Food Web and Trophic Pyramid
Ecology is the study of the various interactions of organisms and the
environment. It is not surprising that we start here, as it is at this level
that humans interact with plants and observe the environment that we share.
We are fortunate to be scheduled for the fall semester. In previous semesters,
this course was scheduled for spring...and the natural approach makes much
less sense when we start in midwinter.
Of course it is essential that we heighten our awareness of how plants
interact with the organisms around them and the environment in which
they are competitive. From our animal perspective it is easy to overlook
important details! We are consumers rather than producers, so our natural
approaches overlook critical interactions involving plants. Nevertheless,
it is essential for us to see the plant perspective of ecology as we depend
so thoroughly upon plants. I will remind you here of the lecture and supporting
pages about what plants provide us...if you have not looked at those pages,
you really should. Let me just summarize here that plants provide us with
the oxygen we breathe, the food we eat, the clothes we wear, the wood in
our homes and furniture, the basis for paints and varnish, the plastic we
surround our lives with, the power in our cars and electrical energy in our
homes, the paper we write on, the medicine that keeps us functioning properly,
the latex that gives us good smooth rides, smooth hands, comfortable clothes,
safe surgery and safe sex. Plants provide cooling shade, remove greenhouse
gas from the air, eliminate much of the mutagenic UV rays, give habitat to
many animals, and bring beauty to our surroundings.
So if plants are important to us, then we need to review what plants
require from the environment. Again, as a review of our brief introduction
to photosynthesis, plants require a source of water, carbon dioxide, and
light for basic metabolism. Plants also require a range of mineral elements
that are supplied by the soil. The temperature needs to be within a certain
range that the plant has evolved to tolerate. These needs, in part, govern
the distribution of plant species on our planet.
Water is abundant upon the planet, covering perhaps two-thirds of
the surface. However deep our human perspective considers this water, the
surface area gives us a wrong impression. The depth of the water surface
averages less than five miles which might seem deep from our dwarf perspective,
but on the global level this depth can be thought of as like the paint
layer on a globe in your elementary classroom. Worse, the plants which
meet most of our human needs are terrestrial, and the salty oceanic waters
are toxic to most plants. So, while we might think of water availability
as almost unlimited, indeed fresh water can be a major limiting factor
for plant growth, development, and reproduction. The global water cycle
therefore becomes critical to plant survival.
As you might observe, terrestrial plants depend upon evaporation of water
from the salty ocean and then precipitation from the atmosphere on the
earth which allows fresh water to run-off the land back into the ocean
or to evaporate back into the atmosphere. This cycle is not too different from
the distillation process that you have done in Chemistry classes.
The driving force for this cycle is solar energy that heats the ocean surface
and the rising humid air causes circulation cells in the atmosphere. As these
humid air cells move onto land and up the mountain slopes, the vapor is cooled
much as in a condenser and the water vapor condenses into droplets and
precipitates onto the land. It is important to remember the contribution of
the transpiration stream in plants to the water that enters the atmosphere
from the terrestrial ecosystem. Indeed a single tree can evaporate thousands
of gallons of water into the atmosphere in a very short time depending upon
Minerals in the soil are dissolved in the precipitation water. The
soil mineral elements essential for plants include: P, K, N, S, Fe, Ca, Mg,
Mn, Cu, Zn, Mo, Al and more. Some of the minerals are provided abundantly by
the soil, while others are often provided in less-than-ideal amounts in certain
soil types. Nitrogen, Phosphorus, and Potassium are among these most-limiting
minerals, explaining why they are the critical constituents of fertilizers or
"plant food" and why we study the nitrogen and phosphorus cycles.
In the nitrogen cycle you will notice that the major pool of nitrogen
is the atmosphere. About 80% of the atmosphere is N2, so you might
wonder how it could possibly be limiting to plant growth. Yet you probably know,
from such human diseases as kwashiorkor, that insufficient nitrogen in
the diet can be debilitating or even lethal, even though we breathe-in (inspire)
vast amounts of nitrogen gas every minute. Nitrogen fixation by bacteria
in soils or in association with plants and by lightning, volcanoes, or industrial
processes is required to convert the nitrogen gas...which is almost inert...to
ammonium or nitrates that can be used in living organisms. Animals, such as
humans, need to eat plants in order to get the required amount of nitrogen
for building proteins and other essential biochemicals. This is yet another
way that we are dependent upon plants for our survival. Here in Connecticut
naturalized lawns are quite poor looking because our soils lack sufficient
nitrogen for lush growth of grasses. Industrial nitrogen fertilizers can make
our land productive and our vegetables nutritious.
In the phosphorus cycle you observe that the major pool of phosphates,
this time in rock, have to be mobilized into soil, assimilated into plants,
to provide the phosphorus needed for animals to survive. You might notice
here, as in the nitrogen cycle, there are also ways for the animal's minerals
to go back into the soil to nourish the plants. This shows us a kind of
interdependence of the flora and fauna in these nutrient cycles. Indeed
guano deposits from sea birds on oceanic islands, and from bats in caves, have
been mined historically as "natural" fertilizer. The US holds a caribbean island,
Navassa, between Jamaica and Haiti, as the remains of one such oceanic island.
