Biological Production

CHAPTER 6

Botkin, D. B., & Keller, E. A. (2014). Environmental science: Earth as a living planet (9th ed.). Hoboken, NJ: John Wiley & Sons, Inc

- Chapter 6: Ecosystems: Concepts and Fundamentals

6.1 the Ecosystem: Sustaining Life on Earth

We tend to associate life with individual organisms, for the obvious reason that it is individuals that are alive. But sustaining life on Earth requires more than individuals or even single populations or species. Life is sustained by the interactions of many organisms functioning together, interacting through their physical and chemical environ- ments. We call this an ecosystem. Sustained life on Earth, then, is a characteristic of ecosystems, not of individual organisms or populations. As the opening case study about Lyme disease illustrates, to understand important environmental issues—such as controlling undesirable species; conserving endangered species; sustaining renew- able resources; and minimizing the effects of toxic sub- stances—we must understand the basic characteristics of ecosystems.

Basic Characteristics of Ecosystems

Ecosystems have several fundamental characteristics, which we can group as structure and processes.

Ecosystem Structure

An ecosystem has two major parts: nonliving and liv- ing. The nonliving part is the physical-chemical environ- ment, including the local atmosphere, water, and mineral soil (on land) or other substrate (in water). The living part, called the ecological community, is the set of spe- cies interacting within the ecosystem.

Ecosystem Functions and Processes

Two basic kinds of processes (sometimes referred to as ecosystem functions) must occur in an ecosystem: a cycling of chemical elements and a flow of energy. These processes are necessary for all life, but no single species can carry out all necessary chemical cycling and energy flow alone. That is why we said that sustained life on Earth is a characteristic of ecosystems, not of individuals or populations. At its most basic, an ecosystem consists of several species and a fluid medium—air, water, or both (Figure 6.4). Ecosystem energy flow places a fun- damental limit on the abundance of life. Energy flow is a difficult subject, which we will discuss in Section 6.4.

Ecosystem chemical cycling is complex as well, and for that reason we have devoted a separate chapter (Chapter 7) to chemical cycling within ecosystems and throughoutthe entire Earth’s biosphere. Briefly, 21 chemical elements are required by at least some form of life, and each chemi- cal element required for growth and reproduction must be available to each organism at the right time, in the right amount, and in the right ratio relative to other ele- ments. These chemical elements must also be recycled— converted to a reusable form: Wastes are converted into food, which is converted into wastes, which must be con- verted once again into food, with the cycling going on indefinitely if the ecosystem is to remain viable.

For recycling of chemical elements to take place, sev- eral species must interact. In the presence of light, green plants, algae, and photosynthetic bacteria produce sugar from carbon dioxide and water. From sugar and inorganic compounds, they make many organic compounds, in- cluding proteins and woody tissue. But no green plant, algae, or photosynthetic bacteria can decompose woody tissue back to its original inorganic compounds. Other forms of life—primarily bacteria and fungi—can decom- pose organic matter. But they cannot produce their own food; instead, they obtain energy and chemical nutrition from the dead tissues on which they feed. In an ecosystem, chemical elements recycle, but energy flows one way, into and out of the system, with a small fraction of it stored, as we will discuss later in this chapter.

To repeat: Theoretically, at its simplest, an ecosystem consists of at least one species that produces its own food from inorganic compounds in its environment and anoth- er species that decomposes the wastes of the first species, plus a fluid medium—air, water, or both (Figure 6.4). But the reality is never as simple as that.

6.2 Ecological Communities

and Food Chains

In practice, ecologists define the term ecological commu- nity in two ways. One method defines the community as a set of interacting species found in the same place and functioning together, thus enabling life to persist. That is essentially the definition we used earlier. A problem with this definition is that it is often difficult in practice to know the entire set of interacting species. Ecologists therefore may use a practical or an operational definition, in which the community consists of all the species found in an area, whether or not they are known to interact. Animals in different cages in a zoo could be called a com- munity according to this definition.

One way that individuals in a community interact is by feeding on one another. Energy, chemical elements, and some compounds are transferred from creature to creature along food chains, the linkage of who feeds on whom. The more complex linkages are called food webs. Ecologists group the organisms in a food web into trophic levels. A trophic level (from the Greek word trephein, meaning to nourish, thus the “nourishing level”) consists of all organisms in a food web that are the same number of feeding levels away from the original energy source. The original source of energy in most ecosystems is the sun. In other cases, it is the energy in certain inorganic compounds.

Green plants, algae, and certain bacteria produce sug- ars through the process of photosynthesis, using only energy from the sun, water (H2O), and carbon dioxide (CO2) from the local environment. They are called auto- trophs, from the words auto (self) and trephein (to nour- ish), thus “self-nourishing,” and are grouped into the first trophic level. All other organisms are called heterotrophs. Of these, herbivores—organisms that feed on plants, al- gae, or photosynthetic bacteria—are members of the sec- ond trophic level. Carnivores, or meat-eaters, that feed directly on herbivores make up the third trophic level. Carnivores that feed on third-level carnivores are in the fourth trophic level, and so on. Decomposers, those that feed on dead organic material, are classified in the highest trophic level in an ecosystem.

