Biology Asked by Nichola on February 16, 2021
According to Bar-on et al. (2018) (see figure 2), the terrestrial consumer/producer biomass ratio is 20/450 while the marine consumer/producer biomass ratio is 5/1 (in Gton of carbon).
Why is the consumer/producer ratio much higher in the oceans?
Is it perhaps because marine animals are more efficient? Or is terrestrial plant biomass underexploited by surface animals?
Bar-On, Y.M., Phillips, R. and Milo, R., 2018. The biomass distribution on Earth. Proceedings of the National Academy of Sciences, 115(25), pp.6506-6511.
The paper you cited suggests an explanation:
Such inverted biomass distributions can occur when primary producers have a rapid turnover of biomass [on the order of days (34)], while consumer biomass turns over much more slowly [a few years in the case of mesopelagic fish (35)]. Thus, the standing stock of consumers is larger, even though the productivity of producers is necessarily higher. Previous reports have observed inverted biomass pyramids in local marine environments (36, 37). An additional study noted an inverted consumer/producer ratio for the global plankton biomass (16).
To explain/clarify this a bit further: there are two main processes that affect the consumer/producer ratio. One, which you have identified, is efficiency (how much of the biomass is transferred from one trophic level to the next). The other is turnover rate (rate at which biomass leaves a trophic level), which is the reciprocal of the residence time (the average length of time that a unit of biomass spends in a trophic level before transitioning to another trophic level/compartment).
At equilibrium, the biomass in a trophic level is equal to (inflow/turnover rate) or (inflow $times$ residence time). If consumers at a particular level consume one tonne of biomass per month (assume 100% efficiency for now) and the residence time is 6 months, the standing stock or quantity of biomass at that level will be 6 tonnes. Taking some of the numbers from the paragraph above: suppose the residence time for phytoplankton is 4 days while of consumers is 400 days. If the consumers have a 10% efficiency for taking up phytoplankton biomass, then the consumer biomass will be $(400~textrm{days}/4~textrm{days}) times 10% = 10$ times higher than the phytoplankton biomass.
Correct answer by Ben Bolker on February 16, 2021
Phytoplankton lives for about a week and is renewed, i.e. Cocolithophoridae which clones itself every 6 days.
Terrestrial biomass lives for about 10 years. i.e. trees
Hence, there is a higher ratio of phototrophic/producer biomass on land.
There were 45 billion tonnes of new phytoplankton each year, 45 times more than their own mass at any given time. The phytoplankton would therefore have had to reproduce themselves entirely, on average, 45 times a year, or roughly once a week. In contrast, the world's land plants have a total biomass of 500 billion tonnes, much of it wood. The same calculations showed that the world's land plants reproduce themselves entirely once every ten years.
Answered by aliential on February 16, 2021
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