Notes on Symbiosis: Mutualism, Commensalism, and Parasitism
Symbiosis
Symbiosis is defined as the interaction between different species.
It can occur in three primary ways:
Mutualism: Both species benefit from their close association; it is a mutually beneficial relationship.
Commensalism: One species benefits from the close association, while the other is unharmed.
Parasitism: One species benefits from the close association, and the other is harmed. The species that benefits in this relationship is specifically called a parasite.
Mutualism
In mutualistic relationships, both species thrive, contrasting with competitive relationships where both struggle and expend extra energy, and predator/prey relationships where one benefits at the expense of the other.
These mutually beneficial relationships are crucial for the survival of both involved species.
Examples of Mutualism:
Leafcutter Ants and Fungi: Leafcutter ants feed leaves to their fungus gardens. The fungi then digest the toxic leaves and convert them into proteins and sugars, which they feed back to the ants. This creates a food source for both, and the ants essentially cultivate their food supply.
Clownfish and Anemone: Clownfish gain shelter from the stinging tentacles of the anemone. In return, the anemone receives protection from predators (as the clownfish defend their territory) and food in the form of waste products from the clownfish, which helps to fertilize the anemone.
Pollination (Bees and Flowers): This is a vital mutually beneficial relationship in nature.
Pollination involves the transfer of pollen (male gametes) to ovules (containing female gametes) to enable plant fertilization.
A significant of all flowering plants depend on animals, known as pollinators, to facilitate this process.
Pollinators, such as bees, feed on the plants they pollinate, obtaining essential resources like pollen and nectar. In doing so, they inadvertently transfer pollen between flowers, allowing the plants to reproduce.
Humans and Bacteria: Humans host a vast array of beneficial bacteria, particularly in their gut.
An interesting example is found in human mother's milk, where the third largest component is not for directly nourishing the baby, but specifically for feeding the beneficial bacteria in the baby's gut.
In return, these bacteria play a crucial role in helping the baby maintain health by aiding digestion and supporting a robust immune system.
Termites and Gut Bacteria: Termites consume wood, but lack the enzymes to digest cellulose directly.
They harbor specific bacteria within their digestive systems that consume and break down the wood, releasing sugars.
Both the termites and the bacteria benefit: the termites gain access to nutrients from wood, and the bacteria get a habitat and food source.
Acacia Tree and Ants: This relationship showcases a dual benefit for both parties.
How ants benefit the tree: The ants actively remove growing vines that could compete with or harm the tree. They also aggressively fight off other insects that might attempt to feed on or damage the tree.
How the tree benefits the ants: The acacia tree provides the ants with nectar, a sugary food source. Additionally, the tree's thorns offer protected shelter, especially for ant larvae, ensuring the ant colony's safety and development.
Obligate Mutualism
Some mutually beneficial relationships are so interdependent that they are absolutely essential for the survival of both species involved.
Example: Yucca Tree and Yucca Moth: The yucca tree relies entirely on the yucca moth for pollination. In turn, the yucca moth lays its eggs exclusively inside the yucca flower, and the developing moth larvae can only feed on parts of the yucca flower. Neither species could survive without the other, highlighting the 'obligatory' nature of their mutualistic link.
Commensalism
In commensalistic relationships, one species benefits, while the other species is neither helped nor harmed.
Examples of Commensalism:
Squirrels and Trees: In temperate forests, squirrels build their homes (nests or dens) within trees. The trees provide shelter for the squirrels, but typically the trees are unharmed by this interaction.
Cattle Egrets and Cattle: Cattle egrets follow grazing cattle. As the cattle move through grass, they disturb insects, making them easier targets for the egrets to catch and eat. Unlike oxpeckers, which might eat parasitic insects directly off the cattle, egrets do not provide a direct benefit to the cattle, nor do they cause any harm.
Golden Jackals and Tigers: Golden jackals sometimes follow tigers. They benefit by eating the leftover portions of prey that tigers abandon after a kill. This behavior provides a food source for the jackals without helping or harming the tiger.
Parasitism
In parasitic relationships, one species (the parasite) benefits at the expense of the other species (the host), which is harmed.
Examples of Parasitism:
Parasitic Plants (e.g., Cuscuta): These plants obtain their nutrition directly from other plants, referred to as host plants. They achieve this by tapping into the host plant's vascular system, drawing out water and nutrients. The yellow Cuscuta plant shown is living off a green acacia plant, weakening its host.
Ticks: Ticks are parasitic arachnids that feed on the blood of their hosts. They provide no benefit to their hosts and cause harm by taking blood. Additionally, ticks are significant vectors for diseases, meaning they can transmit various pathogens to their hosts, further exacerbating the harm they inflict.