Lecture 19: Energy Flow and Food Webs

Question 1

Trophic Interactions

Answer 1

• what they eat and what eats them
• Can change over time

Question 2

Trophic Levels

Answer 2

• Describe the feeding positions of groups of organisms in ecosystems
• All organisms are either consumed by other organisms or enter the pool of dead organic matter (detritus)
• First (primary producers - autotrophs), second (primary consumers - herbivores), third (secondary consumers - primary carnivores), fourth (tertiary consumers - secondary carnivores)

Question 3

Detritus (dead organic matter)

Answer 3

• part of the first level

Question 4

Detrivores (equivalent to herbivores)

Answer 4

• part of the second level

Question 5

Allochthonous

Answer 5

• external energy inputs
• Found in water systems
• Inputs can be important in stream ecosystems (99.8%)

Question 6

Autochthonous

Answer 6

• Energy produced by autotrophs within the system
• Energy input increases from the headwaters toward the lower reaches of a river

Question 7

As you Go Downstream

Answer 7

• velocity decreases
• nutrient concentrations increase

Question 8

Energy flow among Trophic Levels

Answer 8

• Amount of energy transferred from one trophic level to the next depends on food quality and consumer abundance and physiology
• Energy will decrease with each trophic level

Question 9

Trophic Pyramid

Answer 9

• Portrays the relative amounts of energy or biomass of each trophic level
• Very large at base and gets smaller as you go to the top
• Due to some of the biomass at each level not being consumed, so it is dispersed in the transfer to the next level

Question 10

Terrestrial Ecosystems

Answer 10

• Energy and biomass pyramids are similar
• Biomass is closely associated with energy production

Question 11

Aquatic Ecosystems

Answer 11

• Biomass pyramid inverted
• Live and die quickly, energy is produced and then leaves the ecosystem

Question 12

Inverted biomass pyramids

Answer 12

• more common where productivity is lowest, such as nutrient-poor regions of the open ocean.
• Low productivity system = inverted pyramid

Question 13

terrestrial vs aquatic biomass

Answer 13

• terresteral consume a much lower proportion of autotroph biomass than herbivores in most aquatic ecosystems
• On average 13% of terrestrial NPP is consumed
• Aquatic ecosystems average of 35% NPP is consumed

Question 14

Herbivores are Constrained by Predators

Answer 14

• never reach carrying capacity
• Predator removal experiments support this

Question 15

Autotrophs have defences against herbivory

Answer 15

• such as secondary compounds, spines, etc
• Plants of resource-poor environments tend to have stronger defences than plants from resource-rich environments

Question 16

Phytoplankton are more nutritious for herbivores than terrestrial plants

Answer 16

• Terrestrial plants have structural components such wood (with few nutrients)
• Freshwater phytoplankton have carbon
• nutrient ratios closer to those of herbivores than to those of terrestrial plants

Question 17

Trophic Efficiency

Answer 17

amount of energy at on trophic level divided by the amount of energy at the trophic level immediately below it

Question 18

Trophic Efficiency incorporates three types of efficiency

Answer 18

1. Consumption efficiency
2. Assimilation efficiency
3. Production efficiency

Question 19

Consumption Efficiency

Answer 19

-Proportion of available energy that is consumed
- Higher in aquatic ecosystems than in terrestrial ecosystems
- Tends to be higher for carnivores than herbivores

Question 20

Assimilation efficiency

Answer 20

• Proportion of ingested food that is assimilated
• Determined by food quality and the physiology of the consumer
• Food quality of plants and detritus is low because of complex compounds such as cellulose, lignins, and humic acids that are not easily digested and low concentrations of nutrients such as nitrogen and phosphorus

Question 21

Production efficiency

Answer 21

• Proportion of assimilated food that goes into new consumer biomass
• Is strongly related to the thermal physiology and size of the consumer

Question 22

Assimilation Efficiencies of herbivores and Detritivores

Answer 22

20-50%

Question 23

Assimilation Efficiency of Carnivores

Answer 23

80%

Question 24

Endotherms

Answer 24

• Digest food more completely than ectotherms
• Thus have higher assimilation efficiencies
• Allocate more energy to heat production
• Have less for growth and replication than ectotherms
• Body size affects heat loss ( as body size increases, the surface area-tovolume ratio decreases)

