community populations of different species that occupy the same area and interact with one another
ecosystem a natural unit composed of all the living forms in an area, functioning together with all the abiotic components of the environment
biosphere the portion of the planet occupied by living matter
Ecologists study ecosystems at every level. They can ask different types of questions at each level. Examples of these questions are given in Table (below) , using the Zebra as an example.
Ecological Ecosystems Level Question
Individual How do zebras regulate internal water balance?
Population What factors control zebra populations?
Community How does a disturbance influence the number of mammal species in African grasslands?
Ecosystem How does fire affect nutrient availability in grassland ecosystems?
Biosphere What role does concentration of atmospheric carbon dioxide play in the regulation of global temperature?
Lesson Summary
Ecology is the scientific study of how living organisms interact with each other and with their environment.
The study of ecology can be broken down into subdisciplines and can be studied using various methods.
The organism’s environment includes abiotic and biotic factors.
Levels of organization in ecology include the population, community, ecosystem and biosphere.
Review Questions
What are three ways of sub-dividing the study of ecology? Give an example of each.
Name four types of research studies or methods that ecologists use.
Laboratory studies are valuable for studying ecological principles in that certain factors can be isolated and manipulated in a laboratory setting. Give an example of how the effect of an abiotic factor could be evaluated in the laboratory and the response of an organism measured.
A question that an ecologist could ask at the population level is “What factors control Zebra populations?” Think of two examples in which another species might influence the Zebra population.
Further Reading / Supplemental Links
Unabridged Dictionary, Second Edition. Random House, New York, 1998.
http://www.ecokids.ca/pub/index.cfm
http://www.eco-pros.com/ecologykids.htm
http://www.kidsolr.com/science/page12.html
http://www.southplainfield.lib.nj.us/homeworklinks/Ecology.htm
http://www.surfnetkids.com/ecology.htm;
http://en.wikipedia.org/wiki
Vocabulary
abiotic
Physical (nonliving) properties of an organism’s environment, such as sunlight, climate, soil, water and air.
biome
A homogeneous ecological formation that exists over a large region.
biosphere
The portion of the planet occupied by living organisms.
biotic
Biological (living) properties of an organism’s environment, i. e., the other living organisms which share its habitat.
community
Populations of different species that occupy the same area and interact with each other.
ecology
The scientific study of how living organisms interact with each other and with their environment.
ecosystem
A natural unit composed of all the living forms in an area, functioning together with all the abiotic components of the environment.
population
Organisms belonging to the same species that occupy the same area and interact with each other.
Points to Consider
How do you think the study of ecology would be applied at the level of the population and what study methods do you think might be used?
What do you think causes populations to grow?
Lesson 23.2: Populations
Lesson Objectives
Explain what a population is.
Describe how births, deaths and migration affect population size.
Explain how populations grow.
Describe how limiting factors affect population growth.
Describe growth of the human population.
Check Your Understanding
What is ecology?
How does an organism interact with its environment?
Introduction
The study of populations is important to better understand the health and stability of a population. Such factors as births, deaths and migration influence population size. Different models explain how populations grow. Limiting factors can help determine how fast a population grows. All of these aspects of population biology can be applied to the study of human population growth.
What is a Population?
A population is comprised of organisms belonging to the same species, all living in the same area and interacting with each other. Since they live together in one area, members of the same species form an interbreeding unit. Ecologists who study populations determine how healthy or stable they are and how they interact with the environment, by asking specific questions, such as: Is the population stable, growing, or declining, and what factors affect the stability, growth, or decline the population?
In determining the health of a population, one must first measure its size or the population density, the number of individuals per unit area or volume. Population size or density can also be examined with respect to how individuals are distributed. How individuals are spaced within a population is referred to as dispersion. Some species may show a clumped or clustered distribution (Figure below) within an area, others may show a uniform, or evenly spaced (Figure below), distribution and still others may show a random, or unpredictable, distribution.
Figure 23.4
Individuals within this population of purple loosestrife plants show a clumped distribution due to local variation in soils.
Figure 23.5
A population of cacti in the Sonoran Desert generally shows uniform (even) dispersion due to competition for water.