Food chains and food webs are often quite complicat- ed and thus not easy to analyze. For starters, the number of trophic levels differs among ecosystems.

A Simple Ecosystem

One of the simplest natural ecosystems is a hot spring, such as those found in 4geyser basins in Yellowstone National Park, Wyoming. They are simple because few organisms can live in these severe environments. In and near the center of a spring, water is close to the boilingpoint, while at the edges, next to soil and winter snow, water is much cooler. In addition, some springs are very acidic and others are very alkaline; either extreme makes a harsh environment.

Photosynthetic bacteria and algae make up the spring’s first trophic level. In a typical alkaline hot spring, the hottest waters, between 70° and 80°C (158–176°F), are colored bright yellow-green by photosynthetic blue- green bacteria. One of the few kinds of photosynthetic organisms that can survive at those temperatures, these give the springs the striking appearance for which they are famous (Figure 6.5). In slightly cooler waters, 50° to 60°C (122–140°F), thick mats of other kinds of bac- teria and algae accumulate, some becoming 5 cm thick (Figures 6.5, 6.6).

The bacteria living in these hot springs became of spe- cial interest in 1985 when an enzyme was isolated from one species that allows “DNA fingerprinting,” a now ma- jor method in molecular biology.5

Ephydrid flies make up the second (herbivore) tro- phic level. Note that because the environment is so stress- ful, they are the only genus on that entire trophic level, and they live only in the cooler areas of the springs. One of these species, Ephydra bruesi, lays bright orange-pink egg masses on stones and twigs that project above the mat. These larvae feed on the bacteria and algae.

The third (carnivore) trophic level is made up of a dolichopodid fly, which feeds on the eggs and larvae of the herbivorous flies, and dragonflies, wasps, spiders, tiger beetles, and one species of bird, the killdeer, that feeds on the ephydrid flies. (Note that the killdeer is a carnivore of the hot springs but also feeds widely in other ecosystems. An interesting question, little addressed in the ecological scientific literature, is how we should list this partial mem- ber of the food web: Should there be a separate category of “casual” members? What do you think?)

In addition to their other predators, the herbi- vorous ephydrid flies have parasites. One is a red mite thatfeeds on the flies’ eggs and travels by attaching itself to the adult flies. Another is a small wasp that lays its eggs within the fly larvae. These are also on the third trophic level.

Wastes and dead organisms of all trophic levels are fed on by decomposers, which in the hot springs are primarily bacteria. These form the fourth trophic level.

The entire hot-springs community of organisms— photosynthetic bacteria and algae, herbivorous flies, car- nivores, and decomposers—is maintained by two factors: (1) sunlight, which provides usable energy for the organ- isms; and (2) a constant flow of hot water, which provides a continual new supply of chemical elements required for life and a habitat in which the bacteria and algae can per- sist. (See A Closer Look 6.1 and Figure 6.7 for another example of trophic levels and keystone species.)

An Oceanic Food Chain

In oceans, food webs involve more species and tend to have more trophic levels than they do in a terrestrial ecosystem. In a typical pelagic (open-ocean) ecosystem (Figure 6.8), microscopic single-cell planktonic algae and planktonic photosynthetic bacteria are in the first trophic level. Small invertebrates called zooplankton and some fish feed on the algae and photosynthetic bacteria, forming the second trophic level. Other fish and invertebrates feed on these In the abstract or in extreme environments like a hot herbivores and form the third trophic level. The great ba- spring, a diagram of a food web and its trophic levels may leen whales filter seawater for food, feeding primarily on small herbivorous zooplankton (mostly crustaceans), and thus the baleen whales are also in the third level. Some fish and marine mammals, such as killer whales, feed on the predatory fish and form higher trophic levels.

Food Webs Can Be Complex: The Food Web of the Harp Seal

In the abstract or in extreme environments like a hot spring, a diagram of a food web and its trophic levels may leen whales filter seawater for food, feeding primarily on seem simple and neat. In reality, however, most food webs are complex. One reason for the complexity is that many creatures feed on several trophic levels. For example, con- sider the food web of the harp seal (Figure 6.9). This spe- cies is of special interest because large numbers of the pups are harvested each year in Canada for their fur, giving rise to widespread controversy over the humane treatment of animals even though the species is not endangered (there are more than 5 million harp seals.)6 This controversy is one reason that the harp seal has been well studied, so we can show its complex food web. 6

The harp seal is shown at the fifth level. It feeds on flatfish (fourth level), which feed on sand launces (third level), which feed on euphausiids (second level), which feed on phytoplankton (first level). But the harp seal actually feeds at several trophic levels, from the second through the fourth. Thus, it feeds on the prey of some of its prey and therefore competes with some of its own prey.7 A species that feeds on several trophic levels is typi- cally classified as belonging to the trophic level above the highest level from which it feeds. Thus, we place the harp seal on the fifth trophic level.

