Chapter 15- Mutualism and Commensalism

Case Study: Fungus growing ants

started cultivating fungi for food at least 50 million years before the first human farmers

-the ant farms nourish, protect, and eat the fungal species they grow, forming a relationship that benefits both and neither can survive without the other

-the fungi are cultivated in underground gardens. A colongy may contain hundreds of gardens, each the size of a football, they can feed 2-8 million ants

Positive interactions

are those in which one or both species benefit and neither is harmed

Positive interactions have influenced

key events in the history of life and continue to shape communities and influence ecosystems functions

In positive interactions, neither species is

harmed and the benefits are greater than costs for at least one speices

Facilitation

mutualism

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

Commensalism

individuals of one species benefit; individuals of the other species do not benefit but are not harmed (+/0 relationship)

Symbiosis

two species live in close physiological contact with each other

sometimes there is a cost to one or both partners, but the net effect is positive

Symbioses can include

parasitism (±)

commensalism (+/0)
mutualism (+/+)

Mycorrhizae

symbiotic associations between the roots and various fungi

most plants (80% angiosperms and all gymnosperms)

Fungi and plant interaction

-the fungi increase the surface area for the plant to take up water and soil nutrients ( over 3m of fungal hyphae may extend from 1 cm of plant root)

-the plants supply the fungi with carbohydrates

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 net work called an arbuscule

Ex) corals form a mutualism with symbiotic algae (zooxanthellae)

-the coral provides the algae with a home, nutrients (nitrogen and phosphorous), and access to sunlight

-the algae provides the coral with carbohydrates produced by photosynthesis

Coral bleaching

occurs when coral polyps expel algae that live inside their tissues

Coral bleaching causes

• increasing temperature

• pollution (nutrients, sediments)

• increased exposure to sun and air

Ex) Rumen microbes

-behind every successful cow are millions of gut microorganisms- mostly bacteria, protozoa, and some fungi. This bastion of bugs that resides in a cow’s 20 gallon rumen are ultimately responsible for digesting all the plant material the bovine consumes.

the cow supplies the bugs with nutrients and the bugs convert cellulose into usable energy for the cow

Commensalism is everywhere

-millions of species form +/0 relationships with organisms that provide habitat

-linchens grow on tree

bacteria on human skin

in kelp forests, many species depend on the kelp for habitat and do not harm the kelp

Different types of ecological interactions can

-evolve into commensalism or mutualism

Mutualism can arise from a host-parasite interaction

Ex) a strain of amoeba infected by a bacterium was studied. The two species coevolved so that within 5 years, the bacterium was harmless to the amoeba and the amoeba was dependent on the bacterium for metabolic functions

Spanish Moss

-is not actually a moss but a bromeliad ( as are pineapples)

-grows on trees (epiphyte), but no negative effect unless it becomes so abundant it reduces photosynethsis

-native to this region

-absorbs pollutants (used as environmental indicator)

-creates habitat

Positive interactions can be

1. obligate

2. facultative

Obligate

-not optional for either speices

-show clear signs of coevolution

-common among mutualistic relationships

Obligate Example

-the leaf cutter ants and fungus can’t survive without each other, and both have evolved unique features that benefit the other species

Facultative

-show few signs of coevolution

-commensalism is always facultative for species that doesn’t benefit

Facultative Example

-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

Deer consumes seeds of a herbaceous plant. The seeds pass through the deer unharmed and undigested, and are deposited in the feces, becoming an additional dispersal mechanism for the plant. This type of interaction is ____ from the deer perspective

facultative commensalism

Trade offs

interactions, and the balance between costs and benefits can vary in space and time

when interactions no longer provide a net benefit, they may cease to occur

Positive interactions may be more common in

stressful environments (high elevations, intertidal zones, wetlands)

Relative neighbor effect (RNE)

target species’ performance with neighbors present minus its performance with neighbors removed

EX) a group of ecologists looked at effects of neighboring plants on 115 target species in 11 different regions

Positive values= neighbor increased growth

negative values= neighbor decreased growth

-At high elevation was more positive values

Each partner in a mutualistic interaction acts in ways

that serve its own ecological and evolutionary interests

Mutualism are categorized by the type of benefits that result

1. trophic

2. habitat

3. service

Trophic mutualism

mutualist receives energy or nutrients from its partner

trophic mutualism example

leaf cutter ants and fungus, mycorrhizae

Habitat mutualisms

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

Habitat mutualism example

Pistol shrimp dig burrows they share with a goby fish. The goby gets a refuge and in turn serves as a seeing eye fish for the nearly blind shrimp

Service mutualism

one partner performs an ecological service for the other

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

Mutualists are not

altruistic

they are in it for their own benefit

There are costs involved but

net benefits must exceed net costs for both partners

If environmental conditions change

and benefit is reduced or cost increased for either partner, the outcome may change particularly for facultative interactions

How do mutualists avoid overexploitation?

if this happens, the interaction probably will not persist

Penalties may be imposed on cheaters

cheaters

are individuals that increase offspring production by overexploiting their mutualistic partner

Ex) in an obliate mutualism between a yucca and its exclusive pollinator

-the yucca moth, the female moth collects pollen in the one yucca and lay eggs in another, depositing the pollen in this flower

-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

-Penality: yuccas can selectively abort flowers with too many eggs before the moth larvae hatch

• retain 62% with flowers of 0-6 eggs, only 11% of flower with more than or equal to 9 eggs

Positive interactions affect the

the abundances and distributions of populations as well as the composition of ecological communities

Abundance ex) ants and acacia trees

-the ants live in large thorns on the tree and feed on nectar and high-protein Beltian bodies produced by the tree

-in exchange, ant workers patrol the three 24 hours a day, aggressively attacking insects, herbivores, and competitors

Including from the perspective of the ant, as well as the perspective of the acacia tree. What type of mutualism occurs?

-trophic

-habitat

-service

Studies compared acacias with and without ants:

Acacias with ant colonies weighed over 14 times more than those without

-72% with ants survived compared to 43% without

-Bottom line: ant- plant mutualism can increase the abundance of both species

Distribution example

In rocky intertidal zones, many species live under seaweed. The seaweed creates a moist, cool, environment at low tides so crabs, snails, starfish, urchins, barnacles can survive where they otherwise could not

Diversity example

One of the most common interactions in coral reefs is cleaner fish- small fish remove parasites from larger fish (and get a meal in the process)

On the Great Barrier Reef, cleaner fish were experimentally removed from five small reefs

-parasites on client fish increased rapidly

-after 18 months the diversity of fish species on the reef decreased, as did total abundance