Populations change over time. There are a lot of factors that affect the populations within a biological community. Some have to do with organisms themselves. Others relate to the habitat. Any factor or condition that limits the growth of a population in an ecosystem is called a limiting factor. Too much or too little of any biotic or abiotic factor makes an unstable ecosystem and brings about change.

Population Growth and Decline - Reproduction Reproduction rates affect population size. Birth rate is a measure of the number of births in an animal population. It can also be a measure of the stability of an ecosystem. Ex. Black bears reproduce once every two years, however if not enough food is available the female bear's reproductive cycle is delayed. Many organisms also require specific conditions in order to reproduce. The number of locations is limited, the reproduction will be limited Ex. Bank Swallows need sandy cliffs in which to dig nests

Population Growth and Decline – Predation Predation affects population sizes. A large population of predators will limit the population of prey. A small population of prey will limit the population of predators. Diseases limit populations in the same way. A disease organism can enter a population and kill many organisms

Ex. Snowshoe Hare and Lynx We will look closely at the relationship between the Canada lynx and its primary prey, the snowshoe hare. The snowshoe hare is a common species of rabbit found in North America, its range extending throughout the tundra of Canada, Alaska, and into the northern United States. One distinctive quality is its 2 different coloration patterns – brown in the summer, and white in the winter to better camouflage with the snow. Its diet consists of grasses, berries, twigs, bark and leaves. 

Ex. Snowshoe Hare and Lynx The Canada lynx is a wild cat that resembles a large house cat with a short tail and prominent tufts on its ears. It is very secretive and even experienced hunters rarely see one in the wild. Its range overlaps with the snowshoe hare, on which it almost exclusively preys upon.  

Ex. Snowshoe Hare and Lynx For over 300 years, the Hudson Bay Company has been involved in the fur trade in Canada. Detailed company records list the number of snowshoe hare pelts and the number of lynx pelts collected by hunters and trappers every year since the late 1700’s. The data shows a 200 year history of cyclical population booms and busts in the snowshoe hare population and a slightly delayed population boom and bust in the lynx population. There are many competing theories to explain why the populations cycle in so dramatic a fashion.

Ex. Snowshoe Hare and Lynx These theories include: During peak years, the hares devour all the available vegetation and quite literally breed like rabbits until the environment can no longer support their blossoming population. As the hares become weakened by starvation, the lynx are better able to find and kill them, adding to their decline. The population does not reestablish itself immediately because it takes time for the vegetation to grow back. Another theory is that the lynx population determines the hare population. As the number of hares increases, so does the numbers of lynx that survive to eat them. Soon, there are too many lynx for the number of hares and the lynx eat away their favorite food until they too suffer a population decline until the hare population can start growing again. Lastly, there is evidence that at the peak population levels, the hares become so stressed by the increasing numbers of predators that they no longer reproduce at the same rate. Their population falls both as a result of the lowered reproductive success and the sheer number of lynx that are out to eat them.

Snowshoe Hare and Lynx

Population Growth and Decline - Resources Populations are limited by food supply. Lack of food is one of the most important limitations on populations. If an organism cannot acquire the energy needed to survive and reproduce, the population will decline. Ex. If a snake cannot find enough mice to sustain itself, it will starve to death. If the snake lives, it may be so weak that it can’t reproduce.

Population Growth and Decline - Seasons Seasonal changes put pressure on populations. Ex. Rain, snow, and wind during a winter season Some organisms have adapted to this by migrating, becoming dormant/hibernation, or accumulating fat stores. Many populations decline to minimal levels during the winter in temperate and polar populations.

Maintaining a Balance Living things have certain minimum requirements for food, water, and living space. When a population reaches a state where it can no longer grow, the population has reached its carrying capacity. This is the maximum number of number of individuals that an ecosystem can support.

Moose and Wolf Populations, Isle Royale National Park

Interpreting the Graph What was the moose population in 1970? The wolf population? How did the moose population change between 1970 and 1975? How did the wolf population change in the same period? What time period did the wolf population decrease as the moose population decreased?

Carrying Capacity Even if there were no wolves on the island of Isle Royale, the population of moose would still be limited because there is only so much food and space available.

Carrying Capacity An ecosystem’s carrying capacity is different for each population. Ex. Isle Royale supports many more moose than wolves. The moose is a PRIMARY CONSUMER. It is at a lower level or the energy pyramid than the wolf, a SECONDARY CONSUMER.