Relationships and Populations
No organism lives alone. Living things constantly interact, competing for resources, hunting one another, and forming partnerships, and these relationships shape how populations grow and shrink. This topic connects the ways organisms interact with the numbers that result.
This lesson covers the main relationships between organisms and what controls population size.
Organisms interact through competition, predation, and symbiosis. A population is a group of the same species in one area, and its size is shaped by births, deaths, and the limits of its environment. When resources run short, population growth slows and levels off.
How do organisms interact?
Three interactions cover most cases. Competition happens when organisms need the same limited resource, like food or space, and it can occur within a species or between species. Predation is when one organism, the predator, hunts and eats another, the prey. Symbiosis is a close, long-term relationship between two species, and it comes in three flavors.
In mutualism, both species benefit, like a bee getting nectar while pollinating a flower. In commensalism, one benefits and the other is unaffected, like a bird nesting in a tree. In parasitism, one benefits while the other is harmed, like a tick feeding on a dog.
| Relationship | Species A | Species B |
|---|---|---|
| Mutualism | Benefits | Benefits |
| Commensalism | Benefits | Unaffected |
| Parasitism | Benefits | Harmed |
What controls population size?
A population grows when births outnumber deaths and shrinks when the reverse is true. But no population grows forever. The environment can only support so many individuals, a limit called the carrying capacity. As a population nears that limit, food, space, and other resources run short, so growth slows and the population levels off. Predators, disease, and competition are among the factors that keep populations in check.
Watch: A Short Video Lesson
greatpacificmedia walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:
A routine for relationship questions
- Identify the interaction: competition, predation, or symbiosis.
- For symbiosis, decide who benefits and who is harmed or unaffected.
- Both benefit is mutualism; one benefits, one unaffected is commensalism; one benefits, one harmed is parasitism.
- For populations, compare births and deaths.
- Remember carrying capacity limits how large a population can grow.
Practice questions
- What kind of relationship is a bee pollinating a flower while collecting nectar?
- In predation, which organism is the prey?
- Define carrying capacity.
- A tick feeding on a dog is an example of which symbiotic relationship?
- Name one factor that can limit population growth.
- True or false: in commensalism, both species are harmed.
Answers:
- Mutualism, because both the bee and the flower benefit.
- The organism that is hunted and eaten.
- The largest population size an environment can support over time.
- Parasitism.
- Any of: limited food, limited space, predators, disease, competition.
- False. One benefits and the other is unaffected.
Where this fits
Relationships and populations build on ecosystems, habitats, and niches and connect to how energy and matter move through communities in energy flow and the cycles of matter. Symbiosis also appears in symbiosis, disruption, and extinction. All topics live on the ASVAB General Science Learning Hub.
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