The population density is the measure of the population per unit area or volume, and may be measured over land or water.

The carrying capacity refers to the maximum number of organisms that can be sustained by the resources of an environment. The range of a population is simply the area over which the population can be found. Dispersion refers to the different regions where a species is found within a given area. Competition is the use of limited resources between multiple members of a population, or multiple species within a niche.

The earth's habitats and ecosystems are dispersed based on air circulation, rainfall, and topography. For example, when looking at the globe, we may notice that the world's major grasslands are always leeward of major mountain ranges, or that tropical rain forests are located around the equator. Why do grasslands fall on the leeward side of mountain ranges, and tropical rainforests at the equator? As the air travels up the mountain, it condenses and precipitation occurs. Once the air reaches the leeward side, it is dry, resulting in the grasslands and plains. Tropical rainforests are located at the equator, an area that gets constant sunlight and rainfall regardless of the earth's tilt. The dispersion of the earth's habitats and ecosystems is very dependent on such environmental factors.  

A community is a group of populations of different species but live in the same area and interact. Community interactions may display interspecific interactions such as competition, predation, herbivory, symbiosis, or facilitation. A population is a group of organisms of the same species that live together and interact. 

The sea otter is an example of a keystone species. Keystone species are not abundant in the community, however they have a strong control on community structure because their ecological roles are so important. On the other hand, dominant species are those that are most abundant in their communities and have the highest biomass. The dominant species competitively exploits resources and avoid predation, giving them an advantage and allows them to grow large in number. Ecosystem engineers are species that dramatically alter their environment by changing its physical characteristics. 

Competitive exclusion is when an inferior competitor is eliminated. In the moss example, the moss that is able to use resources more efficiently and reproduce quicker will beat out the less competitive moss. An ecological niche is the sum of a species’ use of the biotic and abiotic resources in its environment. Resource partitioning is the differentiation of niches that allows similar species to coexist in a community. If one of the mosses could adapt to use different resources, the two species could coexist together. Character displacement has to do with speciation. It refers to the tendency for character traits to diverge more in sympatric populations rather than allopatric populations.  

Species diversity is the variety of organisms that make up a community. There are two parts of species diversity: species richness and relative abundance. Species richness refers to the number of different species in the community. In this example, forest 1 has greater species richness and diversity over forest 2.  

Invasive species are organisms that become established outside their native range. In this example, the seed’s native range is the countryside. However, when it is carried by the wind to the forest (an environment outside its native range), it plants itself and establishes a population there. The seed is an example of an invasive species. 

Ecosystems are described as a community of organisms and their interactions between themselves and the environment. Ecosystems include abiotic and biotic components of the environment and are not limited to one species.

Ecosystems are communities of abiotic and biotic components that interact together in the same environment. Ecosystems are controlled by both internal and external factors. Internal factors that control ecosystems include the availability of limiting resources, species presence, and disturbance.

Ecosystems are communities of abiotic and biotic components that interact together in the same environment. Ecosystems are controlled by both internal and external factors. External factors that impact ecosystems include but are not limited to the invasion of non-native species, climate, and time.