Chapter 9

Mutualism

Harned Hall 302 963-5782

Everglades Cactus (Stenocereus sp.)

 

Sections

Mutualism

Relationship between two organisms that benefits both

mutualisms carry both costs to each partner and benefits as well

mutualisms are favored when the benefits are greater than the costs, so it is the net benefits (or benefit cost ratio) that determine the outcome of these interactions

  • Obligatory - organisms cannot survive in the absence of the other partner
  • Facultative - organism can lead an independent existence

Note that:

  • mutualistic relationship does not have to be symmetric
    • one organism may be obligated to the mutualism, while the other can live without its mutualistic partner
      • many stony corals do not feed at a rate to sustain themselves when they lose their algal partners
      • the algal partners can usually grow and reproduce outside of the corals
  • mutualisms may or may not be symbiotic
    • lichen fungi and lichen algae are only found together - symbiotic
    • plants and pollinators are only in contact when the pollinator is feeding - not symbiotic

Diffuse Mutualisms

  • these are called diffuse because the strength of the connection between any two species is not as strong as when each species has one and only one other species with which it can form a mutualism
  • may involve many species
    • yeast living together often have mutualisms in which each can feed from the activity of the others present and the interaction may involve three or more species
  • each partner may interact with more than one other partner
    • the species involved in the mutualism may change from place to place or through time
    • some plants have many species as pollinators, including birds, bats, and insects

Modeling Mutualism

We can modify the logistic equation to model mutualism, just as we did for competition. The difference this time is that we assume that the presence of a mutualist has the opposite effect that the presence of a competitor did. A mutualist will increase the carrying capacity of the environment, and the size of the effect will increase as the number of mutualists does.

  and 

 

  • The equations above do this (notice how close to the Lotka-Volterra equations they are). Each equation is identical to the Lotka-Volterra equation but the sign of the alpha term has been changed to positive. Thus, addition of the mutualist species adds to the total number of individuals that can be sustained in the population (K is supplemented, not reduced as it is in competition).
  • These equations can be solved to get a zero-isocline and analyzed in the same fashion as in the competition model.

    and

    • Now the lines have a positive slope, but K1 and K2 haven't changed.
  • Facultative Mutualism - in the model graphs below, both species are facultative mutualists. This means both have a positive carrying capacity because each could exist in the environment without the presence of the other. But the slope of the zero isoclines is positive, so each goes to the right and up from K1 and K2.

    • the + and - signs indicate the growth rate of each species when the system is in a particular region (the first of the pair is always species 1, the second is always specie 2)
      • notice that, in both graphs, the + region is consistent. For instance, all space to the right of K1 is negative for species 1, indicating that in this region the population size of species 1 is too great, it is beyond the carrying capacity, even accounting for the presence of he other species (which increases the populations size in this case).
        • If you go to the notes for the chapter on competition, you will see that the + and - regions are consistent between these graphs and the graphs generated by the Lotka-Volterra equations
      • Now to the key point. What conclusions can we draw from each graph.
        • The Run Away graph on top describes a case in which the zero isoclines do not cross. This means there is no point at which both species are just maintaining their population sizes. In other words, there is no equilibrium point with both species present. What happens, if you follow the vector changes around (see the book), is that, no matter where you begin in this system, you end up with a run-away situation, in which one species always increases the size of another species, which, in turn, increases the size of the first, which increases the size of the second . . . until both species hit infinite population sizes.
        • The Stable graph on the bottom has an intersection point. where both species' growth rates are 0, so that it is an equilibrium point. If you follow a point around, you will see that it is a stable equilibrium point and no matter where you begin, you always end up at the stable point. Thus, we can see that a stable mutualism is possible even with these simple equations.
  • Obligate Mutualisms - in the graphs below, both species are obligate mutualists. This means that neither can exist in the environment unless the other species is present at some critical population size. In this case, the obligate nature of the relationship can be seen in the fact that the carrying capacity for each species is below 0. It is hard to interpret a negative carrying capacity, but I suppose one might take the magnitude of K as a measure of just how unfavorable the environment is.

    • In the Collapsing graph on top, there is no equilibrium point with both species present. If you use the + and - signs to follow a point through time, you will see that, no matter where you start you end up at the origin
      • this means that neither species can exist in this environment, no matter how many of the mutualists are present.
    • In the Unstable graph on the bottom, there is an equilibrium point, where both species can exist in the environment even though neither could be there alone. If you follow a point around, you will see that you never go back to the intersection of the lines, so that this in an unstable equilibrium
      • this means that this obligate mutualism is unstable, and any change in the population size of either member of the mutualism will mean the collapse of both populations.
  • The finding of greater instability for obligate mutualisms may be an outcome of the model that is not well supported in nature, as many obligate mutualisms exist
    • Note that the book has some more sophisticated models, in which the zero isoclines are not straight line but are curved, and that this opens up the possibility of stable obligate mutualisms
    • this might mean that nature is more complex than our simple models, but I think we should at least explore how things might interact with the simple models before going on to more complicated models..