Other factors of importance in the study of populations are age and sex. The proportion of males and females at each age level gives information about birth rate (number of births per individual within the population per unit time) and death rate (number of deaths within the population per unit time), and this age structure may give further information about a population’s health. For example, an age structure with most individuals below reproductive age often indicates a growing population. A stable population would have roughly equal proportions of the population at each age level, and a population with more individuals at or above reproductive age describes a declining population.
Another pattern in populations has to do with how they change with time. Survivorship curves – graphing the population numbers over time - allow us to study how populations grow and change, a topic that will be considered in greater detail in subsequent lessons.
Births, Deaths, and Migration
Births, deaths and migration all affect population density and growth. The population growth rate is the rate at which the number of individuals in a population increases. Population growth rate depends on birth rate and on death rate. The growth rate then is represented by the equation:
growth rate = birth rate – death rate.
According to this equation, if the birth rate is greater than the death rate, then the population grows; if the death rate is greater, then the population declines. If the birth and death rates are equal, then the population remains stable.
Factors which influence a successful reproduction are age at first reproduction, frequency of reproduction, the number of offspring, parental care, reproductive lifespan, and death rate of offspring. In birds, altricial (helpless at birth and requiring much parental care (Figure below)) and precocial (independent at birth or hatching and requiring little parental care (Figure below)) strategies use different reproductive systems to ensure breeding success.
Figure 23.6
A hummingbird nest with young illustrates
an altricial reproductive strategy, with a few, small eggs, helpless and naked young, and intensive parental care.
Figure 23.7
Canada Goose, , adult and young show a precocial reproductive strategy, where they lay a large number of large eggs, producing well-developed young.
Migrations and other movements in and out of populations affect population density as well. Therefore, both birth and immigration (movement of individuals into a population from other areas) rates increase the population growth rate, while death and emigration (movement of individuals out of a population) rates decrease the population rate. The earlier growth rate equation now looks like this:
growth rate = (birth rate + immigration rate) – (death rate + emigration rate)
One type of migration that you are probably pretty familiar with is the direct, often seasonal, movement of a species that results in a predictable change for that population size. Maybe you’ve heard that ‘birds fly south for the winter.” Examples of this migration are the thousands-of-miles migrations that many birds perform in the fall and then again in the spring when they return to their original habitat (Figure below). Another example of a long-distance migration is the movements of Monarch butterflies from their Mexican wintering grounds to the northern summer habitats (in various regions of the United States) and back again. These types of migrations move entire populations from one set of location and environmental conditions to another.
Figure 23.8
A flock of barnacle geese, , fly in formation during the autumn migration in Finland.
Population Growth
Under ideal conditions, given unlimited amounts of food, moisture, and oxygen, and suitable temperature and other environmental factors, oxygen-consuming organisms show exponential or geometric growth - as the population grows larger, the growth rate increases. This is shown as the “J-shaped curve” in Figure below. You can see that the population grows slowly at first, but as time passes, growth occurs more and more rapidly.
Figure 23.9
Growth of populations according to Malthus exponential (or J-curve) growth model (left) and Verhulsts logistic (or S-curve) growth model (right)
These ideal conditions are not often found in nature. They occur sometimes when populations move into new or unfilled areas. If ideal conditions were found all the time, what would you expect to happen to populations?
In nature, limits occur. One basic requirement for life is energy; growth, survival and reproduction all require this. Do you think energy supplies are limited or unlimited?
The answer is they are limited and therefore organisms must use these resources and others wisely. How do you think this affects the way organisms grow and what do you think the growth rate would look like?
In nature, under more realistic conditions, at first populations grow exponentially (J-shaped curve), but as populations increase, rates of growth slow and eventually level off. This is shown as an “S-shaped curve” in Figure below. Why do you think occurs? It's because various factors limit the growth of populations. Can you think of some of those factors?
Limiting Factors
Limiting factors that can lower the population growth rate include reduced food supply and reduced space. These can have the effect of lowering birth rates, increasing death rates, or can lead to emigration. This growth model is known as the logistic (S-curve) model, and looks different than the one for exponential growth (Figure above). In this case, the growth rate begins as proportional to the size of the population, but at higher population levels, competition for limited resources leads to lower growth rates. Eventually, the growth rate stops increasing and the population becomes stable.
