Ch 21- Energy flow/Foodwebs

Define the main trophic levels and the limitations of this framework (i.e. who doesn’t fit)

• 1st→ autotrophs: generate chem energy from sunlight or inorganic chem compounds 

• 2nd→ primary consumers: eat autotrophs, book includes detritivores here 

• 3rd and 4th→ carnivores: eat animals from levels below 

• Detritivores, and omnivores dont fit 

Explain how detritus fits into this framework.

• Detritivores→ eat dead organisms and feces in process called decomposition

• Fit in 2nd level and detritus is in 1st level then

Describe and compare trophic pyramids (both biomass and energy) between terrestrial and aquatic ecosystems. Why is aquatic the biomass pyramid sometimes inversed?

Terrestrial 

• Energy and biomass pyramid similar (both upright) 

• Primary producers= plants→ longer lived

• Biomass closely associated with energy production 

  • Base→ primary producers

  • Second→ herbivores

  • Third→ primary carnivores

  • 4th→ secondary csrnivores 

Aquatic 

• Primary producers= phytoplankton

• Biomass pyramid can be inverted, most common in nutrient poor regions where productivity low (like regions of open ocean)

  • due to phytoplankton having high turnover rate and shorter lifespan

4 factors that impact how much energy transfers between trophic levels. Link this to comparisons of aquatic and terrestrial

BASICALLY

• phsyiology (efficiency of energy transfers)

• food quality

• amount consumed

1. Amount of NPP at base of food web

• ( how much energy plants or algae produce at the base of the food web)

• herbivores on land consume lower amount of autotroph biomass than herbivores in aquatic ecosystems 

  • less goes up then

• Across ecosystems: (+) relationship bw NPP and amount of biomass consumed by herbivores 

  • Terrestrial ecosystems (land):

    • Plants (trees, grasses) have lots of structural tissue (cellulose, lignin).

    • Harder to digest.

    • Less nutritious per gram.

    • So herbivores eat only a small fraction of plant biomass produced.

Aquatic ecosystems:

• Primary producers are mostly phytoplankton:

  • Soft-bodied

  • Nutrient-rich

  • Fast-growing

• Easy for herbivores (zooplankton, small fish) to consume.

• So herbivores eat a much larger fraction of NPP.

4 factors that impact how much energy transfers between trophic levels. Link this to comparisons of aquatic and terrestrial

2. Proportion of each trophic level consumed by the one above it

• More aquatic autotroph biomass is consumed by herbivores compared to terrestrial 

• Terr herbivores don’t eat more bc of 

  • predator constraints, autotroph defenses, cellulose and lignin hard to eat 

4 factors that impact how much energy transfers between trophic levels. Link this to comparisons of aquatic and terrestrial

3. Nutritional content of autotrophs, detritus, and prey 

• unicellular algae lack chem and structural defenses and are very nutritious

• Terrestrial plants more defended 

  • particularly in resource poor envts

  • and have nutrient poor structural compounds like wood 

4 factors that impact how much energy transfers between trophic levels. Link this to comparisons of aquatic and terrestrial

4. Efficiency of energy transfers 

• consumption efficiency, assimilation efficiency, and production efficiency

• Higher efficiency means more of the energy consumed makes it to the next trophic level 

• Consumption efficiency→ higher for aquatic ecosystems, carnivores, lower for terr ecosystems and herbivores 

• Assimilation efficiency→ (how much absorbed, depends on food quality, consumer physiology).

  • Higher for predators, lower for herbs and detrivores.

  • Endotherms higher bc digest food more completely

• Production efficiency→ (% that becomes biomass).

  • Lower for endotherms bc energy used for heat (awuatic mainly ecotherms)

  • low for small animals due to larger SA:vol ratio they lose more heat to envt 

Compare and contrast "bottom-up" vs "top-down" trophic impacts and interpret examples. Link this to comparisons of aquatic and terrestrial ecosystems

• Bottom up view→ energy flow det by resources that limit NPP (nutrients, water, light)

  • ex: fertilizing a grassland increases plant biomass→ more herbivores→ more predators. 

  • aquatic: iron fertilization in oceans→ more phytoplankton biomass

• Top down view→ energy flow det. by rates of consumption/interactions at the highest trophic level→ influences levels below 

  • ex: wolves reduce deer pops→ veg recovers→ affects other animals 

  • ex: removing sharks can lead to overpop of smaller fish→ reduced zooplankton→ algal blooms 

  • less signficant in terrestrial bc causes diffuse impacts, more species specific, productivty more of a major structuring factor

  • the effect cascade downward from top preds 

Describe the three factors that influence trophic complexity (i.e. number of trophic levels).

1. Amount of energy entering via primary production

2. Frequency of disturbance

3. ecosystem size 

Discuss the evidence supporting the idea that food webs become more stable as they become more complex (i.e. have more links

• Portfolio effect: variation in some species cancels out others→ overall stability 

• Tilmans praire plots→ higher plant diversity→ more stable herbivore and predator communities 

Compare and contrast terrestrial and aquatic ecosystems with respect to the following:

a) Primary producer characteristic

Terrestrial 

• Plants 

• larger, long lived, structurally complex 

Aquatic 

• phytoplankton 

• small, short lived 

Compare and contrast terrestrial and aquatic ecosystems with respect to the following:

b) Proportion of each trophic level consumed by the one above it

c) Nutritional content of autotrophs, detritus, and prey

d) Efficiency of energy transfers

Terrestrial 

• Low proportion consumed

• plants are better defended, or harder to digest (lignin, cellulose) and less nutritious (have components like wood)

• Energy lost between trophic levels

  • Low production efficiency (dom by endotherms in terrestrial) low turnover

Aquatic

• High proportion consumed

• Autotrophs like phytoplankton nutritious, digestable, and poorly defended

• Energy transfer more efficient 

Compare and contrast terrestrial and aquatic ecosystems with respect to the following:

e) Importance of "bottom-up" vs "top-down" trophic impacts

Terrestrial

• Bottom up dominates

  • plant growth limited by nutrients, water and light→ affects herbs and predators

• Top down has diffuse impacts, more species specific, and productivty plays a bigger role in structure.

Aquatic 

• Both important 

• Phytoplankton growth often nutrient limited→ bottom up control 

• Predators can influence herbivore (zooplankton) pops→ top down trophic cascades common 

  • top down effects stronger in aquatic ecosystems

  • sea otters declining can cause kelp forests to collapse

Explain how the process of bioaccumulation works, and why it is important. What is the significance of the book “Silent Spring”?

• Bioaccumulation→ chemicals that aren’t metabolized/excreted becoming more concentrated in tissues over an organisms lifetime 

• Important bc it can affect ecosystem and impact human health as well 

Book important bc 

• brought attention to harmful effects of DDT which undergoes bioaccumulation and biomagnification that harmed top predators like eagles 

• LED TO BAN OF DDT