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.

Environmental Requirements

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 conditions.

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 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 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 trophic pyramid.

2° Carnivores: 10 kcal m-2 yr-1
1° Carnivores: 400
Herbivores: 4000
Producers: 21000
<---------------------------------- 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
Herbivores: 0.6
Producers: 470
<---------------------------------- 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
Phytoplankton: 4
<---------------------------- 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.