Ecology
Symbiosis
Living together — mutualism, commensalism, parasitism
Symbiosis is the close, long-term biological interaction between two or more different species. Three main types: (1) Mutualism — both benefit (mycorrhizae, gut microbiome, pollination). (2) Commensalism — one benefits, other unaffected (epiphytes, remoras). (3) Parasitism — one benefits at other's expense (tapeworms, ticks, viruses). Endosymbiosis: special form where one lives inside another (mitochondria origins). Drives biodiversity, ecosystem functioning, evolution. Some species so dependent they cannot survive separately (obligate vs facultative symbiosis).
- DefinitionLong-term close interaction between species
- MutualismBoth benefit
- CommensalismOne benefits, other unaffected
- ParasitismOne benefits at other's expense
- Famous mutualismMycorrhizae (~80% of land plants)
- Term coinedAnton de Bary, 1879
Interactive visualization
Press play, or step through manually. The visualization is yours to drive — try it before reading on.
Watch the 60-second explainer
A condensed visual walkthrough — narrated, captioned, under a minute.
Why symbiosis matters
- Health. Microbiome essential.
- Agriculture. Nitrogen fixation, mycorrhizae.
- Ecosystem functioning. Many critical interactions.
- Disease. Parasites, pathogens.
- Evolution. Symbiosis drives major innovations.
- Conservation. Coral bleaching = lost symbiosis.
- Origin of life. Endosymbiosis enabled eukaryotes.
Common misconceptions
- Symbiosis = mutualism. Includes parasitism.
- Always permanent. Some are.
- Mutualism perfect. Often partial; can shift.
- Strict categories. Continuum; can change with conditions.
- Just biological. Plants, fungi, animals, microbes all involved.
- Symbiosis rare. Pervasive in biology.
Frequently asked questions
What's the difference between symbiosis types?
Three classifications by outcome. Mutualism: +/+ (both benefit). Commensalism: +/0 (one benefits, other unaffected). Parasitism: +/- (one benefits, other harmed). Plus: amensalism (-/0; one harmed, other unaffected; e.g., antibiosis); competition (-/-). Categories sometimes blur — interactions can shift based on conditions.
What are mycorrhizae?
Symbiotic association between fungi and plant roots. ~80% of land plants. Fungi extend roots' reach for nutrients (especially phosphorus); plants give sugars from photosynthesis. Mutualism. Two main types: ectomycorrhizae (cells outside; mostly trees), endomycorrhizae (cells inside; mostly other plants). Critical for plant nutrition; co-evolved over hundreds of millions of years.
What's the human microbiome?
Microbes living in/on humans. Gut: ~100 trillion bacteria; outnumber human cells. Functions: digest food (fiber, complex carbs), produce vitamins (K, B12), train immune system, prevent pathogen colonization. Mostly mutualistic. Disrupted (dysbiosis): linked to obesity, IBD, depression, autoimmune diseases. Critical for health.
What's commensalism?
One benefits, other unaffected (in theory). Examples: (1) Epiphytic plants (orchids, bromeliads on trees) — get height, light; tree unaffected. (2) Remoras hitchhike on sharks — transport, leftover food; shark unaffected (mostly). (3) Bacteria on skin — some species commensal. Strict commensalism rare; often subtle effects. Practical: classification depends on level of analysis.
What's parasitism?
One species benefits at expense of host. Examples: tapeworms, malaria parasite, fleas, ticks, mistletoe, parasitoid wasps. Levels: (1) Endoparasites (inside host) — tapeworms, malaria. (2) Ectoparasites (outside) — ticks, lice. Many subdivide. Tend to balance: too virulent → host dies → no transmission; too gentle → outcompeted. Coevolution: arms race.
What's endosymbiosis?
Special symbiosis where one lives inside another. Includes: mitochondria (originally α-proteobacterium), chloroplasts (cyanobacterium), bacterial endosymbionts of insects (Buchnera in aphids; provides amino acids), zooxanthellae in coral. Often: integrated; can't survive separately. Major evolutionary mechanism; eukaryotes themselves came from endosymbiosis.
How do symbiotic relationships start?
Various paths. (1) Repeated chance encounters → adaptation → tighter association. (2) Infection that becomes mutualistic over time. (3) Niche-sharing organisms gradually intertwine. Examples: lichens (fungus + alga or cyanobacterium); fungal-algal partnerships repeatedly evolved. Endosymbiosis: rare but pivotal events when they occur.