Mutualisms

Question 1

Define a mutualism.

Answer 1

Reciprocally positive interactions between 2 species.

Question 2

Mutualisms can be a) obligate or b) facultative. What do these mean?

Answer 2

a) The species must enter a mutualism to survive

b) Entering into a mutualism is beneficial but not essential for survival

Question 3

Mutualisms can be direct or indirect. True or false?

Answer 3

True.

Question 4

Give 2 examples of negative interactions between organisms.

Answer 4

1. Competition

2. Predation

Question 5

How does indirect mutualism work?

Answer 5

It involves 2 species and the resources they thrive on. Species 1 eats lots of resource 1, so there is less competition for resource 2. Resource 2 then thrives, and species 2 can feed on it. Thus Species 1 has indirectly benefitted species 2.

Question 6

Generally what causes mutualisms to occur?

Answer 6

Negative conditions.

Question 7

Nˆ 1 = K1 +α12 Nˆ2 is the equation for species 1 in a mutualism. Explain the terms.

Answer 7

N^1 = density of 1 at equilibrium
K1 = carrying capacity of 1
α12 = effect of species 1 on species 2
N^2 = density of 2 at equilibrium

Question 8

Nˆ 2 = K2 +α21 Nˆ1 is the equation for species 2 in a mutualism. Explain the terms.

Answer 8

N^2 = density of 2 at equilibrium
K2 = carrying capacity of 2
α21 = effect of species 2 on species 1
N^1 = density of 1 at equilibrium

Question 9

There is a linear relationship between N^1 and N^2. True or false?

Answer 9

True.

Question 10

At equilibrium unbounded growth can be reached. Why is unbounded growth good in a mutualism?

Answer 10

Because it reflects on to both species in a positive multiplier effect - if one species grows so should the other.

Question 11

At equilibrium, Nˆ 1 = K1 + α12K2/1 - α12α21.

Answer 11

okay…

Question 12

If there is no unbounded growth, what does it indicate about the mutualism?

Answer 12

It is weak.

Question 13

In obligate mutualisms one species cannot grow without the other. What does a) below threshold and b) above threshold mutualism lead to?

Answer 13

a) Extinction

b) Instability. Positive feedback density dependence is unstable.

Question 14

Lokta-Volterra models are not accurate when discussing mutualisms. This is because they predict unbounded growth of both populations. What is the problem with this?

Answer 14

A mutualism between 2 species is not always linear. At very high levels the ‘saturation benefit’ is reached in which other factors come into play, like territoriality, thus growth is no longer linear.

Question 15

Give a classic mutualism concerning insects.

Answer 15

Pollination: the pay-off to the insect is nectar and to the plant is seed dispersal.

Question 16

In pollination, insects always have only one plant species they enter into a mutualism with. True or false?

Answer 16

False. For example honey bees are non-specific and enter into mutualisms with hundreds of plants.

Question 17

Give an example of a specific pollinator relationship.

Answer 17

Figs and fig wasps: there are approx. 700 species of fig, each with 1 or 2 species of fig wasp.

Question 18

Plants and fungi enter into mutualisms. What do they both gain from this?

Answer 18

1. The plants receive soil nutrients from the fungi

2. The fungi receive photosynthates from the plant

Question 19

How do fungi transfer nutrients to the plant?

Answer 19

Via the hyphae, a series of branching filaments.

Question 20

What percentage of all known plant species exhibit mutualism?

Answer 20

80%

Question 21

What percentages of plant families exhibit mutualism?

Answer 21

92%

Question 22

Where do mutualistic relationships between plants and fungi most commonly occur? Give examples.

Answer 22

Environments with poor soils, e.g. deserts or tropical rainforests

Question 23

Why is the soil quality poor in a tropical rainforest?

Answer 23

Because there is such a high turnover rate of nutrients. Microbes decompose everything v. quickly in the warm, wet conditions, and there are so many plants that these are used up almost immediately.

Question 24

Some orchids are non-photosynthetic and cannot fix carbon. How do they survive?

Answer 24

They are obligate mutualists with mycorrhiza fungi. The fungi form a network and take carbon from other plants and transfer it to the orchids. Thus the orchids are indirectly parasitic.

N.B. these orchids can/will not germinate without fungal infection.

Question 25

What is an arbuscular mycorrhiza?

Answer 25

A fungal species that penetrates the cortical cells of the roots in vascular plants.

Question 26

What kind of mutualisms do arbuscular mycorrhizal fungi enter into?

Answer 26

Facultative.

Question 27

How much of its fixed carbon does a plant transfer to arbuscular mycorrhiza in mutualism?

Answer 27

20%

Question 28

How do ectomyccorhizal fungi transfer nutrients to plants?

Answer 28

Their hyphae form a sheath around the plant roots.

Question 29

Which is more common, arbuscular mycorrhiza or ectomycorrhiza?

Answer 29

Arbuscular.

Question 30

What percentage of plant families display mutualisms with ectomyccorhizal fungi?

Answer 30

10%

Question 31

What effect do fungal mutualisms have on species diversity? Give an example.

Answer 31

They increase species diversity. It was found via experiments with labelled carbon (to trace nutrient flow) that carbon fixed by birch trees was transferred to Douglas firs via a fungal network. These Douglas firs were in the shade and were unable to photosynthesise themselves, thus the mutualism maintained species diversity.

Question 32

Mycorrhizal fungi can mediate competition between plants. What tends to happen in a) intraspecific and b) interspecific competition?

Answer 32

a) The larger individual is favoured

b) The species in the mutualism is favoured

Question 33

Pinus radiata (pine tree) growth is often limited by shade. In an experiment saplings were kept in 2 types of cylinder, one without holes and one with holes. The ones with holes thus had access to fungi. What happened to the trees?

Answer 33

The ones with access to fungi had more above-ground biomass although height was not affected, and the ones without fungal access had a higher mortality rate.