Population dynamics is the study of how populations change over time. Scientists study the ways populations interact and how those interactions affect the number of individuals in a given area.

The term population density is used to describe how many individuals of given species live in a specific amount of space. For example, one organism may need hundreds of square meters of living space while another may be able to survive in just a few square meters. In a forest, there may be on average less than 1 tree per square meter of land, but there might be 3 bayberry bushes in the same square meter. Think about how densely grass grows compared to trees. Each species has its own unique needs for space.

The needs of living organisms are sometimes referred to as limiting factors. These are environmental elements that stabilize population size and keep species from producing too many offspring. We often think of predators as population stabilizers, and to a certain extent, predators do limit prey population sizes. However, other factors may also affect the size of a population. In the absence of a predator, a population will stabilize when competition for food becomes the limiting factor. For aquatic organisms, dissolved oxygen is a limiting factor. Biodiversity in an aquatic system can change dramatically when dissolved oxygen levels drop.

Rabbits are legamorphs with a big appetite for plants and a rapid reproductive rate. Rabbits were introduced into Australia where they had plenty of space to roam and food to eat. What was missing was a natural predator. The rabbits competed with native herbivores for food. It didn't take long for the rabbit population to grow to the point that it threatened native herbivores that had predators. Do some research to find out what Australia did to try to control the runaway rabbit boom.

Many migratory songbirds make their nests deep in the forest interior. They breed in the forests of the New Jersey highlands. The dense forest canopy protects the songbirds from predators who wait for them on the forest edges. Humans also love to live in forested areas along lakes and rivers. Contiguous (connected) forests are being cut up into smaller pieces by development in the highlands. This has reduced the space available for songbirds to breed. With less deep forest areas, competition for suitable nesting spots has increased among members of the same species. Those who are forced to the edges of the forest are more likely to be subject to attacks by predators. Another space related factor that affects the survival of some of the songbirds is that as the forest is fragmented, the amount of edge space increases. This benefits the edge species whose populations increase. Not only is there less successful nesting, there are also more predators! Do some research to find out what the New Jersey Audubon Society and New Jersey Conservation Foundation are doing to preserve habitat in New Jersey.

Learn more about conservation in New Jersey

For millions of years the horseshoe crab has made an annual migration to New Jersey to mate and lay eggs in the sands of Cape May. In recent years, the horseshoe crab population has been under stress because of harvesting. The crabs, really relatives of spiders, are not eaten by humans, but are used as bait for conch. Each May, during the high tides, millions of shorebirds stopover in Cape May to fatten up on horseshoe crab eggs. A shorebird may fly nonstop from South America to Cape May depleting most of its body fat in the process. The protein rich eggs of the horseshoe crab allow the shorebirds to bulk up for the second leg of their journey to breeding grounds in Canada. The decline in horseshoe crabs has been accompanied by a decline in the number of shorebirds coming to Cape May. Scientists are studying horseshoe crabs to determine if there is a connection between the loss of food during migration and the decrease in shorebirds. It may be possible that if shorebirds are not finding enough food to eat then they may not be successful in breeding. Do some research to find out what scientists are learning about horseshoe crabs and shorebird migration.

More about the Shorebird Crisis

More about Horseshoe Crabs

Farmers noticed that they were losing chicks and suspected that hawks might be cause of the problem. They set about the task of hunting the hawks and destroying their nests. It solved the problem of missing chickens. However, soon after the hawks disappeared, farmers noticed that rodents were all over the grain bins. The farmers really hadn't noticed the mice that much when the hawks were around. Although the mice seemed to be just everywhere, the population did eventually stabilize even without a predator.

1. Why was the rodent population under control when there were hawks in the community?

2. Why did the rodent population stabilize even though the predator was gone?