Day 1: Population Ecology by 1/27 (A) or 1/28(B)
Reading
What Is a Population?
A population is all the members of a species living in the same place at the same time. The bass in an Iowa lake make up one population. A population is a reproductive group because organisms usually breed with members of their own population. For example, daisies in an Ohio field will breed with each other and not with daisies in a Maryland population. The word population refers to the group in general and also to the size of the population—the number of individuals it contains.
Populations
Biotic and abiotic factors determine the population size of a species in an ecosystem.
What are some important biotic factors?
Biotic factors include the amount of food that is available to that species and the number of organisms that also use that food source.
What are some important abiotic factors?
Space, water, and climate all help determine a species population.
When does a population grow?
A population grows when the number of births is greater than the number of deaths.
When does a population shrink?
When deaths exceed births.
What causes a population to grow?
For a population to grow there must be ample resources and no major problems
.What causes a population to shrink?
A population can shrink either because of biotic or abiotic limits. An increase in predators, the emergence of a new disease, or the loss of habitat are just three possible problems that will decrease a population. A population may also shrink if it grows too large for the resources required to support it.
Population Growth Rate
The population growth rate is how fast a population is growing. The letter r stands for the growth rate. The growth rate equals the number of new members added to the population in a year for each 100 members already in the population. The growth rate includes new members added to the population and old members removed from the population. Births add new members to the population. Deaths remove members from the population. The formula for population growth rate is:
r = b - d , where
b = birth rate (number of births in 1 year per 100 population members)
d = death rate (number of deaths in 1 year per 100 population members)
If the birth rate is greater than the death rate, r is positive. This means that the population is growing bigger. For example, if b = 10 and d = 8, r = 2. This means that the population is growing by 2 individuals per year for every 100 members of the population. This may not sound like much, but it’s a fairly high rate of growth. A population growing at this rate would double in size in just 35 years!
If the birth rate is less than the death rate, r is negative. This means that the population is becoming smaller. What do you think might cause this to happen?
Patterns of Population Growth
Populations may show different patterns of growth. The growth pattern depends partly on the conditions under which a population lives.
Exponential Growth
Under ideal conditions, populations of most species can grow at exponential rates. Curve A in the Figurebelow represents exponential growth . The population starts out growing slowly. As population size increases, the growth rate also increases. The larger the population becomes, the faster it grows.
What Is a Population?
A population is all the members of a species living in the same place at the same time. The bass in an Iowa lake make up one population. A population is a reproductive group because organisms usually breed with members of their own population. For example, daisies in an Ohio field will breed with each other and not with daisies in a Maryland population. The word population refers to the group in general and also to the size of the population—the number of individuals it contains.
Populations
Biotic and abiotic factors determine the population size of a species in an ecosystem.
What are some important biotic factors?
Biotic factors include the amount of food that is available to that species and the number of organisms that also use that food source.
What are some important abiotic factors?
Space, water, and climate all help determine a species population.
When does a population grow?
A population grows when the number of births is greater than the number of deaths.
When does a population shrink?
When deaths exceed births.
What causes a population to grow?
For a population to grow there must be ample resources and no major problems
.What causes a population to shrink?
A population can shrink either because of biotic or abiotic limits. An increase in predators, the emergence of a new disease, or the loss of habitat are just three possible problems that will decrease a population. A population may also shrink if it grows too large for the resources required to support it.
Population Growth Rate
The population growth rate is how fast a population is growing. The letter r stands for the growth rate. The growth rate equals the number of new members added to the population in a year for each 100 members already in the population. The growth rate includes new members added to the population and old members removed from the population. Births add new members to the population. Deaths remove members from the population. The formula for population growth rate is:
r = b - d , where
b = birth rate (number of births in 1 year per 100 population members)
d = death rate (number of deaths in 1 year per 100 population members)
If the birth rate is greater than the death rate, r is positive. This means that the population is growing bigger. For example, if b = 10 and d = 8, r = 2. This means that the population is growing by 2 individuals per year for every 100 members of the population. This may not sound like much, but it’s a fairly high rate of growth. A population growing at this rate would double in size in just 35 years!
If the birth rate is less than the death rate, r is negative. This means that the population is becoming smaller. What do you think might cause this to happen?
Patterns of Population Growth
Populations may show different patterns of growth. The growth pattern depends partly on the conditions under which a population lives.
Exponential Growth
Under ideal conditions, populations of most species can grow at exponential rates. Curve A in the Figurebelow represents exponential growth . The population starts out growing slowly. As population size increases, the growth rate also increases. The larger the population becomes, the faster it grows.
