A population is a group of individuals of the same species that live in the same geographic area that interbreed with each other. Many species consist of multiple populations dispersed over a few to many geographic areas. The demographic structure of a population includes its age structure (for example, the number of adults vs. juveniles), sex ratio (female to male ratio), and the rates of reproduction and death for individuals of a given age. The demographic structure of a population is important for understanding how and why population sizes increase or decrease over time, how species are affected by environmental changes, and how traits evolve.
Species with large population sizes and geographic ranges are less likely to go extinct than species with small populations and limited geographic ranges.
Species populations are affected by many Earth system processes and phenomena, including:
- Evolutionary processes that change the growth and reproduction and death rates of organisms over time.
- Species interactions, including how species provide resources for each other, consume each other for food, or compete for resources such as food, water, and/or space.
- Climatic conditions, such as the amount of sunlight absorbed at different latitudes, temperature, and precipitation patterns.
- Nutrients, especially nitrogen and phosphorus, which when limited can decrease productivity and thus species populations, or when abundant can increase population sizes.
- Many other environmental factors, including soil quality (often related to nutrient levels), wildfires, water acidity, and oxygen levels can affect population sizes by affecting the survival, death, growth, and reproduction.
- Events, such as extreme weather or volcanic eruption, which can kill individuals or entire populations of organisms.
Humans have affected species populations through a variety of activities, including:
- Deforestation, habitat destruction, and urbanization, which reduce population sizes of many species, but can also increase population sizes of other species.
- Agricultural activities that increase the amount of livestock and crops available to feed the growing global human population.
- The use of fertilizers for agricultural activities that increase the amount of nutrients, especially nitrogen and phosphorous, in soil or water. These nutrients can increase plant and algae growth. However, increased nutrient does not always increase population sizes. For, example, in aquatic environments, nutrient-rich runoff can cause large amounts of algae grow – when the algae die, they are consumed by bacteria which can reduce oxygen levels in the water, killing fish and other species. This process is known as eutrophication.
- Fishing and hunting, which reduces species populations of the exploited species, but can also lead to increased numbers of other species if competitive interactions are reduced thereby making more resources and space is available for the other species. Additionally, fishing and hunting which often removes large, reproductively mature individuals from species populations, which, in turn, can favor the survival of smaller individuals that can reproduce.
- Human freshwater use, which can limit the amount of water available for other organisms in an ecosystem.
- The release of pollutants and waste which can reduce growth and reproduction or kill organisms.
- Activities that cause global warming, such as the burning of fossil fuels, agricultural activities, and deforestation. Increasing average global land and ocean temperatures have altered temperature and precipitation patterns, as well as the distribution of snow and ice cover, which affects population sizes.
- Activities such as the burning of fossil fuels, agricultural activities, and deforestation. that release carbon dioxide into the atmosphere, which is absorbed by the ocean causing acidification. This decreasing pH of ocean waters (along with ocean warming) causes physiological stress for many species, which can cause decreased growth, decreased reproductive rates, and death, leading to decreased species population sizes.
- Introducing invasive species that compete with native species for food, water, or other resources, reducing the population sizes of native species.
Can you think of additional cause and effect relationships between characteristics of species populations and other parts of the Earth system?
Visit the evolution, species interactions, and species ranges pages to explore more connections between the biosphere and global changes.