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Chapter 31: An Interactive Living World: Populations and Communities in Ecology Overview

Ecology is the study of how living things interact—with each other and with their physical environment. One way of studying these interactions is to focus on populations, meaning all the members of a single species living in one area. Another way of studying these interactions is to look at communities, meaning all the populations of all the species living in one area.

31.1 The Study of Ecology

31.2 Populations: Size and Dynamics

31.3 r-Selected and K-Selected Species

31.4 Thinking about Human Populations

31.5 Communities: Looking at the Interactions of Many Populations

31.6 Types of Interaction among Community Members

31.7 Succession in Communities

31.1 The Study of Ecology

• Ecology is the study of the interactions living things have with each other and with their environment.
• Ecology is not the same thing as environmentalism. The function of ecology is to describe interactions that affect the living world, not to work on behalf of environmentalism.
• There are five scales of life that concern ecology: physiology, populations, communities, ecosystems, and the biosphere. A population is all the members of a single species that live together in a specified geographical area; a community is all the species living in a single area; an ecosystem is a community and all the nonliving elements that interact with it; the biosphere is the interactive collection of all the Earth’s ecosystems.

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31.2 Populations: Size and Dynamics

• Ecologists employ several means to estimate the size of populations of living things, among them counting animal droppings or surveying bird populations as they migrate.
• An arithmetical increase occurs when, over a given interval of time, an unvarying number of new units is added to a population. An exponential increase occurs when the number of new units added to a population is proportional to the number of units that exists. Populations of living things are capable of increasing exponentially, because living things are capable of giving rise to more living things.
• The rapid growth that sometimes characterizes living populations is referred to as exponential growth, or as the J-shaped growth curve. Populations that initially grow, but whose growth later levels out, have experienced logistic growth, sometimes referred to as the S-shaped growth curve.
• The size of living populations is kept in check by environmental resistance, defined as all the forces of the environment that act to limit population growth.
• Exponential growth in living populations can be calculated by subtracting a population’s death rate from its birth rate, which yields the population’s growth rate. Denoted as r, this rate is also known as the population’s intrinsic rate of increase; it can be thought of as the population’s potential for growth.
• Carrying capacity, denoted as K, is the maximum population density of a given species that can be sustained within a defined geographical area over an extended period of time.

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31.3 r-Selected and K-Selected Species

• Different species have different reproductive strategies, meaning characteristics that have the effect of increasing the number of fertile offspring they bear.
• Some species are said to be K-selected, or equilibrium, species. These species tend to be large, to experience their environment as relatively stable, and to lavish a good deal of attention on relatively few offspring. The pressures on K-selected species tend to be density dependent, meaning that as a population’s density goes up, factors that limit the population’s growth assert themselves ever more strongly.
• Other species are said to be r-selected or opportunist species. These species tend to be small, to experience their environment as relatively unstable, and to give little or no attention to the numerous offspring they produce. The pressures on r-selected species tend to be density independent, meaning pressures that are unrelated to the population’s density.
• Survivorship curves describe how soon species members tend to die within the species’ life span. There are three idealized types of survivorship curves: late loss, constant loss, and early loss.

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31.4 Thinking about Human Populations

• Survivorship curves are created from life tables, which set forth the probabilities of a member of a species being alive after given intervals of time.
• An important step in calculating the future growth of human populations is to learn what proportion of the population is at or under reproductive age. A population pyramid displays this proportion. Populations whose pyramids are heavily weighted toward younger age groups are likely to experience relatively large growth.
• Following centuries of explosive increase, the world’s human population is projected to stabilize in the coming decades, going from about 6.3 billion now to a maximum of about 8.9 billion at mid-century. This stabilization is being brought about by a decrease in female fertility, defined as the number of children born, on average, to each woman in a population.
• The global reduction in female fertility masks enormous, ongoing differences between fertility in more-developed and less-developed countries. Fertility in less-developed countries tends to be much higher than that in more-developed countries. The fertility in most European nations is now so low that the continent’s population stands to shrink significantly by mid-century. The population of the United States, however, is projected to grow significantly during this same period. The primary factors bringing the U.S. increase about are immigration and relatively high female fertility.
• Some scientists believe that there is no greater single threat to the environment than the continued growth of the human population. Others argue that a more important concern is the use of natural resources per person.