Question 25

Some Herbivores Have Mutualistic Symbionts

Answer 25

• Help them digest cellulose
• EX. (cattle, deer, camels) have a modified foregut with bacteria and protists that break down cellulose
• They have higher assimilation efficiencies than other herbivores

Question 26

Change in food quality

Answer 26

• Impacts trophic efficiency and can determine consumer population size
• Steller sea lion populations in Alaska declined by about 80% over 25 years
• Smaller body size and decreased birth rates suggested food quantity or quality might be a problem
• Prey quantity was not declining
• Sea lions however had shifted from a diet of mostly herring (high in fats) to more cod and pollock (half as much fat)
• Change in diet reflected a shift in the fish community

Question 27

Trophic Cascades

Answer 27

Changes in the abundances of organisms at one trophic level can influence energy flow at multiple trophic levels

Question 28

Bottom Up Control

Answer 28

production at bottom drives rest of ecosystem

Question 29

Top Down Control

Answer 29

energy flow governed by energy consumption – predators’ impact how many plants get eaten
- highest level affects the trophic levels below it

Question 30

What determines # of trophic levels in an ecosystem?

Answer 30

Number of trophic level may change due to:
- Addition or loss of a top predator
- Predator in the middle of the food chain
- Omnivore may change food preference

Question 31

Several factors may control the number of trophic levels:

Answer 31

• Amount of energy entering via primary production
• Frequency of disturbances
• Ecosystems size

Question 32

Amount of energy entering via primary production

Answer 32

more production should allow more trophic levels
Appears to be important in resource poor ecosystems

Question 33

Frequency of Disturbances

Answer 33

• higher trophic levels depend on lower levels, and take time to re-establish after disturbance
• If disturbance is frequency, higher level may never establish

Question 34

Food Webs

Answer 34

• conceptual models of the trophic interactions of organisms in an ecosystem
• an important tool for modelling ecological interactions
• As more organisms are added, the complexity increases to reflect the complexity of real ecosystems
• Static descriptions of energy flow and trophic interactions
• Normally don’t include other types of interactions, such as pollination

Question 35

Stability Of Food Webs

Answer 35

• Measured by how much the populations change over time
• How an ecosystem responds to species loss or gain is strongly related to the stability of food webs

Question 36

How does diversity at one trophic level affect stability at other trophic levels?

Answer 36

1. > plant diversity = > plant biomass and stability.
2. > plant biomass and stability = > arthropod diversity.
3. > arthropod diversity = a greater portfolio effect, in which variation in one herbivore population can cancel out variation in that of another.

Question 37

Toxins in Remote Places

Answer 37

• Arctic has been thought of as one of the most remote and pristine areas on Earth
• Studies of inuit women showed they have PCB concentration in their breast milk
• Seven times higher than in women in Quebec

Question 38

PCB’s

Answer 38

• Belong to a group of industrial chemical compounds called persistent organic pollutants (POPs)
• because they remain in the environment for a long time

Question 39

How do these toxins make their way to the Arctic?

Answer 39

• POPs produced at low latitudes are in gaseous form and enter the atmosphere
• Atmospheric circulation patterns carry them to the Arctic
  where they condense to liquid and fall from the atmosphere

Question 40

Manufacture and Use of POPs

Answer 40

• banned in North America but some developing countries still use them
• Emissions of POPs have decreased, but they can remain in Arctic snow and ice for decades
• Everytime the snow melts you get contaminants in the air during spring and summer

Question 41

Corelation between POPs and Diet

Answer 41

• Communities that eat marine mammals tend to have the highest levels of POPs
• Communities that eat herbivorous caribou tend to have lower levels

Question 42

Understanding Energy Flow in Ecosystems

Answer 42

Important in understanding the effects of POPs

Question 43

Bioaccumulation

Answer 43

• Some chemicals are not metabolised or excreted
• Become progressively more concentrated in tissues over and organisms life

Question 44

Biomagnification

Answer 44

• Concentration of these compounds increases in animals at higher trophic levels
• as animals at each trophic level consume prey with higher concentrations of the compounds