Species Distributions and Biodiversity
Communities and Habitats
Earth's biosphere can be broadly divided into different ecosystems. These ecosystems can be further divided into communities, which are made up of interacting populations of living plants and animals that co-exist in a particular place.
Communities can be identified in various ways - physical appearance, what species are present, how common certain species are, patterns of their interdependence (e.g. trophic levels), and so on. For instance, in a rainforest you may have one community on the forest floor, another in the tree canopy above. Likewise, a lake ecosystem may have community at the bottom of the lake which is distinct from the community near the water's surface.
Different species also have a habitat, which is the environment in which an organism resides or is biologically adapted to live. Habitats contain both biotic and abiotic elements, with habitat size and character varying based on the species' needs. For instance, Great Blue Herons are large wading birds that live in shoreline habitats throughout North America such as riverbanks, freshwater marshes, and tidal flats. They hunt fish in the water, but like to nest near the tops of trees near the water to keep hatchlings safe from terrestrial predators.
Ecological Niche
Organisms have specific functions or occupations within their given communities, a concept known as their ecological niche. This function can be determined by the organism's physical, chemical, and/or biological needs. Their ecological niche is different from their habitat. While a habitat is the environment shared by many species, an ecological niche is a highly specific and unique role played by a species within that habitat.
For example, the white-breasted nuthatch is a small bird living throughout North America that lives in forest habitats, particularly deciduous forests with trees that drop their leaves in winter. Its ecological niche involves foraging for insects up and down tree trunks, using its sharp beak to extract insects and turning upside down and sideways to find food overlooked by other birds. The nuthatch occupies a similar habitat to woodpeckers, another bird in the region that forages for insects in tree bark using its beak. However, the nuthatch's unique foraging behavior gives it a distinct niche different from the woodpecker.
No two species can occupy the same ecological niche (meaning eating the same food and/or space) because one species will always outcompete the other. This idea is known as the competitive exclusion principle. For this reason, closely related species tend to be either spatially separated by distance, or - as in the case of the nuthatch and woodpecker - use different species-specific strategies. In terms of evolution, the competitive exclusion principle means that species seek to reduce competition and maximize their own reproductive rate to help ensure successful reproduction and, ultimately, species survival.
Importantly, the competitive exclusion principle also helps result in greater species biodiversity, as species shift and adapt to fill different ecological niches across habitats.
Abiotic Factors
Species distribution is highly influenced by numerous abiotic factors, or the nonliving environmental variables that influence how species interact and grow.
As mentioned on previous pages, solar energy is a vitally important abiotic factor for the functioning of Earth's biosphere. However, plants have adapted widely to accommodate differing levels of sunlight and darkness, or photoperiod, across Earth's surface. For instance, some plants require a certain amount of sunlight in order to germinate seeds or to flower. Other plants need longer nights; for example, the poinsettia plant needs at least 2 months of 14 hours of darkness to flower.
Some additional abiotic factors can include:
- Air and soil temperatures, which can determine the rates of chemical reactions
- Precipitation and water availability
- Water quality, including mineral content, salinity, and levels of pollution or toxicity
- Earth's magnetic field is detected by birds, bees, and some other life forms to find direction and assist with migration. For instance, sea turtles are imprinted with magnetic data unique to their birth location upon hatching, which assists them as they spend their adult lives traveling Earth's oceans. Female sea turtles use this magnetic data to return to their birthplaces to lay their own eggs.
Limiting Factors
Certain characteristics within an environment can limit the distribution and/or population size of a species. These limiting factors can be physical, chemical, or biological. Plant limiting factors can include precipitation, temperature, light levels, and soil nutrients, all of which impact vegetation patterns and abundance. Animal limiting factors can include the number of predators, the availability of suitable foods and/or habitats, the availability of breeding sites, and the prevalence of disease.
Taken together, all possible limiting factors determine a species' environmental resistance which will eventually stabilize their population within an ecosystem. Sometimes environmental resistance is dominated by one specific limiting factor, and sometimes it is determined through a combination of factors. For example, the snail kite is a tropical raptor with a very small habitat limited to the Florida Everglades and only eats one specific type of snail. The presence of this snail is a crucial limiting factor.
The limiting factors impacting a species will create their range of tolerance, which each organism possesses for the physical and chemical environmental characteristics they need to thrive. Within that range, the species will be abundant. At the edges of that range, the species will be infrequent. Outside the range, the species will be absence. For instance, the coastal redwood is a tree that has a narrow range along the coast of California and Oregon in the United States. The foggy conditions provide sufficient condensation to meet the tree's need for water. At the edges of this range, the trees are shorter, smaller, and less common. Other trees such as maples, oaks, and junipers have much wider tolerance ranges, meaning they have much wider species distributions and can tolerate more varying moisture and temperature conditions.