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Population Ecology

There are many sub-fields of ecology, which focus on distinct aspects of this branch of science. Population ecology focuses on the dynamics of populations, particularly as it comes to how a population changes over time.

Carrying Capacity

The availability of resources has a large impact on the growth of a population. The maximum population that an ecosystem can support is known as the carrying capacity, and is denoted as K in these graphs. Carrying capacity can change as the amount of resources change, and it is not constant from one environment to the next.

Should a population exceed its carrying capacity, overshoot occurs. When this happens, resources are depleted, which typically results in dieback - a sharp decrease in the population - due to a lack of resources. Increased competition, famine, and increased rate of disease transmission (due to population density increasing) are common results of this.

A species' biotic potential refers to its maximum reproductive rate in ideal conditions. If you look at the two carrying capacity graphs above, they reach the carrying capacity at vastly different rates. Populations have distinct reproductive strategies that favor different rates of growth, with the two extremes being r-selected and K-selected species. Many species do not fall into one of these two categories, but rather somewhere in-between, and some species can change their reproductive strategies under certain conditions, but these are the two general strategies that we will be discussing.

r-selected Species

Selection favors organisms with high biotic potential; r-selected species have rapid population growth. These are typically smaller, shorter-lived organisms that reach sexual maturity quickly. These species have a lot of "energetically cheap" offspring; they are large in number but the parents do not invest energy into taking care of them. This is favorable in low-density conditions where there is little competition for resources.

As these organisms reproduce so rapidly, they are more likely to recover from a disturbance, more likely to be invasive, and have a higher chance of rebounding and adapting after a disturbance.

K-selected Species

Selection favors organisms with low biotic potential; K-selected species are stable near the carrying capacity (K). These organisms are typically larger, longer-lived organisms that take significantly longer to reach sexual maturity. These species have fewer offspring, but dedicate a lot of energy to their offspring in the form of parental care. This is favorable in high-density conditions where there is high competition for resources.

Due to their low biotic potential, K-selected species are more likely to be adversely affected by a disturbance, making them more likely to face extinction.

Survivorship Curves

Different reproductive strategies of species lead to differential survival rates within the species. As r-selected species have "cheap" offspring with little parental care, they tend to have a high growth rate, but low survivability. K-selected species, on the other hand, have "expensive" offspring where there is a low growth rate, but much higher survivability. The relative survival rates of a population can be displayed by survivorship curves.

Type I Survivorship: There is high survivorship in early and mid-age, with a rapid decrease in old age.

Type II Survivorship: There is a steady decline in survivorship throughout life; survivorship is independent of age.

Type III Survivorship: Lack of parental care leads to low survivorship in early age, with few making it to mid-age. Survivorship decrease greatly slows in old age.

K-selected species usually follow a Type I or Type II curve, while r-selected species usually follow a Type III curve.

Population Growth

Growth Limiting Factors

Population growth is limited by environmental factors, which contribute to the carrying capacity of a population. There are two broad types of these limiting factors: density-dependent and density-independent.

Density-dependent factors will inhibit the growth rate, by increasing the death rate or decreasing the birth rate, based on the population density. An example of this is with limited resources: as the population density increases, food may become more scarce, increasing competition and limiting the population. Factors such as territory, water, food, light, etc. are examples of density-dependent factors. As infectious diseases experience increased transmission with higher population density, they too are a density-dependent factor.

Density-independent factors will change the birth and death rates, but they will not change with the population density. An example of this could be a drought, which would limit the population regardless of its size. Such factors are typically abiotic and include things such as natural disasters, pollutants, and climate.

It is sometimes important to calculate the change in size of a population. There is a fairly simple equation to do so, which is seen to the right where:

N = population size

t = time

B = births

D = deaths

That may not always be the entire story, however, as individuals can leave the population without being born or dying. So this equation can be extended to the lower one, where:

I = immigrations

E = emigrations

At its essence, this equation can be simplified to:

change in population = those coming into the population - those leaving the population

If the answer you get is positive, the population has grown. If it is negative, the population is shrinking.

Let's look at a quick example.

A population of 86 penguins had 17 births and 8 deaths this past summer. Two penguins were taken from the flock to be added to a zoo and no new penguins migrated in. How has this population changed over the past year?

Click here for the solution

(population change)/(1 year) = (births + immigrations) - (deaths + emigrations)

(population change)/(1 year) = (17+0) - (8+2)

(population change)/(1 year) = (17) - (10)

population change in one year = 7 penguins

Reproduction without constraints can result in the exponential growth of a population.

N = population size

t = time

rmax = maximum per capita growth rate of the population (the biotic potential)

When populations are limited due to growth limiting factors and carrying capacity, it is common to see logarithmic growth models, especially in K-selected species.

N = population size

t = time

rmax = maximum per capita growth rate of the population (the biotic potential)

K = carrying capacity

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