle" of human development and birth from a microscopic event of conception. How does a fetus get energy and matter? Does a fetus produce entropy as it becomes more complex?

Critical materials are recycled within food webs, but energy cannot be recycled; 9/10 of potential energy is converted to entropy at each typical "link" in the food web.   food web links

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answers 

1. [d] herbivores.
2.  [a] lost to the system
3. [b] phosphorus (it's at the bottom of the ocean)
4.  [c] water (it's frozen)

LINKS

NEWS  STORIES which could be on the exam

 

       


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