Module 6: Producers: The Basis of Ecosystems

Flow of Matter and Energy

By Dr. James A. Danoff-Burg, Columbia University

Most of the study of ecology is the study of interactions among organisms on the Earth. However, the origin of most of our research is not terrestrial. Nearly every terrestrial and aquatic organism and process derives energy from the sun. Therefore, the basis of life on Earth is an object 150 million kilometers away. How does this solar radiation energy first enter organic matter?

Of course, the answer is that plants and other producer species use solar energy, carbon dioxide, and water to produce glucose — a sugar that is the beginning chemical component of most life. Glucose is produced via photosynthesis in the chloroplasts of leaves and is used by plants and photosynthetic bacteria as an energy repository. Glucose is later processed to make all the other tissues in plants, which in turn is consumed by animals, which are in turn consumed by other animals, and so on. In this manner, plants begin the process of most life on Earth.

Not all energy enters ecosystems via photosynthesis. The above statements have all included qualifiers such as ‘most’ or ‘nearly all’. The photosynthetic plants (there are parasitic plants as well) and bacteria are the main way that energy enters ecosystems. However, chemosynthetic bacteria are the basis of many aquatic ecosystems, as in the hot springs of Yellowstone Park in Wyoming USA and in the hydrothermal vents in the Mid-Atlantic Ridge.

Organisms that create their own food from pure energy, whether from solar or geothermal energy, are collectively referred to as autotrophs. As you should be able to gather from the word, autotrophs are self (auto-) feeders (-troph) and collectively begin ALL life on Earth. During this module, we will focus on terrestrial, photosynthetic autotrophs, otherwise known as plants.

Productivity

Were we to weigh all the organic matter that is present in any given terrestrial ecosystem, we would find that the vast majority of this is due to plants. All of the organic matter that exists, whether living or dead, is collectively referred to as the standing crop and can be referred to in terms of energy (J / m2) or dry organic matter (tonnes / ha). If we factor out the dead trees, animal carcasses, and other dead organisms from the standing crop, we would only have the living organisms, or biomass, remaining in our total weight. Surprisingly for many people, most biomass is actually dead. The bulk of plant weight is due to the heartwood and bark, both of which are dead tissue. This tissue, with the addition of the dead organisms that are present, can be collectively referred to as necromass. These terms are essential when tracking chemical and energy flows through ecosystems, as we will discuss today and then again in Module 11.

When we track the production of novel dry organic matter in standing crop by the action of plants, we are tracing the primary productivity of the ecosystem. As with standing crop calculations, this value is expressed in either energy (J / m2 / day) or in dry organic matter (kg / ha / yr). The total productivity of novel organic matter that occurs through photosynthesis or chemosynthesis is called the gross primary productivity (GPP). Plants use energy during respiration (R) to make novel tissue and to maintain old tissue. They thereby reduce the net gain to the ecosystem. Therefore, to get the net gain to an ecosystem due to producer species (the net primary productivity or NPP) we need to subtract R from the GPP.

Primary production via autotrophs are not the only way that tissue is added to the ecosystem. Heterotrophs, or organisms that feed on other organisms such as fungi, animals, and most protists, also produce tissue and therefore weight. Because they do not produce novel tissue from raw energy, their productivity is called secondary productivity. This is usually such a small number relative to primary productivity that it is ignored in many calculations of the productivity of a site.

Net primary productivity is not uniformly distributed around the globe. Terrestrially, it is concentrated along the Equator with additional isolated peaks in productivity along the northwest coast of North America, the southwest coast of South America, in western Europe, and in eastern Australia. In general, it seems that a combination of temperature and available solar radiation are limiting factors to primary productivity. See the Global Analysis Interpretation and Modelling Task force Summer 98 bulletin for more information on how to calculate these estimates.

The most productive areas in marine environments are primarily isolated to either the continental shelves or in areas with many small adjacent islands and are not concentrated along the Equator. In contrast to land, marine environments tend to be most productive where nutrients are most abundant. This is particularly true in areas where there are upwellings of nutrient-rich waters, as off the western coast of South America.

In general where NPP is greatest, overall biodiversity will be greatest as well. We will discuss this further during Module 7.

Forest Structure

Let us now focus at a bit smaller scale — at the level of a forest. Forests have a very characteristic structure, with several canopy layers stacked on top of each other. At the lowest level of a mature forest is the herbaceous layer of plants. These do not get more than a meter or so in height and include the lichens, mosses, and flowering plants. The lichens and mosses are collectively referred to as thallophytes and are occasionally included as a separate layer below the herbaceous layer. Above the herbaceous layer is the shrub layer, composed of small trees and, well, shrubs. This is typically the first layer to produce wood during succession and as we go up from the ground in a mature forest.