6.3 ecosystems as Systems

Ecosystems are open systems: Energy and matter flow into and out of them (see Chapter 4). As we have said, an ecosystem is the minimal entity that has the properties required to sustain life. This implies that an ecosystem is real and important and therefore that we should be able to find one easily. However, ecosystems vary greatly in structural complexity and in the clarity of their bound- aries. Sometimes their borders are well defined, such as between a lake and the surrounding countryside (Figure 6.11a). But sometimes the transition from one ecosystem to another is gradual—for example, the transition from deciduous to boreal forest on the slopes of Mount Wash- ington, New Hampshire (Figure 6.11b), and in the subtle gradations from grasslands to savannas in East Africa and from boreal forest to tundra in the Far North, where the trees thin out gradually and setting a boundary is difficult and usually arbitrary.

A commonly used practical delineation of the bound- ary of an ecosystem on land is the watershed. Within a watershed, all rain that reaches the ground from any source flows out in one stream. Topography (the lay of the land) determines the watershed. When a watershed is used to define the boundaries of an ecosystem, the ecosys- tem is unified in terms of chemical cycling. Some classic experimental studies of ecosystems have been conducted on forested watersheds in U.S. Forest Service experimen- tal areas, including the Hubbard Brook experimental for- est in New Hampshire (Figure 6.12) and the Andrews experimental forest in Oregon. In other cases, the choice of an ecosystem’s boundary may be arbitrary. For the purposes of scientific analysis, this is okay as long as this boundary is used consistently for any calculation of the exchange of chemicals and energy and the migration of organisms. Let us repeat the primary point: What all eco- systems have in common is not a particular physical size or shape but the processes we have mentioned—the flow of energy and the cycling of chemical elements, which give ecosystems the ability to sustain life.

6.4 Biological production and Ecosystem Energy Flow

All life requires energy. Energy is the ability to do work, to move matter. As anyone who has dieted knows, our weight is a delicate balance between the energy we take in through our food and the energy we use. What we do not use and do not pass on, we store. Our use of energy, and whether we gain or lose weight, follows the laws of physics. This is true not only for people but also for all populations of living things, for all ecological communi- ties and ecosystems, and for the entire biosphere (which we defined in Chapter 4 as the planetary system that in- cludes and sustains life, and that sometimes is referred to loosely as the global ecosystem).

Ecosystem energy flow is the movement of energy through an ecosystem from the external environment through a series of organisms and back to the exter- nal environment. It is one of the fundamental processes common to all ecosystems. Energy enters an ecosystem by two pathways: Energy that is fixed by organisms and moving through food webs within an ecosystem; and heat energy that is transferred by air or water currents or by convection through soils and sediments and warms living things. For instance, when a warm air mass passes over a forest, heat energy is transferred from the air to the land and to the organisms.

Energy is a difficult and an abstract concept. When we buy electricity, what are we buying? We cannot see it or feel it, even if we have to pay for it. At first glance, and as we think about it with our own diets, energy flow seems simple enough: We take energy in and use it, just like machines do—our automobiles, cell phones, and so on. But if we dig a little deeper into this subject, we discover a philosophical importance: We learn what distinguishes Earth’s life and life-containing systems from the rest of the universe.

Although most of the time energy is invisible to us, with infrared film we can see the differences between warm and cold objects, and we can see some things about energy flow that affect life. With infrared film, warm objects ap- pear red and cool objects blue. Figure 6.13 shows birch trees in a New Hampshire forest, both as we see them, us- ing standard film, and with infrared film. The infrared film shows tree leaves bright red, indicating that they have been warmed by the sun and are absorbing and reflecting energy, whereas the white birch bark remains cooler. The ability of tree leaves to absorb energy is essential; it is this source of energy that ultimately supports all life in a forest. Energy flows through life, and energy flow is a key concept.

The Laws of Thermodynamics and the Ultimate Limit on the Abundance of Life

When we discuss ecosystems, we are talking about some of the fundamental properties of life and of the ecological systems that keep life going. A question that frequently arises both in basic science and when we want to produce a lot of some kind of life—a crop, biofuels, pets—is: What ultimately limits the amount of organic matter in living things that can be produced anywhere, at any time, forever on the Earth or anywhere in the universe?

We ask this question when we are trying to improve the production of some form of life. We want to know: How closely do ecosystems, species, populations, and individuals approach this limit? Are any of these near to being as productive as possible?

The answers to these questions, which are at the same time practical, scientifically fundamental, and philosophical, lie in the laws of thermodynamics. The first law of thermodynamics, known as the law of con- servation of energy states that in any physical or chemi- cal change, energy is neither created nor destroyed but merely changed from one form to another. (See Work- ing It Out 6.1.) This seems to lead us to a confusing, contradictory answer—it seems to say that we don’t need to take in any energy at all! If the total amount of energy is always conserved—if it remains constant— then why can’t we just recycle energy inside our bodies? The famous 20th-century physicist Erwin Schrödinger asked this question in a wonderful book entitled What Is Life? He wrote:

In some very advanced country (I don’t remem- ber whether it was Germany or the U.S.A. or both) you could find menu cards in restaurants indicating, in addition to the price, the energy content of every dish. Needless to say, taken lit- erally, this is . . . absurd. For an adult organism the energy content is as stationary as the material content. Since, surely, any calorie is worth as much as any other calorie, one cannot see how a mere ex- change could help.

