Mutualism

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Mutualism is any relationship between two species of organisms that benefits both species. This is the relationship most people think of when they use the word "symbiosis."

Exampes of Mutualism

Pollination

Seed Dispersal

Corals

Lichens

Mycorrhizal Fungi

Ants and Aphids

 

Flowers and their pollinators are a common and ubiquitous form of mutualism. Pollination is a term for the sexual reproduction process in plants. In essence the male gametes (pollen) from the stamens of the flower are moved (by some process) to the female reproductive organs (the style and ovary) to create the plant seed. Pollination can occur in a number of ways. One common method of moving pollen to a receptive female flower is by wind. Conifers and grasses are typically pollinated by wind-carried pollen. In the higher plants (Angiosperms) pollination is more commonly carried out by a second species (pollinator) such as a moth, bee, wasp, or beetle. Pollinators are most commonly insects, but even birds, bats, and small mammals sometimes play a part. Wind pollination is, by its nature, haphazard and wasteful. The use of a pollinator, however, can be much more exact if the plant species can attract a pollinator, attach its pollen to it, and then get the pollinator to go to another individual of the same plant species.

To achieve this accuracy, a plant species needs to fulfill two requirements.

1. The plant must attract a pollinator. The flower is a structure designed to attract a pollinator by its shape, color, and smell. The features of a flower that we as humans appreciate so much are designed to attract a pollinator. It's doubtful that pollinators have an appreciation for the beauty of a flower. Instead the flower and its aroma provide a series of visual clues the pollinator associates with a food reward (see below). If the reward is sufficiently large, the pollinator will most likely visit other plants of the same species and pass on the pollen they received at the first plant. Pollinators pick up the pollen from the male plant organs while feeding, move on to another individual of the same species, and pass the pollen to the female organs (pistil) of the next flower of the same species.

2. A mutualistic relationship requires a positive interaction between both species of the pair. A flower gets its pollen passed from one individual to another. In turn the flower provides a food reward in the form of nectar (a sugar rich solution), or pollen as a solid food source. If the food reward is large enough, the pollinator is likely to go looking for flowers of the same plant species after it is done visiting the flowers of the first individual.

Higher plants sometimes have another mutaulistic relationship with animals, in this case birds and mammals. Once fertilization has occurred and the plant has developed seeds, the seeds must be moved away from the mother plant to germinate and grow into new individuals. If the seeds just drop near the mother plant, developing seedlings and new plants will compete with the mother plant for space and nutrients.

Some seeds are dispersed by the wind. Consider the parachute-like seeds of the dandelion. Others are moved by passively sticking to passing mammals. The burrs and sticktight seeds you gather on your clothes as you wander through fields and woods are examples. Examples of this commensalistic relationship can be found under the commensalism segment of Nearctica. A third mechanism higher plants have evolved to disperse their seeds are berries and fruits such as the raspberries in the photograph to the left of the screen. The berries or fruits are eaten by birds or mammals. They pass through the digestive system. However the seeds contained within the fruit are resistent to the animal's digestive system. Therefore they are (there is no polite way to say this) pooped by the bird or mammal some hours after the animal has moved away from the mother plant.

The photograph on the left with the purple spots is not a modern artistic masterpiece, but bird poop with seeds kindly left on the windshield of our car by a neighborhood bird.

The mutualistic relationship is clear. The birds and mammals derive a food benefit by eating the berries and fruits. The plant, in turn, disperses it seeds.

Scattered throughout the shallow waters of the tropical and subtropical oceans of the world are unique, complex, and massive ecosystems called coral reefs (learn more about coral reefs). These ecosystems are rich in species involved in a multitude of mutualistic and commensalistic relationships. However the fundamental and most important mutualistic relationship is between the corals that form the reef and the dinoflagellates (learn more about dinoflagellates) that live within the coral polyps. Corals are among the simplest of animals (learn more about corals). The coral individual (polyp) is basically a hollow cylinder fringed on top with small tentacles. The tentacles capture small organisms or detritus and sweep it into the cylinder where it is digested. Coral polyps live in massive colonies. The polyps secrete calcium carbonate (limestone), and over time the buildup of calcium carbonate creates the coral reef.

Coral reefs usually occur in nutrient poor waters and must be shallow enough for sunlight to reach them. Contained between the cells of the coral polyp cylinder are single-celled green algae called dinoflagellates. The coral polyp and the dinoflagellates form a complex mutualistic relationship. The coral polyp sweeps organic material from the water and metabolizes this material forming carbon dioxide and nitrogenous wastes. The dinoflagellates use the carbon dioxide and nitrogenous wastes in photosynthesis to form oxygen and sugars that are in turn used by the coral polyps as well as the dinoflagellates in their metabolism, reforming the carbon dioxide and nitrogenous wastes. This cyclical exchange of nutrients, oxygen, and carbon dioxide is beneficial to both the coral polyps and the dinoflagellates, a mutualistic relationship.

One usually thinks of mushrooms as organisms as decomposers living on dead and decaying plant matter. However up to a quarter of the mushrooms you see while walking through the woods actually make their living through a mutualistic relationship with the trees in the forest. A mushroom is just the reproductive structure of a far more extensive organism consisting of a highly intertwined mass of fine white threads called a mycelium. Mutualistic fungi are called Mycorrhizal Fungi. The mass of the mycelium of the fungus envelopes the roots of the tree in effect greatly increasing the soil area covered by the tree root system. The fungus aids the tree in absorbing water from the soil, increases the stability of the root system, and protects the roots from drying out and the effects of heavy metals. In return the tree provides sugars and starches to the fungus that the fungus uses in its metabolism.

The relationship between fungi and trees can be general with one fungus species entering into a relationship with several tree species. In other cases some tree and fungal species have highly specific relationship between one tree species and one fungus species. It's possible trees and forest, as we know them today could not exist without the mycorrhizal relationships between trees and fungi.

The photograph on the left consists of a mycorrhizal relationship between the fungus Amanita muscaria and a white pine tree.

Aphids are small, soft-bodied, near defenseless insects that feed on plant sap. They feed by inserting a pointed, strawlike mouth structure called a stylus into the vascular tissues (internal piping) of the plant and sucking the plant juices out. Plant sap, a combination of water and sugars, is low in other nutrients, however, and the aphid must process a great deal of plant sap in order to get the amino acids and other nutrients it needs. Most of the sugars and water, therefore, are excreted as waste through a pair of structures called cornicles located near the rearend of the insect. If you own a car and have parked it under a tree during the summer, the sticky sap you find on the car is likely to be the waste plant sap produced by aphids.

Some ant species use this excess plant sap for their own nutrition. Ants find a colony of aphids and milk the waste plant sap from the cornicles. In return the ants protect the aphids from predators and parasites. In some cases ants tend colonies almost like ranchers with their cattle, not only protecting the aphids, but moving them around from plant to plant.

In the photograph to the left a group of ants are tending a colony of aphids (the almost invisible gray spots) living on a weedy plant species.