Module 8 Lesson 5 - 2

Biotic Potential

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Read pages 709 and 711 - 712

What factors determine the biotic potential (r) of a species? Biotic potential refers to the maximum growth rate of a population. At least four differences occur between field mice and humans to account for the differences in biotic potential. Differences in biotic potential are responsible partly for how fast population numbers change.

Four factors determine the biotic potential of a population:

1. Number of offspring per reproductive cycle
   
2. Number of offspring that survive until their reproductive age
3. Age at first reproduction and the number of reproductive cycles in a year
   
4. Life span
   

A pair of houseflies lay approximately 120 eggs in each reproductive cycle. With about 7 reproductive cycles in a year, a single pair of houseflies can reproduce 5 x 10¹² offspring in a single year. With such a huge biotic potential, why haven't houseflies taken over Earth?

Exponential Growth

When a population, such as houseflies, can reproduce without any limits, exponential growth can be expected. A brief period of lag phase occurs in which the initial population is adjusting to the environment. Afterward, the population size increases dramatically, forming a steep J-shaped curve.

Carrying Capacity

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How much growth can an environment sustain without depletion of already scarce resources? Most natural populations have limits to how many organisms the environment can sustain. This is called the carrying capacity (K) of the environment. At a certain point, a growing population reaches the limits on food, space, sunlight, and water. These limits imposed on the population determine its maximum size. The population size stabilizes at this point and fluctuates slightly in a stable equilibrium.

Logistic Growth

When a population reaches the carrying capacity, the growth rate slows and forms an S-shaped curve, known as the logistic growth pattern. Initially, the logistic growth pattern looks very similar to a J-shaped curve. An initial lag phase occurs when the population is adjusting to the new environment, and an exponential growth phase occurs when no limits such as food and space are imposed on the population size. However, as the population nears the carrying capacity of the environment, the population feels the effect of environmental resistance on its size. At this point, the S-shaped curve enters the stationary phase at a steady population size.

Density Dependent Factors vs. Density Independent Factors

Environmental resistance is the limiting factors that prevents the population growth at its biotic potential. Environmental resistance is of two types: density-dependent factors and density-independent factors.

The limiting factors influenced by the population density and size are called density-dependent factors. As a population grows larger, the competition for food and water can increase, leading to lower birth rates and higher mortality rates. When a population is large, disease and parasites can spread quickly, increasing the mortality rate.

The limiting factors not influenced by the population density and size are density-independent factors. These factors are abiotic and can include drought, volcanic eruptions, flood, fires, and extremely cold weather. The density-independent factors limit population growth regardless of the population size.

Can a habitat not have a carrying capacity? Is some species capable of changing its habitat to increase continually the carrying capacity? Does this carrying capacity have an upper limit? Consider the following figure that illustrates the human population curve, and think about whether the human species has a carrying capacity.

Try This

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This Population Density Factors activity will help you sort density-dependent and density-independent factors.