Logistic Growth
Most populations do not live under ideal conditions. Therefore, most do not grow exponentially. Certainly, no population can keep growing exponentially for very long. Many factors may limit growth. Often, the factors are density dependent (known as density-dependent factors ). These are factors that are influential when the population becomes too large and crowded. For example, the population may start to run out of food or be poisoned by its own wastes. As a result, population growth slows and population size levels off. Curve B in the Figure above represents this pattern of growth, which is called logistic growth .
At what population does growth start to slow in the logistic model of growth? That depends on the population’s carrying capacity (see Figure below). The carrying capacity (K) is the largest population size that can be supported in an area without harming the environment. Population growth hits a ceiling at that size in the logistic growth model.
Carrying Capacity
When the number of births equals the number of deaths, the population is at its carrying capacity for that habitat. In a population at its carrying capacity, there are as many organisms of that species as the habitat can support. The carrying capacity depends on biotic and abiotic factors. If these factors improve, the carrying capacity increases. If the factors become less plentiful, the carrying capacity drops. If resources are being used faster than they are being replenished, then the species has exceeded its carrying capacity. If this occurs, the population will then decrease in size.
Most populations do not live under ideal conditions. Therefore, most do not grow exponentially. Certainly, no population can keep growing exponentially for very long. Many factors may limit growth. Often, the factors are density dependent (known as density-dependent factors ). These are factors that are influential when the population becomes too large and crowded. For example, the population may start to run out of food or be poisoned by its own wastes. As a result, population growth slows and population size levels off. Curve B in the Figure above represents this pattern of growth, which is called logistic growth .
At what population does growth start to slow in the logistic model of growth? That depends on the population’s carrying capacity (see Figure below). The carrying capacity (K) is the largest population size that can be supported in an area without harming the environment. Population growth hits a ceiling at that size in the logistic growth model.
Carrying Capacity
When the number of births equals the number of deaths, the population is at its carrying capacity for that habitat. In a population at its carrying capacity, there are as many organisms of that species as the habitat can support. The carrying capacity depends on biotic and abiotic factors. If these factors improve, the carrying capacity increases. If the factors become less plentiful, the carrying capacity drops. If resources are being used faster than they are being replenished, then the species has exceeded its carrying capacity. If this occurs, the population will then decrease in size.
Limiting Factors
Every stable population has one or more factors that limit its growth. A limiting factor determines the carrying capacity for a species. A limiting factor can be any biotic or abiotic factor: nutrient, space, and water availability are examples (Figurebelow). The size of a population is tied to its limiting factor.
Every stable population has one or more factors that limit its growth. A limiting factor determines the carrying capacity for a species. A limiting factor can be any biotic or abiotic factor: nutrient, space, and water availability are examples (Figurebelow). The size of a population is tied to its limiting factor.
What happens if a limiting factor increases a lot? Is it still a limiting factor? If a limiting factor increases a lot, another factor will most likely become the new limiting factor.
This may be a bit confusing so let’s look at an example of limiting factors. Say you want to make as many chocolate chip cookies as you can with the ingredients you have on hand. It turns out that you have plenty of flour and other ingredients, but only two eggs. You can make only one batch of cookies, because eggs are the limiting factor. But then your neighbor comes over with a dozen eggs. Now you have enough eggs for seven batches of cookies, and enough other ingredients but only two pounds of butter. You can make four batches of cookies, with butter as the limiting factor. If you get more butter, some other ingredient will be limiting.
Species ordinarily produce more offspring than their habitat can support (Figure below). If conditions improve, more young survive and the population grows. If conditions worsen, or if too many young are born, there is competition between individuals. As in any competition, there are some winners and some losers. Those individuals that survive to fill the available spots in the niche are those that are the most fit for their habitat.
This may be a bit confusing so let’s look at an example of limiting factors. Say you want to make as many chocolate chip cookies as you can with the ingredients you have on hand. It turns out that you have plenty of flour and other ingredients, but only two eggs. You can make only one batch of cookies, because eggs are the limiting factor. But then your neighbor comes over with a dozen eggs. Now you have enough eggs for seven batches of cookies, and enough other ingredients but only two pounds of butter. You can make four batches of cookies, with butter as the limiting factor. If you get more butter, some other ingredient will be limiting.
Species ordinarily produce more offspring than their habitat can support (Figure below). If conditions improve, more young survive and the population grows. If conditions worsen, or if too many young are born, there is competition between individuals. As in any competition, there are some winners and some losers. Those individuals that survive to fill the available spots in the niche are those that are the most fit for their habitat.
Questions
- What are some biotic factors that determine population size?
- What are some abiotic factors that determine population size?
- Whe does a population grow? What are the causes?
- When does a population shrink? What are the causes?
- How can you calculate the population growth rate? What is the growth rate if a population has 12 births and 9 deaths per 100 members?
- Compare exponential and logistic growth rates.
- What is carrying capacity?
- What is a limiting factor?