Lecture: Productivity
and Biogeochemical Cycles
WHY DO PLANTS MAKE SUGAR? FOR YOU?
NO. Plants make sugar for their own use. As they collect solar energy and store it as
sugar, they are also *burning* that sugar and using the energy to run their own chemical
reactions.
Fortunately for the consumers (heterotrophs), plants usually have some energy left over
after photosynthesis. This is what becomes the BIOMASS (dry weight) of the plant, and
it's what the heterotrophs eat, stealing the plant's hard-won energy!
The total amount of light energy converted by producers into chemical energy (organic
molecules, such as sugars) is known as GROSS PRIMARY PRODUCTION (GPP).
Not all of this is stored as plant biomass, of course. What's left over after the plants
have used the sugars they've made for themselves is called NET PRIMARY
PRODUCTION (NPP). This can be calculated by subtracting the energy required for
cellular respiration from the GPP.
NPP = GPP - respiration
The productivity of various ecosystems can be calculated by measuring the
biomass of vegetation per unit area per unit of time.
Now we get back to that pesky Second Law of Thermodynamics.
Because
unfortunately for us consumers, energy transfers between trophic levels (you remember
those!) are NOT 100% efficient. In fact, about 90% of the energy of one trophic level is
LOST AS ENTROPY when it is eaten by the next higher trophic level! (This varies
among ecosystems, with some being more efficient or less efficient. The 90% loss per
trophic level is a rough average).
In other words…
It takes 1,000,000J of sunlight for a plant to store 10,000J worth of biomass.
If grasshoppers (primary consumers) eat those 10,000J of plant biomass, they will
convert it into only 1000J of grasshopper biomass.
If field mice eat 1000J of grasshopper biomass, they will convert it into only 100J
of field mouse biomass.
If foxes eat 100J of field mouse biomass, they will be able to convert only 10J
into fox biomass.
…and so on. It's this PYRAMID OF PRODUCTIVITY that shows us
why environmentalists urge us to "eat low on the food chain." The higher you go in
trophic levels, the more energy is wasted at lower levels.
To make 1 kg of human biomass, it takes 10kg of grain.
But if you eat beef, it takes 10kg of beef to make 1 kg of human, and 100kg of grain
to make that 10kg of beef! So if you skip the cow step, you can feed more people!
Biogeochemical Cycles
As plants collect solar energy, they also take up nutrients from the soil, water, and
atmosphere. When animals eat the plants, those elements and compounds enter the
animal body and become part of the animal. And when those animals are eaten or
decomposed and consumed, the nutrients take yet another trip.
The cycling and recycling of inorganic nutrients in ecosystems can be represented
as biogeochemical cycles. Here's a generalized cycle...
...can be superimposed on Biogeochemical Cycles for other substances
o The Water Cycle
o The Carbon Cycle
o The Nitrogen Cycle
o The Phosphorus Cycle
o ...and any other element.
Your activities any day have profound effects on these cycles, and on ENERGY
cycling in the biosphere. To test how much impact you have on our biosphere, take the
following test of your Ecological Footprint at
http://www.earthday.net
Go forth and spread the word.