Ecology Lectures
Intro Biology 1102
Spring 2002
Ecology - Relevance to Today’s Society
Ecology is a relatively new science, only about 150 years old, and in reality, perhaps less than
100 in terms of doing experimental research. The early work was pretty much natural history and
descriptive, not mechanistic.
Since we only have three lectures left, I am going to concentrate on three major areas, all of
them mostly applied ecology. They are: Ecosystem Ecology, Biodiversity, and Overpopulation. These
are all inter-related, and cover only a small fraction of the material you should know, but these
subjects are all critically important to society today, and will be so for the foreseeable future, thus
they will affect you and your children as well.
I think John Sawhill, former President of The Nature Conservancy, said it best when he
wrote:
“In the end, our society will be defined not only by what we create,
but by what we refuse to destroy.”
Ecosystem Ecology
We can imagine an ecosystem as the sum total of all the creatures and the abiotic (non-living)
portions of the landscape. This means that we are looking at the fluxes of elements, such as nutrients,
and energy as affected by living organisms. Nutrients can be recycled, but not energy.
Ecologists define ecosystems by their trophic levels:
Base Level
Autotrophs
Plants (primary producers)
Next
Heterotrophs
Animals that consume plants (primary consumers)
Next
Heterotrophs
Secondary consumers
Next
Heterotrophs
Tertiary consumers
Detritivores
Animals that consume dead material
(Fungi, bacteria, dung beetles, etc.)
Source of Energy
The ultimate source of energy is the sun. About 1% of the sun’s energy is used to convert
sunlight into chemical energy through the process of photosynthesis. Some of the sun’s energy is used
to melt snow, move ocean currents, etc. Even with only 1% of the sun’s energy, that is enough to
produce 170 B metric tons of plant material per year.
Annual Productivity of the Earth
Plant productivity known as annual net primary productivity (ANPP). Total productivity would
also include energy used in respiration, and is gross primary productivity. The relationship between
the two is: ANPP = GPP + R
But respiration releases energy off as heat and as CO2. What’s left is the net primary
productivity. For most systems, ANPP is about 50-60% of GPP.
Productivity by Ecosystem
Tropical Rainforests
Savannah
Temperature Deciduous Forest
Tundra
Coral Reefs
(g m- 2 yr-1 )
2200
900
1200
200
2500
Productivity a function of T, rainfall, nutrients, and seasonality.
This represents the amount of energy stored in plant biomass each year. That is energy
available to consumers. But as food moves up the trophic levels, energy is lost due to various
processes, such as heat, indigestibility, feces, etc. What ends up in the consumer is only a small
fraction of what is consumed.
Raymond Lindemann was the first to work out these relationships, after studying food chains
(actually webs) in a small pond, back in the 1940's. Here is an example of how energy is transferred
from the leaves of a plant to a caterpillar:
Energy Content in Joules
Percent
Caterpillar eats 48 gms of leaves
200
100%
Amount consumed (edible parts)
100
50%
Respiration uses 2/3 of absorbed energy
67
Remaining for growth
33
16%
Generally, as food moves up trophic levels, approximately 90% of the energy is lost. That
means that if we start with 100 J of energy, by the time it gets into a bird, only 1% of the energy of
the original plant material remains. Obviously, this constrains how many food levels one can have most food chains are 4-6 lengths long at most.
This also limits the number of higher order consumers - can’t have as many lions as mice!!
See below:
Number of organisms
3
355,000
710,000
5, 800,000
Trophic Level
Tertiary consumer
Secondary consumer
Primary consumer
Primary producer
Implications of Food Web and Trophic Energy Transfer
Obviously, the more links food goes through, the less there is available for the next trophic
level. If we eat lower down on the trophic chain, there will be more energy available. Thus, eating
vegetarian will stretch the food over more people than eating meat. This has important implications
for agriculture - raising cows for meat consumes a lot of energy and less is available for consumption
by humans. China is now largest consumer of pork, and as a result, can no longer raise enough grain to
feed itself, whereas before, when Chinese ate lower on the trophic scale, they were self-sufficient.
