Population Growth — Definition

Definition

Imagine a group of organisms, like a pack of wolves or a colony of bacteria, living in a specific area. The 'population' is simply the total number of individuals of that species in that area. 'Population growth' is how this number changes over a period. Does it get bigger, smaller, or stay roughly the same? This change isn't random; it's driven by four main factors, which are like the 'ins and outs' of the population.

First, we have births, or 'natality' in scientific terms. When new individuals are born or hatched, they add to the population, making it grow. The more births there are, the faster the population tends to increase. Second, there are deaths, or 'mortality'. When individuals die, they are removed from the population, causing it to shrink. A high death rate can slow down or even reverse population growth.

Beyond births and deaths, populations can also change due to movement. Immigration is when individuals from other populations move into our study area. Think of new birds flying into a forest. These newcomers add to the existing population. Conversely, emigration is when individuals leave our study area to go somewhere else. If some wolves leave their pack to find new territory, that's emigration, and it reduces the local population.

So, population growth is a balance: (Births + Immigration) - (Deaths + Emigration). If the 'ins' are greater than the 'outs', the population grows. If the 'outs' are greater, it declines. If they're roughly equal, the population remains stable.

Scientists often visualize these changes using 'growth curves'. There are two main types you'll encounter. The exponential growth curve (often called a 'J-shaped' curve) shows a population growing very rapidly, almost unchecked, as if resources are limitless.

This happens when a species colonizes a new, abundant habitat. However, in the real world, resources are limited. This leads to the logistic growth curve (an 'S-shaped' curve). Here, the population initially grows fast, but as it approaches the maximum number of individuals the environment can sustain (called the 'carrying capacity'), its growth slows down and eventually levels off.

This S-shaped curve is a more realistic representation of how most populations grow in nature, as environmental factors like food, space, and predators start to limit expansion.