Ecosystems in the natural world are comprised of living organisms that interact with one another in assorted ways. The term mutualism refers to a type of relationship that mutually benefits two species sharing an environment.

Living creatures have adapted interesting and unusual ways of helping each other out, although their motives are self-serving.

Symbiosis in biology refers to a close connection between different species that evolved together. A one-sided relationship that helps one species without affecting the other is called commensalism.

A one-sided relationship that benefits one species to the detriment of the other is called parasitism. A useful two-way relationship is referred to as mutualism.

Mutualism in biology refers to symbiotic species interactions that are mutually beneficial, or even essential, for survival. A mutualistic relationship forms when two different species each benefit by working closely together.

The relationship can be a bit complicated, however. For instance, one species may derive greater benefit, and the interaction could border on parasitism.

Mutualism is common in all ecosystems, including the human body. For instance, Harvard Medical School estimates that trillions of bacteria called gut microbiota live in the human intestine and aid in digestion and overall health. When a mutually beneficial relationship is close and longstanding, it is an example of mutualistic symbiosis.

Not all symbiotic relationships are mutualistic.

Mutualistic symbiosis came about through evolution. Mutualism between partner species enhances fitness to the environment and bolsters reproductive success. Organisms of different species that have adapted to adapt to each other’s behavior and traits are called symbionts. Some species have become so interdependent that they cannot survive without the other one.

When the growth, reproduction or sustenance of living organisms is intertwined, the relationship represents obligate mutualism. For instance, certain types of Yucca plants and moth species have come to depend on each other to complete their reproductive life cycle. When a regularly occurring interaction benefits organisms but isn’t essential for survival, that is facultative mutualism.

There are countless examples of mutualism on Earth. Mutualistic interactions can develop between two animals, two plants, animals and plants, and bacteria and plants, for instance.

Interspecific interactions help maintain stable populations and vice versa. Loss of one species can lead to the loss of others because of the interdependent nature of the food web.

The oxpecker is a little bird that has strong toes to grip animals’ coats, and a colorful beak perfectly shaped for dislodging parasites. Although elephants want nothing to do with the bird, the oxpecker has a longstanding mutualistic relationship with zebras, giraffes and rhinoceroses in South Africa. The birds are always on the lookout for lice, blood-sucking ticks and fleas that jump on an animal's hide.

Along with eradicating pests, oxpeckers clean wounds. Some scientists have questioned whether such behaviors are mutual or parasitic because pecking at the wound delays healing. Nonetheless, feeding on bugs, grease and earwax is a helpful grooming service.

Thus, the oxpecker and certain hoofed species are generally considered mutualistic. Further, oxpeckers sound the alarm with a screeching hissing sound when a predator is lurking in the grass, giving bird and beast more time to flee.

Flowering plants need a plant-pollinator like nectar-craving bees for reproductive success during their life cycle. Some plants and trees even need a species-specific insect for fertilization.

For instance, the fig tree and small Agaonidae wasps peacefully co-exist and gain from their interaction. Fig trees and their mutualistic species of wasps are great examples of mutualism and coevolution.

Figs are modified stems with many flowers inside that mature into seeds if fertilized. Fig flowers emit on odor that attracts a fertilized female wasp that will bring pollen and lay eggs in the fig flower before she dies. Some seeds ripen, and others provide nourishment for growing wasp grubs. Wingless male wasps mate and die, and winged females leave in search of a new fig.

Legumes, like soybeans, lentils and peas, offer an excellent source of protein in the diet. Therefore, legumes need an optimal amount of nitrogen to synthesize amino acids and build protein.

Legumes have a species-specific mutualistic relationship with bacteria. Legumes and certain bacteria meet each other’s needs without causing harm, unlike pathogenic bacteria.

Rhizobium bacteria in the soil form bumpy nodules on plant roots and “fix” nitrogen by converting N₂ in the air to ammonia, or NH₃. Ammonia is a form of nitrogen that plants can use as a nutrient. In turn, plants provide carbohydrates and a home for nitrogen-fixing bacteria.

Reliance on bacteria when growing crops like soybeans reduces the use of chemical fertilizer that can seep into the waterways and cause toxic algal blooms.

Many ecological studies have shown that birds and animals play a role in seed dispersal. Now scientists are taking a closer look at the mutualistic interactions of plants and reptiles, especially in island ecosystems. Fruit-eating lizards, skinks and geckos play a key role in plant biodiversity and viability.

Because plants cannot move, they are dependent on external means for seed dispersal. Some species of lizards gorge on pulpy fruit, along with arthropods, and excrete undigested seeds at another location. Seed dispersal reduces competition with the parent plant for nutrients and facilitates gene exchange within the plant population.

Sea anemones are an ancient species that has characteristics of a plant and animal. When unsuspecting small fish swim by, the sea anemone uses its deadly tentacles to paralyze its prey.

Surprisingly, the orange and white clownfish makes its home within the sea anemone. Clownfish have adapted a thick coating of mucus that offers protection from the sea anemone's deadly sting.

Brightly colored clown fish lure other fish to the sea anemone’s clutches, and subsequently benefit from the leftovers of the sea anemone’s meal. Clown fish also provide air circulation to the sea anemone by swimming between the tentacles. They keep the sea anemone clean and healthy by getting rid of excess food.

American researchers at Binghamton University, State University of New York recently studied the mechanisms of how mutually beneficial relationships between small organisms improve their odds of survival.

The study showed that advantages are greatest when the small organisms live in an ecosystem dominated by large organisms. Further benefit can be gained from mutualistic partnerships between three symbionts.

For example, the whistling thorn acacia tree of Africa provides nectar and habitat for ants that bite elephants who nibble on the tree. During dry spells, ants feed on honeydew excreted by scale insects that live off tree sap.

A change in one symbiont would set off a chain reaction. For example, if the ants died off, elephants would destroy the tree, and the scale insect would lose its habitat and main food source.

The various types and examples of mutualism are not fully understood. Many questions remain about coevolution and the persistence of the various types of interspecific interactions.

Much of the work to date has focused on beneficial plant and microbe relationships. Mathematical modeling may deepen understanding of the genetics and physiology of co-evolutionary phenomena in the natural world.

Predictive modeling also looks at how factors such as resource availability and proximity may influence cooperative behaviors. Data at the cellular, individual, population and community levels can be integrated with mathematical models for comprehensive analysis of ecosystem interactions. Models can be tested and reconfigured as data accumulates.