Module 8
1. Module 8
1.39. Page 2
Module 8—Populations, Individuals, and Gene Pools
Explore
J curve: population growth with a brief lag phase and a steep increase in the growth curve
S curve: a logistic growth pattern with a lag phase, growth phase, and stationary phase in which limiting factors become significant
exponential growth: a growth pattern exhibited by a population that doubles with each generation; results in a J-shaped curve; begins with a lag phase, followed by a steep rise
density-independent factors: abiotic factors that limit a habitat’s carrying capacity (e.g.,fire, flood, frost); the impact is not affected by the density of the population
lag phase: the first phase of a growth graph showing little increase in population numbers
carrying capacity (K): the theoretical maximum population size that the environment can sustain over an extended period of time; may change as the quality of the habitat changes; the population is at carrying capacity when it levels off during logistic growth
logistic growth: a population increase resulting in an S-shaped curve; begins with slow growth, steepens to exponential growth, and then levels to a carrying capacity due to competition because of environmental resistance
exponential phase: the second phase of a growth graph in which population size increases significantly because limiting factors are not yet significant
density-dependent limiting factors: biotic factors that limit a habitat’s carrying capacity (e.g., parasites, disease, increase in predators, lack of water, food, or territory); the impact increases with the density of the population
environmental resistance: the combined effects of factors that limit population growth; prevents a population from growing at its biotic potential; determines the carrying capacity of the environment
stationary phase: the third stage of a population growth graph in which population size stabilizes because of the balance between environmental resistance and biotic potential
Exponential Versus Logistic Growth Patterns
When a population is introduced into a new area and population growth is plotted on a graph, one of two shapes can result: a J curve or an S curve. Although it may seem strange that all of Earth’s organism’s fall into one of the two patterns of growth, that does appear to be the case.
If a population has a constant birth rate through time and is minimally limited by food or disease, it is an example of exponential growth. In exponential growth, the birth rate is constant despite the size of the population. The only factors that limit exponential growth are density-independent factors such as climate (e.g., temperature, water, wind, light), natural disasters, or other abiotic factors.
Examples of organisms that reproduce in this way are bacteria, most insects, and fast-growing plants like dandelions. For example, bacteria always double themselves (by binary fission) with each generation. They are reproducing at their biotic potential. An easy way to predict the population size for organisms that double with each generation is to use the exponential term 2n, where n is the number of generations involved. For example, bacteria that are in their 7th generation will have 128 individuals from one original organism.
The population growth curve of exponentially reproducing species has certain distinct phases. Initially, in the first few generations following introduction to a new environment, population growth is slow and the curve stays fairly flat. This portion of the graph is called the lag phase. Thereafter the population size can increase dramatically from generation to generation, forming an ever-steepening J-shaped curve. As time goes on, the difference in population size from one generation to the next becomes quite astounding. Applied to bacterial populations with a generation time of 20 minutes, it would not take long for the progeny of one bacterial cell to cover the face of Earth!
However, in most natural populations both food and disease factors become significant as conditions become more crowded. In natural conditions there is an upper limit to the number of individuals that any habitat can support. Ecologists refer to this number as the carrying capacity of the environment, symbolized as K. Populations that increase in number to K are examples of logistic growth and form an S-shaped curve if they are introduced into a new environment.
Unlike species that grow exponentially, in logistic growth the birth rate does change over time. If a logistically growing species is introduced into a new environment and the changes in population size are graphed, you wouldn’t see much difference between the logistic growth curve and the growth curve of an exponentially growing species. Initially, food and space are plentiful and nothing limits growth rate. The growth curve will show the same lag phase as the J curve and then a steep exponential phase, where resources are abundant and organisms reproduce at their biotic potentials.
At this point however, the J and S curves begin to diverge. In logistic growth, density-dependent limiting factors become an issue as the population grows. The habitat’s food sources become depleted, nesting space and territory become scarce, wastes accumulate, and disease becomes significant, spreading faster as individuals become more crowded. Predators become more numerous, and life becomes more dangerous.
The effect of all these density-dependent factors is called environmental resistance, and it indicates that the habitat can not sustain this rate of growth. With environmental resistance, birth rate begins to fall and the growth curve begins to flatten, unlike exponential growth in which the birth rate never changes. Consequently, the curve drops and eventually flattens into a steady population size that the habitat is able to support—a number called the carrying capacity of the environment. At this point the S-shaped curve enters the stationary phase.
The population stays at carrying capacity because of the continued action of density-dependent limiting factors, like food supply, disease, predation, waste accumulation, and lack of space. The only factor that could cause the population size to change would be a fluctuating carrying capacity of the environment. For example, if the habitat were degraded (perhaps by human activity or natural catastrophe) and the environment could no longer support the same number, carrying capacity would find a lower level.
Logistic growth is found in any species that changes its reproductive rates depending on the availability of resources and the action of density-dependent limiting factors.
Read
To understand and study graphs representing the concepts of exponential and logistic growth patterns, read from “Factors That Affect Population Growth” on page 709 of the textbook to “Life Strategies” on page 712. Include in your notes drawings of the J- and S-curve graphs showing the characteristic shapes of both kinds of growth. Label the two graphs with the distinct phases of growth.
Save these notes in your course folder. You will note that interpreting graphs is an important skill in Biology 30. If you use The Key to prepare for the diploma exam, you will have the opportunity to see many types of questions based on graphs, or you can go to the Alberta Education website to view samples of Diploma Exam questions.