Lecture week 13 (11-14) - Notes Mutualistic Interactions.docx

Biol 114 Fall 2023

Lecture week 13 (11/14)

Mutualistic Interactions

1. Introduction
   1. 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 (+ / +).
   2. 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.
   3. 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.
   4. 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.
2. Commensalism
   1. 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.

• 1. Examples: Burdocks,
  2. 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.
  3. Examples: Brown-headed Cowbird

1. Mutualism
   1. 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.
         1. 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.
         1. 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).
   2. 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
   3. What would the world be like without mutualisms?