Species Interactions - Mutualism and Commensalism

Lecture 11 (Page 3 on exam 2)

Symbiosis

A relationship between two species in very close physical contact (can be positive, negative or neutral)

Examples:

• Pea aphids and their bacterial symbionts; humans and bacteria

• Can include parasitism (+/–), commensalism (+/0), and mutualism (+/+).

• Symbiotic organisms do not cause disease, but parasitic ones can and do

Commensalism

A relationship between two species where one species benefits and the other is unaffected by the interaction (+/0 relationship)

Examples:

• Bacteria live on human skin

• Cattle egrets eat insects stirred up by cattle when grazing

• Owls use tree holes as shelter

• Tree frogs use plants for protection

Mutualism:

Mutually beneficial interaction between individuals of two species (+/+ relationship)

Habitat mutualisms

A relationship between two species where both benefit and one of the species is provided with shelter, living space, or a favorable habitat.

Example: Pistol shrimp dig burrows that they share with goby fish. The goby gets a refuge and, in turn, serves as a “seeing-eye fish” for the nearly blind shrimp

Trophic mutualisms

A relationship between two species where both benefit and one or both are provided with energy, nutrients, or food in some form

Facilitation

A sometimes transient (temporary) interaction between two species where one allows for the existence of the other by amending the environmental conditions

Mycorrhizae

Symbiotic associations between the roots and various fungi

Ectomycorrhizae

The fungus grows between root cells and forms a mantle around the root

Arbuscular mycorrhizae

The fungus penetrates the cell walls of some root cells, forming a branched network called an arbuscule

Symbionts

Organisms that live in or on other organisms.

Some positive interactions are highly species-specific and obligate

not optional for either species

Relative Neighbor Effect (RNE)

the growth of the target plant species when neighboring plants are present, minus its growth when neighbors are removed

Cheaters

Individuals that increase offspring production by overexploiting their mutualistic partner. If this happens, the interaction probably will not persist

Describe the difference between mutualism and commensalism

Mutualism and commensalism are both symbiotic relationships, meaning two distinct species live in close association.

The main difference between mutualism and commensalism is that, in mutualism, both associated species benefit, whereas in commensalism, only one species benefits while the other is neither harmed nor helped.

An Ant–Plant Mutualism

The ant–acacia partnership is an example of an obligate and coevolved mutualism.

Acacia ants (Pseudomyrmex spinicola) tending to larvae and pupae inside an acacia thorn. (B) A nectary at the base of a leaf and Beltian bodies at the leaflet tips. (C) Ants have removed the plants that grew near this acacia, creating a competitor-free zone for the plant. Acacias without ant colonies are often killed by herbivores in 6–12 months and the ants can’t survive without the trees.

Neighbors Increase Plant Growth at High-Elevation Sites

At high-elevation sites, neighbors also tended to increase survival and reproduction of the target species.

Neighbors had the opposite effect at low-elevation sites.

RNE was negatively related to maximum summer temperature, suggesting that positive interactions were more common in colder, more stressful environments and that competition was more common in warmer, less stressful environments.

Soil temperature determines whether wetland plants are commensals or competitors:

In an experiment, cattails (Typha) and a forget-me-not (Myosotis) that lacks air spaces were grown at different temperatures.

At low temperatures, soil oxygen increased, allowing growth of Myosotis when cattails were present, but at higher temperatures, Typha decreased the growth of Myosotis.