Lecture 18: Production Flashcards
Ecosystem
- 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
Primary production
- 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
Gross primary production (GPP)
- total amount of carbon fixed by autotrophs
- depends on photosynthetic rate
Photosynthetic rate
- is influenced by climate and leaf area index (LAI)—leaf area per unit of ground area
Plants use about half of the carbon fixed in photosynthesis for cellular respiration.
- 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
Net primary production (NPP)
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)
Allocation of NPP to storage products (starch)
- provides insurance against loss of tissues to herbivores, disturbances such as fire, and climatic events such as frost
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.
- Anything put into NPP is pulled out of the atmosphere
Measuring below ground NPP is 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
Harvest techniques
- are impractical for large or biologically diverse ecosystems
Chlorophyll concentrations
- 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.
NDVI (normalized difference vegetation index)
- 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)
High and low NDVI values
- Vegetation has a high NDVI value
- water and soil have low NDVI values.
NEE (Net ecosystem production or exchange)
- More carbon coming out (source) or is carbon going in (carbon sink)
- Wants ecosystems to be sinks rather than sources
- 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
- At night, level near plants, soil respiring, lot of CO2
- Daytime, plants sucking up CO2, see drop in CO2
Phytoplankton
- 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
Environmental controls on NPP
- 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.
What nutrient controls NPP in terrestrial ecosystems?
- 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.
Co-limitation
two nutrients limiting growth
resource-poor vs resource-rich communities
- 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.
NPP in rivers and streams
- 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
NPP in ocean
- In the open ocean, NPP is mostly limited by nitrogen.
Global Patterns of NPP
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.
NPP in terrestrial biomes
variation is associated mostly with leaf area index and length of growing season.
NPP in aquatic ecosystems
- variation is primarily related to variation in nutrient inputs.
- Upwellings bring nutrient-rich deep water to the surface
Herbivores
eat plants and algae
Secondary production
- Heterotrophs get their energy by consuming organic compounds that were produced by other organisms
Carnivores
eat other live animals
Detritivores
eat dead organic matter
Omnivores
eat both plants and animals
Stable isotopes
- used to determine an organism’s diet
Net secondary production
= 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.