This plateau in growth is known as the carrying capacity, or the maximum population size that can be supported in a particular area without degradation of the habitat. Limiting factors determine what the carrying capacity is.
In general, a limiting factor is a living or nonliving property of a population’s environment, which regulates population growth. There are two different types of limiting factors: density-dependent factors and density-independent factors.
Density-dependent factors, such as food supply, promote competition between members of the same population for the same resource, as the population increases in size and there is more crowding. Therefore, the population size is limited by such factors.
In the example of food supply, when population size is small, there is plenty of food for each individual and birth rates are high. As the population increases, the food supply decreases and birth rates decline, causing the population growth rate to decrease. Food shortages can eventually lead to an increase in death rates or emigration, therefore leading to a negative growth rate and lower population size. With a lower population size, each individual has more food and the population begins to increase again, reaching the carrying capacity. Can you think of some other density-dependent limiting factors?
Such factors could include light, water, nutrients or minerals, oxygen, the ability of an ecosystem to recycle nutrients and/or waste, disease and/or parasites, temperature, space, and predation. Can you think of some other factors that limit populations, but seldom regulate them? That means that these factors act irregularly, regardless of how dense the population is. Populations limited by such factors seldom reach carrying capacity.
An example of this other kind of factor, a density-independent factor, is weather. For example, an individual Agave americana (century plant) has a lifespan dependent at least in part upon erratic rainfall. Rainfall limits reproduction, which in turn limits growth rate, but because the rainfall is unpredictable, it cannot regulate Agave populations. Can you think of some other factors like this?
Human activities, for example, act in this way. These include use of pesticides, such as DDT, and herbicides, and habitat destruction. See if you can think of explanations as to why these factors are considered density-independent factors.
We will next be examining the growth of human populations. What kind of growth rate do you think humans follow?
Growth of the Human Population
There are two major schools of thought about human population growth. One group of people, sometimes known as the “Neo-Malthusians,” believes that human population growth cannot continue without dire consequences. Another group, the “Cornucopians,” believes that the Earth can provide an almost limitless amount of natural resources and that technology can solve or overcome low levels of resources and degradation of the environment caused by the increasing population. Which do you think is correct?
If we look back again at the growth curves that we examined in the last two sections, we might ask ourselves if human growth resembles the exponential J-shaped model or the logistic S-shaped model? In other words, are we built, as a population, to keep growing and to use up all our resources, and thus become extinct, or will we efficiently use our resources so that the Earth can sustain our growth?
We don’t know all the answers yet, but by looking at population growth through history and by examining population growth in different countries we may see some patterns emerge. For example, if we look at worldwide human population growth from 10,000 BCE through today, our growth, overall, resembles exponential growth, increasing very slowly at first, but later growing at an accelerating rate and which does not approach the carrying capacity (Figure below).
However, by looking at different countries’ population growth over history, we see more complexity. The history of human population growth can be divided into four stages and we can see snapshot views of these stages in countries today. Human populations pass through these four or five predictable stages of growth (Table below):
The Stages of Human Population Growth Stage of Human Population Growth Description
Stage 1
Birth and death rates are high and population growth is stable (i.e. early human history)
Stage 2
Significant drop in death rate, resulting in an increasingly rapid rise in popula
tion size (exponential growth)(i.e. 18th and 19th century Europe)
Stage 3
Population size continues to grow
Stage 4
Birth rates equal death rates and populations become stable
Stage 5
Total population size may level off
In looking ahead to the future, projections by the United Nations and the US Census Bureau predict that by 2050, the Earth will be populated by 9.4 billion people. Other estimates predict 10 to 11 billion. The Cornucopians believe that more people are good for technology and innovation. The 5-stage model above predicts that when all countries are industrialized, the human population will eventually reach stability and a carrying capacity of sorts. However, many scientists and other Neo-Malthusians believe that humans have already gone over the Earth’s carrying capacity for resources and habitat, and that this will eventually lead to famine, epidemics, or war, thus causing a population crash or even extinction.
Which of the above theories makes sense to you? What ways can you think of that people might use to avoid reaching Earth’s carrying capacity?
Figure 23.10
Worldwide human population growth from 10,000 BCE through today
Lesson Summary
CK-12 Life Science Page 63
