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Populations

- Population—members of the same species living in a specific geographic area.
- For example: Alligators living in a swamp make up a population

Defining Populations

- Some populations change in size dramatically over time, increasing rapidly and then decreasing again.
- Other populations are more stable, with only small increases or decreases in the number of members over time.

Defining Populations

- Several factors influence a population's size and how much it changes over time.
- They include:
- the availability of food
- the availability of space
- weather conditions
- breeding patterns

Defining Populations

- In studying how these factors affect a population, ecologists need to define the population's geographic boundaries.
- These boundaries might be natural, such as the edges of a lake where a particular species of catfish lives.
- The boundaries might be chosen to make the population easy to study, such as the walls of an aquarium in which algae are growing.
- A researcher exploring the effects of hunting on a deer species might define the study population as all the deer within a particular state.

Population Density

- Ecologists often describe a population in terms of its density.
- Population density is the number of individuals of a particular species per unit area or volume.
- The number of alligators per square kilometer of swamp, the number of bacteria per square centimeter of an agar plate, and the number of earthworms per cubic meter of soil are all examples of population density measurements.

Population Density

- On rare occasions you can count all the individuals in a population, such as the number of beech trees in a forest measuring 50 square kilometers (km2). If there were 1000 beech trees in this forest, the population density would be 1000 trees per 50 km2. Reducing this fraction allows you to express the density of trees in a single square kilometer (20 trees per km2).

Population Density

- Population density is a helpful measurement for comparing populations in different locations. For example, beech trees are more dense in the above forest (20 trees/km2) than in a 100 km2 forest with 500 beech trees (5 trees/km2).

Sampling Techniques

- It usually isn't practical to count every member of a population. There may be too many individuals, or they may move around too quickly to be counted accurately, as with many species of insects, birds, and fish. In such cases, ecologists use a variety of sampling techniques to estimate the size of the population.

Sampling Techniques

- Quadrates
- One method is to mark off a particular area, then count the number of a particular species within this boundary, called a quadrate. After repeating this procedure in several locations within the ecosystem, ecologists average their results to estimate the population density of this species in the ecosystem. The more quadrates they study, the more accurate the estimate.

Sampling Techniques

- Indirect Counting
- A sampling technique for organisms that move around a lot or are difficult to see is indirect counting. This method involves counting nests, burrows, or tracks rather than the organisms themselves.

Sampling Techniques

- Mark-Recapture
- Another technique commonly used to estimate animal populations is the mark-recapture method. The biologist traps animals in the study area and marks them, such as with a drop of colored dye. The researcher then releases the marked individuals. After a period of time, the researcher again captures animals from the population and counts the marked and unmarked individuals in the second sample.

Sampling Techniques

- The following formula then gives an estimate of the total population size:
- Total population = number in first capture x number in second capture

number of marked animals recaptured

Sampling Techniques

- Limits to Accuracy
- Most sampling techniques involve making some assumptions about the population being studied.
- If these assumptions are not valid, then the estimate will not be accurate.

Sampling Techniques

- For example, the quadrate method assumes that organisms are distributed fairly evenly throughout the study area. But some populations may be arranged in clumps, such as cottonwood trees clustered near sources of water in a dry ecosystem. If the quadrate includes a clump, the estimate for the total population may be too high. If the quadrate does not include a clump, the estimate may be too low. To minimize this problem, biologists must consider how a study population is distributed when choosing an appropriate quadrate size.

Sampling Techniques

- The mark-recapture technique assumes that both marked and unmarked animals have the same chance of surviving and of being caught in the second capture. In reality, animals that have been captured once may be wary of traps and avoid being recaptured. This behavior change in previously captured animals could lead to overestimating the population size. To lessen this problem, researchers try to minimize the effects of trapping on the captured animals.

Exponential Growth of Populations

- A population's ability to grow depends partly on the rate at which its organisms can reproduce. Bacteria are among the fastest-reproducing organisms. A single bacterium can reproduce every 20 minutes under laboratory conditions of unlimited food, space, and water. After just 36 hours, this rate of reproduction would result in enough bacteria to form a layer almost half a meter deep covering the entire planet.

Exponential Growth of Populations

- In general, larger mammals reproduce at slower rates than smaller mammals. For example, elephants produce fewer offspring than mice and have longer time intervals between offspring. But even so, in theory, the descendants of a single pair of mating elephants would number 19 million elephants within 750 years.

Exponential Growth of Populations

- In these theoretical examples, the bacteria and elephant populations are undergoing exponential growth, in which the population multiplies by a constant factor at constant time intervals. Consider the bacteria example. The constant factor for this population is 2, because each parent cell splits, forming two offspring cells. The constant time interval is 20 minutes. So every 20 minutes, the population is multiplied by 2. time.

