A typical rainforest can support thousands of trees of the same species. A desert may not be able to support a single tree. Each of these environments has different amounts of resources like food, water, and shelter and this impacts the population size.
The number of births compared to the number of deaths determines if a population is increasing or decreasing.
If resources are available a population will continue to grow until the resources will be used up. At this point, the carrying capacity, which is the maximum number of individuals of one species that the environment can support, will be reached.
For example, owls eat mice, small birds, and other small rodents in order to stay alive. As a result, the carrying capacity of owls depends on the number of mice, birds, and rodents available to eat. Owls also need trees to live in and this also impacts the carrying capacity.
The amount of food available for the owl and the amount of shelter are examples of limiting factors. Limiting factors are living and nonliving items that restrict a population's size.
Nonliving parts of an environment are called abiotic factors. Examples include sunlight, temperature, water, and rocks.
Living factors are called biotic factors. Biotic factors will interact with other biotic factors. For example, zebras will eat grass, but lions will prey on the zebra.
As the environment changes so does the carrying capacity. After a period of abundant rain, there may be extra plants available for insects and animals like zebras to eat which allow their population to grow.
Conversely, natural disasters like fires flooding, or droughts can cause populations to decrease or even crash.
Let’s take a look at a real-world example. Yellowstone National Park was established on March 1, 1872. At this time wolves were considered dangerous and were hunted and driven completely out of the park. As a result, the elk population began to grow because one of their main predators was eliminated. On average the population of Elk in Yellowstone National park was around 17,000. On January 12, 1995 wolves were reintroduced in the park. As a result, the Elk population fell below 10,000 in 2003, and in 2017 the number of Elk was 5349 Elk. This shows how the population of Elk is impacted by the number of predators. In addition, the Elk are affected by high fluoride and silica levels in the water, which are abiotic factors they drink in Yellowstone park. These high levels impact enamel formation and cause their teeth to wear out quickly and shortens their life.
So in summary, population size can be impacted by both biotic and abiotic factors found in the ecosystem.
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