The populations of some species are always growing exponentially. These are populations that usually do not reach their carrying capacity because they frequently suffer catastrophes that knock them back or they continually colonize new, resource-rich areas. Many insect species, weedy plant species, and bacteria display this type of growth. Other species have populations that are usually at or near their carrying capacity. These two different types of species are called r-selected and K-selected, respectively. Their reproductive strategies are quite different.