Symbiosis

Essential Question

How do close relationships between species help or harm each partner?

Many species live in close, long-term contact with another species. In this lesson, you’ll learn the three main types of symbiosis, how to classify a relationship by its effect on each partner, and how the costs and benefits can change with conditions.

Types of Symbiosis

• Symbiosis: two species living in close contact
• Mutualism: both partners benefit
• Commensalism: one benefits, one is not clearly affected
• Parasitism: one benefits, one is harmed

Symbiosis means two species living in close, long-term contact. Biologists classify these relationships by the effect on each partner. In mutualism, both partners benefit. In commensalism, one partner benefits and the other is not clearly helped or harmed. In parasitism, one partner, the parasite, benefits while the other, the host, is harmed. These categories are useful, but real relationships are not always clean. A relationship can be hard to measure, and the effects can change with conditions. Some textbooks use the word symbiosis only for mutualism, so check how your class defines it.

Mutualism

• Pollinators get food, plants get pollen moved
• Mycorrhizal fungi help roots take up nutrients
• Gut microbes help digest food
• Benefits depend on conditions

Many mutualisms involve a trade. Bees get nectar and plants get pollen moved. Mycorrhizal fungi grow on or in plant roots and help the plant take up water and nutrients such as phosphorus, while the plant supplies the fungi with sugars. Microbes in the digestive systems of many animals help break down food while living in a supply of nutrients. The graph shows sample data from an illustrative experiment. The fungi helped seedlings a lot in low-nutrient soil but made little difference in nutrient-rich soil, which shows that the benefit of a mutualism can depend on conditions.

Commensalism and Parasitism

• Commensalism: hard to prove that the other partner is unaffected
• Parasites feed on a host
• Examples: tapeworms, ticks, fleas, mistletoe
• Parasites usually depend on a living host

In commensalism, a barnacle attached to a whale gets a place to live and access to food-rich water, and the whale is usually not clearly affected. Scientists often find it hard to show that the other partner is truly unaffected, so some relationships labeled commensal may have small costs or benefits. A parasite lives on or in a host and takes nutrients from it. Examples include tapeworms, ticks, fleas, and mistletoe. A parasite that kills its host very quickly loses its source of food, so many parasites harm their hosts without killing them right away, although some do kill them.

When Relationships Shift

• Costs and benefits can change
• Nutrient levels, other species, and climate matter
• A helpful partner can become less useful
• Classify by the effect in the conditions studied

The effect of one species on another is not fixed. If nutrients in the soil rise, plants may get less benefit from mycorrhizal fungi, because they can take up enough on their own, while still paying the cost of feeding the fungi. Changes in climate or the arrival of another species can also shift the balance of costs and benefits. For this reason, scientists describe a relationship by its measured effects in the conditions studied, and they are careful about saying that a partnership is always helpful or always harmful.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Classify the Relationships

Use the three types of symbiosis. Classify each: (a) a tick feeds on a deer’s blood, (b) bees collect nectar while moving pollen between flowers, (c) a barnacle attaches to a whale and the whale is not clearly affected.

Show the solution

1. (a) Tick and deer: the tick benefits and the deer is harmed, so this is parasitism.
2. (b) Bees and flowers: the bees get food and the flowers get pollen moved, so both benefit. This is mutualism.
3. (c) Barnacle and whale: the barnacle benefits and the whale is not clearly affected, so this is commensalism.
4. Answer: (a) parasitism, (b) mutualism, (c) commensalism.

Example 2: Fungi and Seedlings

Use the graph. Do the fungi benefit the seedlings in both soils? Write a claim, evidence, and reasoning.

Show the solution

1. Read the graph: in low-nutrient soil, seedlings had 2.1 g without fungi and 3.4 g with fungi. In nutrient-rich soil, they had 4.0 g without and 4.2 g with.
2. Claim: the fungi helped the seedlings a lot in low-nutrient soil but made little difference in nutrient-rich soil.
3. Evidence: the mass rose by 1.3 g in low-nutrient soil but by only 0.2 g in nutrient-rich soil.
4. Reasoning: the fungi help roots take up nutrients. When the soil already has plenty, the plant gains less from the help, so the benefit of the partnership depends on conditions.
5. Answer: The fungi helped a lot in low-nutrient soil (+1.3 g) but little in nutrient-rich soil (+0.2 g).

Example 3: How Much Did the Fungi Help?

In low-nutrient soil, seedlings had a dry mass of 2.1 g without fungi and 3.4 g with fungi. Calculate the percent increase. Round to the nearest tenth of a percent.

Show the solution

1. Change: 3.4 − 2.1 = 1.3 g.
2. Percent increase: change ÷ starting value × 100 = 1.3 ÷ 2.1 × 100 = 61.9%.
3. Answer: 61.9% increase (1.3 g more).

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Practice

Classify symbiotic relationships, use experiment data as evidence, calculate a percent change, and explain how conditions can change a relationship. Try it on your own first, then check your work with the answer key.

• Symbiosis Practice Worksheet (PDF)
• Symbiosis Practice Answer Key (PDF)

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Lesson Slides

• 3.6 Symbiosis Slides (PDF)

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Big Ideas

• Symbiosis is a close, long-term relationship between two species
• Mutualism helps both, commensalism helps one without clear effect on the other, parasitism harms one
• Parasites feed on a host, and many do not kill it right away
• The benefit of a partnership can depend on conditions
• Classify a relationship by its measured effects

Back to the essential question: close relationships between species can help both partners, help one without a clear effect on the other, or help one at the other’s expense. The effects depend on conditions, so scientists measure them rather than assuming them. Next, you’ll look at how communities change over time through ecological succession.

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