Describing Populations
What Are the Differences between Exponential and Logistical Growth?
Exponential Population Growth
Exponential growthglossary term (opens in a new window) of a populationglossary term (opens in a new window) occurs when the growth rate is density-independent and constant. Exponential growth causes a population to increase rapidly over time. The increase in population size can be seen as a J-shaped curve on a graph. The steep slope is due to the continually increasing population. As an example, imagine a population of bacteria that reproduces by each cell dividing once per hour, so that the population doubles every hour. The table below shows the number of bacterial cells over a period of time, starting with a single cell.
This table shows the exponential growth of a group of cells that double during a specific time period. What circumstances are necessary for this theoretical pattern to occur in a natural setting?
Populations growing in ideal conditions and without limitations will initially show exponential growth. Invasive speciesglossary term (opens in a new window) are an ecological problem in part because they undergo exponential growth. When these organisms are first brought to a new environment, they often do not have natural predators or parasites. They can grow unchecked if the new environment is favorable. These organisms will increase in number very quickly. Invasive organisms tend to take over unoccupied niches or out- compete native organisms. This can drive native organisms to extinction.
Many species rely on exponential growth as their main reproductive strategy. These organisms are referred to as r-selected. The r refers to rate of increase because this strategy relies on producing many offspring. In general, r-selected species are adapted to unstable habitats. Fungi are r-selected. Decomposers grow when a tree or other organism dies. They cannot plan for a new food source and will eventually exhaust a given source. As a result, fungi grow quickly and reproduce large numbers of spores before the food source is exhausted. In general, r-selected organisms tend to be small, grow quickly, produce large numbers of offspring, and provide very little parental care to offspring.
Logistical Population Growth
Populations tend to grow exponentially until they are regulated by lack of resources or other factors. Once the population size nears the ecosystemglossary term (opens in a new window)’s carrying capacityglossary term (opens in a new window), the rate of increase will slow. The carrying capacity is the largest population size that can be supported by the ecosystem. Logistical growth is density-dependent because the density of the population is limiting its growth.
Populations show logistic growth as the population nears the ecosystem’s carrying capacity. What is the name given to the shape of a logistic growth curve?
Some species use logistical growth as a reproductive strategy. These organisms are called K-selected. K is the abbreviation for carrying capacity. K-selected organisms are adapted to stable environments. Their population sizes tend to remain near the carrying capacity of the environment. These populations are generally regulated by density-dependent factorsglossary term (opens in a new window) such as predator-–prey interactions and resource availability. K-selected organisms tend to be larger, grow slowly, produce fewer offspring, provide parental care to offspring, and are adapted to stable environments.
Species that produce fewer offspring tend to provide more parental care. Each individual organism has a finite energy supply and cannot allocate energy to both high reproductive rates and extensive parental care of each offspring. These reproductive strategies apply to plants as well as to animals. In plants, parental care can be defined as energy allocation to the seeds. Plants that produce large seeds with abundant nutrient storage tend to produce fewer seeds overall.