Above the shrubs begins the canopy layers involving trees. These layers can be divided up into many layers and sublayers, but for the purposes of this class and eBiome, we will use the undercanopy layer (5-10 m high), mid-canopy layer (10-30 m), and the canopy (30-50 m). The numbers of each of these layers is dependent on the overall height of the forest and change accordingly. Additionally not all of these layers are present, particularly at shorter forests, as we find in the Mata Atlantica along the southeast coast of Brazil. In shorter tropical forests such as this and in temperate forests, we tend to find only the undercanopy and canopy layers.

Towering above all other plants in the forest are the emergent layer. These colossal trees break through the canopy layer and have most of their leafy tops exposed to the wind and sun. These are the local giants. They usually have the largest root system, hardest wood, and are among the oldest trees in the forest. As a consequence many of these trees are valued for furniture and construction.

Although we’ve presented these layers as being relatively clear-cut, there are many plants that extend from the emergent layer all the way to the ground, with active leaves the entire way. These plants usually grow on top of other plants and occupy a variety of niches. They include lianas (vines), epiphytes (plants that live on the surface of others, such as bromeliads and orchids), and parasitic plants that draw their water and much of their nutrition from the larger trees.

Plant Morphology

Irrespective of the canopy layer in which they live, all of these types of plants share several basic structural (morphological) features. All photosynthetic plants have some sort of supportive structure to elevate and orient their leaves towards the sun. All have some sort of structure to capture the solar energy and to perform photosynthesis — usually this occurs in the leaves or needles, but it also occurs in the stem, some roots, and some tree trunks. Gas exchange (CO₂ in, and O₂ out) happens on those photosynthetic structures as well, usually on the underside of the leaf. Lastly, fluid transport occurs with water and minerals coming up the plant from the roots to the leaves and the products of photosynthesis (primarily glucose) going from the leaves down the plant.

In the past some people have viewed the world as an herbivore’s paradise. In this view, the world is replete with greenery and all of it is edible. In truth we know that nearly all plants protect themselves, using either or both mechanical or chemical defenses. Mechanical defenses include spines, urticating hairs (spines with irritating chemicals), thorns, hooked hairs, dense hairs, thick waxy leaves, among others. You have probably had direct experience with some or all of these!

Chemical defenses are produced by many plants to make themselves distasteful and thus to discourage herbivory. These chemicals are usually referred to as secondary chemicals because they are not essential to the growth and reproduction of the plant. In contrast, chemicals which are essential to plant growth and reproduction are considered primary chemicals. Secondary chemicals are the basis of many of the spices that we derive from plants, including capsaicin (what gives chili peppers their kick), pepper, and mustard — but also many other chemicals that we consume including nicotine, caffeine, and many others. There are many other types of secondary chemicals that are used by humans for many medical, culinary, and well, let’s just say recreational purposes. In many ways, your life as a college student has benefited because of the threat that herbivores pose to plants!

Additional Relevant Online Resources

Ms. J. Stein Carter, of The University of California, Clermont College, has several good introductory biology discussions (click on the Class Notes link to get to the subjects treated), including the one available for photosynthesis.

Read more about photosynthetic bacteria from the University of California Berkeley's Museum of Paleontology Phylogeny Wing Website.

Dr. Thomas D. Brock of The University of Wisconsin has several interesting discussions of life at extremely high temperatures, particulary those at Yellowstone Park, USA.

The American Museum of Natural History has a great site discussing the biology and ecology of the hydrothermal vents in the Mid-Atlantic Ridge.

The Introduction to Global Change Course of the Global Change Project, based at the University of Michigan has many excellent informative lectures or resource pages about a diversity of subjects including The Flow of Energy: Primary Production.

See the Global Analysis Interpretation and Modelling Task force Summer 98 bulletin for more information on how to calculate estimates of primary and secondary productivity.

The World Wildlife Fund, UK has an interesting page discussing the structure of tropical forests and the layers of canopy within.

More information on plants, their utility, basic biology, and other processes can be obtained from the Kew Botanical Gardens' Information Sheets.

The Rainforest Explorer from REMedia is an excellent exploration of many subjects related to tropical rainforests, including an excellent glossary.

From Wayne's World, a page discussing the chemical defenses in plants and their effects on humans - Plants that make you loco.

A great overview of plant defenses against herbivory is available from the University of Maryland's Department of Plant Biology, which also has a separate page discussing Psychoactive Plants

All Materials Copyright © 2000 by James Danoff-Burg

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