Understanding Populations Chapter 8 Ms. Cuthrell Ecology 2nd Period

Populations A population in a group of organisms of the same species living in the same place at the same time. These organisms can interbreed to make fertile offspring and are therefore reproductively successful. Examples: humans in a school, bullfrogs in a pond, E. coli in a drop of water

Properties of Populations Populations are described in relation to size, density, and dispersion. Size: number if individuals in a population, Density: number of individuals of a population in a certain area. Dispersion: how the population is arranged throughout the environment. (even, clumped, or scattered) These factors will determine how a population will change over time.

Change in Population Populations change constantly with the addition and removal of individuals to the population. Growth rate: birth rate minus death rate. A population is said to be growing if the birth rate is higher than the death rate. A population is said to be declining if the death rate is higher than the birth rate. A population is said to be in equilibrium if the birth rate and death rate are equal.

Population growth Populations stay relatively stable from year to year with slight changes in the population size. Various factors keep these population from growing to rapidly by killing some members of the population before they can reproduce. No reproduction = no population growth

Population Growth Biotic potential: the fastest rate at which a population can grow. Populations can only reach biotic potential when there is nothing else limiting the growth of the population, or an absence of limiting factors. One of these limiting factors is reproductive potential, or the maximum amount of offspring each individual in the population can produce.

Population growth Reproductive potential varies between different species. For example, you might expect an adult elephant to produce less offspring in a lifetime than an adult mosquito. Reproductive potential increases if an organism reproduces a large number of offspring, reproduces more often, or reproduces earlier in life. Reproduction early in life has the most effect on reproductive potential because it will shorten generation times (the average time it takes an individual in a population to reach age of reproduction.)

Population Growth Exponential growth: When an population is able to reproduce the maximum number of offspring in each generation, the population will undergo a time of extreme population growth. For example, if a dog has 6 puppies and each of those 6 puppies also has 6 puppies and so on… Eventually, the population will grow at an extremely high rate. This only happens when there are no limits on population growth.

Population Growth Logistic growth: When a population is limited by certain factors, it will also limit the growth of the population. Limiting factors include resources, space, sanitation, environmental effects, etc. When the population can no longer grow, it reaches carrying capacity, or the limit of individuals that are able to be supported by a certain environment.

Population Growth Once a population has reached carrying capacity, it shows a dynamic equilibrium. If the population goes over carrying capacity, natural factors will be more scarce and some the population will die out through natural selection. When the population declines to a number of individuals the environment can again sustain, the population will start to grow again and repeat the trend. Over time this keeps the population from overusing or running out of resources and also keeps predator/prey relationships in balance.

Competition In their environments, organism compete with each other in a population and between population for common resources. Some organisms will define a specific territory, or area where there are resources that organism protects, to ensure the continuation of their species. Same species competition usually involves fighting for space, mates, and food. Different species compete for resources that are shared such as the same kind of food or a common water resource.

Population Regulation Population is regulated by other natural factors as well. Density-dependent factors: factors that limit a population and depends on the number of individuals in a population. Usually this happens in a closely packed population. Ex: competition for food, space, sanitation

Population Regulation Density-independent factors: factors that will limit a population but are not affected by the number of individuals in that population. The whole population will be affected whether there are 3 or 30 individuals in an area. Ex: severe weather, natural disasters