Population Ecology

numbers of organisms within an ecosystem. Climate is always changing, which changes the ecosystem over time.

Despite the dynamic nature of ecosystems, they are always in equilibrium, in which there is a balance of organisms in the ecosystem with relatively steady levels of the different organisms in the system. The problem happens when there is a disturbance to an ecosystem, which disrupts the balance of organisms. A disruption can be a major fire, acid rain, algal blooms, deforestation, and the introduction of a new species into the system. Some ecosystems will recover, while others will fall apart.

Ecosystems can be resistant to a disturbance and will be able to recover despite the change. The system can also be resilient so that it stays steady over a long period of time. A key factor in the ecosystem is the biodiversity in the ecosystem. It means that, while some organisms do not recover after a disturbance, others will survive and will continue the ecosystem, even though it may take a while to recover.

POPULATION ECOLOGY

There are many factors in the study of ecology, including the effects of the environment and the changes in the genetics of the ecosystems. Population ecology is a branch of the study of ecology and involves the different biological populations within an ecosystem, such as their structure and dynamics. This study is similar to community ecology, which studies the structure and dynamics of the different communities in an ecosystem as well as the study of population genetics, which is the study of gene frequencies within the ecosystem.

Populations will differ in their level of stability. Some will be stable for many thousands of years, while others will die off and become extinct. Other populations will be transient, such as larval forms of insects that survive only until they get past that stage of their life cycle and leave the ecosystem to go to another ecosystem.

Populations are groups of different individual organisms that share the same characteristics. Even though the organisms are unique within the population, they are considered ecologically “equivalent”. This means basically that they have the same life cycle, live in the same ecological environment, and react the same to the environment.

The population density is the number of individuals in a population per unit area. It can be numbers of organisms or weight of organisms per area (or volume in oceanic ecosystems). There is the crude density, which is the number or biomass per unit space. The ecological density, on the other hand, is the density per unit of habitat space (which is the available space that can be colonized by the members of the population). This is an important distinction because not all space in an ecosystem reflects an appropriate habitat for a specific organism.

Population dispersal is possible. There are three possible dispersal patterns. In regular dispersion the individuals are regularly spaced. This is primarily seen in crops and with territorial animals. There is also random dispersion, in which there is no relationship between the different members of the population. This is also rare. Clumped dispersal is the most common, with individuals aggregated into patches with less populated areas in between.

Populations are made from organisms of different age. The proportion of different animals or plants of different age groups is called the age structure of the population. This determines the reproductive status of the population. The population can be mainly pre-reproductive, reproductive, or post-productive. The duration of these different stages depends on the organism. There are three different age structures, including the following:

• Pyramidal shape—this involves a broad base of young individuals and a narrow apex of older individuals. The birth rate needs to be high and is something seen in many lower-order animals, like paramecium and insects.

• Bell-shaped polygon—this is an equal group of young and middle-aged individuals with a small number of older, post-productive organisms.

• Urn-shaped structure—this is a low percentage of young individuals and a low birth rate so that the population dwindles because many are in the postproductive years.

The number of individuals in a population depends on the birth rate and the death rate of the different organisms in the population. There is a wide variation among species as