Mutualism and evolution

  • Coevolved mutualisms have become more specialized through time because a change in one partner may lead to a change in the other, which may lead to a change in the first partner, which then . . . etc.
    • Coevolution is the process of evolutionary change in two species in which each changes in response to change in the other species
  • Coadaptation is a characteristic which helps enable the mutualism by interacting with some feature of the other partner
    • example is the communication that goes on between roots and nodulating bacteria
  • Coadaptations need not be the product of coevolution
    • Serendipity - good fortune due to chance - can also bring together two organisms that already have features that make their mutualism possible
  • Conflict within Mutualisms
    • Stable mutualisms must prevent cheating by a partner (getting benefit, bearing no cost)

Importance of Mutualisms

  • Mutualism once thought to be important in the way nature worked
    • Allee and the isopods
      • Showed that terrestrial isopods (pillbugs or rolly-pollys), which are very susceptible to desiccation, survived longer in groups than when alone when the soil got dry
      • Allee effect is still used to indicate an situation in which animals are better able to survive and reproduce in groups than when alone
      • interpreted this to mean that organisms often cooperated for mutual benefit
      • makes cooperation as important as competition and predation (more negative interactions)
  • Mutualism fell out of favor:
    • Competition/predation studies became more common
    • Theory predicted either that mutualist populations became infinite in size or that an equilibrium was unstable (tended to go to extinction when perturbed from equilibrium point)
      • Correlated point is that you never see three-way mutualism (where there must be three partners present) and theory predicts that instability goes up very sharply as the number of partners increases
  • Many feel that some mutualisms get their start as parasitic relationships and that evolution of the system may, under certain conditions, favor mutualism as the final outcome

Examples of Mutualisms

Pollination

  • Pollinator may get:
    • Food (nectar, pollen- high energy or high protein food)
    • Mating advantage - some bees get scent molecules
    • Nesting materials - some bees get wax for their nests
  • Flowering plant gets:
    • Efficiency of pollen transfer (compared to wind)
    • Mixing of pollen from many plants and prevention of inbreeding
  • Pollinators include flies, bees, wasps, bats, beetles, birds
    • any animal that visits the flower regularly may be a pollinator

May be a very "tight", highly coevolved relationship or a diffuse relationship

  • Examples of diffuse systems
    • Many flowers in the fields in Tennessee are visited by more that a dozen species of insect, all of which may act as pollinators (I have seen 10+ species of insect visiting a flowering fruit tree at the same time)
  • Examples of highly coevolved systems
    • Orchids and pollinators
      • many orchids are pollinated by a single species of insect
      • flowers of orchids are often shaped so that only the correct insect can get to the nectar and so will carry the pollen
    • Fig- wasp
      • there are many species of fig - they produce many flowers enclosed in a capsule (we call the capsule and its contents a fig)
      • each has its own species of wasp (called Agaonid wasps)
      • the female wasp lives all of its larval life in fig and only spends enough time out of one as an adult to disperse to the next fig, where she will deposit her eggs and never leave (only its progeny will)
      • males never leave the fig in which they hatched, grew as larvae, and pupated
      • fertilize females in same fig and die there, never having left it
      • the fig must supply food for its wasps or it will not produce a new generation
      • wasps must not overexploit the resource or they will eat the fig and it will never produce the next generation of fig plants
      • neither species can enter a new environment without the other
    • Yucca - moth
      • similar to fig story - each species of yucca is pollinated by a single specie of moth which lives only on the species of yucca that it pollinates

Some plants and some animals cheat

  • some animals may take nectar but do not carry pollen
    • some insects are unable to get to the bottom of deep, vase-like flowers but simply drill through the base of the flower to steal nectar
  • some plants look just like other, nectar-producing flowers, and so trick the pollinator into visiting them without the cost of rewarding it
    • some plants have flowers that look and smell like females of insects. They attract the males, who mate with the flower and carry away pollen

Dispersal Mutualisms

  • Fruits are plant rewards for animal dispersal of seeds
  • Seeds often pass through the guts of dispersers without harm
    • some seeds even benefit from this by being deposited with the manure as a fertilizer
    • some seeds use the passage as a signal to germinate and will not do so without this
    • some plants protect the seed with toxins while making the fruit palatable
      • peach seeds (pits) are full of cyanide
    • some plants sacrifice some seeds to dispersers (seeds are usually very good food - lots of vitamins, protein and lipids)
  • Lots of cheaters in this system (whenever seeds are eaten as food and are not just passing through the gut)
  • Fruit colors are important signals
    • make fruit apparent to dispersers (advertisements)
    • green fruit often contain same toxins as other part of plant to stop herbivory
      • when ripe, color change signals readiness in that the fruit has:
        • lost it toxins
        • been stocked with sugars

