Lecture 18: Production

Question 1

Ecosystem

Answer 1

• refers to all the components of an ecological system, biotic and abiotic, that influence the flow of energy and elements
• integrates ecology with other disciplines such as geochemistry, hydrology, and atmospheric science

Question 2

Primary production

Answer 2

• is the chemical energy generated by autotrophs during photosynthesis and chemosynthesis
• the rate of primary production
• Energy assimilated by autotrophs is stored as carbon compounds in plant tissues, thus carbon is the currency used to measure primary production

Question 3

Gross primary production (GPP)

Answer 3

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

Question 4

Photosynthetic rate

Answer 4

• is influenced by climate and leaf area index (LAI)—leaf area per unit of ground area

Question 5

Plants use about half of the carbon fixed in photosynthesis for cellular respiration.

Answer 5

• All plant tissues lose carbon via respiration, but not all tissues are photosynthetic (e.g., tree trunks). Trees tend to have higher respiratory losses.
• Respiration rate increases with temperature, so tropical forests have higher respiratory losses

Question 6

Net primary production (NPP)

Answer 6

GPP - Respiration
- GPP: all carbon that’s been fixed
- NPP: is the energy left over for plant growth, and for consumption by detritivores and herbivores
- Where water is plentiful (tropics), less NPP to roots, more to shoots to get more light (lots of competition)

Question 7

Allocation of NPP to storage products (starch)

Answer 7

• provides insurance against loss of tissues to herbivores, disturbances such as fire, and climatic events such as frost

Question 8

It is important to be able to measure NPP

Answer 8

• NPP is the ultimate source of energy for all organisms in an ecosystem.
• Variation in NPP is an indication of ecosystem health.
• Anything put into NPP is pulled out of the atmosphere

Question 9

Measuring below ground NPP is more difficult

Answer 9

• 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 10

Harvest techniques

Answer 10

• are impractical for large or biologically diverse ecosystems

Question 11

Chlorophyll concentrations

Answer 11

• Can be a proxy for GPP and NPP. They can be estimated using remote sensing methods that rely on reflection of solar radiation
• Chlorophyll absorbs blue and red wavelengths and has a characteristic spectral signature.

Question 12

NDVI (normalized difference vegetation index)

Answer 12

• is measured over large spatial scales and can estimate CO2 uptake and NPP, deforestation, desertification, and other phenomena.
  NDVI = (NIR – red)/ (NIR + red)
  NIR – near-infrared wavelengths (700-1000nm)
  Red = red wavelengths (600-700nm)

Question 13

High and low NDVI values

Answer 13

• Vegetation has a high NDVI value
• water and soil have low NDVI values.

Question 14

NEE (Net ecosystem production or exchange)

Answer 14

• More carbon coming out (source) or is carbon going in (carbon sink)
• Wants ecosystems to be sinks rather than sources
• NEE = GPP - (AR + HR)

Question 15

Eddy correlation or eddy covariance

Answer 15

• NEE is estimated by measuring CO2 at various heights in a plant canopy
• Instruments are mounted on towers to take continuous CO2 measurements
• At night, level near plants, soil respiring, lot of CO2
• Daytime, plants sucking up CO2, see drop in CO2

Question 16

Phytoplankton

Answer 16

• do most of the photosynthesis in aquatic habitats
• Phytoplankton have short life spans, so biomass at any given time is low compared with NPP; harvest techniques are not used

Question 17

Environmental controls on NPP

Answer 17

• NPP varies substantially over space and time. Much of the variation is correlated with climate.
• NPP increases as precipitation increases, up to a point. At very high precipitation levels, there is usually heavy cloud cover, so less light, and wet soils can become hypoxic.
• NPP increases with increasing average annual temperature
• ecosystem carbon storage (NEE) does not necessarily increase. Warmer temperature also increase respiration rates and loss of carbon.

Question 18

What nutrient controls NPP in terrestrial ecosystems?

Answer 18

• Nitrogen
• Phosphorus
• More nitrogen/phosphorus leads to greater plant growth
  Experiment: Dry and wet meadows
• In the dry meadow, nitrogen limited NPP.
• In the wet meadow, both nitrogen and phosphorus limited NPP.

Question 19

Co-limitation

Answer 19

two nutrients limiting growth

Question 20

resource-poor vs resource-rich communities

Answer 20

• Plants from resource-poor communities tend to have low growth rates, and require less nutrients, and thus respond less to fertilization, than plants from resource-rich communities.
• When nutrient-poor communities are fertilized, there is often a change in species composition.

Question 21

NPP in rivers and streams

Answer 21

• NPP is often low. Most of the energy is derived from terrestrial organic matter.
• Water flow limits phytoplankton growth: most NPP is from macrophytes and attached algae as opposed to free-flowing algae

Question 22

NPP in ocean

Answer 22

• In the open ocean, NPP is mostly limited by nitrogen.

Question 23

Global Patterns of NPP

Answer 23

reflect climatic constraints and biome types
- Average rate of NPP for the land surface is higher than for oceans: 54% of carbon is taken up by terrestrial ecosystems, 46% by the oceans.

Question 23

NPP in terrestrial biomes

Answer 23

variation is associated mostly with leaf area index and length of growing season.

Question 24

NPP in aquatic ecosystems

Answer 24

• variation is primarily related to variation in nutrient inputs.
• Upwellings bring nutrient-rich deep water to the surface

Question 24

Herbivores

Answer 24

eat plants and algae

Question 24

Secondary production

Answer 24

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

Question 25

Carnivores

Answer 25

eat other live animals

Question 26

Detritivores

Answer 26

eat dead organic matter

Question 27

Omnivores

Answer 27

eat both plants and animals

Question 28

Stable isotopes

Answer 28

• used to determine an organism’s diet

Question 29

Net secondary production

Answer 29

= Ingestion – Respiration – Egestion
- Not everything converted to energy, some waste products
- depends on the quality of the heterotroph’s food (digestibility and nutrient content), and physiology
- a small fraction of NPP. The fraction is greater in aquatic ecosystems than terrestrial.