Lecture 18 Flashcards
The Term Ecosystem
- 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
Primary Production
- the chemical energy generated by autotrophs during photosynthesis and chemosynthesis
- the rate: primary productivity
Energy Assimilated by Autotrophs
- stored as carbon compounds in plant tissues
Carbon
- the currency used to measure primary production
Gross Primary Production (GPP)
- total amount of carbon fixed by autotrophs
- depends on the photosynthetic rate
Photosynthetic Rate is Influenced By
- climate and leaf area index (LAI)
Leaf Area Index (LAI)
- Leaf area per unit of ground area
LAI Varies Among Biomes
- 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
Plants
- 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
Net Primary Production (NPP)
- 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
Allocation of NPP to Storage Products
- 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
It is Important to be Able to Measure NPP
- 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
Harvest Techniques
- Measure biomass before and after the growing season
- This is a reasonable estimate of aboveground NPP if corrections are made for herbivory and mortality
Measuring Belowground NPP
- 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
Mechanism for Measuring Belowground NPP
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
Harvesting Techniques and Chlorophyll
- 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
Normalized Difference Vegetation Index
- 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
Example with High NDVI Value
- vegetation
Example with Low NDVI Value
- water
- soil
Net Change in CO2
- GPP- total respiration
- Which can also be referred to as Net ecosystem production or exchange (NEE)
Heterotrophic Respiration
- Must be subtracted from NEE to obtain NPP
- NEE is a more refined estimate of ecosystem carbon storage than NPP
- NEE = GPP - (AR + HR)
Eddy Correlation or Eddy Covariance
- 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
Phytoplankton
- 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
Photosynthesis in Phytoplankton
- measure in a water sample collected and incubated with light