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
Respiration in Phytoplankton
- measure in a water sample collected and incubated without light
Environmental Controls on NPP
- 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
NEE- Ecosystem Carbon Storage
- 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
In a Dry Meadow
- nitrogen limited NPP
- Change in NPP resulted from change in species composition
- The dominant plant biomass did not increase as much as others
In a Wet Meadow
- both nitrogen and phosphorus limited NPP
- The dominant’s biomass increased more than the others
NPP in Lake Ecosystems
- use enclosures called “limnocorrals”
- NPP is measured as a change in chlorophyll concentrations or the number of phytoplankton cells
David Schindler
- Declining water quality in the 1960s motivated him to determine wether inputs of nutrients in wastewater were causing increases in the growth of phytoplankton
NPP in the Ocean
- 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
Global Patterns of NPP
- 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
What % of Carbon is Taken up by Terrestrial Ecosystems
- 54%
What % of Carbon is Taken up by the Ocean
- 46%
Highest Rates on NPP on Land
- found in the tropics
Secondary Production
- Heterotrophs get their energy by consuming organic compounds that were produced by other organisms
Heterotrophs
- 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)
Stable Isotopes
- often used to determine an organism’s diet
Net Secondary Production
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