Schrödinger was saying that, according to the first law of thermodynamics, we should be able to recycle energy in our bodies and never have to eat anything. Similarly, we can ask: Why can’t energy be recycled in ecosystems and in the biosphere?

Let us imagine how that might work, say, with frogs and mosquitoes. Frogs eat insects, including mosquitoes. Mosquitoes suck blood from vertebrates, including frogs.

Consider an imaginary closed ecosystem consisting of wa- ter, air, a rock for frogs to sit on, frogs, and mosquitoes. In this system, the frogs get their energy from eating the mosquitoes, and the mosquitoes get their energy from bit- ing the frogs (Figure 6.15). Such a closed system would be a biological perpetual-motion machine. It could continue indefinitely without an input of any new material or energy. This sounds nice, but unfortunately it is impossible. Why? The general answer is found in the second law of thermody- namics, which addresses how energy changes in form.

To understand why we cannot recycle energy, imagine a closed system (a system that receives no input after the initial input) containing a pile of coal, a tank of water, air, a steam engine, and an engineer (Figure 6.16). Suppose the engine runs a lathe that makes furniture. The engineer lights a fire to boil the water, creating steam to run the en- gine. As the engine runs, the heat from the fire gradually warms the entire system.

When all the coal is completely burned, the engineer will not be able to boil any more water, and the engine will stop. The average temperature of the system is now higher than the starting temperature. The energy that was in the coal is dispersed throughout the entire system, much of it as heat in the air. Why can’t the engineer recover all that en- ergy, recompact it, put it under the boiler, and run the en- gine? The answer is in the second law of thermodynamics. Physicists have discovered that no use of energy in the real (not theoretical) world can ever be 100% efficient. Whenever useful work is done, some energy is inevitably converted to heat. Collecting all the energy dispersed in this closed system would require more energy than could be recovered.

Our imaginary system begins in a highly organized state, with energy compacted in the coal. It ends in a less organized state, with the energy dispersed throughout the system as heat. The energy has been degraded, and the system is said to have undergone a decrease in order. The measure of the decrease in order (the disorganization of energy) is called entropy. The engineer did produce some furniture, converting a pile of lumber into nicely ordered tables and chairs. The system had a local increase of order (the furniture) at the cost of a general increase in disorder (the state of the entire system). All energy of all systems tends to flow toward states of increasing entropy.

The second law of thermodynamics gives us a new un- derstanding of a basic quality of life. It is the ability to create order on a local scale that distinguishes life from its nonliving environment. This ability requires obtaining energy in a usable form, and that is why we eat. This principle is true for every ecological level: individual, population, commu- nity, ecosystem, and biosphere. Energy must continually be added to an ecological system in a usable form. Energy is inevitably degraded into heat, and this heat must be re- leased from the system. If it is not released, the tempera- ture of the system will increase indefinitely. The net flow of energy through an ecosystem, then, is a one-way flow.

Based on what we have said about the energy flow through an ecosystem, we can see that an ecosystem must lie between a source of usable energy and a sink for de- graded (heat) energy. The ecosystem is said to be an inter- mediate system between the energy source and the energy sink. The energy source, ecosystem, and energy sink to- gether form a thermodynamic system. The ecosystem can undergo an increase in order, called a local increase, as long as the entire system undergoes a decrease in order, called a global decrease. (Note that order has a specific meaning in thermodynamics: Randomness is disorder; an ordered system is as far from random as possible.) To put all this simply, creating local order involves the production of organic matter. Producing organic matter requires energy; organic matter stores energy.

With these fundamentals in mind, we can turn to prac- tical and empirical scientific problems, but this requires that we agree how to measure biological production. To complicate matters, there are several measurement units involved, depending on what people are interested in.

6.5 Biological production and Biomass

The total amount of organic matter in any ecosystem is called its biomass. Biomass is increased through biologi- cal production (growth). Change in biomass over a given period is called production. Biological production is the capture of usable energy from the environment to produce organic matter (or organic compounds). This cap- ture is often referred to as energy “fixation,” and it is often said that the organism has “fixed” energy. There are two kinds of production, gross and net. Gross production is the increase in stored energy before any is used; net pro- duction is the amount of newly acquired energy stored after some energy has been used. When we use energy, we “burn” a fuel through respiration. The difference between gross and net production is like the difference between a person’s gross and net income. Your gross income is the amount you are paid. Your net income is what you have left after taxes and other fixed costs. Respiration is like the expenses that are required in order for you to do your work. (See Working It Out 6.2.)