Carbon dioxide also moves through an ecosystem in a cyclic or
web-like manner. In this web you can see that sedimentary rocks and deep ocean
hold the largest reserves of carbon. These are released for use by living
organisms through combustion of fossil fuels and melting rocks (as in volcanoes).
There is a biotic cycle of carbon too in which plants remove carbon dioxide
from the water or atmosphere by photosynthesis. This process produces
carbohydrates and other compounds that can then be used as a carbon source
by animals. Respiration by plants and animals return carbon dioxide to either
the water or the atmosphere, depending on whether we are considering aquatic
or terrestrial ecosystems.
The representation of how plants and animals interact in an ecosystem is sometimes
referred to as a food web. In a food web you get an appreciation of the
feeding relationships. The organisms depicted in the food web together constitute a
community, for example, the arctic tundra community. We should find the
movement of carbon trapped by the photosynthesis in plants moving into herbivores,
such as caribou, lemmings, voles, insects, and birds. The carbon held briefly by
these herbivores is then transferred to carnivores such as weasels, owls, and foxes.
In many food webs, the top carnivore eats just about every organism in the ecosystem;
in the example of your book, the wolf fits this description quite well. In our
arboretum visit I hope that you will be looking for the signs of the food web that
exists in our local ecosystem.
As you might expect, considering this food web, there must be a lot of other
organisms around to feed just one wolf. Each level must eat enough organisms of
a certain size lower in the food-web (or food-chain) to allow it to survive and
reproduce, as well as to feed the next-higher level in the web. Survival and
reproduction require expenditure of energy which comes at a cost in terms of
carbon...as the energy in all organisms rests primarily in carbon-carbon bonds
in organic molecules. This relationship is depicted in the trophic pyramid.
The energy of the plants (the primary producers) is greater than the energy
found in all other levels of the pyramid combined. Each level above plants
has successively less entrapped energy...the energy trapped by plants in the food
web is progressively lost by motion, biochemistry, and heat by each layer in the
2° Carnivores: 10 kcal m-2 yr-1|
1° Carnivores: 400
Amount of Energy Processed
The relationship to carbon is then further shown in another adaptation of the
trophic pyramid in units of grams per square meter of area in the
terrestrial ecosystem. Again, the biomass of plants might be expected to outweigh
all other parts of the trophic pyramid combined.
Carnivores: 0.1 g DW m-2|
Standing Biomass (grams Dry Weight)
But take a look at the aquatic ecosystem pyramid shown below. How can we explain
having more zooplankton herbivore biomass than the phytoplankton producers?
Clearly these figures are based upon standing crops and in the case
of carbon analysis we have to be careful to correct for rate of reproduction.
It is very understandable that a small biomass of phytoplankton that reproduces
rapidly could support a larger standing-crop biomass of zooplankton that
reproduces very slowly. But this trophic pyramid has a basic instability, it
is not standing upon a large base. Any perturbation of the environment that
reduces the reproductive rate of the phytoplankton will wipe out the populations
of zooplankton. So if a local landowner wants a "pretty" lawn and hires ChemLawn
to treat his lawn, herbicide is applied to the lawn, it leaches into the local
stream, reduces the reproduction rate of the phytoplankton, and the trophic
pyramid in the stream crashes. When the zooplankton die, the rest of the pyramid
falls with it: the primary carnivorous fish that feed on the zooplankton, the
secondary carnivorous fish that feed on other fish, and the top carnivorous birds
that feed on all the fish.
Zooplankton: 21 g DW m-2|
Standing Biomass (grams Dry Weight)
It is true that in terms of number of individuals (the population), a
trophic pyramid might reflect the expected shape as shown here. In a typical
grassland ecosystem, the plants outnumber all other levels in the pyramid combined.
Number of individuals
However, in the forest ecosystem the producers are quite outnumbered by virtually each
of the other trophic levels! How do we explain that? Well again the key is to examine
the elements of the ecosystem...the figure itself gives you a good hint at the answer.
The tree is how large compared to the webworm and the birds that feed upon it?
What would you predict for the shape of the forest biomass and forest energy pyramids?
Would they be more typically broad-based? Is the forest community, dependent upon a
small population of trees, relatively unstable? If we cut down one tree, how does this
trophic pyramid respond? When we think about tropical forests, where the diversity of
trees is high, but the local population of each species is extremely small, what
could be the impact of cutting just one rare hardwood tree out of that dense jungle?
There may be special relationships that have evolved within the community in which
one particular species grows in obligate association with one other particular species,
upon which still others depend.
Number of individuals
Light is an energy source that ultimately drives virtually all of the biochemistry
on earth. As we think of the energy trophic pyramid we realize that something is missing...
of course it is light! How much light energy should be depicted here to go with this
pyramid? Does 100% of the solar energy illuminate plants in the ecosystem? Do plants
trap 100% of the light energy that they intercept? Do plants use 100% of the light
energy that they trap? If you think about the physics of the distribution of plants,
the structure of leaves, the structure of chloroplasts, and the absorption spectrum
of chlorophyll I think you can answer these questions. But I think you can also answer
them by simply taking a walk to and through the woods. We will do that in laboratory.
I hope you will be observant and use all of your senses while we visit the arboretum.