Chapter 8 - 4

04 Biology

Boom-and-Bust Cycles in Population Growth

The population of any given species has natural growth and reduction cycles. Through these natural cycles, most species and populations tend to remain relatively stable.

However, due to environmental factors, some species can experience large fluctuations between an enlarged population and greatly decreased numbers in their population.

This occurrence has been coined the “boom-and-bust cycle” in population growth.

The changes in the boom-and-bust cycle are perpetual.

They build upon and lead into each other.

A) When a population has an increased number of members, competition for a set amount of resources begins.

B) Also, the number of predators tends to increase as the prey population increases: more food for predators means they will reproduce in greater numbers as well.

C) Conversely, with fiercer competition for resources and an expanded group of predators, the population will begin to experience higher death rates.

D) As the population dies, the number of predators will correspondingly shrink due to the lack of food. Resources replenish during the bust period.

With less predation, the population has a chance to repopulate their environment, and birth rates increase, thus beginning the cycle again.

Favorable conditions such as warmer weather and rainfall foster an environment in which species are free to reproduce and increase in number.

A) Some species experience these kinds of favorable conditions on a yearly basis.

B) They produce many offspring during the spring and summer, but then the entire population experiences a crash when the colder months of the year hit.

C) Many populations experience this cycle at a much slower rate.

D) Their growth and subsequent decline are influenced by environmental factors such as the availability of food and living space. The lemming populations of the arctic are a good example.

Lemmings thrive as long as there is ample vegetation for all members of the population. As the population grows and the food supply starts to dwindle, lemmings will often emigrate from highly crowded areas.

It is during these times of exodus that many lemmings die.

Some accidentally drown when they try to swim across large bodies of water to find new homes.

Also, the lemmings become easy targets for predators when they are on the move. In areas where numbers are depleted, the land and its resources will be replenished.

Vegetation grows, and, before long, lemmings migrate again to these areas. In other animal populations, predatory behavior plays a bigger role in carrying on this cycle.

Rabbit populations are known for their large numbers and ability to reproduce at a swift rate.

They are preyed upon by bigger mammals, which can help maintain equilibrium in their population in most cases.

In one Canadian study, biologists theorized that the rabbit population might experience the boom-and-bust cycle it a particular species was their only predator.

To test their theory, they studied the availability of animal pelts, or skins, on the market between 1845 and 1935.

The number of pelts available would correspond with population size.

Throughout the ninety years of the study, the numbers of rabbit pelts and the pelts of their predator, the lynx, rose and fell in a dramatic fashion.

A) When the number of rabbit pelts grew to be quite large, it meant that the rabbits were thriving and reproducing.

B) Not long after the rabbit population reached a peak, the number of pelts began to decrease, while the number of lynx pelts began to increase.

C) Likewise, as the lynx pelts reached a peak, lynx populations decreased, and the rabbit population began to re-establish itself.

D) Scientists believe that this example of the boom-and-bust cycle is the exception rather than the norm.

Generally speaking, populations of different species maintain a certain balance in their growth patterns.

The boom-and-bust periods within this cycle are special circumstances that only occur for a limited amount of time.

Just the same, there are times when environmental factors can lead to a cycle of great growth and consequent diminution in the species populations of the world.