Exponential Growth of Populations

- Graphing these data forms the J-shaped curve in the figure. Notice that the larger the population of bacteria, the faster the population grows—the curve gets steeper with

Carrying Capacity

- In nature, a population may start growing exponentially, but eventually one or more environmental factors will limit its growth. The population then stops growing or may even begin to decrease.
- For example, consider lily pads growing and spreading across the surface of a pond. Once the pond is covered in lily pads, no more can grow.
- Space is one example of a limiting factor, a condition that can restrict a population's growth. Other limiting factors include disease and availability of food.

Carrying Capacity

- The fig shows the growth of a population of fur seals on Saint Paul Island off the coast of Alaska.
- Until the early 1900s, hunting kept the seal population small and fairly stable. Then hunting on the island was reduced, and the seal population began to increase almost exponentially. By about 1935, the population leveled off again. Ecologists hypothesized that the population became limited by a variety of factors, including disease and competition for food.

Carrying Capacity

- When such environmental factors limit a population's growth rate, the population is said to have reached its carrying capacity.
- The carrying capacity is the number of organisms in a population that the environment can maintain, or carry, with no net increase or decrease.
- As a growing population approaches carrying capacity, the birth rate may decrease or the death rate may increase (or both), until they are about equal. Over time the balance in births and deaths keeps the change in the population size close to zero. Notice the S-shaped curve of the seal population graph in previous figure. The seal population increased rapidly for a time, but then stabilized when it reached the carrying capacity of the environment.

Factors Affecting Population Growth

- In the laboratory, you can observe the effects of environmental factors such as food availability or temperature on population growth. For example, if you put fruit flies in a container and add the same amount of food each day, the population rapidly increases until the daily food supply cannot support more flies. If you place another container of fruit flies on a sunny window ledge, the heat may cause that population to decrease despite a sufficient food supply.

Factors Affecting Population Growth

- Density-Dependent Factors
- Factors similar to those affecting laboratory fruit flies affect natural populations.
- For example, the best nutrition for white-tailed deer is the new leaves and buds of woody shrubs. When deer population density is low, this high-quality food is abundant, and a large percentage of the females bear offspring. On the other hand, when the population density increases, the nutritious food supply becomes scarce due to overgrazing, and many females do not reproduce at all.
- The availability of high-quality food is one example of a density-dependent factor, a factor that limits a population more as population density increases.
- Another example of a density-dependent factor is a disease that spreads more easily among organisms in a dense population than in a less dense population.

Factors Affecting Population Growth

- Density-Independent Factors
- Factors that limit populations but are unrelated to population density are called density-independent factors.
- Extreme weather events, such as hurricanes, blizzards, ice storms, and droughts, are examples of density-independent factors. These conditions have the same effect on a population regardless of its size.

Factors Affecting Population Growth

- A population of aphids typically grows exponentially in the wet springs months. The population nearly dies off in the hot, dry summer. Weather is a density-independent factor that limits the aphid populations

Population Growth Cycles

- Some populations have "boom-and-bust" growth cycles: They increase rapidly for a period of time (the "boom"), but then rapidly decline in numbers (the "bust").
- Populations of various rodents exhibit boom-and-bust cycles. A striking example is lemming populations, which can cycle dramatically every three to five years. Some researchers hypothesize that natural changes in the lemmings food supply may be the underlying cause. Another hypothesis is that stress from crowding during the "boom" may affect the lemmings' hormonal balance and reduce the number of offspring produced, causing a "bust."

Population Growth Cycles

- Some populations' growth cycles appear to be influenced by those of other populations in their environments. For example, in the forests of northern Canada, both the lynx and the snowshoe hare follow boom-and-bust cycles

Population Growth Cycles

- The cycling populations of snowshoe hares and their predators, the lynx, appear to be related. Increases in the hare population are followed closely by increases in the lynx population.

Population Growth Cycles

- About every 10 years, the hare population reaches a high point, followed by a sharp decrease. The lynx, which feeds on the hare, has a population cycle that seems to follow that of the hares. When the hare population increases, the lynx population follows closely. You might hypothesize that the greater availability of food enables the lynx population to grow. Then, as more and more lynx feed on the hares, the hare population decreases again, which in turn becomes a limiting factor for the lynx. This complicated relationship is still not fully understood. Are the two species directly influencing each other's population growth? Or is there another underlying cause for the changes, such as a cycle in the hares' food supply? The causes of boom-and-bust cycles vary among species.

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