Unit Eight. The Living Environment


35. Populations and Communities



Often the most significant ecological events that occur in a particular ecosystem involve the organisms that inhabit it. The swarming insects you see here are migratory locusts, Locusta migratoria, moving across farmland in North Africa in 1988. In most years, the locusts are not plentiful and do not swarm. In particularly favorable years, however, when food is plentiful and the weather mild, the abundance of resources leads to greater-than- usual growth of locust populations. When high population densities are reached, the locusts exhibit different hormonal and physical characteristics and take off as a swarm. Moving over the landscape, the swarm eats every available plant, denuding the landscape. Swarming locusts, although not common in North America, are a legendary plague of large areas of Africa and Eurasia. In this chapter, we examine how natural populations grow, and what factors limit this growth. The organisms of the living world have evolved many accommodations to facilitate living together, creating complex evolutionary arrangements. When these arrangements are disturbed by unusual weather—or human intervention—the consequences can be catastrophic.


35.1. What Is Ecology?


Ecology is the study of how organisms interact with each other and with their environment. Ecology also encompasses the study of the distribution and abundance of organisms, which includes population growth and the limits and influences on population growth. The word ecology was coined in 1866 by the great German biologist Ernst Haeckel and comes from the Greek words oikos (house, place where one lives) and logos (study of). Our study of ecology, then, is a study of the house in which we live. Do not forget this simple analogy built into the word ecology—most of our environmental problems could be avoided if we treated the world in which we live the same way we treat our own homes. Would you pollute your own house?


Levels of Ecological Organization

Ecologists consider groups of organisms at six progressively more encompassing levels of organization. As mentioned in chapter 1, new characteristics called emergent properties arise at each higher level, resulting from the way components of each level interact.

1. Populations. Individuals of the same species that live together are members of a population. They potentially interbreed with one another, share the same habitat, and use the same pool of resources the habitat provides.

2. Species. All populations of a particular kind of organism form a species. Populations of the species can interact and affect the ecological characteristics of the species as a whole.

3. Communities. Populations of different species that live together in the same place are called communities. Different species typically use different resources within the habitat they share (figure 35.1).

4. Ecosystems. A community and the nonliving factors with which it interacts is called an ecosystem. An ecosystem is affected by the flow of energy, ultimately derived from the sun, and the cycling of the essential elements on which the lives of its constituent organisms depend. The redwood forest community pictured in figure 35.1 is part of an ecosystem, where the giant trees and other organisms interact with each other and with their physical surroundings.

5. Biomes. Biomes are major terrestrial assemblages of plants, animals, and microorganisms that occur over wide geographical areas that have distinct physical characteristics. Examples include deserts, tropical forests, and grasslands. Similar types of groupings occur in marine and freshwater habitats.

6. The biosphere. All the world’s biomes, along with its marine and freshwater assemblages, together constitute an interactive system we call the biosphere. Changes in one biome can have profound consequences for others.



Figure 35.1. The redwood community.

(a) The redwood forest of coastal California and southwestern Oregon is dominated by the population of redwoods (Sequoia sempervirens). Other species in the redwood community include, (b)        redwood sorrel (Oxalis oregana), (c) sword ferns (Polystichum munitum), and (d) ground beetles (Scaphinotus velutinus), this one feeding on a slug on a sword fern leaf.


Although we include biomes and the biosphere as higher levels of ecological organization in this list of organizational levels, the ecosystem is viewed as the “basic functional unit,” in much the same way the cell rather than tissues or organs is considered the basic unit of living organisms.

Some ecologists, called population ecologists, focus on a particular species and how its populations grow. Other ecologists, called community ecologists, study how the different species living in a place interact with one another. Still other ecologists, called systems ecologists, are interested in how biological communities interact with their physical environment.

We will begin our study of ecology at the basic levels, by examining populations and communities. We will then work our way up the hierarchy by examining ecosystems, biomes, and ending with a critical look at the conditions of the biosphere. Although we break these topics into separate chapters, we should not overlook the fact that an organism does not live in a vacuum. Individuals interact with each other and with their physical environment and these interactions introduce challenges and obstacles to survival.


The Environmental Challenge

The nature of the physical environment determines to a great extent which organisms live in a particular climate or region. Key elements of the environment include:

Temperature. Most organisms are adapted to live within a relatively narrow range of temperatures and will not thrive if temperatures are colder or warmer. The growing season of plants, for example, is strongly influenced by temperature.

Water. All organisms require water. On land, water is often scarce, so patterns of rainfall have a major influence on life.

Sunlight. Almost all ecosystems rely on energy captured by photosynthesis, and so the availability of sunlight influences the amount of life an ecosystem can support, particularly below the surface in marine environments.

Soil. The physical consistency, pH, and the availability of minerals in the soil often severely limit plant growth, particularly the amount of nitrogen and phosphorus present in the soil.

During the course of a day, a season, or a lifetime, an individual organism must cope with a range of living conditions. Many organisms are able to adapt to environmental change by making physiological, morphological, or behavioral adjustments. For example, you sweat when it is hot, increasing heat loss through evaporation and thus preventing overheating. Morphological adaptations in some mammals may include growing a thicker coat of fur in winter (figure 35.2). And, many animals deal with variations in the environment through behavior, such as moving from one place to another, thereby avoiding areas that are unsuitable. For example, a tropical lizard manages to maintain a fairly uniform body temperature by basking in the sunlight but then retreating to the shade when it becomes too hot (figure 35.3). These physiological, morphological, or behavioral abilities are a product of natural selection acting in a particular environmental setting over time, which explains why an individual organism that is moved to a different environment may not survive.



Figure 35.2. Wolf in winter.

This gray wolf grows a thicker coat of fur in the winter to insulate its body. Escaping body heat is trapped in the air surrounding the hairs of its coat, holding in heat and thus helping to maintain the wolf's body temperature in the cold winter.



Figure 35.3. Costa Rican lizard.

This green iguana escapes to the shade in the heat of the day, helping to keep its body cooler as the temperature outside rises.


Key Learning Outcome 35.1. Ecology is the study of how the organisms that live in a place interact with each other and with their physical environment. An ecosystem is a dynamic ecological system that challenges organisms to adjust to its changing physical conditions.