BIO120 Fall 2023
Lecture 9: Mutualism and Symbiosis
BIO120
- Symbiosis means two species live together while mutualism means two species
benefit each other
- Mutualism typically involve reciprocal exchange of goods or services between
species
• Nutritional mutualisms involve the provision of limiting nutrients to one species by
another, such as when legumes exchange carbohydrates for rhizobia’s fixed
nitrogen
• Defensive mutualisms occur when one species protects the other species against
its enemies, such as when plants exchange food for ants’ protection
• Dispersive mutualisms occur when one species moves the offspring or gametes of
another, such as when plants exchange food for animals’ seed dispersal
- One example of mutualism between humans and wild animals is that the locals in
Mozambique harvest wild honey but can’t find bees’ nests easily while honeyguides
eat bee wax and know where nests are but can’t access them easily
- The Lotka-Volterra model can be manipulated to model mutualism - instead of a
negative interspecific break term, a positive term is included
d N1 N1 α12 N2 d N2 N2 α21N1
• dt = r1 N1 (1 − + ) and = r2 N2 (1 − + )
K1 K1 dt K2 K2
• One of the problems about this model is that the positive interspecific term will
lead to unbounded exponential growth
- Therefore there are factors that limit the growth of mutualists
• A strong intraspecific competition or a third species such as a predator or a
competitor
• There may be diminishing returns to mutualism as the population grows such
that when population size is small but not when it becomes large
- Invasional meltdown is the process by which two invasive species facilitate one
another’s spread, leading to an exponential growth of both species
18
,BIO120 Fall 2023
- Mutualism can influence what species will live in a community
• Experiments have shown that if cleaner fish is removed from a cleaning station, the
area where the cleaner fish congregate and remove ectoparasites from alien fish,
then parasite abundance increases on client fish
• Similar experiments have also focused on the impacts of such mutualism on fish
diversity across the entire reef: when cleaners were removed or naturally left, the
reef fish diversity went down, when cleaners were added, the diversity went up
- Mutualists often adapt to each other in order to extract greater benefits from the
interaction, known as reciprocal adaptation
• Darwin famously predicted that epiphytic orchids must be pollinated by an insect
with a long proboscis (sucking mouthpart), such insect was later discovered to be
a Sphinx moth with a 25 cm long proboscis
- Nancy Moran’s work is one of the best studied examples of a microbial mutualism -
she intensively studied the bacterial endosymbionts of aphids
• Endosymbionts is a type of symbiosis in which one of the organism living inside
the other
• Aphids feed on phloem sap that is rich in sugars but lack amino acids, which is
provided by the intracellular bacteria Buchnera
• Buchnera are vertically transmitted, they are passed in aphid eggs from mothers to
offsprings
- Vertically transmitted endosymbionts often have tiny genomes
- Endosymbiotic bacteria lose genes that they no longer need
• Some functions are unnecessary because bacteria are no longer free-living
• Bacteria are protected inside host cells, so they no longer need genes for
immune or defence
• Other functions are outsourced to host genome
- Some mutualisms are highly specialized but most are not tightly coevolved species-
specific interactions
• Most mutualism are horizontally transmitted, partners are acquired anew each
generation
19
, BIO120 Fall 2023
• Mutualisms are rarely one-to-one interactions, usually many-to-many
- Two current areas of mutuals research include: 1) understanding networks of
interactions and microbiomes (either all the microbes living together or their
collective genome)
• Ecolgoists often visualize interactions among species as a network with the nodes
being the different species and the lines being the interactions between species
• Scientists had to culture microbes in order to study them - but new sequencing-
based methods allow the sequencing of a highly conserved genes to be used to
identify microbes
- Microbiome research is a rapidly moving field as accumulating evidence
suggests that a host’s microbiome affects its metabolism, immune system, etc.
