How Symbiosis Connects Different Species

Mutualism is a relationship in which both species gain a benefit. A bee receives sugary nectar from a flower while the flower gains a chance for its pollen to be carried to another flower. The two species do not need to be equally dependent, but both gain something useful from the interaction.

Commensalism describes a relationship in which one species benefits while the other is neither clearly helped nor harmed. Barnacles can attach to whales and gain transport through food-rich water. The whale usually experiences little effect from carrying them, although real biological relationships can be more complicated than a neat classroom label.

Parasitism benefits one species while harming the other. A tick feeds on a mammal's blood, and a tapeworm takes nutrients from an animal's digestive system. Parasites usually depend on keeping their host alive long enough to feed, grow or reproduce, so harming a host does not always mean killing it quickly.

Symbiotic relationships are not fixed contracts. The effect of one species on another can change with food supply, climate, population size or the health of the organisms involved. A relationship that usually appears harmless may become costly when resources are scarce.

Some partnerships are so close that the organisms depend heavily on each other. Many corals contain microscopic algae that live inside their tissues. The algae provide sugars made through photosynthesis, while the coral provides shelter and access to materials the algae need. If stressful conditions cause the partnership to break down, the coral can lose colour and energy in a process called bleaching.

Other relationships are flexible. An organism may cooperate with one species while competing with another, and a single species can take part in several different interactions at once. Symbiosis is therefore one piece of a much larger ecological network.

When you meet an example, focus on the effect each species experiences rather than memorising the names first. Ask whether each participant gains a benefit, experiences a cost or is mostly unaffected.

A simple comparison helps:

• •Both species benefit in mutualism.
• •One benefits and the other is mostly unaffected in commensalism.
• •One benefits while the other is harmed in parasitism.
• •The same relationship may change under different conditions.
• •Close relationships can influence survival and reproduction.

One of the most important symbiotic relationships in the history of life is hidden inside your own cells. Mitochondria, the structures that release usable energy from food, are widely accepted to be descended from bacteria that came to live inside another cell more than a billion years ago. Chloroplasts in plants have a similar origin in photosynthetic bacteria. This idea, called endosymbiotic theory, was championed in the late twentieth century by the biologist Lynn Margulis, and it was controversial until the evidence accumulated.

The evidence is strong because it comes from several directions. Mitochondria and chloroplasts contain their own small circular DNA, similar to bacterial DNA. They are surrounded by two membranes, consistent with one cell having been engulfed by another. They divide by splitting, much as bacteria do, rather than being built from scratch by the cell, and their ribosomes resemble bacterial ones. Comparing their DNA sequences places them close to particular groups of living bacteria. Over time most of their original genes moved to the host nucleus, turning a partnership into permanent dependence.

Plants need nitrogen to build proteins and DNA, and the air is mostly nitrogen gas, yet plants cannot use it directly because the two atoms in each molecule are held by a very strong triple bond. Plants in the pea and bean family solve this through mutualism with soil bacteria often called rhizobia. The bacteria enter the roots and live inside small swellings called nodules. There they use an enzyme to convert nitrogen gas into ammonia, which the plant can use. In return, the plant supplies the bacteria with sugars made by photosynthesis.

The partnership has a delicate detail. The enzyme that fixes nitrogen is damaged by oxygen, so the plant makes a protein related to haemoglobin that binds oxygen and keeps levels low around the bacteria. It gives healthy nodules a pink colour inside when cut open. Farmers have used the benefits long before anyone understood them. Crop rotations that included clover, beans or peas left soil richer for the next crop, and today legumes are still planted partly to reduce the need for manufactured nitrogen fertiliser.

Fig trees have one of the most tightly linked partnerships known. A young fig is not really a fruit but an enclosed cluster of tiny flowers turned inward, with only a narrow opening at one end. Bees cannot reach those flowers. Instead, each fig species is pollinated mainly by its own species of tiny fig wasp, often only a millimetre or two long. A female wasp carrying pollen squeezes through the opening, often losing her wings on the way, pollinates the flowers inside and lays eggs in some of them, then dies inside the fig.

Her offspring develop inside the flowers where eggs were laid, while other pollinated flowers produce seeds. Wingless males hatch first, mate with the females and chew exit tunnels. The young females gather pollen and fly off to find another fig of the right species at the right stage, carrying the partnership forward. Both sides pay a cost. The tree gives up some flowers to wasp larvae, and the wasp depends entirely on the tree. Because different fig trees fruit at different times of year, many tropical forest animals rely on figs when other food is scarce.

Some parasites do more than feed on a host. They change its behaviour in ways that help the parasite complete its life cycle. In tropical forests, a fungus called Ophiocordyceps infects carpenter ants. As the infection develops, the ant leaves its usual trails, climbs into low vegetation and clamps its jaws onto a leaf vein before dying. The fungus then grows a stalk from the ant's head and releases spores onto the forest floor below, where other ants are likely to walk. Researchers have found that the final bite tends to happen at a fairly consistent height and time of day, suggesting remarkably precise control.

Snails have their own version. A flatworm called Leucochloridium develops inside land snails and pushes swollen, brightly striped sacs into their eye stalks, where the sacs pulse and twitch rather like small caterpillars. Infected snails also tend to spend more time in exposed, well-lit places instead of hiding. Birds that peck at the pulsing stalks swallow the parasite, which matures inside the bird, and its eggs leave in the bird's droppings, ready to be eaten by another snail. Cases like these show that parasitism can reach beyond stealing nutrients into the nervous system and behaviour of the host.

Symbiosis shows how strongly the lives of different species can become connected. Mutualism, commensalism and parasitism describe common patterns, but real interactions can shift with changing conditions. Follow the benefits and costs for each organism, and the labels become much easier to understand.