Ecological systems are governed by physical and biological principles

Although ecological systems are complex, they are governed by a few basic principles. Life builds on the physical properties and chemical reactions of matter. The diffusion of oxygen across body surfaces, the rates of chemical reactions, the resistance of vessels to the flow of fluids, and the transmission of nerve impulses all obey the laws of thermodynamics. Within these constraints, life can pursue many options and has done so with astounding innovation. In this section we briefly review the three major biological principles that you may recall from your introductory biology course: conservation of matter and energy, dynamic steady states, and evolution.

Conservation of Matter and Energy

The law of conservation of matter states that matter cannot be created or destroyed, but can only change form. For example, as you drive a car, gasoline is burned in the engine; the amount of fuel in the tank declines, but you have not destroyed matter. The molecules that comprise gasoline are converted into new forms including carbon monoxide (CO), carbon dioxide (CO₂), and water (H₂O).

Another important biological principle, the first law of thermodynamics—also known as the law of conservation of energy—states that energy cannot be created or destroyed. Like matter, energy can only be converted into different forms. Living organisms must constantly obtain energy to grow, maintain their bodies, and replace energy lost as heat.

The law of conservation of matter and the first law of thermodynamics imply that ecologists can track the movement of matter and energy as it is converted into new forms through organisms, populations, communities, ecosystems, and the biosphere. At every level of organization, we should be able to determine how much matter enters the system and account for its movement. For example, consider a field full of cattle (Bos taurus) eating grass. At the organismal level, we can determine how much energy an individual animal consumes and then calculate the proportion of this energy that is converted into growth of its body, the maintenance of its physiology, and waste. At the population level, we can calculate how much energy the entire herd of cattle consumes by eating grass. At the community level, we can evaluate how much energy each species of grass creates via photosynthesis and how much of this energy is passed on to cattle and other plant-eating species, such as rabbits that might coexist with the cattle. At the ecosystem level, we can estimate how elements such as carbon flow from the grasses to herbivores (cattle and rabbits) and then on to predators. We can then track how dead grass, the waste products of herbivores and predators, and the dead bodies of herbivores and predators decompose and return to the soil.

Dynamic Steady States

Although matter and energy cannot be created or destroyed, ecological systems continuously exchange matter and energy with their surroundings. When gains and losses are in balance, ecological systems are unchanged and the system is said to be in a dynamic steady state. For example, birds and mammals continuously lose heat in a cold environment. However, this loss is balanced by heat gained from the metabolism of foods, so the animal’s body temperature remains constant. Similarly, the proteins of our bodies are continuously broken down and replaced by newly synthesized proteins, so our appearance remains relatively unchanged.

The principle of the dynamic steady state applies to all levels of ecological organization, as illustrated in Figure 1.4. For individual organisms, assimilated food and energy must balance energy expenditure and metabolic breakdown of tissues. A population increases with births and immigration, and it decreases with death and emigration. At the community level, the number of species living in a community decreases when a species becomes extinct, and increases when a new species colonizes the area. Ecosystems and the biosphere receive energy from the Sun, and this gain of energy is balanced by heat energy radiated by Earth back out into space. One of the most important questions ecologists ask is how the steady states of ecological systems are maintained and regulated. We will return to this question frequently throughout this book.

Figure 1.4 Dynamic steady states. At all levels of organization, the inputs to the systems must equal the outputs.

An understanding of dynamic steady states helps provide insights regarding the inputs and outputs of ecological systems. Of course, ecological systems also change. Organisms grow, populations vary in abundance, and abandoned fields revert to forest. Yet all ecological systems have mechanisms that tend to maintain a dynamic steady state.

Evolution

Although matter and energy cannot be created or destroyed, what living systems do with matter and energy is as variable as all the forms of organisms that have ever existed on Earth. To understand the variation among organisms—the diversity of life—we turn to the concept of evolution.

An attribute of an organism, such as its behavior, morphology, or physiology, is the organism’s phenotype. A phenotype is determined by the interaction of the organism’s genotype, or the set of genes it carries, with the environment in which it lives. For example, your height is a phenotype that is determined by your genes and the nutrition you received in the environment where you were raised.

Over the history of life on Earth, the phenotypes of organisms have changed and diversified dramatically. This is the process of evolution, which is a change in the genetic composition of a population over time. Evolution can happen through a number of different processes that we will discuss in detail in later chapters. Perhaps the best known process is evolution by natural selection, which is a change in the frequency of genes in a population through differential survival and reproduction of individuals that possess certain phenotypes. As outlined by Charles Darwin in his book On the Origin of Species, evolution by natural selection depends on three conditions:

When these three conditions exist, an individual with higher survival and reproductive success will pass more copies of its genes to the next generation. Over time, the genetic composition of a population changes as the most successful phenotypes come to predominate. As a result, the population becomes better suited to the surrounding environmental conditions. Phenotypes that are well suited to their environment and, in turn, confer higher fitness are known as adaptations. Consider the example in Figure 1.5 in which some individuals in a population of caterpillars are colored in such a way that they blend in with their surroundings and escape the notice of predators while other individuals are not. If color is inherited, over time the population will consist of a progressively larger proportion of caterpillars that blend in with their environment.

Figure 1.5 Evolution by natural selection. In this example, the caterpillar population is initially quite variable in color (a). Individuals that better match the twig are less obvious to the bird hunting for food and therefore more likely to survive. If color is genetically inherited, the next generation (b) of the caterpillar population will be better matched to resemble twigs. As this natural selection continues over many generations, the color of the caterpillar population will closely match the twigs (c). At this stage, the color of the caterpillar represents an adaptation against predation.

Species do not evolve in isolation. Evolution in one species opens up new possibilities for evolutionary change, both in the evolving species and in other species with which the evolving species interacts. For example, caterpillars of the monarch butterfly (Danaus plexippus) have evolved the ability to eat the leaves of milkweed plants, which are toxic to caterpillars of other species. Monarch caterpillars not only tolerate these toxic compounds, but also sequester these compounds in their bodies and use the toxins to defend themselves, as larvae as well as adults, against bird predators. In addition, both larvae and adults have evolved conspicuous “warning” coloration to advertise their toxicity. After the caterpillar species evolved these defensive abilities, predatory birds evolved a new ability to discriminate between caterpillars and butterflies that were toxic and those that were edible.

The complexity of ecological communities and ecosystems builds on, and is fostered by, existing complexity. Ecologists seek to understand how these complex ecological systems came into being and how they function in their environmental settings.