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Food Webs and Energy Flow. Chapter 17. 1. What is a food chain? Ex: midwest deciduous forest & Artic & 1 other ecosystem. Gulf of Mexico: Osprey as top carnivore. Ex 2. Midwest. Ex 3. For each example, be able to list 2 – 3 food chains.

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Food Webs and Energy Flow

Chapter 17


1. What is a food chain? Ex: midwest deciduous forest & Artic & 1 other ecosystem.

  • Gulf of Mexico: Osprey as top carnivore


Ex 2. Midwest


Ex 3. For each example, be able to list 2 – 3 food chains.

  • Summerhayes and Elton studied feeding relations on Bear Island in High Arctic.

    • Primary producers were terrestrial plants and aquatic algae.

      • Fed on by several kinds of terrestrial and aquatic invertebrates.

        • Consumed by birds.

          • Attacked by arctic foxes.


Arctic Food Web


2.  What is a food web? 

  • Make a food web for grasslands, using symbols provided in handout


3.  Describe each of the following trophic levels  and give an example of each.

  • Trophic Level: Position in a food web determined by number of energy transfers from primary producers to current level:

    • Primary producers occupy first level. Autotrophic, energy from sun or chemicals.

    • Primary consumers occupy second level.

      • herbivores

    • Secondary consumers occupy third level.

      • Omnivore or carnivore

    • Tertiary consumers occupy fourth level.

      • Carnivore


4.   What are energy or biomass pyramids?  What are they used for?  Draw ex.


5. Keystone Species

  • If keystone species reduce likelihood of competitive exclusion, their activities would increase the number of species that could coexist in communities.


5. Keystone species

  • A keystone species is a species that has a disproportionate effect on its environment relative to its abundance.

  • organism plays a role in its ecosystem that is analogous to the role of a keystone in an arch.

  • While the keystone feels the least pressure of any of the stones in an arch, the arch still collapses without it.

  • ecosystem may experience a dramatic shift if a keystone species is removed, even though that species was a small part of the ecosystem by measures of biomass or productivity.


5. Keystone example – yellowstone Nat. Park

  • Before wolvesafter wolves

  • High coyotelow coyote

  • High red foxlow red fox

  • Low weaselseveral weasel sp.

  • Low rabbithigher rabbit

  • Low rodentmore rodent species

    • (mice, rat)

  • High mule deerlower deer

  • High elklower elk

  • Less diverse vegmore diverse vegetation

  • Fewer hawksmore hawks


Effects of Predation by Birds on Herbivory

  • Birds reduce densities of insect larvae feeding on dominant understory shrub.

    • Higher densities of insect larvae translated directly into higher levels of damage to shrubs.

  • Marquis and Whelan found higher densities of herbivorous insects on tress from which birds were excluded.


Effects of Predation by Birds on Herbivory

  • Atlegrim studied influence of birds on herbivorous insects and insect-induced plant damage.

    • Insectivorous birds may act as keystone species via effects on herbivorous insects.

      • Larval insect densities peak when many insectivorous birds are feeding their young.


Keystone Species: Summation

  • Power :Keystone species exert strong effects on their community structure, despite low biomass.


6. Consumers’ Effects on Local Diversity

  • to resolve the effect herbivores have on plant diversity, you need to know:

    • Herbivore food preference.

    • Competitive relationships between plant species in the local community.

    • Variance in feeding preferences and competitive relationships across environments. (Lubchenko)


6. Consumers’ Effects on Local Diversity


6. Consumers’ Effects on Local Diversity

  • When snails are present in high densities, Littorina grazes down Enteromorpha, releasing Chondrus from competition.

    • Green crabs (Carcinus maenus) prey on young snails, preventing juveniles from colonizing tide pools.

    • Populations of Carcinus are controlled by seagulls.

    • 1) Seagull --- green crabs --- algae species

    • 2) seagulls--- snails -----algae species


6. Consumers’ Effects on Local Diversity

  • Low snail density - Enteromorpha dominates tide pool.

  • Medium snail density - Competitive exclusion eliminated, and algal diversity increased.

  • High snail density - Feeding requirements are high enough that snails eat preferred algae and less-preferred algae.

