4/10 pt.2: Mutualisms (REC.2)(DOC)

• Introduction to Mutualisms

  • Last lecture topic on species interactions focusing on mutualisms.

  • Key questions to consider during the lecture:

  • How did mutualistic relationships start?

  • What factors influence species to engage in such relationships?

  • How common and important are these interactions?

• Definition of Mutualisms

  • Mutualisms are defined as plus-plus relationships where both species benefit.

  • Benefits: Increased fitness from the relationship; may include additional resources like nutrients, food, or reproductive opportunities.

  • Contrast with cooperation:

  • Cooperation occurs within a species and is not technically a mutualism, despite both parties benefiting.

• Mental Exercise (Pause and Reflect)

  • Consider what the world would look like without mutualisms.

• Types of Species Interactions

  • Predator-prey (Plus-minus), parasite-host, and commensalism.

  • Commensalism is a plus-zero relationship where one species benefits while the other is unaffected.

  • Amensalism:

  • A minus-zero relationship where one individual suffers a cost with no effect on the other.

  • Example: A large herbivore crushing insects while feeding, benefiting from no cost.

• Examples of Commensalism

  • Burdock plant as an example:

  • Seeds stick to mammals for dispersal; the plant benefits but the mammal does not.

• Confusion between Mutualism and Commensalism

  • Case study: Brown-headed cowbird

  • Engages in nest parasitism, appearing commensal but may have mutual benefits.

• Distinction of Symbiosis

  • Symbiosis: Close living relationship between two organisms; not necessarily mutualism.

  • Example of symbiosis: Parasitism (obviously a plus-minus interaction).

• Examples of Mutualistic Relationships

  • Honeybee and Goldenrod Flower:

  • Bee gets nectar/pollen while aiding plant reproduction; not symbiotic as they do not live together continuously.

  • Goby Fish and Pistol Shrimp:

  • Goby acts as the shrimp’s eyes, shrimp provides shelter; more symbiotic but not strictly – they can be separated.

  • Clownfish and Sea Anemone:

  • Clownfish cleans anemone; gets protection; not strictly symbiotic but closely related.

  • Lichens:

  • Fungus provides protection to algae/cyanobacteria, which provide carbohydrates; true mutualistic and symbiotic.

• Obligate vs. Facultative Mutualisms

  • Obligate mutualism: Required for both individuals’ survival (e.g., herbivores and their gut bacteria).

  • Facultative mutualism: Not required for survival; both can live independently (e.g., honey guide and Boran people).

• Example of Obligatory Mutualism

  • Greater honey guide: Guides people to honey; both benefit, but the relationship is not essential for their survival.

• Examples of Mutualism in Plants

  • Acacia trees and ants:

  • Ants provide protection from herbivores, while acacia trees offer protein packets (Beltian bodies) as food.

  • Coral reefs:

  • Coral polyps and zooxanthellae exemplify mutualism; coral provides protection and nutrients, algae offers energy through photosynthesis.

• Nitrogen-Fixing Bacteria and Legumes

  • Rhizobium bacteria in legumes help acquire nitrogen; legumes offer carbohydrates.

• Frugivory

  • Dispersal of seeds by fruit-eating animals benefits both parties.

• Conclusion: The Importance of Mutualism

  • Revisit initial thoughts about the world without mutualisms; emphasize their importance in survival and ecosystem stability.

Lecture week 12 (4/10)

Mutualistic Interactions

I. Introduction

A. Until now, our survey of interactions has been where costs have been sustained by

one or both parties. Mutualism, on the other hand, is a relationship where both

individuals benefit from the interaction (+ / +).

B. Knowledge of mutualistic interactions has existed since the time of Aristotle.

However, mutualism has received much less attention from biologists than

interactions like competition and exploitation (predation). Is this because mutualisms

are less common than those other interactions? Is it because mutualistic interactions

are less important than them? You can be the judge.

C. A word of warning: The nature of mutualism is one of both individuals benefiting.

However, it is unwise to think of the members involved as willingly generous

individuals. Each party has evolved to provide something to the other only because

the resulting benefit to themselves is greater than the cost associated with the

production of the benefit to the partner. As soon as the costs of producing the “gift”

outweigh the benefits of receiving something from the partner, the individual should

no longer take part in the relationship. Overall, it might be better to consider

mutualism more as reciprocal parasitism.

D. Another word of warning: The word Symbiosis is often used to describe (and is often

considered synonymous with) mutualism. However, they are not necessarily the

same. A symbiotic relationship is one where the two parties are together all the time.

A symbiotic relationship may be one of mutualism, but where the partners are in

almost constant contact. For example, lichens are a symbiotic relationship of algae

and fungi. That is, they live together to form a single organism. A plant and

pollinator may form a mutualistic relationship around the pollination event, but the

two do not live together if the pollinator visits other plants. Therefore, they are not

symbionts.

II. Commensalism

A. Commensalism is a relationship where one individual benefits from the interaction,

and the other sees no benefit or cost ( +/ 0 ). Essentially, commensalisms occur

where one takes advantage of an existing behavior of another without affecting that

individual.

B. Examples: Burdocks,

C. Frequently, scientists make incorrect assumptions about the relationships between

organisms because they do not know the full details of the relationship. Sometimes

truly mutualistic relationships are considered commensalistic because we do not

know that the one member (who is not affected) is receiving some benefit. We also

may assume that both members of a relationship are benefitting, even though one is

not receiving a benefit or a cost (commensalism). This latter scenario occurs when

the relationship seems intentional by both parties, and is complex enough that we feel

that there must be benefits that we are not able to detect.

D. Examples: Brown-headed Cowbird

III. Mutualism

A. Can partners in a mutualistic relationship survive without it? Yes and no.

1. Facultative mutualism is where the individuals in the relationship can exist

without their mutualistic partner.

a. Example: Honeyguides in Africa

2. Obligate mutualism is where the individuals in the partnership are so

dependent on the relationship that they cannot survive without it.

a. Example: Cellulose-digesting bacteria.

3. Survive? The word “survive” may literally mean to live, or it may refer to the

ability to pass genes onto the next generation (i.e., reproduce).

B. More mutualisms:

1. For each of the following examples, consider the costs and benefits for each

partner. Also, think about what situations would lead to the dissolving of the

relationship (i.e., what may cause the costs of providing a benefit to a partner

to outweigh the benefits gained from the partner?)

2. Ants on Acacia Trees

3. Coral Reefs

4. Mycorrhizae

5. Frugivory

6. Rhizobium Nitrogen Fixation

C. What would the world be like without mutualisms?