The carrying capacity of a certain habitat is the maximum number of individuals that it can support. Population numbers are limited by resources such as space, food, and water. Current estimates of Earth's carrying capacity are around 10 billion. 

The population's new size at the start of 2012 was = original population + (births + immigrations) - (deaths + emigrations) = 120 + (18 + 2) - (2 + 3) = 135. Then, the net population growth rate is equal to (net change in population / size of old population) = 15/120 = .125.

Species designated as K-strategists thrive through longevity and have a higher survival rate at birth, but produce far fewer offspring. Examples include elephants and whales. Consider an elephant; it may produce relatively few baby elephants, but these baby elephants each have the potential to live relatively long lives. On the other hand, contrast K-strategists like elephants with r-strategists like rats, locusts, and flies. These species are designated as r-strategists because in contrast, they produce numerous offspring, few of which may survive to adulthood, and each one of which reaches maturity quickly and lives a relatively short life. K-strategists in general spend more time than r-strategists caring for and raising their offspring; they have fewer offspring, so they put more care into ensuring the survival of each one. Contrast elephants with frogs to see this difference; elephants care for their young, while frogs lay their eggs and care for their offspring very little, if at all.

Species described as "r-strategist" have a survival strategy of producing large numbers of offspring, a short life expectancy, and typically smaller body sizes. Examples include mice, locusts, and frogs. These species survive by producing lots of offspring, since many individuals don't survive to adulthood.

R-selected individuals are characterized by the following: early age of sexual maturity, many lifetime reproductive events, large numbers of offspring per event, and minimal parenting. Many of the offspring of r-selected species will die before adulthood. On the other hand, K-selected individuals have only one or few offspring at once and invest larger amounts of time and energy into parenting to ensure survival. The rabbit is the only species listed whose life traits can be categorized as being r-selected.

Use the Rule of 72:

 is rate of growth

 is time (in years) it takes to double

By manipulating the equation to divide 72 by the given growth rate, you can find the number of years it takes for the population to double.

Mutations occur at low rates. They are rare and not very common in most individuals. This explains why the process of evolution and adaptation occurs over long periods of time. 

The use of child laborers leads to higher birth rates. More children are needed to support families and households. Educating women also decreases birth rates. This is the reason why developing countries tend to have higher birth rates than developed countries. A lower average marriage age leads to an increase in birth rates. Religion can influence birth rates, either raising or lowering (generally it raises the birth rate).

Corals are r-selected species, categorized by producing many offspring at once, which individually have low chances of survival, and short lifespans. Other r-strategists include rodents, insects, and other small-bodied, early-maturing animals. K-selected species produce few offspring and invest lots of parental care to ensure a higher chance of survival. Humans are an example of a K-selected species.

Elephants are large-bodied and give birth to only one offspring every few years, and mothers have a large role as caregivers to ensure the survival of their young. The other species listed here are r-selected species and have many offspring at a time. In some cases, r-selected species do not parent. In others, offspring mature very quickly and parents do not have to provide care for very long.