Mutualism

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Mutualism

• Chapter 17, pages 321-334, 6th Ed.
• Chapter 27, pages 579-594, 5th Ed.

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So, What is MUTUALISM? Mutualism is a
relationship in which reciprocal selection
pressures benefit two species. This reciprocal
selective pressure on 2 interacting populations
us called co-evolution.
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Types of Mutualism

• Mutualism can be symbiotic or non-symbiotic,
  facultative, or non-facultative
• In symbiosis 2 organisms live together in close
  physical association from which 1 or both derive
  benefit and at least 1 cannot live an independent
  life (this relationship is obligate).
• E.g. intestinal parasites of birds mammals.

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Types of Mutualism

• In non-symbiotic mutualism both members of the
  pair benefit, but they dont live together. They
  can survive w/o each other but it benefits both
  to have a relationship.

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Types of Mutualism

• obligate-symbiotic mutualism at least one member
  cannot exist without the other.
• Classic example is between algae and fungi
  symbiosis in the lichens.

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• obligate-symbiotic mutualism at least one member
  cannot exist without the other.
• Classic example is between algae and fungi
  symbiosis in the lichens.
• A thin layer of algae 2-32 cells form between the
  two layers of a fungal mass.
• Algae receive only protection from damaging solar
  radiation dessication.
• Fungi lives off of algal production (fungi grow
  slower in absence of algae)
• There are many other examples involving
  Obl.Symb.mutualism with root sheaths and fungi.

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Types of Mutualism
Obligate non-symbiotic mutualism mutualists live
physically separate lives, but cant survive w/o
each other. E.g. pollination seed dispersal
offer many examples.
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• Ants Acacia
• Ants live in swollen thorns of acacia.
• Ants get about 100 of diet from sugar secreting
  nectories on the plant.
• Ants in turn protect the plant from herbivores
  like cattle. At the slightest disturbance they
  swarm, emitting repulsive odors and attack the
  intruder until it is driven off.

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Types of Mutualism
facultative mutualism most mutualisms are
nonobligatory and most are facultative
(opportunistic) at least on one side. Widely
evolved in seed dispersal and pollination.
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Types of Mutualism
defensive mutualism perennial rye grass and tall
fescue are infected by certain fungal endophytes
that live inside plant tissues. fungi produce
alkaloides that give grasses bitter taste and
toxicity to grazers. Fungi also stimulate
plant growth and seed production.
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Types of Mutualism
Indirect mutualism Example If predation of X
on A favors B, then predator Y benefits. Fiel
d studies by Wilburt Fauth (1980) supported
this, if for e.g. species X went extinct, species
y also declined.
prey
A
B
inhibits
X
Y
predator
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Pollination

• Goal of pollination is to transfer its pollen
  from the anthers of one plant to the stigma of a
  con-specific.
• Some disperse by wind works well and costs
  little when plants grow in large homogenous
  stands (e.g. corn fields). It is unreliable when
  plants are scattered.

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Pollination

• Scattered plants generally rely on animals for
  pollen transfer, mostly insects with some
  nector-feeding birds and bats.
• Plants attract bees, hummingbirds, etc. by
  putting out attractive smells, bright flower
  colors, and usually a promise of nectar or oil
  reward that animals use as food.
• In return, the plants get their pollen spread to
  conspecifics by the animals.
• Some orchids have no reward, but they may have
  the shape color pattern of female bees to
  attract males.

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Seed Dispersal

• Plants with seeds too heavy to disperse by wind
  rely on animals to carry seeds some distance from
  the parent plant and deposit them on sites
  favorable for development.
• Some of these animals are seed predators (plants
  must produce high numbers of seeds knowing most
  will be sacrificed to get a few to germinate).
• Some such as (stick-tights) attach to animal fur
  and later are dropped or rubbed off.
• Some animals cache seeds for winter, but dont
  find all of them before they germinate.
• Some seeds require a processing trip through an
  animals digestive tract to achieve optimal
  viability.

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Fruit

• An alternate to seed dispersal is fruit.
• Enclosing seeds in nutritious fruit that is
  attractive to fruit-eating animals (frugivores).
• Eat only the endocarp around the seeds.
• Unripe fruits are discouraged from consumption by
  cryptic coloration (green fruit like apples among
  green foliage), unpalatable texture, hard
  outercoats.

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Population Effects of Mutualism

• It is easy to consider mutualism at the
  individual level, but difficult at the population
  level.
• We can model the population theoretically like
  the Lotka-Volterra model for competition.
• Due to the strong and feedback of one species
  on the other, this could be de-stabilizing.
• Therefore a strong, negative feedback of
  density-dependence, or a 3rd species that is a
  competitor or predator of one of the mutualists
  is needed to stabilize the populations.