Measuring Biomass and Production

Three measures are used for biomass and biological pro- duction: The quantity of organic material (biomass), en- ergy stored, and carbon stored. We can think of these measures as the currencies of production. Biomass is usu- ally measured as the amount per unit surface area—for example, as grams per square meter (g/m2) or metric tons per hectare (MT/ha). Production, a rate, is the change per unit area in a unit of time—for example, grams per square meter per year. (Common units of measure of production are given in the Appendix.)

The production carried out by autotrophs is called primary production; that of heterotrophs is called secondary production. As we have said, most autotrophs make sugar from sunlight, carbon dioxide, and water in a process called photosynthesis, which releases free oxygen (see Working It Out 6.1 and 6.2). Some autotrophic bac- teria can derive energy from inorganic sulfur compounds; these bacteria are referred to as chemoautotrophs. Such bacteria live in deep-ocean vents, where they provide the basis for a strange ecological community. Chemoauto- trophs are also found in muds of marshes, where there is no free oxygen.

Once an organism has obtained new organic mat- ter, it can use the energy in that organic matter to do things: to move, to make new compounds, to grow, to reproduce, or to store it for future uses. The use of energy from organic matter by most heterotrophic and autotro- phic organisms is accomplished through respiration. In respiration, an organic compound combines with oxy- gen to release energy and to produce carbon dioxide and water (see Working It Out 6.2). The process is similar to the burning of organic compounds but takes place within cells at much lower temperatures through enzyme-medi- ated reactions. Respiration is the use of biomass to release energy that can be used to do work. Respiration returns to the environment the carbon dioxide that had been re- moved by photosynthesis.

6.6 Energy Efficiency and transfer Efficiency

How efficiently do living things use energy? This is an im- portant question for the management and conservation of all biological resources. We would like biological resourc- es to use energy efficiently—to produce a lot of biomass from a given amount of energy. This is also important for attempts to sequester carbon by growing trees and other perennial vegetation to remove carbon dioxide from the atmosphere and store it in living and dead organic matter (see Chapter 20).

As you learned from the second law of thermody- namics, no system can be 100% efficient. As energy flows through a food web, it is degraded, and less and less is usable. Generally, the more energy an organism gets, the more it has for its own use. However, organisms differ in how efficiently they use the energy they obtain. A more ef- ficient organism has an advantage over a less efficient one.

Efficiency can be defined for both artificial and natu- ral systems: machines, individual organisms, populations, trophic levels, ecosystems, and the biosphere. Energy efficiency is defined as the ratio of output to input, and it is usually further defined as the amount of useful work obtained from some amount of available energy. Efficiency has different meanings to different users. From the point of view of a farmer, an efficient corn crop is one that con- verts a great deal of solar energy to sugar and uses little of that sugar to produce stems, roots, and leaves. In other words, the most efficient crop is the one that has the most harvestable energy left at the end of the season. A truck driver views an efficient truck as just the opposite: For him, an efficient truck uses as much energy as possible from its fuel and stores as little energy as possible (in its exhaust). When we view organisms as food, we define efficiency as the farmer does, in terms of energy storage (net production from available energy). When we are en- ergy users, we define efficiency as the truck driver does, in terms of how much useful work we accomplish with the available energy.

Consider the use of energy by a wolf and by one of its principal prey, moose. The wolf needs energy to travel long distances and hunt, and therefore it will do best if it uses as much of the energy in its food as it can. For itself, a highly energy-efficient wolf stores almost nothing. But from its point of view, the best moose would be one that used little of the energy it took in, storing most of it as muscle and fat, which the wolf can eat. Thus what is efficient depends on your perspective.

A common ecological measure of energy efficiency is called food-chain efficiency, or trophic-level efficiency, which is the ratio of production of one trophic level to the pro- duction of the next-lower trophic level. This efficiency is never very high. Green plants convert only 1–3% of the energy they receive from the sun during the year to new plant tissue. The efficiency with which herbivores convert the potentially available plant energy into herbivorous energy is usually less than 1%, as is the efficiency with which carnivores convert herbivores into carnivorous en- ergy. In natural ecosystems, the organisms in one trophic level tend to take in much less energy than the potential maximum available to them, and they use more energy than they store for the next trophic level. At Isle Royale National Park, an island in Lake Superior, wolves feed on moose in a natural wilderness. In one study, a pack of 18 wolves killed an average of one moose approximately every 2.5 days,9 which gives wolves a trophic-level efficien- cy of about 0.01%. (In 2012, the wolf population had dropped to nine individuals on the island, for reasons not yet understood.10)

The rule of thumb for ecological trophic energy ef- ficiency is that more than 90% (usually much more) of all energy transferred between trophic levels is lost as heat. Less than 10% (approximately 1% in natural ecosystems) is fixed as new tissue. In highly managed ecosystems, such as ranches, the efficiency may be greater. But even in such systems, it takes an average of 3.2 kg (7 lb) of vegetable matter to produce 0.45 kg (1 lb) of edible meat. Cattle are among the least efficient producers, requiring around 7.2 kg (16 lb) of vegetable matter to produce 0.45 kg (1 lb) of edible meat. Chickens are much more efficient, using approximately 1.4 kg (3 lb) of vegetable matter to produce 0.45 kg (1 lb) of eggs or meat. Much attention has been paid to the idea that humans should eat at a low- er trophic level in order to use resources more efficiently. (See Critical Thinking Issue: Should People Eat Lower on the Food Chain?)

