Populations – Flashcards

A population is made up of a group of organisms of the same species that live together in one place at one time and interbreed. Understanding population growth is important because populations of different species interact and affect one another, including human populations.

Whether a population grows or shrinks depends on births, deaths, immigration, and emigration. Immigration is the movement of individuals into a population. Emigration is the movement of individuals out of a population. So, a simple population model describes the rate of population growth as the difference between birthrate, death rate, immigration, and emigration. Plotting population changes against time on a graph creates a model in the form of a curve. Two major models of population growth are exponential growth and logistic growth.

One important part of a population model is the growth rate. When more individuals are born than die, a population grows. In exponential growth, there are always more births than deaths. As time goes by, more and more individuals enter the population. Exponential growth occurs when numbers increase by a certain factor in each successive time period.n exponential growth, population size grows slowly when it is small. But as the population gets larger, growth speeds up.

An ecosystem can support only so many organisms. The largest population that an environment can support at any given time is called the carrying capacity. Density-dependent factors are variables affected by the number of organisms present in a given area. An example of a density-dependent factor is the availability of nesting sites. Logistic growth is population growth that starts with a minimum number of individuals and reaches a maximum depending on the carrying capacity of the habitat. When a population is small, the growth rate is fast because there are plenty of resources. As the population approaches the carrying capacity, resources become scarce. Competition for food, shelter, and mates increases between individuals of a population. As a result, the rate of growth slows. The population eventually stops growing when the death rate equals the birthrate.

Water, food, predators, and human activity are a few of many factors that affect the size of a population.

onliving factors that affect population size are called abiotic factors. Weather and climate are the most important abiotic factors.The amount of water available can also influence populations

A factor that is related to the activities of living things is called a biotic factor. Food, such as grass or other animals, is a biotic factor.Diseases and parasites, when they infect many individuals, can also cause populations to decline. Biotic factors are often density dependent because they can have a stronger influence when crowding exists. As the density of a population increases, the effects of starvation, predators, and disease often also increase. Humans affect populations of many species. Most of the time, humans cause populations to drop by disrupting habitats, introducing diseases, or introducing nonnative species. But some organisms do better around humans.

Better sanitation and hygiene, disease control, and agricultural technology are a few ways that science and technology have decreased the death rate of the human population.

For most of human history, there have been fewer than 10 million people. Once agriculture was developed, the population began to grow, but relatively slowly. Two thousand years ago, there were only 10 million people. Around the time of the Industrial Revolution, the human population started to accelerate rapidly.

Science and technology are major reasons why the human population is growing so rapidly. Advances in agricultural technology have allowed efficient production of crops and other foods. More food supports more people. As a result, the human population has begun to grow faster. Medical advances have also allowed the human population to increase. Vaccines have lowered the death rate. More children are surviving to adulthood. Other medical advances have allowed adults to live longer lives.

One of the most common interactions in communities is that between predators and their prey. Predation is the act of one organism killing another for food.Species that involve predator-prey or parasite-host relationships often develop adaptations in response to one another. For example, predators evolve to be more cunning to catch their prey. In response, prey evolve to be faster runners to escape more easily. Back-and-forth evolutionary adjustment between two species that interact is called coevolution.

In parasitism, one organism feeds on another organism called a host. The host is almost always larger than the parasite and is usually harmed but not killed. Parasites often live on or in their host. Therefore, the parasite depends on its host not only for food but for a place to live as well. For example, tapeworms live in the digestive system of their hosts. Fleas that live on the skin of their host are another example. Hosts try to keep parasites from infecting them. Hosts can defend themselves with their immune systems or behaviors such as scratching. In response, parasites may evolve ways to overcome the host's defenses.

Herbivores are animals that eat plants. Unlike predators, herbivores do not often kill the plants. But plants do try to defend themselves. Some plants have thorns or spines that cause pain for herbivores that try to eat them. Other plants have chemical compounds inside them that taste bad. Some chemical compounds can make an herbivore sick or kill the herbivore. Some herbivores have evolved ways to overcome plant defenses. For example, monarch butterfly caterpillars feed on milkweed, which is a plant that is toxic to many herbivores. Not only can the caterpillars survive eating the toxic milkweed but the plant toxins then make the monarch butterfly inedible to bird predators.

is a relationship in which two species live in close association with each other. In some forms of symbiosis, a species may benefit from the relationship. Mutualism and commensalism are two kinds of symbiotic relationships in which at least one species benefits.

A relationship between two species in which both species benefit is called mutualism.

commensalism, two species have a relationship in which one species benefits and the other is neither harmed nor helped.