    • Algal diversity decreased.


6. Con’t Example: Consumers’ Effects on Local Diversity

  • Lubchenko studied influence of intertidal snail (Littorina littorea) on structure of an algal community.

    • Snails fed on green (Enteromorpha spp.) and red (Chondrus crispus) algae.

      • Under normal conditions, Enteromorpha out-competes Chondrus in tide pools, and Littornia prefers Enteromorpha.

        • In the absence of snails, Chondrus is competitively displaced.


7. Exotic Predators

  • Exotic species have dramatic impacts on communities because they were outside the evolutionary experience of local prey populations.

    • Nile Perch (Lates nilotica) exotic fish predator in Lake Victoria.

      • Fish fauna dramatically reduced.

        Local examples: gobi, zebra mussels


7. Exotic Predators

Species diversity decreased after nile perch introduced.


Exotic Predators

  • Kaufman pointed out changes in Lake Victoria fish community coincide with other ecosystem changes.

    • Dissolved oxygen concentrations significantly decreased.

    • Cultural eutrophication.


8. Define terms associated with energy production and energy flow.

  • Primary production: Fixation of energy by autotrophs in an ecosystem.

    • Rate of primary production: Amount of energy fixed over a given period of time.

      • Gross primary production: Total amount of energy fixed by autotrophs.

      • Net primary production: Amount of energy leftover after autotrophs have met their metabolic needs.


9. What is AET? How do ecosytems compare?

  • annual actual evapotranspiration (AET).

    • AET: Annual amount of water that evaporates and transpires off a landscape.

      • Cold dry ecosystems tend to have low AET.

  • positive relationship between net primary production and AET.

    • east-west variation in primary production correlated with rainfall. (east = wetter)


9. Con’t

  • Draw

  • Compare ecosystems


10.   Why do coral reefs and marine ecosytems have the highest primary productivity?

  • Highest rates of primary production by marine phytoplankton are generally concentrated in areas with higher levels of nutrient availability.

  • Highest rates found along continental margins.

    • Nutrient run-off from land.

    • Sediment disturbance

  • Open ocean tends to be nutrient poor.

    • Vertical mixing main nutrient source.


10. Marine productivity con’t

  • Highest marine productivity.


10. Con’t terrestrial productivity.


10. Con’t grasslands

  • McNaughton estimated Serengeti grazers consume an average of 66% of annual primary production.

    • Rate of primary production in the Serengeti is positively correlated with rainfall quantity.

  • Found grazers can increase primary production.

    • Increased growth rate.

      • Lower respiration rate due to lower biomass.

      • Reduced self-shading.

      • Improved water balance due to reduced leaf area.


10. Con’t

  • Primary Production in the Serengeti


11. What is Lindeman’s efficiency concept?

  • Lindeman concluded the ecosystem concept is fundamental to the study of energy transfer within an ecosystem.

    • Suggested grouping organisms within an ecosystem into trophic levels.

      • Each feeds on level immediately below.

        • As energy is transferred from one trophic level to another, energy is degraded.


12. Calculate Energy flow in an ecosystem

  • Gosz studied solar energy flow:

    • 15% reflected

    • 41% converted to heat

    • 42% absorbed during evapotranspiration

    • 2.2% fixed by plants as gross primary production

    • 1.2% used in plant respiration

    • 1% left for primary production


12. Energy flow con’t

  •    sun           

  • 30% reflected, 20% absorbed by atmosphere

  • 50% absorbed by ground, water or vegetation

  • 1% left for photosynthesis


Producer level

  •    Producer      ex 2.

  • 15,000 stored                     10,000 calories

  • 60% of 15,000 =9000 stored____________

  • 40%  waste (6000)                    ____________  


primary consumer

  • 20% of 9000 = 1800 eaten              ____________

  • 10% stored = 180                            ____________

  • 90% lost = 620                                ____________


secondary consumer

  • 30% of 180 = 54 cal       ____________

  • 10% stored = 5.4 cal ____________


  tertiary consumer 

  • 30% of 5.4 = 1.6  cal         ____________

  • 10% stored = .16           ____________


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