MUTUALISMS
A mutualism is an interaction between two different species that benefits both participants in very different ways. The more accurate description of this interaction would be a reciprocal exploitation because not only are the goods exchanged very different but the two individuals are participating solely for their own benefit. It should be remembered that mutualisms are always sliding on a spectrum and can turn parasitic. The loss of a mutualist, which is becoming increasingly common, could be detrimental to a species and put it at risk of extinction depending on the severity of dependence on the lost mutualist. This page will go through some of the categories of mutualisms as well as provide examples for each.
POLLINATION
Pollination is a classic example of an extremely common mutualism. The two participants are a flowering plant and a pollinator, which you can learn about the many species that act as a pollinator on the pollination page. The pollinator participating in exchange for food while the plant is receiving sexual reproduction via the transfer of pollen to conspecifics (same species). It is important to remember that neither species is doing their task for the benefit of the other species. While mutualisms by nature benefit both species they can still have an unbalanced dependence on each other. For example, plants that have coevolved with a pollinator are much more dependent on the presence of that pollinator than the pollinator is on the presence of that plant species. This is because birds, like the swordbilled hummingbird, can be generalized to feed on nectar from a variety of plants but since that swordbilled is the only pollinator of the trumpet flower, then the plant could not reproduce if the bird suddenly went extinct.
Swordbilled Hummingbird
Image by Cornell Lab of Ornithology
Swordbilled about to feed on nectar
Image by National Audubon Society
Bacteria
Bacteria are probably surprising mutualists of plants but they can be comparable to the microbiome of our gut. Even though nitrogen makes up the majority of our atmosphere, it is not biologically available, so organisms need different ways to absorb nitrogen that is essential for proteins and enzymes that organism's need to survive. Plants need a lot of enzymes to photosynthesize efficiently which in turn means they need a ton of nitrogen for those enzymes. Bacteria have a lot of metabolic diversity and today there are 19 genera of bacteria that can make nitrogen gas available to plants. The bacteria are housed in nodules on the roots of the plants, these nodes do not have oxygen because the enzyme that bacteria use to convert nitrogen cannot come into contact with oxygen. The bacteria receive products of photosynthesis (sugar) as a food source in exchange for the nitrogen it gives to the plant. However, if nitrogen suddenly becomes abundant in the soil, nitrogen fixing bacteria can become parasitic and simply take in the sugars the plant needs to grow without exchanging nitrogen. This parasitism can be prevented by the plant because it can allow oxygen to enter the nodes thus killing off the bacteria.
Nitrogen Fixing Bacteria Nodules
Image by Khan Academy
FUNGI
Fungi is a similar mutualist to plants but it provides phosphorous and water in exchange for sugars. When a species of fungus provides this exchange its term is "mycorrhizae." Mycorrhizae are believed to have evolved as soon as plants colonized land because they suddenly faced the challenge of acquiring nutrients and water from the soil. The fungus grows either on the outside (ectomycorrhizae) or infects the inside (endomycorrhizae) of the roots. This fungi grows rapidly and provides an extension from the roots of the plant which increases surface are allows for the plant to have more access to water. In contrast to nitrogen-fixing bacteria, it is not extremely costly to the plant to host the mycorrhizae; since a mycorrhizal relationship is not as costly it tends not to turn parasitic.
Mycorrhizal Fungi
Image by Pennington
Seed Dispersers
As you may have learned on the seed dispersal page, seeds can be dispersed by mammals through consumption of fruit and dispersal via feces. However a wide variety of animals can act as seed dispersers such as ants, fish, and elephants. This mutualism is extremely important to the persistence of many flowering plants. In the tropics, frugivorous fish are responsible for the dispersal of many tropical trees however, the over-harvesting of certain species could put the future of the tree species at risk of extinction. Ants act as seed dispersers for seeds that have something called an elaiosome, which is a fat rich structure on a seed that serves as a food source. The ant will disperse the seed to a location where it will feed only on the elaiosome and leave the seed to germinate in the future. This relationship is being threatened for many species that are negatively impacted by climate change and human alteration of the landscape, and it is important that we conserve these important mutualisms.
Frugivorous Fish
Image by Jill Anderson