6.7 Ecological Stability and Succession

Ecosystems are dynamic: They change over time both from external (environmental) forces and from their inter- nal processes. It is worth repeating the point we made in Chapter 4 about dynamic systems: The classic interpreta- tion of populations, species, ecosystems, and Earth’s entire biosphere has been to assume that each is a stable, static system. But the more we study these ecological systems, the clearer it becomes that these are dynamic systems, always changing and always requiring change. Curiously, they persist while undergoing change.11 We say “curi- ously” because in our modern technological society we are surrounded by mechanical and electronic systems that stay the same in most characteristics and are designed to do so. We don’t expect our car or television or cell phone to shrink or get larger and then smaller again; we don’t expect that one component will get bigger or smaller over time. If anything like this were to happen, those systems would break.

Ecosystems, however, not only change but also then recover and overcome these changes, and life continues on. It takes some adjustment in our thinking to accept and understand such systems.

When disturbed, ecosystems can recover through ecological succession if the damage is not too great. We can classify ecological succession as either primary or secondary. Primary succession is the establishment and development of an ecosystem where one did not exist previously. Coral reefs that form on lava emitted from a volcano and cooled in shallow ocean waters are examples of primary succession. So are forests that develop on new lava flows, like those released by the volcano on the big island of Hawaii (Figure 6.17a), and forests that develop at the edges of retreating glaciers (Figure 6.17b).

Secondary succession is reestablishment of an eco- system after disturbances. In secondary succession, there are remnants of a previous biological community, includ- ing such things as organic matter and seeds. A coral reef that has been killed by poor fishing practices, pollution, climate change, or predation, and then recovers, is an ex- ample of secondary succession. Forests that develop on abandoned pastures or after hurricanes, floods, or fires are also examples of secondary succession.

Succession is one of the most important ecological processes, and the patterns of succession have many man- agement implications (discussed in detail in Chapter 12). We see examples of succession all around us. When a house lot is abandoned in a city, weeds begin to grow. Af- ter a few years, shrubs and trees can be found; secondary succession is taking place. A farmer weeding a crop and a homeowner weeding a lawn are both fighting against the natural processes of secondary succession.

Patterns in Succession

Succession follows certain general patterns. When ecologists first began to study succession, they focused on three cases involving forests: (1) on dry sand dunes along the shores of the Great Lakes in North America; (2) in a northern fresh- water bog; and (3) in an abandoned farm field. These were particularly interesting because each demonstrated a repeat- able pattern of recovery and each tended to produce a late stage that was similar to the late stages of the others.

Dune Succession

Sand dunes are continually being formed along sandy shores and then breached and destroyed by storms. In any of the Great Lakes states, soon after a dune is formed on the shores of one of the Great Lakes, dune grass invades. This grass has special adaptations to the unstable dune. Just under the surface, it puts out runners with sharp ends (if you step on one, it will hurt). The dune grass rap- idly forms a complex network of underground runners, crisscrossing almost like a coarsely woven mat. Above the ground, the green stems carry out photosynthesis, and the grasses grow. Once the dune grass is established, its run- ners stabilize the sand, and seeds of other plants have a better chance of germinating. The seeds germinate, the new plants grow, and an ecological community of many species begins to develop. The plants of this early stage tend to be small, grow well in bright light, and withstand the harsh environment—high temperatures in summer, low temperatures in winter, and intense storms.

Slowly, larger plants, such as eastern red cedar and east- ern white pine, are able to grow on the dunes. Eventually, a forest develops, which may include species such as beech and maple. A forest of this type can persist for many years, but at some point a severe storm breaches even these heavily vegetated dunes, and the process begins again (Figure 6.18).

Bog Succession

A bog is an open body of water with surface inlets— usually small streams—but no surface outlet. As a result, the waters of a bog are quiet, flowing slowly if at all. Many bogs that exist today originated as lakes that filled depres- sions in the land, which in turn were created by glaciers during the Pleistocene ice age. Succession in a northern bog, such as the Livingston Bog in Michigan (Figure 6.19), begins when a sedge (a grasslike herb) puts out floating runners (Figure 6.20a, b). These runners form a complex, matlike network similar to that formed by dune grass. The stems of the sedge grow on the runners andcarry out photosynthesis. Wind blows particles onto the mat, and soil, of a kind, develops. Seeds of other plants, instead of falling into the water, land on the mat and can germinate. The floating mat becomes thicker as small shrubs and trees, adapted to wet environments, grow. In the North, these include species of the blueberry family.

