Lecture 5

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12 Feb 2011

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Lecture 5
We discussed running out of crop space to grow food for the human race. I mentioned the use of
human high rise structures (vertical gardening) to furnish food on site and bypass the need to
transport food items and eliminate the need for tractors etc. It sounds ridiculous at first but it is
not and it has to be entertained to serve human and livestock needs. One has to be aware that we
are encroaching of every type of eco system on earth and the most land intense activity is land
agriculture. The concept of the ecological footprint should be revisited in your minds. Who
remember how that was defined?
Ecological footprint the biggest part of this equation is agriculture production and
furnish materials i.e. pulp and paper materials calculated on a per capita basis. Remember
the number for the average Canadian is over 4.27 hectares/capita. A group from Simon
Fraser came up with this number. So this concept is very important here because it draws the
radius of resource consumption. The 4.27 isn’t sustainable in the sense that our planet isn’t large
enough and doesn’t have a large enough land base. We are dealing with 6.5 billion people and
they can’t all enjoy 4.27 hectares. So the crisis is more a demand for land than any other crisis
i.e. extinction of habitat and species. Water tables around Beijing drop by 2 metres on average
per year because humans have to pump from underground aquifers to supply agricultural lands
and to flush toilets etc. If we took all humans alive on the planet and said they required 4.27
hectares that would give us a requirement of 3 planets not taking into consideration that the
human population is still growing. We are not that far off from 12 billion that each expert says
will be on this earth soon and that will truly be unsustainable. A reduction in family sizes/human
offspring size correlates linearly with the welfare of a given nation. The better to do people will
have fewer children and it is the law of demographic law. Remember the demographic
transition model. On average each Canadian child consumes as much energy as 1,000 children
in Bangladesh. Countries like China (mainly), India, and Indonesia per capita are beginning to
consume a larger and larger ecological footprint that resembles the footprint of Canadians.
Canadians are on the top of the chart, nobody consumes as many resources as Canadians.
In these vertical gardens we can supply temperature control, moisture control, and nutrients in an
indoor environment. In Canada we wouldn’t have to shut down agriculture of this kind each
year. We could grow all through the winter months as well. We might have to do this no matter
what the price. This was written up in Popular Science and we will touch on this again in the
next few weeks.
Table #10-4
This is the average transit time of energy in living plant biomass for representative ecosystems.
The biomass sitting there is called the standard biomass (gm-2). Once again not surprisingly the
largest amount of standing biomass per area is in the tropical rain forests and then the list goes
Tropical rain forest
BGYC61H3F.October.6.2008 Lecture 5 1
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Temperate deciduous forest
Boreal forest
Temperate grassland
Desert scrub
Swamp and marsh
Lake and stream
Algal beds and reefs
Open ocean
Eventually it has to enter the food chain and this depends on the eco system. This is how the
transit time/energy of biomass is calculated. We divide the standing biomass by the net primary
production we looked at in the previous lecture and also inB50. The standard has become square
meter and year (net primary production = gm-2 yr-1). Transit time is in years. Each morsel of
biomass produced by photosynthesis sits around for an amount of time in each of the ecosystems
listed above. Example: tropical rain forest is 22.5 years, temperate deciduous forest is 25 years,
and then record low levels in algal beds and reefs which is 1 year and then in the open ocean it is
1 to 2 weeks. The consequences for the global carbon balance is significant, in forests the
biomass keeps the carbon dioxide out of the troposphere. All types of forests come in around the
same number and yet the transit time of energy is similar and that has to do with how trees
defend themselves.
(Overhead) We saw for grasses the transit time is much shorter. Here is a moose in Algonquin
Park on its way to feast for several miles and it will destroy ¾ of this grass area and then move
on. A typical herbivore in the wild that evolved to live in this area won’t graze everything to the
ground. They have reasons to leave like they don’t want to be eaten by another predator as well
as other reasons.
Energy Flow through an Ecosystem
You saw this in B50. The primary production of the biomass that was fixed by photosynthesis
(the standing biomass - net primary production) is the only material available to feed everything
else including humans. Over 3 billion people get 70% of their food calories from 1 food source
can that is tropical lowland rice. Energy dilutes as we go up the trophic chain and it is called
trophic transfer efficiency and 10% is the number. On average a given trophic level cannot
clear more than 10% of the energy that is contained in the trophic level just below it. 90% of all
valuable material (carbon) is wasted.
Average number of trophic levels in various ecosystem types calculated from primary
production, consumer energy flux, and ecological efficiencies. Only 7 levels could sustain
and that is where the transit time of biomass is short i.e. the open ocean, coastal marine, etc.
Energy travels quickly and efficiently. Not so much in forests, 3 or sometimes 4 trophic levels
are possible but the rule of thumb is 5 levels across all ecosystems around the world with the
exception of the water ecosystems.
BGYC61H3F.October.6.2008 Lecture 5 2
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