Lecture 10: Dispersal, Metapopulations and Island
Biogeography
BIO120
- Since real populations are not closed system, individuals can disperse which allows
them to colonize new areas, escape competition and avoid inbreeding depression
• Many organisms have evolved traits that help them to disperse: animals rely on
active movement while plants disperse seeds
- Plants have evolved sweet and fleshy fruits to attract animals, which they
discard the seeds as they eat or pass the intact seed when they defecate
• Dispersal is important for postglacial colonization as most of Canada was under
ice 12000 years ago
- A metapopulation is a collection of spatially distinct populations, or patches, that are
connected via dispersion
• Metapopulation structure can allow population persistence even when local
populations go extinct as they can be reestablished by colonists
• This leads to source-sink dynamics - sinks are populations in small habitat
patches that would go extinct, except migrants from source populations rescue
these population
20
Lecture 9: Mutualism and Symbiosis
BIO120
- Symbiosis means two species live together while mutualism means two species
benefit each other
- Mutualism typically involve reciprocal exchange of goods or services between
species
• Nutritional mutualisms involve the provision of limiting nutrients to one species by
another, such as when legumes exchange carbohydrates for rhizobia’s fixed
nitrogen
• Defensive mutualisms occur when one species protects the other species against
its enemies, such as when plants exchange food for ants’ protection
• Dispersive mutualisms occur when one species moves the offspring or gametes of
another, such as when plants exchange food for animals’ seed dispersal
- One example of mutualism between humans and wild animals is that the locals in
Mozambique harvest wild honey but can’t find bees’ nests easily while honeyguides
eat bee wax and know where nests are but can’t access them easily
- The Lotka-Volterra model can be manipulated to model mutualism - instead of a
negative interspecific break term, a positive term is included
d N1 N1 α12 N2 d N2 N2 α21N1
• dt = r1 N1 (1 − + ) and = r2 N2 (1 − + )
K1 K1 dt K2 K2
• One of the problems about this model is that the positive interspecific term will
lead to unbounded exponential growth
- Therefore there are factors that limit the growth of mutualists
• A strong intraspecific competition or a third species such as a predator or a
competitor
• There may be diminishing returns to mutualism as the population grows such
that when population size is small but not when it becomes large
- Invasional meltdown is the process by which two invasive species facilitate one
another’s spread, leading to an exponential growth of both species
18
,BIO120 Fall 2023
- Mutualism can influence what species will live in a community
• Experiments have shown that if cleaner fish is removed from a cleaning station, the
area where the cleaner fish congregate and remove ectoparasites from alien fish,
then parasite abundance increases on client fish
• Similar experiments have also focused on the impacts of such mutualism on fish
diversity across the entire reef: when cleaners were removed or naturally left, the
reef fish diversity went down, when cleaners were added, the diversity went up
- Mutualists often adapt to each other in order to extract greater benefits from the
interaction, known as reciprocal adaptation
• Darwin famously predicted that epiphytic orchids must be pollinated by an insect
with a long proboscis (sucking mouthpart), such insect was later discovered to be
a Sphinx moth with a 25 cm long proboscis
- Nancy Moran’s work is one of the best studied examples of a microbial mutualism -
she intensively studied the bacterial endosymbionts of aphids
• Endosymbionts is a type of symbiosis in which one of the organism living inside
the other
• Aphids feed on phloem sap that is rich in sugars but lack amino acids, which is
provided by the intracellular bacteria Buchnera
• Buchnera are vertically transmitted, they are passed in aphid eggs from mothers to
offsprings
- Vertically transmitted endosymbionts often have tiny genomes
- Endosymbiotic bacteria lose genes that they no longer need
• Some functions are unnecessary because bacteria are no longer free-living
• Bacteria are protected inside host cells, so they no longer need genes for
immune or defence
• Other functions are outsourced to host genome
- Some mutualisms are highly specialized but most are not tightly coevolved species-
specific interactions
• Most mutualism are horizontally transmitted, partners are acquired anew each
generation
19
, BIO120 Fall 2023
• Mutualisms are rarely one-to-one interactions, usually many-to-many
- Two current areas of mutuals research include: 1) understanding networks of
interactions and microbiomes (either all the microbes living together or their
collective genome)
• Ecolgoists often visualize interactions among species as a network with the nodes
being the different species and the lines being the interactions between species
• Scientists had to culture microbes in order to study them - but new sequencing-
based methods allow the sequencing of a highly conserved genes to be used to
identify microbes
- Microbiome research is a rapidly moving field as accumulating evidence
suggests that a host’s microbiome affects its metabolism, immune system, etc.
Lecture 10: Dispersal, Metapopulations and Island
Biogeography
BIO120
- Since real populations are not closed system, individuals can disperse which allows
them to colonize new areas, escape competition and avoid inbreeding depression
• Many organisms have evolved traits that help them to disperse: animals rely on
active movement while plants disperse seeds
- Plants have evolved sweet and fleshy fruits to attract animals, which they
discard the seeds as they eat or pass the intact seed when they defecate
• Dispersal is important for postglacial colonization as most of Canada was under
ice 12000 years ago
- A metapopulation is a collection of spatially distinct populations, or patches, that are
connected via dispersion
• Metapopulation structure can allow population persistence even when local
populations go extinct as they can be reestablished by colonists
• This leads to source-sink dynamics - sinks are populations in small habitat
patches that would go extinct, except migrants from source populations rescue
these population
20