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31.5 Communities: Looking at the Interactions of Many Populations

• An ecological community is all the populations that inhabit a given area, though the term can be used to mean a collection of populations in a given area that potentially interact with one another.
• Most communities tend to be dominated by only a few species; the few species that are abundant in a given area are called ecological dominants.
• A keystone species is a species whose impact on the composition of a community is disproportionately large relative to its abundance within that community. Some keystone species are top predators in a community, meaning species that prey upon other species but that are not preyed upon themselves. Keystone species need not be top predators, however, nor does every community have a keystone species.
• Biodiversity is variety among living things. It takes three primary forms: a diversity of species in a given area; a distribution of species across the Earth; and genetic diversity within a species.
• Recent research indicates that species diversity tends to enhance a community’s productivity, defined as the amount of solar energy photosynthesizing organisms are able to capture and transform into living material or biomass. There is disagreement about whether species diversity also enhances community stability, meaning the ability of a community to retain its characteristics in the face of environmental disruption.

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31.6 Types of Interaction among Community Members

• There are three primary types of interaction among community members: competition; predation and parasitism; and mutualism and commensalism.
• Habitat is the physical surroundings in which a species can normally be found. Niche can be defined metaphorically as an organism’s occupation, meaning what the organism does to obtain the resources it needs to live.
• The competitive exclusion principle states that when two populations compete for the same limited, vital resource, one always out competes the other and thus brings about the latter’s local extinction.
• There are numerous instances in nature in which two related species use the same kinds of resources from the same habitat over an extended period of time, but will divide the resources up, such that neither of species undergoes local extinction. This phenomenon is called coexistence through resource partitioning.
• Predation is defined as one free-standing organism feeding on parts or all of a second organism. Parasitism is a variety of predation in which the predator feeds on prey, but does not kill it immediately, if ever.
• The population dynamics of a predator and its prey can be linked, but predator-prey interaction generally is only one of several factors controlling the population level of either group.
• The prey of a parasite is known as the host. A parasite can use a host not only as a food source but as a vehicle to facilitate its reproduction.
• Mimicry is a phenomenon by which one species has evolved to assume the appearance of another. A mimic species evolves to match the appearance of a model species. In general, the value of mimicry is that the mimic species suffers less predation as a result of its resemblance to the model species. Batesian mimicry is the evolution of one species to resemble a species that has a superior protective capability. In Müllerian mimicry, several species that have protection against predators come to resemble each other.
• Mutualism is an interaction between individuals of two species that is beneficial to both individuals. Commensalism is an interaction in which an individual from one species benefits while an individual from another species is neither harmed nor helped.
• Coevolution is the interdependent evolution of two or more species. Flowers have evolved colors and fragrances that attract bees, for example, while bees have evolved vision that is most sensitive to the colors in the flowers they pollinate.

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31.7 Succession in Communities

• Parcels of land or water that have been abandoned by humans or devastated by physical forces will almost always be reclaimed by nature to some degree. The process by which this takes place is called succession: a series of replacements of community members at a given location until a relatively stable final state is reached.
• Primary succession proceeds from an original state of little or no life and soil that lacks nutrients. Secondary succession occurs when a final state of habitat is first disturbed by some outside force, but life remains and the soil has nutrients. The final community in any process of succession is known as the climax community.
• A common set of developments occurs in most instances of primary succession, including the arrival of “pioneer” photosynthesizers, facilitation of the growth of some later species through the actions of earlier species, and the competitive driving out of some species by the actions of later species.
• The rejuvenation of the Mount St. Helens area that has occurred since 1980 has provided ecologists with a wealth of information regarding both primary and secondary succession. One of the chief lessons learned concerns the importance of biological legacies, meaning living things, or products of living things, that survive a major ecological disturbance. Biological legacies have been critical to the rapid renewal of habitat at Mount St. Helens.

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