Positive interactions

15

Mutualism and

Commensalism

Chapter 15 Mutualism and Commensalism

CONCEPT 15.1 In positive interactions, neither species is harmed and the benefits are greater than the costs for at least one species.

CONCEPT 15.2 Each partner in a mutualistic interaction acts in ways that serve its own ecological and evolutionary interests.

Chapter 15 Mutualism and Commensalism

CONCEPT 15.3 Positive interactions affect the abundances and distributions of populations as well as the composition of ecological communities.

Introduction

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

CONCEPT 15.1

In positive interactions, neither species is harmed and the benefits are greater than the costs for at least one species.

Concept 15.1

Positive Interactions

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).

Concept 15.1


Symbiosis : Two species live in close physiological contact with each other.

• Examples: pea aphids and their bacterial symbionts; humans and bacteria

• Symbioses can include parasitism

(+/ –), commensalism (+/0), and mutualism (+/+).

Concept 15.1


Benefits of positive interactions can take many forms —food, shelter, transport, etc.

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

For each species, the benefits are greater than the costs.

Concept 15.1


Mutualistic associations are everywhere.

Most plants form mycorrhizae — symbiotic associations between the roots and various fungi.

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

Figure 15.3 Mycorrhizal Associations Cover Earth

’s Land Surface

Concept 15.1


The fungi may also protect the plants from pathogens.

The fungi improve plant growth and survival in a wide range of habitats.

The plants supply the fungi with carbohydrates.

Concept 15.1


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 network called an arbuscule.

Figure 15.4 Two Major Types of Mycorrhizae

Concept 15.1


Commensalism is also everywhere.

Millions of species form +/0 relationships with organisms that provide habitat.

• Lichens grow on trees

• Bacteria on human skin

• In kelp forests, many species depend on the kelp for habitat and do no harm to the kelp.

Concept 15.1


Countless insect and understory plant species live in tropical rainforests and depend on the forests for habitat, yet many have little or no effect on the trees.

Concept 15.1


Different types of ecological interactions can evolve into commensalism or mutualism.

Mutualism can arise from a host –parasite interaction.

Concept 15.1


In a strain of Amoeba proteus that was infected by a bacterium, the bacterium initially caused the host cells to be smaller, grow slowly, and it often killed them (Jeon 1972).

But parasites and hosts often coevolve, each in response to selection pressure imposed by the other.

Concept 15.1


Five years later, the bacterium had evolved to be harmless to the amoeba; the amoeba had evolved to be dependent on the bacterium for metabolic functions.

Various tests showed that the two species could no longer exist alone.

Concept 15.1


Many mutualisms and commensalisms are facultative and show few signs of coevolution.

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.

Concept 15.1


One species of nurse plant may protect the seedlings of many other species.

Desert ironwood serves as a nurse plant for 165 different species.

The nurse plant and beneficiary species may evolve little in response to one another.

Concept 15.1


Interactions can be categorized by the outcome for each species:

• Positive (benefits > costs)

• Negative (costs > benefits)

• Neutral (benefits = costs)

But costs and benefits can vary in space and time.

Concept 15.1


Example: Soil temperature determines whether wetland plants are commensals or competitors:

• Wetland soils can be anoxic. Cattails aerate soils by passively transporting oxygen through continuous air spaces in the leaves, stems, and roots.

• Some of this oxygen becomes available to other plants.

Concept 15.1


In an experiment, cattails ( Typha ) and a forget-me-not ( Myosotis ) that lacks continuous air spaces, were grown at different temperatures.

At low temperatures, soil oxygen increased when cattails were present, but not at the higher temperatures.

Figure 15.8 A Wetland Plant Aerates the Soil under Some Conditions

Concept 15.1


At low temperatures, growth of Myosotis increased when Typha was present

( Typha had a positive effect on

Myosotis) .

At the higher temperatures, presence of

Typha decreased growth of Myosotis

( Typha had a negative effect on

Myosotis).

Figure 15.9 From Benefactor to Competitor

Concept 15.1


Studies to assess the importance of positive interactions:

• Performance of a target species with neighbors present is compared to performance when neighbors are removed.

Concept 15.1


A group of ecologists looked at effects of neighboring plants on 115 target species in 11 different regions.

