Symbiosis, Disruption, and Extinction

Organisms in an ecosystem are constantly interacting, and some live in especially close, long-term relationships called symbiosis. Sorting these relationships by who benefits, plus understanding how ecosystems get disrupted and how species go extinct, rounds out the ecology topics on the test.

Three Kinds of Symbiosis

Symbiosis just means a close relationship between two different species. There are three main types, sorted by who is helped and who is harmed. 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 and the other is harmed — like a tick feeding on a dog.

The quick way to classify any relationship is to ask two questions: does species A benefit, and does species B benefit or get harmed? Both benefit is mutualism; one benefits and the other is unaffected is commensalism; one benefits at the other’s expense is parasitism.

When Ecosystems Are Disrupted

Ecosystems stay balanced through these relationships and through food webs, but they can be thrown off. A disruption — a new predator, a disease, a drought, pollution, or the loss of a habitat — can change population sizes throughout the system. Because organisms are connected, a change to one species ripples out to others. An invasive species, for example, can crowd out native species that have no defenses against it.

Extinction

Extinction happens when a species dies out completely. It can result from natural events or from rapid changes a species cannot adapt to fast enough — habitat loss, climate change, or new competitors and predators. The link to evolution is important: species survive when they can adapt to change, and go extinct when the environment changes faster than the population can. On the test, look for the cause of a change and trace its effect through the ecosystem.

Watch: A Short Video Lesson

StoneAgeMan (Untamed Science) walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:

A Routine for These Questions

1. Classify symbiosis by who benefits: both (mutualism), one/neutral (commensalism), one/harmed (parasitism).
2. A disruption to one species ripples through connected populations.
3. Invasive species can crowd out natives that lack defenses.
4. Extinction follows when change outpaces a species’ ability to adapt.

Practice

1. What is symbiosis?
2. A bee gets nectar and pollinates a flower. Which type of symbiosis is this?
3. A tick feeds on a dog. Which type is this?
4. Name one thing that can disrupt an ecosystem.
5. What is extinction?
6. Why can an invasive species harm an ecosystem?

Answers

1. A close, long-term relationship between two different species.
2. Mutualism (both benefit).
3. Parasitism (one benefits, the other is harmed).
4. Any of: a new predator, disease, drought, pollution, or habitat loss.
5. When a species dies out completely.
6. Native species may have no defenses and get crowded out.

Where This Fits in Your Science Prep

These interactions build on ecosystem roles and energy flow, and they connect to natural selection. See all topics on the Science Topics Hub.