The bog also fills in from the bottom as streams carry fine particles of clay into it (Figure 6.20b, c). At the shore, the floating mat and the bottom sediments meet, forming a solid surface. But farther out, a “quaking bog” occurs. You can walk on this mat; and if you jump up and down, all the plants around you bounce and shake because the mat is really floating. Eventually, as the bog fills in from the top and the bottom, trees grow that can withstand wet- ter conditions—such as northern cedar, black spruce, and balsam fir. The formerly open-water bog becomes a wet- land forest. If the bog is farther south, it may eventually be dominated by beech and maple, the same species that dominate the late stages of the dunes.

Old-Field Succession

In the northeastern United States, a great deal of land was cleared and farmed in the 18th and 19th centuries. Today, much of this land has been abandoned for farming and allowed to grow back to forest (Figure 6.21). The first plants to enter the abandoned farmlands are small plants adapted to the harsh and highly variable conditions of a clearing—a wide range of temperatures and precipitation. As these plants become established, other, larger plants en- ter. Eventually, large trees grow, such as sugar maple, beech, yellow birch, and white pine, forming a dense forest.

Since these three different habitats—one dry (the dunes), one wet (the bog), and one in between (the old field)—tend to develop into similar forests, early ecolo- gists believed that this late stage was in fact a steady-state condition. They referred to it as the “climatic climax,” meaning that it was the final, ultimate, and permanent stage to which all land habitats would proceed if undis- turbed by people. Thus, the examples of succession were among the major arguments in the early 20th century that nature did in fact achieve a constant condition, a steady state, and there actually was a balance of nature. We know today that this is not true, a point we will return to later.

Coral Reef Succession

Coral reefs (Figure 6.22) are formed in shallow warm waters by corals—small marine animals that live in colo- nies and are members of the phylum Coelenterata, which also includes sea anemones and jellyfishes. Corals have a whorl of tentacles surrounding the mouth, and feed by catching prey, including planktonic algae, as it passes by. The corals settle on a solid surface and produce a hard polyp of calcium carbonate (in other words, limestone). As old individuals die, this hard material becomes the surface on which new individuals establish themselves. Coralline algae, a kind of red algae that also uses calcium carbonate, is also important in the formation of the reefs. In addition, other limestone-shell-forming organisms such as certain snails and sea urchins live and die on the reef and are glued together by coralline algae. Eventu- ally a large and complex structure results involving many other species, including autotrophs and heterotrophs, creating one of the most species-diverse of all kinds of ecosystems. Highly valued for this diversity, for produc- tion of many edible fish, for the coral itself (used in vari- ous handicrafts and arts), and for recreation, coral reefs attract lots of attention.

Succession, in Sum

Even though the environments are very different, these four examples of ecological succession—dune, bog, old field, and coral reef—have common elements found in most ecosystems:

1. An initial kind of autotroph (green plants in three of the examples discussed here; algae and photosynthetic bacteria in marine systems; algae and photosynthetic bacteria, along with some green plants in some freshwater and near-shore marine systems). These are typically small in stature and specially adapted to the unstable conditions of their environment.

2. A second stage with autotrophs still of small stature, rapidly growing, with seeds or other kinds of reproductive structures that spread rapidly.

3. A third stage in which larger autotrophs—like trees in forest succession—enter and begin to dominate the site.

4. A fourth stage in which a mature ecosystem develops.

Although we list four stages, it is common practice to combine the first two and speak of early-, middle-, and late-successional stages. The stages of succession are de- scribed here in terms of autotrophs, but similarly adapt- ed animals and other life forms are associated with each stage. We discuss other general properties of succession later in this chapter.

Species characteristic of the early stages of succes- sion are called pioneers, or early-successional species. They have evolved and are adapted to the environmen- tal conditions in early stages of succession. In terrestrial ecosystems, vegetation that dominates late stages of suc- cession, called late-successional species, tends to be slower growing and longer lived, and can persist under intense competition with other species. For example, in terrestrial ecosystems, late-successional vegetation tends to grow well in shade and have seeds that, though not as widely dispersing, can persist a rather long time. Typical middle-successional species have characteristics in between the other two types.

6.8 Chemical Cycling and Succession

In Chapter 4, we introduced the biogeochemical cycle. One of the important effects of succession is a change in the storage of chemical elements necessary for life. On land, the storage of chemical elements essential for plant growth and function (including nitrogen, phospho- rus, potassium, and calcium) generally increases during the progression from the earliest stages of succession to middle succession (Figure 6.23). There are three reasons for this:

Increased storage. Organic matter, living or dead, stores chemical elements. As long as there is an increase in organic matter within the ecosystem, there will be an increase in the storage of chemical elements.

Increased rate of uptake. For example, in terrestrial ecosystems, many plants have root nodules containing bacteria that can assimilate atmospheric nitrogen, which is then used by the plant in a process known as nitrogen fixation.

