Factors That Influence Population Growth (College Board AP® Biology)

The Definition of a Population

• A population can be defined as a group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed and produce fertile offspring

  • For example, the Bornean orangutan (Pongo pygmaeus) and Müller's gibbon (Hylobates muelleri) live in the same area at the same time but are two species with two different populations

  • They cannot be part of the same population as they are different species (they cannot interbreed and produce fertile offspring)

• Some organisms have very small populations, whereas others have very large populations, for example:

  • The Amur leopard (Panthera pardus orientalis), now found only in the Amur River basin of eastern Russia (having already become extinct in China and the Korean Peninsula) has a remaining population of around 60 individuals

  • Although humans (Homo sapiens) used to live in separate populations, we are now widely considered as one global population of around 8,100,000,000 individuals (and counting...)

Population Growth Factors

• The study of populations and how they change over time is called population dynamics

• Populations can grow or shrink depending on a wide range of abiotic and biotic factors

• The growth or shrinkage of a population is a function of the species' reproductive fitness

  • Which in turn comes from how well suited an organism's alleles are to the environmental conditions at the time

• Population growth can be quantified as the change in the number of individuals per unit time

  • Negative growth means shrinkage

• Limits to population growth are collectively called constraints

• Reproduction without constraints gives rise to an exponential growth in the population

  • Constraints lead to population stability or decline

  • A habitat will have a certain stable population level, called its carrying capacity 

  • Carrying capacity is dealt with in more detail in topic 8.4

Population Growth Curve with Carrying Capacity Diagram

• Population growth can be defined in the following formula

where

N = population size
dN = change in population size
B = birth rate
D = death rate
dt = change in time

Exponential Growth

• The rate of exponential growth of a population is defined by the equation

where:

dt = change in time
N = population size
r_max = maximum per capita growth rate of a population

• This would occur for a population that can reproduce without constraints e.g. 

  • Has access to abundant food

  • Can reproduce at their physiological capacity

• Exponential growth gives rise to a J-shaped curve: