BIOB50 CHAPTER 15 NOTES - Mutualism and Commensalism UTSC

Chapter 15

Mutualism and Commensalism

Ecology Biob50 

Positive Interactions -  those in which one or both species benefit and neither is harmed. 2 types

1.     Mutalism – is a mutually beneficial interaction b/w individuals of 2 species (a+/+) relationship

2.     Commensalism – is an interaction between individuals of 2 species in which individuals of one species benefit, while those of the other species don’t benefit and are not harmed (a+/0) relationship

Symbiosis – a relationship in which individuals of the 2 species live in close physiologica;l contact with each other.  This can also be a parasite and host relationship.

So symbiotic relationships can range from parasitism (+/-) commenalism (+/0), to mutalism (+/+).

Mycorrhizae – symbiotic associations between plant roots and various types of fungi that are usually mutalistic. Most plants have these.  The funus increases the surface area over which the palnts can extract water and nutrients from the soil.  2 major types

1.     Ectomycorrhizae – the fungal partner typically grows between root cells and froms a mnatle around the exterior of the root

2.     Arbuscular Mycorrhizae – the fungal partner also grows into the soil and it grows between some root cells while penetrating the cells walls of others.

The algae provide the coral with carbs produced by photosynthesis. All the numerous vertabrae and vertebrate species live in and on coral reefs depend directly or indirectly on the coral-alga mutualism.

Parasites and hosts often coevolve.

Trophic Mutualisms  -in which a mutalist receives energy or nutrients from its partner. In the leaf cutter ant –fungus mutualism, each partner feeds the other.

Habitat Mutalisms – one partner provides the other with shelter, a place to live, or a favourable habitat.

The grass example with Dichantehlium....with the curvalaria protuberata. In the lab 100% of the grass plants that had this survived intermitten soil temperatures of 60 degrees. While non eof the plants without the endophyte survived. Plants with endophytes had greater root and elaf mass than plants without the endophytes. Thus curvularia increased the ability of its grass host to tolerate high soil temperatures.  

Service Mutalisms – includes interactions in which one partner performs an ecological service for the other. Polliantion, defence against herbivores predators and parasites.

Cheaters – natural selection might favour these. Individuals that increase their production of offspring by overexploiting their mutualistic partner. If this happens, it becomes less likely that the mutalism will persist.

Penalties – may be imposed on cheaters that overexploit their partners.

Female Yuca mothj lays eggs on flower. But she lays too many eggs which eat too many seeds, the flower then aborts them if this happens.

The partners in a mutualism are not altruistic. In general, a mutalism evolves and is maintained because its net effect is advantageous to both parties.

Ants in the trees example – the plant has unusually large thorns, which provide a home for the ants. The ants burrow in the body of the plant and establish a colony. Soon they lay eggs and the colony grows so that it occupies all the thorns, and feeds on the nectar the plant secretes. The ants aggressively attack insect and even mammalian herbivores that attempt to eat the plant. Thus providing the acacia with a competitor free zone in which to grow.

The experiment showed that acacias with ant colonies on average weighed 14 times as much as acacias that lacked ant colonies. They also had higher survival rates. Thus the ant-acacia mutualism has considerable effects on the abundance of each partner. Furthermore the ant and the palnt have each evolved unseals characterstics that benefit their partner.

The effects that commensalism and mutalism have on the abundances and distributions of species can affect interactions among species.

One of the most common interactions among these diverse coral reef fish is a service mutalism in which a small species (cleaner) remove parasites from a larger fish 9the client).

Grutters work shows that a service mutalism can have a major effect on the diversity of species found in a community.

Hartnett and Wilson suggested that big bluestems dominance may have come from a competitive advantage conferred by its association with mycorrhia;l fungi, and that removal of those fungi removed that advantage and released the inferior competitors tfrom the negative effects of the competition.