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Big Idea:
Interdependence in Nature
Key Question:
What factors contribute to changes in populations?
Describing Populations
Ecologists are interested in answering questions about ecosystems
Ecologists study a population’s:
Geographic Range
Density & Distribution
Growth Rate
Age Structure
The geographic range describes the spaces where it lives
Range can be very small (bacteria) or very large (migratory fish/birds)
Population density is the number of individuals of a species per unit of area
This describes how the organisms are spread across their range
The distribution may be random, uniform, or clumped
A population’s growth rate tell how the population is changing
Positive growth rate leads to an increase in individuals
Negative growth rate lead to a decrease in individuals
Age Structure
The numbers of males and females in different age groups of a population
This is important because in animals only the females between certain ages can reproduce
The size of a population changes based on how many individuals are added to it or removed from it
How are individuals added or removed?
A population may grow if more individuals are born than die in any period of time
If the death rate is higher, populations may shrink
If birthrate and death rate are equal, then a population remains the same in overall number
Immigration
The movement of individuals into an area occupied by an existing population
Emigration
The movement of individuals out of an area
If immigration is higher than emigration, a population will grow
Imagine a population has and endless supply of food and space, has protection from predators and disease, and all waste products are removed – under these ideal conditions, a population will grow
A single bacterium can divide to form two cells every 20 minutes
This means that in the span of an hour, a single bacterium can divide to become 8 individual cells
This becomes 64 cells in 2 hours, 512 cells in 3 hours, 4096 cells in 4 hours, and so on and so forth
Exponential Growth
A growth pattern in which the individuals in a population reproduce at a constant rate
Populations in the real world don’t grow exponentially for very long
What causes population growth to eventually slow down over time?
Phases of Growth
Phase 1: Exponential Growth
Phase 2: Growth Slows Down
Phase 3: Growth Stops
Logistic Growth
A growth pattern in which a population’s growth rate slows or stops, following a period of exponential growth
These growth curves are shaped like the letter S
Carrying Capacity
The largest number of individuals of a population that a given environment can support
This is shown as a broken horizontal line on a logistic growth graph
Population size may increase or decrease just a little, but will mainly stay very near the carrying capacity
Limiting Factors
Why do populations grow as they do?
What factors cause the growth to slow down?
What determines the carrying capacity of an environment for a species?
Limiting Factor
A factor that causes the growth of a population to decrease
Could include the number or predators or how much food is available
They determine the carrying capacity of an environment for a species
Density-Dependent Limiting Factor
A limiting factor that depends on population size
Several different kinds
Predators
Herbivores
Parasites and Disease
Competition
Overcrowding
Density-Independent Limiting Factor
A limiting factor that affects all populations in similar ways, regardless of population size
Examples include: bad weather events such as hurricanes or drought, natural disasters such as wildfires, or human caused disasters like farming/logging
Historical Overview
Human population grew very slowly for most of human history
Food was hard to find and the death rate was high
Exponential Human Population Growth
The Industrial Revolution led to a boom in human population growth
Goods could be shipped across the world, food supplies became more reliable, healthcare and medicine improved, sanitation and nutrition became important, and the death rate steadily decreased as the birthrate increased
Demography
The scientific study of human populations
Birthrates, death rates, and the age structure of the population are all studied in order to find reasons why populations are growing or that growth is slowing down
Demographic Transition
A change in a population from high birthrates and death rates to low birthrates and death rates
Stage I: Birthrate and death rate are equally high
Stage II: Death rate begins to fall but birthrate remains high
Stage III: Birthrate falls to meet the death rate