Ecology focuses on how organisms interact with their environment and with other organisms within their species and of other species. It is a broad field with many different areas of study and applications, from managing natural resources to mitigating the effects of pollution to preserving endangered species.
There are a few important terms that will come in handy when learning about various types of ecology. An ecosystem comprises all the living and nonliving things present in a particular geographical area, including plants, animals, fungi, microorganisms, soil, air, and water. Biomes are types of terrestrial ecosystems that are categorized primarily by the types of plant life they support. Examples include deserts, grasslands, tundras, tropical rainforests, and temperate forests. A population is a group of organisms of the same species that interact regularly with one another. Biodiversity is a broad term for the variety of species found in a particular habitat or ecosystem (or other unit of area).
Population ecology is the subfield of ecology that analyzes groups of organisms within a species that live in a particular geographic area. Population ecologists study the demographics of these groups—or populations—and look at how they change over time.
A population’s size (the number of individuals it contains) and density (how many individuals there are per unit of area) are key pieces of demographic information that population ecologists use. It’s difficult to directly count each organism in a population, so ecologists usually use a representative sample to estimate the size and density of the population in its habitat.
A population’s distribution throughout its habitat is another important source of data. In a random distribution, organisms in a population are randomly scattered throughout their habitat. A clumped distribution is typical of organisms that are found in groups or clumps throughout the habitat. Uniform distribution, in which organisms can be found in a regular pattern throughout the habitat, is common for organisms that maintain and defend their territory.
Population ecologists also look at patterns of population growth and the factors that regulate that growth. There are two basic models of population growth. Exponential growth occurs when a population can increase with unlimited natural resources. This isn’t really possible in nature, but species can show exponential growth for short periods of time if resources are sufficiently abundant (bacteria in a flask are typically a good example). When represented graphically, exponential growth looks like a J-shaped curve.
In contrast, logistic growth accounts for limited resources by determining an ecosystem’s carrying capacity for a given population, which is the maximum number of individuals of a population that an ecosystem can accommodate given the amount of available resources. Logistic growth starts off as exponential, but as resources are depleted, the growth rate slows down. Eventually, the growth rate levels off once the carrying capacity has been reached. Logistic growth looks like an S-shaped curve, and it is a more realistic model than exponential growth of how populations grow over time.
Demographic-based population models are at the intersection of evolutionary biology and ecology. Life history strategies—particularly the number of offspring and the amount of parental resources devoted to ensuring their survival—evolve as species adapt to their environment. Species that have adapted to stable, predictable environments, like elephants, produce small numbers of offspring and expend lots of resources to ensure their survival. At the other end of the spectrum, organisms that have adapted to unpredictable and/or unstable environments, like jellyfish, produce a large number of offspring and do not invest a lot of resources in ensuring their survival.
Community ecology looks at how species within an ecosystem interact with one another. At a small scale, this type of ecology analyzes interactions and relationships between particular species within a given ecosystem or habitat. At a larger scale, community ecologists study the dynamics of whole communities and how those communities change as a result of disturbances, such as severe weather, natural disasters, and climate change.
Predator-prey relationships are a good example of how increases or decreases in the population of one species can affect another in the same habitat. This often takes the form of a cycle: when a prey animal’s population increases, the population of the predator that eats them also increases, because more prey animals means more food. However, the predator population eventually grows to a point where they eat or kill enough of the prey population that it begins to decrease. In turn, the predator population also decreases, because there is no longer enough food to support the entire population. Then, when fewer predators means that the prey population can grow again, the cycle begins once more.
In addition, predator-prey relationships (and relationships between plants and the herbivores that eat them) frequently drive the evolution of new traits. Traits that provide chemical, behavioral, mechanical, and physical defenses against being eaten are advantageous for prey. Examples include thorns, toxins, and coloration patterns that either hide the prey organism from predators or warn predators that it is toxic. In contrast, traits that augment organisms’ abilities to find and kill prey are advantageous for predators.
Not all relationships between species are adversarial. Symbiotic relationships occur when one or more species benefits from a series of interactions over a long period of time. Commensal relationships occur when one species benefits from the relationship or interaction, and the other is neither harmed nor benefitted. One example of this is how burs—the seed heads of the burdock plant—hook on to passing animals and then fall off later. The seeds get where they need to go, and the animals with burs in their fur are minimally affected.
In mutual relationships (mutualism), both organisms in the relationship or series of interactions benefit. A good example of this is oxpeckers and zebras (or rhinos). Oxpeckers are birds that eat parasites, like ticks, off of zebras’ skin.
Parasitism is when one organism benefits but another is harmed. Usually, this involves the parasite feeding off of another organism and depending on it for survival without killing it (at least not until the parasite is able to complete its reproductive cycle). Plasmodium falciparum, the protist that causes malaria in humans, is one well-known example. These microorganisms reproduce sexually in mosquitoes and are transferred to humans, where they reproduce asexually in the blood. Then, when an infected human is bitten by mosquitoes, the protists are transferred to the mosquito and complete their reproductive cycle.
Ecosystem ecologists study ecosystems—how they change over time and how resources (such as nutrients and energy) move through them and the organisms that inhabit them.
Food chains and food webs track how nutrients and energy move through the organisms within an ecosystem. Food chains are linear and represent which organisms eat and are eaten by others in the same ecosystem. A particular level of a food chain is called a trophic level. Organisms of higher trophic levels consume animals of lower ones, but with each trophic level, less energy is available. Food webs are complex interactions between multiple food chains.
Organisms at different trophic levels have specific roles. Producers are organisms that make their own food. Usually, these are bacteria, plants or phytoplankton. They are at the bottom of the food chain. Primary consumers—typically herbivores—eat the producers. Secondary consumers are carnivores that eat the primary consumers, and tertiary consumers are carnivores that eat the secondary consumers. The highest level of consumer in an ecosystem is the apex consumer. Although they are not always shown on food chain diagrams, decomposers—typically bacteria, fungi, and invertebrate animals—consume the remains of other organisms.
When it comes to depicting the breadth of trophic interactions between species within an ecosystem, a food web is generally more accurate than a simple food chain. Food webs use arrows to show how organisms of a particular species eat (and/or are eaten by) multiple other species, which might be at different trophic levels.
A single ecosystem is often represented by two food webs: a grazing food web, which shows producers at the bottom and various levels of consumers above them, and a detrital food web, which shows the decomposers that feed on dead organisms.
In addition to examining how energy moves through ecosystems and what trophic relationships exist between species, ecosystem ecologists also observe how materials such as water, carbon, and nitrogen move through ecosystems.
Article from the CDC: "About Malaria"
Article from the Ecological Society of America
Article from Khan Academy: "Life history strategies"
Article from New England Complex Systems Institute: "Mutualistic Relationships"
Article from the Oregon Institute of Marine Biology: "Symbiosis"
CrashCourse Ecology Video: Population Ecology
Definition from National Geographic Society: "Carrying Capacity"
"Mutualism: eight examples of species that work together," from the UK Natural History Museum
OpenStax Concepts of Biology, Ch 19: Population and Community Ecology
OpenStax Concepts of Biology, Ch 20: Ecosystems and Biosphere
Video from the California Academy of Sciences