Lecture 18

Question 1

The Term Ecosystem

Answer 1

• First used by Tansley (1935)
• refer to all the components of an ecological system (biotic and abiotic) that influence the flow of energy and elements
• ecosystem integrates ecology with
  geochemistry, hydrology, and atmospheric science

Question 2

Primary Production

Answer 2

• the chemical energy generated by autotrophs during photosynthesis and chemosynthesis
• the rate: primary productivity

Question 3

Energy Assimilated by Autotrophs

Answer 3

• stored as carbon compounds in plant tissues

Question 4

Carbon

Answer 4

• the currency used to measure primary production

Question 5

Gross Primary Production (GPP)

Answer 5

• total amount of carbon fixed by autotrophs
• depends on the photosynthetic rate

Question 6

Photosynthetic Rate is Influenced By

Answer 6

• climate and leaf area index (LAI)

Question 7

Leaf Area Index (LAI)

Answer 7

• Leaf area per unit of ground area

Question 8

LAI Varies Among Biomes

Answer 8

• Less than 0.1 in arctic tundra
• 12 in boreal and tropical forests
• Determines how many layers of leaves you can have in a competitive environment

Question 9

Plants

Answer 9

• Fix a lot of carbon in photosynthesis but lose carbon via respiration
• Allocate carbon to plants that aren’t photosynthetic
• Evolve to optimize allocation strategies

Question 10

Net Primary Production (NPP)

Answer 10

• represents biomass gained by the plant
• the energy left over for plant growth and for consumption by detritivores and herbivores
• Total photosynthesis- respiration (NPP=GPP- respiration)
• the starting point for carbon and energy

Question 11

Allocation of NPP to Storage Products

Answer 11

• Provides insurance against loss of tissues to herbivores, disturbances such as fire, and climatic events such as frost
• Substantial amounts of NPP (up to 20% may be allocated to defensive secondary compounds) to avoid being eaten
• Reproduction and growth are optimized by natural selection

Question 12

It is Important to be Able to Measure NPP

Answer 12

• NPP is the ultimate source of energy for all organisms in an ecosystem
• Variation in NPP is an indication of ecosystem health
• NPP is associated with the global carbon cycle

Question 13

Harvest Techniques

Answer 13

• Measure biomass before and after the growing season
• This is a reasonable estimate of aboveground NPP if corrections are made for herbivory and mortality

Question 14

Measuring Belowground NPP

Answer 14

• more difficult
• Fine roots turn over more quickly than shoots- they die and are replaced quickly
• Roots may exude carbon into the soil, or transfer it to mycorrhizal or bacterial symbionts
• Harvests must be more frequent and additional correction factors are needed

Question 15

Mechanism for Measuring Belowground NPP

Answer 15

minirhizotrons
- pump a clear tube into the ground and it does a 360-degree rotation (the mechanism is the same as those inside a photocopier)
- Allows direct observation of root growth and death
- Have an advanced understanding of belowground production processes

Question 16

Harvesting Techniques and Chlorophyll

Answer 16

• Impractical for large or biologically diverse ecosystems
• Chlorophyll concentrations can be a proxy for GPP and NPP
• Vegetation, dry bare soil, and clear water are present in large landscapes

Question 17

Normalized Difference Vegetation Index

Answer 17

• NDVI
• NDVI = (NIR - red) / (NIR + red)
  NIR = near-infrared wavelengths (700-100 nm)
  Red = red wavelengths (600-700 nm)
• measured over large spatial scales and can estimate CO2 uptake and NPP, deforestation, desertification

Question 18

Example with High NDVI Value

Answer 18

• vegetation

Question 19

Example with Low NDVI Value

Answer 19

• water
• soil

Question 20

Net Change in CO2

Answer 20

• GPP- total respiration
• Which can also be referred to as Net ecosystem production or exchange (NEE)

Question 21

Heterotrophic Respiration

Answer 21

• Must be subtracted from NEE to obtain NPP
• NEE is a more refined estimate of ecosystem carbon storage than NPP
• NEE = GPP - (AR + HR)

Question 22

Eddy Correlation or Eddy Covariance

Answer 22

• NEE is estimated by measuring CO2 at various heights in a plant canopy
• Instruments are mounted on towers to take continuous CO2 measurements
• During the day: photosynthesis
• At night: respiration

Question 23

Phytoplankton

Answer 23

• Do most of the photosynthesis in aquatic habitats
  Short lifespans so biomass at any given time is low compared to NPP; with harvest techniques aren’t used

Question 24

Photosynthesis in Phytoplankton

Answer 24

• measure in a water sample collected and incubated with light

Question 25

Respiration in Phytoplankton

Answer 25

- measure in a water sample collected and incubated without light

Question 26

Environmental Controls on NPP

Answer 26

- NPP varies substantially over space and time - NPP increases as precipitation increases up to a point - At very high precipitation levels there is usually heavy cloud cover - Wet soil can become hypoxic so many organisms can’t survive - Need water for plants to produce a lot - The warmer you get = the more NPP

Question 27

NEE- Ecosystem Carbon Storage

Answer 27

- Does not necessarily increase with the temperature - Warmer temperature also increases respiration rates and loss of carbon - This can be indirect by affecting nutrient availability

Question 28

In a Dry Meadow

Answer 28

- nitrogen limited NPP - Change in NPP resulted from change in species composition - The dominant plant biomass did not increase as much as others

Question 29

In a Wet Meadow

Answer 29

- both nitrogen and phosphorus limited NPP - The dominant’s biomass increased more than the others

Question 30

NPP in Lake Ecosystems

Answer 30

- use enclosures called “limnocorrals” - NPP is measured as a change in chlorophyll concentrations or the number of phytoplankton cells

Question 31

David Schindler

Answer 31

- Declining water quality in the 1960s motivated him to determine wether inputs of nutrients in wastewater were causing increases in the growth of phytoplankton

Question 32

NPP in the Ocean

Answer 32

- mostly limited by nitrogen - NPP in the equatorial Pacific Ocean appears to be limited by iron - Large-scale experiments with iron sulphate additions were done in 1993 (suppports the iron limitation hypothesis) - Iron is lost quickly from the photic zone - Sinking to deeper layers where it is unavailable to support phytoplankton growth

Question 33

Global Patterns of NPP

Answer 33

- reflect climatic constraints and biome types - Remote sensing data now provides direct measurements of NPP - Global NPP has been estimated to be 105 petagrams (1 Pg = 1015 g) of carbon per year - Avg rate of NPP for the land surface is higher than for oceans

Question 34

What % of Carbon is Taken up by Terrestrial Ecosystems

Answer 34

- 54%

Question 35

What % of Carbon is Taken up by the Ocean

Answer 35

- 46%

Question 36

Highest Rates on NPP on Land

Answer 36

- found in the tropics

Question 37

Secondary Production

Answer 37

- Heterotrophs get their energy by consuming organic compounds that were produced by other organisms

Question 38

Heterotrophs

Answer 38

- Herbivores eat plants and algae. - Carnivores eat other live animals. - Detritivores eat dead organic matter (detritus) - Omnivores eat both plants and animals - Not all the organic matter consumed by heterotrophs becomes heterotroph biomass. some used in respiration and some are egested (lost in urine/feces)

Question 39

Stable Isotopes

Answer 39

- often used to determine an organism’s diet

Question 40

Net Secondary Production

Answer 40

ingestion - respiration - egestion - depends on the quality of the heterotroph’s food, and physiology - Animals with high respiration rates (endotherms) have less energy left over to allocate to growth