Ecology: mutualism and commensalism

Explain positive interactions in ecology

In positive interactions, no species is harmed and the benefits are greater than the costs for at least one species.

How does each partner in a mutualistic interaction act?

Each partner in a mutualistic interaction acts in ways that serve its own ecological and evolutionary interests.

How do positive interactions effect the abundances and distributions of populations as well as the structure of ecological communities.

Mutualism and commensalism can increase growth, survival, or reproduction of individuals of interacting species; thus impacting species distribution and abundance, and community composition.

Mutualism (+/+)

A relationship between two species in which both species benefit. Net benefits must exceed net costs for both partners.

Commensalism (+/0)

A relationship between two organisms in which one organism benefits and the other is neither benefited nor harmed.

Symbiosis

Two species live in close physiological contact with each other. Can be (+/+), (+/-), (+/0)

Paratism

(+/-) one organism lives on or in another and harms it

mycorrhizae

symbiotic associations between the roots and various fungi

How do the mutualistic relationship between fungi and plants (mycorrhizae) benefit each other?

Mycorrhizae fungi increases the surface area for the plant to take up water and soil nutrients. The plants provide the fungi with carb.

Ectomycorrhizae

A type of mycorrhizae in which the mycelium forms a dense sheath, or mantle, over the surface of the root. Hyphae extend from the mantle into the soil, greatly increasing the surface area for water and mineral absorption.

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

Arbuscular mycorrhizae

The fungus grows between and penetrates the cell walls of some root cells, forming a branched network called an arbuscular.

Describe obligate positive interactions

Both species cannot live without each other

common in mutualistic relationships

clear signs of co-evolution

Provide an example of obligate mutualism

The leaf cutter ants and fungus cannotsurvive without each other, and both haveevolved unique features that benefit the otherspecies

facultative positive interactions (not obligatory)

Show few signs of coevolution

Commensalism is always facultative for the species that does not benefit

Provide an example of facultative positive interaction

In deserts, the shade of adult plants creates cooler, moister conditions. Seeds of many plants can only germinate in this shade. The adult is called a nurse plant and does not benefit.

Interactions can be categorized by the outcome for each species (+, -. or 0). But costs and benefits are not constant, they change over time and space. An interaction that's beneficial today can turn into harm tomorrow as conditions change. Give an example of this.

At low temperatures, growth of Myosotis increased when Typha was present (Typha had a positive effect on Myosotis; commensalism)

At the higher temperatures, presence of Typha decreased growth of Myosotis (Typha had a negative effect onMyosotis; competition).

As the condition changes, the interaction is no longer beneficial to Myosotis.

How do studies assess the relative importance (magnitude) and frequency (how often) of positive interactions in species that are neighbors of each other?

Using Relative Neighbor Effect (RNE)

The difference between the performance of a target species when its neighbors present (PNP) vs when its neighbors absent (PNA)

=> RNE = PNP - PNA

If RNE is positive, the present of neighboring species benefits the target species. vice versa.

What is performance a measured of?

Performance is a measured of change in plant biomass or leaf number in the presence/absence of neighbors.

What is the value of RNE at high-elevation sites?

+

What is the value of RNE at low-elevation sites? Explain.

-

Take plant for an example. Low-elevation sites provide a relatively warm and low stress environments comparing to high-elevation sites, allowing plants to grow and compete with one another, competition occurs instead of commensalism.

Trophic mutualisms

Mutualist receives energy or nutrients from its partner.

Habitat mutualisms

One partner provides the other with shelter, living space, or favorable habitat.

Service mutualisms

One partner performs an ecological service for the other.

Services include: pollination, dispersal, and defense against herbivores, predators, or parasites

Are both species in a mutualistic relationship provide benefits to the other because they are altruistic?

No. Both partners take actions that promote their own best interests. A mutualism evolves and is maintained because the net effect is advantageous to both partners.

Describe cheaters in a mutualistic relationship

Cheaters are individuals that increase offspring production by overexploiting their mutualistic partner.

Provide an example that explain how mutualists avoid overexploitation.

In an obligate mutualism between a yucca plant and its exclusive pollinator, the yucca moth, the female moth collects pollen in one yucca and lays eggs in another, depositing the pollen in this flower. BUT, larvae complete development by eating the seeds in the flower. Cheating can occur if moths lay too many eggs and the larvae consume too many seeds.

Penalty for cheating: Yuccas can selectively abort flowers with too many eggs before the moth larvae hatch, killing all larvae present.

How do positive interactions influence the distribution of species?

When species A (dominant species) provides species B with a favorable habitat, it allows species B to live, grow, and reproduce there. So it can be said that where there is species A, there is species B => this positive interaction b/w A and B affect the distribution of species B.

Example: seaweed creates a moist, cool environment that allow tide so crabs can survive where they otherwise could not.

Seaweed, in this example, is the dominant species.

Provide an example that explain how positive interactions influence community composition and diversity.

In the service mutualisms between cleaner fishes and other species, the cleaners remove parasites for other fishes (clients), enhancing their survival. Without the cleaners, the number of client species and the number of individuals within a species decrease because of parasites => negatively affecting community diversity and species abundance.