Elemental Cycling
When ecosystems function to cycle elements, we often call this biogeochemical cycling.
Plants, in fact, are the largest miners of minerals in the world. They routinely extract from the
ground nearly 10X what humans do, and they do it in pure form every day!!
Some elements cycle as gases, such as SO2, CO2, and N. Others cycle primarily as solids,
mainly
dissolved in water.
component
ecosystem
Major
s of an
include:
Living Plants
Dead Material
Atmosphere
Soil
Water
We
can better
understand
some of these
cycles by
studying individual
elements. Let’s look at three such cycles: C, N, and water.
Carbon has a complex cycle - it can be taken out of the atmosphere by photosynthesis, stored
in living biomass, or dissolved in the oceans, where some is deep-sixed to great depths. There, it may
remain for many millions of years. Carbon is also released whenever fossil fuels are burned. See
cycle below:
Man’s
been altering the
activities have
C cycle. The
burning of fossil fuels, as well as the clearing of forests, is putting more C in the air than can be
taken out by photosynthesis or dissolution in the ocean. Charles Keeling began measuring atmospheric
carbon dioxide in the late 1950's in Mauna Loa, HI, and the measurements continue to today. We can
see that it is rising rapidly, at about 2-3 ppm per year. The jiggles in the line reflect the seasonal
differences in atmospheric carbon dioxide - going down in the summer, and up in the winter.
What has been the historical trend? The graph below, obtained from ice core data (ice cores
can trap bubbles of air and preserve them for thousands of years) shows that this increase is a fairly
recent phenomenon, mainly associated with the industrial revolution.
Now, you
might wonder, so
what? Is
there any reason
to be
concerned about
this rising carbon dioxide? Look at the next graph. It shows the correlation between changes in
atmospheric carbon dioxide and global mean temperatures.
Here, it is very easy to see that whenever carbon dioxide goes up, so does global mean
temperatures. Thus, carbon dioxide is the atmospheric gas of most concern with regards to rising
global temperatures. However, other gases, such as methane, and nitrous oxides, also contribute to
global warming, but carbon dioxide is the one of most concern.
We see very clearly that humans are altering the global mean temperature and the cycling of
C. Even if we were to stop putting C into the atmosphere today, the time lags of mixing are such that
the concentration would continue to rise for several decades afterwards.
The Nitrogen Cycle
Nitrogen cycles similarly to C, but has several other pathways not available to C. For example,
there are organisms, particular bacteria, that can fix N out of the atmosphere into forms used by
plants. The main forms of N are: N=N or N2, nitrate (NO3-), ammonium ion (NH4+), and urea. Plants
can’t fix N out of the air, but legumes have bacteria in their roots that can.
Humans have been altering the N cycle by adding fertilizers to agricultural fields. Normally,
growth in most ecosystems is strongly limited by N. Now, N is less limiting, and there is greater
cycling than there used to be. In addition, NOx, added to the atmosphere by burning fossil fuels,
contributes to acidic deposition. This is converted to nitric acid, which then falls to the ground, and
contributes to acidification of lakes and streams, and forests.
An diagram of the N cycle is shown on the next page.
N
al
rces of N include lightening, which can add nitrate to ecosystems. Bacteria can also release N by a
process called denitrifying, which turns nitrate N or ammonium N back into N2. When too much N
enters an ecosystem, it causes N saturation. When more comes in than can be taken up by the plants,
the only place for it to go, if not into the atmosphere, is the stream water. This can cause nitrates to
build up in the water supply, which is toxic to humans.
atur
sou
Water Cycle
The global water cycle is fairly simple. The drawing below shows the major pathways. Water
will most likely become the most limiting element constraining further growth of the human population.
Plants can
substantially alter the
global water cycle. In the Amazon, for instance, nearly 50% of the water that falls on the forest is
transpired comes from the trees below. Eliminating the trees greatly reduces evaporation into the
atmosphere, and can lead to drying out of the ecosystem.
Humans currently use nearly 60% of all available freshwater. How will we deal with the
situation when the population doubles, if it can?
