According to Duckster.com (a popular learning website for grade-schoolers), biology is the branch of science that studies life and living organisms. Including such subjects as the cell, genes, inheritance, microorganisms, plants, animals, and the human body.

Around 8.7 million species of plants and animals share the planet. Species cross paths with species in a complex web of interactions—some beneficial, others not. Evolution on earth has shaped many close and sometimes unusual alliances between the different creatures and plants that inhabit it and share or compete for the same resources. 

These relationships can take many forms that are collectively known as symbiosis. 

The term “symbiosis” comes from the Greek words “symbios,” which means living together with another, and “bios,” or life. It describes the close ecological relationship or interaction of two or more different species, which results in both species benefiting from the relationship, only one species benefiting at the other’s expense or neither species benefiting.

Based on this definition, ecologists generally classify symbiotic relationships into five types.

The Main Types of Symbiotic Relationships

1. Mutualism

Mutualism is a symbiotic relationship where both species benefit. These species were part of each other’s environment and adapted to it, evolving together by “making use of” each other in a mutually beneficial way.

This symbiotic relationship’s interaction patterns occurs in three forms: 

• Obligate mutualism

In this relationship, one species requires contributions from the other and can’t survive apart from the other. It is entirely dependent or rely on the other for its survival.

For instance, lichen is a composite organism (organisms made up of two or more independent organisms) and is actually made up of photobiont (i.e., algae or cyanobacteria) living among the filaments of a mycobiont partner (fungus or multiple fungi species). In this relationship, the fungus provides a stable structure where the algae or cyanobacteria can thrive, and in return, the photobiont provides photosynthetic energy for the fungus.

Another example is the relationship between yucca moth and yucca plants. Yucca fruits can only develop from flowers pollinated by female yucca moths, which have special tentacles on its mouth to gather and carry pollen. Then yucca moths lay their eggs in yucca flowers, and when these turn into fruits, the moth larvae feed on it. 

• Diffusive mutualism

In this symbiotic relationship, one species can live with more than one symbiotic partner. It is not entirely dependent on the other and can survive without its partner because it derives benefits from multiple species.

This is the case with plants that have many species as pollinators, including insects, birds and bats. So, if one species stopped pollinating the plants, it will lose the benefit it derived from that particular species but will still survive, thanks to its relationship with the other species.

• Facultative mutualism

One species can survive on its own under certain favorable conditions. That is, this species derives benefits from the relationship but isn’t fully dependent on the other and can survive without the other. 

This is the case of bees pollinating flowers. While the bees can pollinate multiple plant species, each plant species is also pollinated by other animal species. 

Mutualism can also be categorized according to its general purpose: 

• Trophic mutualism

A typical relationship between a heterotroph and an autotroph, trophic mutualism involves the transfer of energy and nutrients between the symbiotic partners. Trophic mutualism is also known as resource-to-resource mutualism. 

A good example of this is the relationship between cows and the bacteria living in their rumens. The bacteria helps digest plant cellulose, which cows do not have the ability to do, and in return, the bacteria receive food and a warm environment to grow.

• Defensive mutualism

This relationship occurs when one species provides protection from predators and the other provides food or shelter. In the case of ants and aphids, ants provide protection while aphids produce honeydew for the ants.

• Dispersive mutualism

This is a service-resource mutualism where one species transports the pollen of the other species in exchange for food. This relationship is typically observed between insects and other animals that acquire nectar from flowers and facilitate pollination to new areas. In the case of honey bees, pollen spreads from plant to plant as the honey bee searches for nectar from one flower to the next. 

2. Commensalism

Commensalism is a symbiotic relationship where one species benefits and the other doesn’t benefit but isn’t harmed. Examples include the remoras which get leftover food, a free ride and protection from hanging onto a shark. Barnacles, which attach to humpback whales, also get a free ride to areas rich in plankton; both feed on this microorganism. Neither the shark nor the whale received any benefit from the relationship but neither were they harmed.

Commensalism is classified into four types: 

• Chemical commensalism—A species of bacteria produces a chemical that sustains another bacteria species.
• Inquilinism—A species lives in the nest, burrow or other dwelling place of another species.
• Metabiosis—One species relies on the other to survive.
• Phoresy—A species temporarily latches onto another species for free transportation.

3. Parasitism

Parasitism is a symbiotic relationship where one species benefits while the other is harmed. The parasite lives in, on or with a host, relying on the latter for its nutrition. Though the host suffers from the loss of nutrients, it rarely dies. 

Parasites are classified as: 

• Ectoparasites—These parasites live on the surface of their host. Examples are ticks, fleas, lice, bedbugs and leeches.
• Endoparasites—They live inside the host (such as tapeworms, roundworms and flukes) and can be further categorized as: 
  • Intercellular parasites—They live in the space between cells.
  • Intracellular parasites—They live inside cells.

4. Competition

Competition describes the struggle within species (intraspecific) or between species (interspecific) for the same limited environmental resource, such as food, space, etc. Both parties are negatively affected by this relationship. 

For example, sponges are abundant in coral reefs. However, if they become too many, they compete with corals for food and other resources. Sponges may outcompete corals in the short term, but this will lead to corals dying and ultimately damaging the reef, which is bad for the sponges in the long term.

There are three types of competition: 

• Consumptive or exploitative competition—competition for a resource
• Interference competition—competition for territory
• Preemptive competition—competition to arrive first in a new territory 

5. Predation

Predation refers to the behavior of one species feeding on another to obtain nutrients. The one that gets eaten is the prey and the one that hunts and kills is the predator. Examples of predators include orcas or killer whales and lions.