In nature, the logistic growth model is more common.
Because interaction between species and the environmental conditions needed for an exponential growth model are not always possible. In fact, they are hardly possible.
Explanation:
Exponential growth model: A population that exhibits an exponential growth model increases in proportion to its size. The increase in the population size depends on the individual´s reproduction rate: a population that grows at constant exponential rate gains individuals faster as the population increases in size.
This model states that there is unlimited resource availability. As there are guaranteed resources and space, there is no competition. Population growth does not depend on density, nor Natality and mortality rate. There is constant growth, which is proportional to the population size. The exponential growth model is represented by an exponential curve.
In nature, this is almost impossible. Just a few organisms, such as bacteria species, might be represented by this model. But in most species, this is not feasible because available resources are not infinite, space is not infinite, and there is always an interaction between species that affects their population sizes.
Logistic growth model: The population density affects its growth. Natality and mortality depend on the population size, which means that there is no independence between population growth and population density.
When a population grows in a limited space, density rises gradually and eventually affects the multiplication rate. The population's per capita growth rate decreases as population size increases. The population reaches a maximum point delimited by available resources, such as food or space. This point is known as the carrying capacity, K.
K is a constant that coincides with the size of the population at the equilibrium point when the natality rate and the mortality rate get qual to each other.
The logistic growth model is represented by a sigmoid curve. In the beginning, the population grows slowly. Its size N is inferior to K.In the second stage, the population size increases rapidly but N, is still inferior to K. Gradually, the population reaches the equilibrium point, K, which depends on resource availability. N equals k. Resources become harder to find, and much of the living space is unavailable. Finally, when N is superior to K, the population must decrease in size because there are not enough resources to maintain that size.
Species have different strategies to survive in different environments, reproduce, disperse, and compete. These strategies are r strategies and k strategies.
"r" species: Their growth rate per capita is very high, which makes possible the colonization of new environments. These species are mostly small-sized organisms with short life cycles. They reach sexual maturity at early stages and have numerous offspring. They hardly spend energy in parental care. Many "r" species exhibit the exponential growth model, but not all of them.
"k" species: Their growth rate per capita is low. They are often big-sized and long-lasting species. They reach sexual maturity in advanced stages and do not have numerous offspring. They spend much time of their lives in parenting to ensure reproductive success. They are important competitors, being capable of displacing other species and monopolizing the available resources. "k" species exhibit the logistic growth model.