Cleaning Mutualisms

  • one species gets food by removing (and eating) ectoparasites of another
  • partner loses its parasites without having to clean itself
    • happens on reefs where cleaner shrimp clean parasites from fish at "cleaning stations"
    • also on reefs, cleaner fish perform same function as shrimp
    • oxpecker birds eat parasites from outside of large herbivores (cattle, antelope, rhinoceros)
      • although they keep the ticks, etc. off, this may not be a mutualism, as the oxpecker will peck a vulnerable area (often an ear) and drink blood when parasites are not available

Defense Mutualisms

  • one species gets food and/or shelter from another species
  • other partner gets protection from being eaten
    • Ant-Acacia system
      • Bull Thorn Acacia provides:
        • place for ants to live in swollen base of thorns (hence the name bull-thorn)
        • food for ants in form of special extension of leaves call Beltsian bodies
      • ants are aggressive and attack almost anything that comes into provide protection from
        • other insect herbivores
        • large, vertebrate herbivores (including you, if you happen to lean on the tree)

Bacteria - Aphid, Leaf Hopper Mutualism

  • Aphids and leaf hoppers feed on sugary sap sucked  directly from the phloem tubes of plants
    • sap is a poor diet that is high in sugars, low in amino acids
    • insects have essential amino acids, just like us, and so they cannot live on this diet without help
  • Bacteria live inside special cells called bacteriocytes in the fat bodies of the aphid and leaf hopper
    • Bacteria receive sugars from plant via the aphid and supply the aphid with amino acids
    • Bacteria also receive easily-made amino acids from insect and transform them into essential amino acids that the insect cannot make
  • Without bacterial mutualists, aphids and leaf hoppers could not live as they do, so this is an obligate mutualism for the insects
  • Bacteria are adapted for only one environment, inside insect cells, and so they are also obligate mutualists but they might have begun the relationship as parasites

Ant - Aphid Mutualism

  • Aphids are protected by ants
  • Ants get sweet plant sap from aphids
  • Ants are like ranchers, as they move the aphids from place to place on the plant to take advantage of where most sap is available

Ant - Fungus Mutualism

  • Leaf Cutter Ants cut pieces of vegetation and carry it back to their nest
    • Chew the plant into a mush on which the fungus grows
    • Ants eat the fungus, not the plant that they cut
  • fungus not found anywhere else
    • grows best at temperature maintained in the center of the nest
  • ants remove competing fungi and bacteria
    • fungus is a monoculture - whereas most fungi must live with other, competing species of fungi
  • when the young queens found new nests, they carry a inoculum of fungi to start the new fungal "farm"
  • obligate mutualism

Lichens

  • Many fungi are lichenized, each one needs a particular species of algae
    • each algae species usually can form a lichen with several different species of fungi
    • because the fungus is the unique partner in each lichen, it is the fungal name that becomes the lichen's name
  • Fungi get photosynthate from algae
  • algae get minerals and some desiccation protection and dispersal from fungi
  • Obligate mutualism

Plants - Mycorrhizae (and some bacteria)

  • Very common and very important mutualism - these fungi can be 50% of the microbial biomass in soils
    • two important types:
      • Ectomycorrhizae - many species of both Ascomycota (ascus-forming fungi) and Basidiomycota (club-spored fungi), the two largest fungal groups - many common mushrooms are the reproductive structures of ectomycorrhizal fungi
        • wrap hyphae around roots, do not penetrate cell walls of plant cells
        • hosts are trees (many conifers) in temperate or boreal systems
      • Vesicular-Arbuscular Mycorrhizae (VAM, sometimes the Vesicular part is dropped and they are called just AM) - come from a few genera of Zygomycota (the group bread molds belong to)
        • hyphae have no walls (septae), so the entire mycelium (all the thread-like hyphae) are essentially a single cell (this condition is called coenocytic).
        • hyphae penetrate the cell walls and split into lots of bifurcations that end in vesicles (swollen tips), but the hyphae do not penetrate the cell membrane, which folds inward to accommodate the fungal growth
        • almost any plant that does not have an ectomycorrhizal association will have a VAM association (the majority of plants by far)
        • have BLO's inside their hyphae - first called Bacteria-like organelles, now known to be intercellular bacteria - role in the system not known at this time
  • Plants get several benefits -
    • minerals from absorptive power of fungi
      • hyphae of fungi increase the absorptive area of roots by penetrating the soil much more finely than the roots can
      • growth rate and reproduction of plants often much lower if mycorrhizae are removed
    • protection from pathogens, both bacterial and fungal
    • some plants  even get their carbohydrates from mycorrhizae (see orchid section below)
  • Fungi get photosynthate from plant
  • Facultative mutualism, except  for orchids
  • Orchids and Orchid Mycorrhizae
    • all Orchids have important pollinator mutualisms with insects (see above) and also important fungal mutualisms
    • orchid seeds are tiny and have little stored resources (fats, carbohydrates, proteins) for the germinating embryo
    • Orchid mycorrhizae in soil (or on surface of a plant for epiphytic orchids) penetrate the seed coat and trigger germination of the seed, then supply the young plant with sugars and proteins until it becomes  photosynthetic and can return the favor
    • some orchids are non-photosynthetic and the mycorrhizae continue to supply sugars and proteins that they get by penetrating the plants the orchid is growing on - in this case the fungus is a parasite of one plant (the tree) and a mutualist of another (the orchid) at the same time!!!!!

