Species that can thrive under a variety of conditions; they’re able to easily adapt to a wide range of circumstances
Generalist species have an advantage in habitats that undergo change: while specialist species aren’t able to adapt quickly, generalists are able to quickly respond if any changes happen
Invasive species tend to be generalist species because they can quickly adapt to new locations, allowing them to repopulate and outcompete native species
Specialist species
Species that are suited only for a specific set of environmental conditions
They are only at an advantage in habitats that remain constant and don’t undergo any changes
If a change happens that causes an environment to deviate from its normal conditions, then the specialist species that relied on those specific conditions for survival will no longer be able to thrive
Specialist species are well-adapted to the specific habitats in which they live, but can be outcompeted by generalists if the habitats change
Reproduce less frequently and produce less offspring
Parents care for the offspring long-term following the birth
K-selected species are more stable, and organisms are more likely to live to a longer age
With K-selected species, most newborn organisms survive into adulthood, where they then die near the end of the organism’s maximum potential life span
R-selected species
Reproduce more frequently and produce lots of offspring at once
Parents do not care for offspring following the birth, instead leaving them to fend for themselves
When r-selected organisms give birth; they don’t expect most of their offspring to make it to adulthood
Instead, most of the offspring will die off, but the few who survive and make it into adulthood will repopulate quickly and continue the species’ survival
Invasive species tend to be r-selected because a population with fast reproduction and copious amounts of organisms is more capable of “invading” a habitat and taking its resources
If the number of individuals in a population exceeds an ecosystem’s carrying capacity, then the members of that population will go into competition over the limited resources
This causes members of the population to die until the population count is once again under the carrying capacity
After an overshoot, the number of individuals alive will typically dip below the carrying capacity before the population adjusts to the now-sufficient amount of resources
This causes the population to reproduce at a rate higher than the ecosystem can support, which contributes to the cycle of a population continuously “overshooting” and then dying off, as shown in the graph above
3.5 Population Growth and Resource Availability
Density-dependent factor
A factor that limits population growth and is related to the size of the population
Food, water, and habitat space are density-dependent factors
If the population size is too “dense” or large, then organisms have to compete for limited resources, slowing the population growth
Light is also a density-dependent factor for organisms such as plants
Disease is a density-dependent factor because it limits the population’s size more drastically when the organisms are too densely packed together
Density-independent factor
A factor that limits population growth and is not related to the size of the population
Natural disasters are density-independent because they negatively affect both large/dense populations and small populations
Sex ratio
The ratio of males to females in a population
After an event such as a natural disaster, the sex ratio of a population might lean too heavily in one direction
If there are too many members of one sex, then those excess members won’t be able to mate, which restricts the potential population growth
The general groupings that individuals fall into based on their age
Ages 0-14 are considered “pre-reproductive”
Ages 15-44 are considered “reproductive”
Ages 45+ are considered “post-reproductive”
If the “pre-reproductive” cohort is larger than the “reproductive” cohort, then the population is growing
This is because the upcoming generation has more people than the current one
If the “pre-reproductive” cohort is smaller than the “reproductive” and “post-reproductive” cohorts, then the population is shrinking
This is because the upcoming generation has less people than the current one
Populations that are in earlier developmental stages tend to have higher growth rates and have more “triangular” age structure diagrams
Women are reproducing more frequently
This gives the age structure diagram a wider base due to more people falling into younger cohorts
Life expectancies are lower in less developed countries
This is why there are fewer people in the older-aged cohorts
Populations that are highly developed have a more consistent growth rate and have more “rectangular” age structure diagrams
Some well-developed countries even have shrinking populations
3.7 Total Fertility Rate
Total fertility rate
The average number of children that a woman in a country will give birth to during her lifetime
Developing countries tend to have much higher TFR’s than well-developed countries
Well-developed countries have better access to education for women, so more women will pursue education and a career instead of prioritizing raising children
Less-developed countries have higher infant mortality rates, so women will give birth to more children as a means of ensuring that some of them survive to adulthood
Infant mortality rate
The number of deaths of chilren under the age of 1 per 1000 live births
Less-developed countries have higher infant mortality rates because they have less access to healthcare, food, water, and shelter
Replacement-level fertility rate
The average number of children that a couple must have in order to “replace” themselves in the next generation (to keep the population number stable)
Since less-developed countries have a higher likelihood of children not surviving to adulthood, their replacement-level fertility rates will be higher
A diagram that displays the stages at which a country’s population growth changes
High stationary
This “pre-industrial” stage has a high birth rate and a high death rate
The high death rate is due to a lack of food, water, shelter, healthcare, and sanitation
The high birth rate happens because women have less access to education, so they focus more on reproduction and raising children
There is also lower access to contraceptives, leading to more reproduction and births
Early expanding
This “developing” stage experiences a declining death rate due to higher access to advanced sanitation and healthcare
During this stage, a country begins industrializing and developing, which gives people better access to resources that can prevent illnesses and death
The population grows at a high rate because the birth rate is high and the death rate is low
Late expanding
This “developed” stage experiences a declining birth rate
Families choose to have less children because the infant mortality rate is going down
Women don’t need to have as many children to guarantee that at least 1-2 survive to adulthood; instead, most, if not all, children survive to adulthood
Women also gain access to education, so more of them pursue schooling and careers over childbearing
Population growth begins to slow down because the birth rate is declining and the death rate remains low
Low stationary
This “highly developed” stage experiences more stable population growth, a low birth rate, and a low death rate
Well-developed countries are represented by this stage
Declining?
Some countries are experiencing a declining population because birth rates are lower than the rate needed to maintain the population
The number of children born is less than the number of adults present in the population
Countries such as Japan have fewer children and younger adults than older adults and elderly people