How To Study Ecosystems
Hubbard Brook - Herb Bormann and Gene Likens (see overheads)
Hubbard was one of the first great ecosystem studies, done by two ecologists, Herb Bormann from
Yale, and Gene Likens from Cornell. They studied how nutrients and energy are moved through and
around a forested ecosystem in New Hampshire. With a host of graduate students and collaborators,
they followed nutrients entering, being stored, and leaving the Hubbard Brook watersheds, a USFS
site. Their strategy was this:
Nutrients Entering - Nutrients Stored = Nutrients Leaving (in stream water)
So, they measured the nutrients coming in the precipitation, how much was being stored in the
forest each year, and the concentration going out in the stream water. The idea was that since the
forests were underlain by solid granite, no water could leach out except in the streams. So it was a
tight system.
What did they find? There were many findings. Of importance were:
1. pH of the rain coming in was acidic. It was Herb Bormann and colleagues who coined the
term “acid rain”.
2. Forests were crucial to the cycling of nutrients. When trees were cut, the ecosystem could
not retain nutrients. They were washed out in the stream water.
3. As forests mature, their ability to retain nutrients varies - young forests are good at it, but
as they mature, ecosystems tend to get “leaky” - growth slows down, and uptake of nutrients does also.
Comparison of Temperate and Tropical Rainforest Nutrient Cycles
Tropical rainforests cover about 6% of the earth’s surface (about the size of the lower 48
states in the US). But they may contain nearly 50% or more of the world’s species. Very important
ecosystems. But they are being chopped down at alarming rates - nearly a football field each second,
or 1% per year. Or the equivalent of half of Florida per year. Some think the rates even higher, at
2% per year, or all of Florida per year. Ecuador, Venezuela, and Bolivia, as well as Brazil, have very
high deforestation rates. Already, 14% of the Amazon has been destroyed in one way or another, and
most of it will likely disappear in your lifetime.
What was the historical use history of the Amazonian rainforest? How did it persist for so
long without being destroyed? In large part, it was because it was used up at a rate that balanced it’s
ability to regenerate. Let’s look at how indigenous peoples used the Amazon.
Slash and Burn
Indigenous peoples used to burn a small area, maybe 1-2 hectares in size. Then, they planted
native crops, like manihot (a starchy crop). The burning turned the downed trees to ash, which acted
like a fertilizer. This promoted crop growth for a few years. But then, due to the high rainfall, the
ash was leached out of the soil, and crop productivity dropped. Since the soils were low in nutrients
to begin with, further production was fruitless. So, the natives moved to another portion of the
forest, and repeated the process. This nomadic life was necessary in this ecosystem. They would not
return to an area for at least a century or more. This gave the cutover forest time to recover it’s
nutrient capital.
But today, due to the loss of available forest land, they are returning at less than 75 year
intervals, and the forest has not had a chance to recover fully. This is slowly wearing down
productivity in the forest. Farmers who raise cattle in the Amazon do so only by applying large
amounts of fertilizers to the land -otherwise the forest would encroach back in, or, the pasture
productivity without fertilizers would be so low as to not be able to sustain cattle on it.
Thus, the plants in the Amazon are used to low nutrients in the soil - how do they deal with it?
Faced with high rainfalls, one might suppose that all the nutrients would be leached out of the system.
But the plants have many adaptations that retain the nutrients in their bodies, and keep them from
being leached out.
Comparison of Temperate and Tropical Nutrient Pools
Temperate
Tropical
40-60% nutrients in soil
20-30% in soil
60-40% nutrients in plants
70-80% in plants
moderate rainfall
very high rainfall
moderate productivity
very high productivity
low leaching rates
very high potential leaching rates
Recycle rates slow to moderate
recycle rates very high
Efficiency of recycling moderate
Efficiency of recycling very high
Adaptations to reduce leaching:
1. roots above ground, called a root mat or aerial roots
2. high uptake efficiency - take up nutrients at low concentrations
3. drip tips on leaves - dries leaves out so nutrients don’t leach from leaves
4. mycorrhizae - very efficient in tropics. Figure to left shows mycorrhizal roots
Figure to right shows
aerial roots