Limiting Factors and Carrying Capacity

Why isn't the world covered in rabbits?

A female eastern cottontail rabbit can have several litters of babies in one summer, with several babies in each litter. If every baby survived and had babies of its own, in a few years there would be millions of rabbits in your neighbourhood alone.

But that doesn't happen. Something keeps the number of rabbits in check. Scientists call these things limiting factors.

Remember from Grade 4

In Grade 4 you learned that every habitat has limits, and that the largest population a habitat can support is called its carrying capacity. In Grade 6 you saw how predators and prey, like Canada lynx and snowshoe hares, rise and fall together. Now you will sort limiting factors into biotic and abiotic, read population graphs, and model population growth with code.

Biotic and abiotic limiting factors

A limiting factor is anything that keeps a population from growing larger.

Biotic (living) limiting factors:

• Food supply (plants or prey)
• Predators
• Disease and parasites (like ticks on moose)
• Competition with other members of the same species, or with other species, for food, nesting sites or territory

Abiotic (non-living) limiting factors:

• Water (droughts)
• Temperature (very cold winters, heat waves)
• Sunlight (for plants)
• Space and shelter (nesting holes, dens)
• Soil nutrients, oxygen in water, snow depth

Often the factor in shortest supply is the one that limits the population. A pond might have plenty of food for frogs, but if it dries up in July before the tadpoles become frogs, water is the limiting factor.

Carrying capacity and the population graph

When a few individuals arrive in a new place with plenty of resources, the population grows slowly at first (there are only a few parents). Then it grows faster and faster. As it gets crowded, food, space and other resources run short, and the growth slows down. Finally the population levels off around the carrying capacity, going up and down a little from year to year.

The graph of this pattern looks like a stretched S.

Crowding matters: two kinds of limiting factors

• Density-dependent factors get stronger as the population gets more crowded. When there are many deer, food runs out faster, diseases and parasites spread more easily from animal to animal, and predators find prey more easily. These factors tend to push a population back toward its carrying capacity.
• Density-independent factors affect a population no matter how crowded it is: a forest fire, a flood, a late spring frost, or a very harsh winter.

Example: white-tailed deer in Ontario. In winter, deep snow makes it hard for deer to walk and reach food, and it costs them a lot of energy. Many deer gather in deer yards, areas of dense conifers like cedar and hemlock where the branches catch snow, so the snow underneath is shallower and there is shelter from wind. After a winter with very deep snow that lasts a long time, many deer may die, even if there were not many deer to start with (density-independent). In a crowded deer yard, the deer also compete for the twigs and branches they can reach (density-dependent).

Carrying capacity can change

Carrying capacity is not a fixed number. It changes when conditions change:

• A wet summer with lots of berries and seeds can raise it for animals that eat them.
• A drought, a fire, or land cleared for buildings can lower it.
• Overshoot: if a population grows past the carrying capacity (for example, deer with no predators), the animals may damage their food supply by overeating. Then the population crashes, and the carrying capacity may be lower for years until the plants recover.

A coded population model

Here is a simple model. The population doubles each year, but it can never go higher than the carrying capacity.