Photo-autotrophs & productivity
- Photosynthetic organisms only grow when GPP > R
- In deep waters, autotrophs cannot survive at depths below the compensation point, where the light intensity allows only a GPP = R
- Above this point is the ‘Euphotic Zone’
- On land, a lack of sunlight or lack of water also limits GPP and plants may reduce maintenance costs by shedding tissues (e.g. deciduous trees in winter).
= rate of production of new biomass by heterotrophic organisms
- 1st trophic level = plants (primary producers)
- 2nd trophic level = primary consumers
- 3rd trophic level = secondary consumers
Secondary productivity relies on primary productivity, therefore, would expect positive relationship between the two variables
if there is a delay in response - its dependant on the feeding and reproductive limitations of the consumer species
Pyramidal structure - plant productivity is the base for smaller productivity of primary consumers and even smaller productivity of secondary consumers. Illustrates rapid loss of energy as one moves from plants to herbivores and carnivores
Although the higher consumers are physically larger, their low numbers means that total biomass is reduced.
Pyramid of numbers is variable depending on the primary producer (e.g. algea – many are often required to support a single primary consumer)
Where has the missing energy gone?
- Not all plant biomass is consumed by herbivores (some dies – ‘necromass’)
- Not all biomass eaten is assimilated by consumers (undigested material, poop!)
- Not all energy assimilated is converted to biomass (needed for movement, body heat regulation, metabolism etc)
Measurement of secondary production
In lab => we can measure assimilation (metabolizable energy) by measuring gross intake and ***** / urine production
In field => more difficult. Indirect methods need to be used...
In the vast majority of animals, an animal's metabolic rate scales to the ¾ power of the animal's mass.
Basal metabolism = minimum rate of metabolism measured under resting conditions with no food in stomach at a temperature where doesn’t need to expend energy for temp control.
Can be measured by growth of individuals in population and reproduction of new animals
Usually measured as biomass and converted to energy by determining calorific value of unit weight of the species
Production = growth + natality (birth rate)
Production = net change in biomass + losses by mortality
Assimilation efficiency – how much of the food eaten is used by the body (for anything)
Herbivores have low assimilation efficiencies (~10%) - have plenty of food but mostly cellulose
Animals higher up the food chain eat easily digestible proteins and fats to have high assimilation efficiency (~90%) so they don’t need to eat as much as herbivores.
Energy flow through a community
We can map out the relative importance of energy pathways for trophic groups once we know:
- net primary…