Performance was measured as change in biomass or leaf number.

Relative neighbor effect (RNE) = target species ’ performance with neighbors present minus its performance with neighbors removed.

Concept 15.1


RNE was generally positive at highelevation sites: neighbors had a positive effect on the target species.

RNE was generally negative at lowelevation sites.

Figure 15.10 Neighbors Increase Plant Growth at High-Elevation Sites

Concept 15.1


At high-elevation sites, neighbors also tended to increase survival and reproduction of the target species.

Neighbors had the opposite effect at lowelevation sites.

Figure 15.11 Negative Effects at Low Elevations, Benefits at High Elevations

Concept 15.1


Because environmental conditions tend to be more extreme at high-elevation sites, these results suggest that positive interactions may be more common in stressful environments.

Similar results have been found in intertidal communities.

CONCEPT 15.2

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

Concept 15.2

Characteristics of Mutualism

Mutualisms are categorized by the type of benefits that result.

Trophic mutualisms : Mutualist receives energy or nutrients from its partner.

Examples:

• Leaf-cutter ants and fungus

• Mycorrhizae

Concept 15.2


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

Example: Pistol shrimp dig burrows that that 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.

Figure 15.12 A Seeing-Eye Fish

Concept 15.2


Service mutualisms : One partner performs an ecological service for the other.

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

Example: The fig –fig wasp pollination mutualism.

Concept 15.2


Although both partners in a mutualism benefit, there are also costs.

In the coral –alga mutualism, cost to the coral includes supplying nutrients and space; cost to the alga is giving up some carbohydrates it could use for itself.

Concept 15.2


Sometimes the cost is providing a

“ reward ” for a service.

Example: During flowering, milkweeds use up to 37% of the energy gain from photosynthesis to produce nectar to attract insect pollinators.

Concept 15.2


In a mutualism, 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.

Concept 15.2


The plant Medicago truncatula can discriminate among mycorrhizal fungi, allocating more carbohydrates to the fungal hyphae that are supplying the most nutrients.

Figure 15.14 Rewarding Those Who Reward You

Concept 15.2


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

If this happens, the interaction probably will not persist.

Several factors contribute to the persistence of mutualisms.

Concept 15.2


“ Penalties ” may be imposed on cheaters.

In an obligate mutualism between a yucca 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.

Larvae complete development by eating the seeds in the flower.

Figure 15.15 Yuccas and Yucca Moths

Concept 15.2


Cheating can occur if moths lay too many eggs and the larvae consume too many seeds.

But yuccas can selectively abort flowers with too many eggs before the moth larvae hatch.

Figure 15.16 A Penalty for Cheating

Concept 15.2


The partners in a mutualism are not altruistic.

Both partners take actions that promote their own best interests.

In general, a mutualism evolves and is maintained because the net effect is advantageous to both partners.

CONCEPT 15.3

Positive interactions affect the abundances and distributions of populations as well as the composition of ecological communities.

Concept 15.3

Ecological Consequences of Positive Interactions

Mutualism and commensalism can increase growth, survival, or reproduction of the interacting species.

Concept 15.3


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 tree

24 hours a day, aggressively attack insect and mammal herbivores, and even destroy plant competitors.

Figure 15.18 An Ant

–Plant Mutualism

Concept 15.3


To determine the benefits for the acacias,

Janzen (1966) removed ants from some and compared them to trees with ants.

Acacias with ant colonies weighed over

14 times as much as plants without ant colonies.

They also survived better and were attacked by insect herbivores less frequently.

Concept 15.3


Acacias without ant colonies are often killed by herbivores in 6 –12 months.

The ants also cannot survive without the trees.

Both species have evolved unusual characteristics that benefit the other species.

Concept 15.3


Positive interactions also influence community composition.

Many coral reef fish have service mutualisms with smaller organisms

(cleaners) that remove parasites from the fish (clients).

The benefit the client receives is greater than the energy benefit it could gain by eating the cleaner.

Concept 15.3


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

Parasites on client fish increased rapidly.

After 18 months, the number of fish species on the reef decreased, as did total abundance.

Figure 15.19 Ecological Effects of the Cleaner Fish Labroides dimidiatus

Positive interactions

Mutualism and

Commensalism

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Mutualism and

Commensalism

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