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A guided journey · 5 stops

Living together

Four partnerships across four kingdoms — and the discovery that none of them is a friendship.

The route

  1. Symbiosis
  2. Lichens
  3. Mycorrhizal networks
  4. Coral and its symbionts
  5. Mutualism

Symbiosis is the most sentimentalised word in biology. It arrives already implying that the arrangement is good for everyone, which is the thing an investigation is supposed to establish.

This route takes it apart. It starts with the definition, which never meant mutual benefit and includes parasitism. Then three partnerships in turn: one where the partners built a shared body, one where they hold a market with sanctions, and one where the partner lives inside the animal's own cells. Each is remarkable. None of them is cooperation in the sense the word suggests.

It ends with the case where a partnership was watched failing — not from betrayal, but because the conditions that made it worth paying for went away.

  1. Stop 1 of 5. Ecology

    Start with what the word actually means

    Because everything after this depends on it. Symbiosis means different species living closely together — outcome unspecified, parasitism included. The popular narrowing to "both benefit" makes a standard category read as a contradiction and lends every close association an unearned warmth. Clearing that up first is what lets the next three stops be read as arrangements rather than friendships.

    Read SymbiosisSymbiosis never meant "both benefit". A tapeworm is a symbiont.

  2. Stop 2 of 5. Fungi

    A partnership that built a body

    The most complete integration short of merging. A fungus, photosynthetic partners, and — as sequencing later showed — usually another fungus as well, in a structure none of them makes alone. It also carries a warning the route needs: the "fungus farming algae" phrase was coined by opponents to make the discovery sound ridiculous, and outlived the argument it lost.

    Read LichensNo roots, no waterproofing. That is why a lichen can live on bare rock — and why it tells you what the air has been like.

  3. Stop 3 of 5. Fungi

    A partnership that behaves like a market

    Where the lichen looks like fusion, this looks like trade — and it has been tested as trade. Partners that deliver more get more in return, and the exchange runs from mutual benefit through to net cost depending on soil conditions. Reading it after the lichen shows that closeness and cooperation are different things: this partnership is looser and considerably more calculating.

    Read Mycorrhizal networksThe fungi are real, the connections are real, and carbon does move. Whether anybody is sharing is a different question, and it is still open.

  4. Stop 4 of 5. Marine life

    A partnership inside the cells, and what heat does to it

    The tightest of the three: photosynthetic cells living inside the animal’s own, supplying most of its energy, which is how a reef is productive in nutrient-poor water. It is also the one with a visible failure mode. Bleaching is this partnership dissolving under heat — the animal alive and starving — and it introduces the theme the final stop completes.

    Read Coral and its symbiontsA reef sits in nutrient-poor water and is one of the most productive places on earth. It holds its nutrients rather than taking them.

  5. Stop 5 of 5. Ecology

    End where one was watched breaking

    The acacia experiment is the strongest evidence in this corpus that a partnership is conditional. Fence the browsers out and the tree stops paying its guard ants; a species that does not defend takes over; and the trees behind the fence do worse than the ones being eaten. After three partnerships that look like devotion, this is the one that shows what holds them together — and it is not devotion.

    Read MutualismA cleaner fish would rather eat its client’s mucus than its parasites. Mutualism is not friendship — it is an arrangement that needs holding in place.

Where this leaves you

You end with a way of reading any close relationship between species: not "do they help each other" but "what does each get, on what terms, and what would change those terms".

The last stop is the one that reframes the rest. Acacias pay ants to defend them; fence out the browsers and within a decade the payments fall, a non-defending ant species moves in, and the trees do worse than the ones still being eaten. Nothing about either partner changed.

What the route does not settle: how often partnerships are this conditional. One experiment shows it can happen; it does not establish how common it is.

Last reviewed 2026-09-04. All guided journeys