Decreased rate of loss. The presence of live and dead organic matter helps retard erosion. Both organic and in- organic soil can be lost to erosion by wind and water. Veg- etation and, in certain marine and freshwater ecosystems, large forms of algae tend to prevent such losses and there- fore increase total stored material.

Ideally, chemical elements could be cycled indefinite- ly in ecosystems, but in the real world there is always some loss as materials are moved out of the system by wind and water. As a result, ecosystems that have persisted con- tinuously for the longest time are less fertile than those in earlier stages. For example, where glaciers melted back thousands of years ago in New Zealand, forests developed, but the oldest areas have lost much of their fertility and have become shrublands with less diversity and biomass. The same thing happened to ancient sand dune vegetation in Australia.11

6.9 how Species Change Succession

Early-successional species can affect what happens later in succession in three ways: through (1) facilitation, (2) interference, or (3) life history differences (Figure 6.24).

Facilitation

In facilitation, an earlier-successional species changes the local environment in ways that make it suitable for another species that is characteristic of a later successional stage. Dune and bog succession illustrate facilitation. The first plant species—dune grass and floating sedge—prepare the way for other species to grow. Facilitation is common in tropical rain forests, where early-successional species speed the reappearance of the microclimatic conditions that occur in a mature forest. Because of the rapid growth of early-successional plants, after only 14 years the tempera- ture, relative humidity, and light intensity at the soil surface in tropical forests can approximate those of a mature rain forest. Once these conditions are established, species adapt- ed to deep forest shade can germinate and persist.

Facilitation also occurs in coral reefs, mangrove swamps along ocean shores, kelp beds along cold ocean shores such as the Pacific coast of the United States and Canada’s Pacific Northwest, and in shallow marine ben- thic areas where the water is relatively calm and large algae can become established.

Knowing the role of facilitation can be useful in the restoration of damaged areas. Plants that facilitate the presence of others should be planted first. On sandy areas, for example, dune grasses can help hold the soil before we attempt to plant shrubs or trees.

Interference

In contrast to facilitation, interference refers to situations where an earlier-successional species changes the local envi- ronment so that it is unsuitable to another species character- istic of a later-successional stage. Interference is common, for example, in American tall-grass prairies, where prairie grasses like little bluestem form a mat of living and dead stems so dense that seeds of other plants cannot reach the ground and therefore do not germinate. Interference does not last forever, however. Eventually, some breaks occur in the grass mat—perhaps from surface-water erosion, the death of a patch of grass from disease, or removal by fire. Breaks in the grass mat allow seeds of trees to germinate. For example, in the tall-grass prairie, seeds of red cedar can then reach the ground. Once started, red cedar soon grows taller than the grasses, shading them so much that they can- not grow. More ground is open, and the grasses are eventu- ally replaced.

The same pattern occurs in some Asian tropical rain forests. The grass, Imperata, forms stands so dense that seeds of later-successional species cannot reach the ground. Imperata either replaces itself or is replaced by bamboo, which then replaces itself. Once established, Imperata and bamboo appear able to persist for a long time. Once again, when and if breaks occur in the cover of these grasses, other species can germinate and grow, and a forest eventually develops.

Life History Differences

In this case, changes in the time it takes different species to establish themselves give the appearance of a succes- sion caused by species interactions, but it is not. In cases where no species interact through succession, the result is termed chronic patchiness. Chronic patchiness is char- acteristic of highly disturbed environments and highly stressful ones in terms of temperature, precipitation, or chemical availability, such as deserts. For example, in the warm deserts of California, Arizona, and Mexico, the ma- jor shrub species grow in patches, often consisting of ma- ture individuals with few seedlings. These patches tend to persist for long periods until there is a disturbance. Simi- larly, in highly polluted environments, a sequence of spe- cies replacement may not occur. Chronic patchiness also describes planktonic ecological communities and their ecosystems, which occur in the constantly moving waters of the upper ocean and the upper waters of ponds, lakes, rivers, and streams.

SUMMARY

• An ecosystem is the simplest entity that can sustain life. At its most basic, an ecosystem consists of several species and a fluid medium (air, water, or both). The ecosystem must sustain two processes—the cycling of chemical elements and the flow of energy.

• The living part of an ecosystem is the ecological com- munity, a set of species connected by food webs and trophic levels. A food web or food chain describes who feeds on whom. A trophic level consists of all the organ- isms that are the same number of feeding steps from the initial source of energy.

• Community-level effects result from indirect interac- tions among species, such as those that occur when sea otters influence the abundance of sea urchins.

• Ecosystems are real and important, but it is often diffi- cult to define the limits of a system or to pinpoint all the interactions that take place. Ecosystem management is considered key to the successful conservation of life on Earth.

Energy flows one way through an ecosystem; the sec- ond law of thermodynamics places a limit on the abun- dance of productivity of life and requires the one-way flow.

Chemical elements cycle, and in theory could cycle forever, but in the real world there is always some loss.

Ecosystems recover from changes through ecological succession, which has repeatable patterns.

Ecosystems are nonsteady-state systems, undergoing changes all the time and requiring change.