Populations IB Topic 5.3

Populations How do populations grow and maintain themselves? Recap:  A population is a group of individuals of the same species that have the potential to breed with each other Some populations are stable, some have rapid growth, some crash  Populations can vary greatly  Why? What factors in an ecosystem control population size?

Population growth Think of the situation where a small number of young rabbits enter a large, well stocked meadow  What would happen to the population size?  What would eventually happen to the amount of available resources?  How might population growth be affected?

Population growth curve If we were to graph this situation (# of individuals vs. time), the shape of the graph would be sigmoidal (S-shaped curve) Figure 5.6 in your packet

Lag phase

Stages of population growth 1. Lag phase  Little or no growth  Period of adaptability 2. Exponential growth or log phase  Number of individuals increases at a faster and faster rate Lag phase

Stages of population growth 3. Transitional phase or linear growth  Growth rates slows down considerably  Population is still increasing 4. Plateau phase  Number of individuals has stabilized  No more growth

Causes for exponential growth Plentiful resources  Food, space, light Little or no competition from other inhabitants Favorable abiotic factors  Temperature, dissolved oxygen Little or no predation or disease

Causes for the transitional phase Increasing competition Predators, attracted by a growing food supply, start to move into the area Limited space leads to opportunities for disease to spread

Causes for the plateau phase Less space Limited food supply  Results in less offspring Predators and disease In this phase, the number of births = the number of deaths  No growth

Exponential growth cannot be maintained Populations cannot continue to grow and grow forever As a population increases, it begins to experience environmental resistance  Space and resources are reduced  Competition increases There comes a time in growth when its numbers stabilize

Carrying Capacity The maximum number of individuals that a particular habitat can support is called the carrying capacity  Represented by the letter K If numbers start to increase above K, shortage of resources reduces the numbers of offspring produced  Population regulates itself at the carrying capacity  Populations tend to be naturally self-regulating

Population Fluctuation Most populations do not show significant fluctuations, but show varying degrees with time Why?

4 main factors that affect population size The birth rate (natality) varies The death rate (mortality) varies Mobile members move away (emigration) New members move in (immigration) Which two factors cause population growth? Which two factors cause population decline?

Limiting factors can affect population size Limiting factors define the carrying capacity of a habitat Examples:  Availability of resources (water, sunlight, shelter, space, oxygen)  Build up of waste (excrement or excess carbon dioxide)  Predation  Disease

Two categories of limiting factors Density-dependent limiting factors  Related to population size Competition for resources Predation Disease Density-independent limiting factors Unrelated to population size Drought, intense cold

What’s our carrying capacity? Many biologists, environmental groups, economists, and governments wonder what is the carrying capacity of Earth for human population We are in exponential growth phase Will climate change, competition, and disease lead to a transitional phase or plateau phase?

In conclusion The size of a population depends on births, deaths, immigration, & emigration Populations tend to follow sigmoidal-shaped growth  Lag phase  Exponential phase  Transitional or log phase  Plateau phase The carrying capacity (K) is the max number of individuals that the region can support

Exit Slip You need a sheet of notebook paper Proper heading Title: Populations Exit Slip

Exit Slip 1. Draw and label a population growth curve (also be sure to label the axis). 2. Suggest what factors might lead to an increase in a population of songbirds. 3. Suggest what factors might eventually limit the growth of that population.