Plants - Nitrogen-Fixing Bacteria

  • Nitrogen is a form useable by plants (nitrate, nitrite, or ammonium) is the product of the metabolism of other organisms
    • N2 is plentiful in atmosphere but useless to plants
    • the process of making N2 into organic nitrogen (as the above forms of N are collectively called) takes lots of energy
    • bacteria (Azotobacter, Azobacter, some Pseudomonas species, some blue-green algal species), and some soil fungi are free-living microbes that can fix nitrogen
      • they are all anaerobes and live in regions of the soil where oxygen has been depleted
  • Some plants have a mutualism with bacteria to transform atmospheric nitrogen into organic nitrogen -a very important mutualism
    • Lack of nitrates (and derived compounds) often limits plant growth in terrestrial ecosystems
    • Ability to produce organic N locally is a great advantage in  nitrogen-poor soils
    • many plants in the Fabaceae (also called the Leguminosae - the pea family that includes peas, beans, clover, alfalfa, honey locust trees, and many more trees) and other families can nodulate
    • Rhizobium is the genus of bacteria that participate in nodulation
  • Presence of bacteria causes plant roots to nodulate
    • Nodules provide bacteria with a place to live and an environment conducive to their growth
    • Plant responds to chemical signals produced by bacteria
      • secretes chemical attractants for the bacteria, which migrate to root and enter it
      • presence of the bacteria and their secretions promotes cell proliferation by plant to make the nodule
  • Plants pay a price for a ready supply of organic N
    • supply photosynthate to bacteria for growth and for the expense of fixing N
    • must also maintain the proper, oxygen-depleted environment for fixation
      • nitrogenase, the enzyme that catalyzes the fixation, is sensitive to the presence of oxygen
        • oxygen fits into its active site as well (or even better) than does nitrogen, so it poisons the process if it is present
      • the plant and bacteria have a coadaptation that produces the low oxygen environment needed
        • oxygen is soaked up by the presence of a compound, Leghemoglobin, that binds to oxygen
          • Leghemoglobin is related to our hemoglobin, both through structure and ancestry
          • the protein portion is produced by the plant from genes in its nucleus
          • the heme portion is produced by the bacterium with enzymes encoded by genes on its chromosome
  • this is a Facultative mutualism
    • pea family plants all grow without nodules (but more slowly)
    • bacteria grow in soil without pea plants (but much more slowly)

Hard Corals - Algae

  • Corals get photosynthate from algae
  • algae get minerals extracted from sea by animals
    • free-floating algae are "trapped" in the water drop in which they float
      • only get nutrients that diffuse into their neighborhood and diffusion is a very slow process
    • algae in corals are fixed and waves pass by
      • they extract nutrients from many gallons of water each day, not just from the drop in which they are floating
    • the difference is huge
      • waters surrounding reef are usually very clear - indicating that they have little algal growth (low productivity)
      • reefs are as productive as tropical rain forests, among the most productive systems on earth
  • Facultative/Obligate symbiosis
    • Algae can leave when conditions not right (bleaching of coral)
    • Coral can feed by predation on plankton (but growth is slow or even negative)

Giant Clam - algae

  • Clam gets photosynthetic output of algae
  • algae get minerals absorbed by clam and protection from herbivores

Yeast-Drosophila Mutualism

  • Yeast need to disperse from habitat patch to patch
    • Yeast spores are not resistant to desiccation so they must be carried
  • Insects need high protein diet
    • plants often low in protein, which are needed for making eggs as adults (even when eaten by larvae)
    • most yeast grow in dead plant material
      • yeast are much higher in protein than the plant tissue they eat and so are high quality food for insects
  • Cactophilic yeast- Drosophila
    • 10-20 species of fly, all found only in cacti
    • 20-30 species of yeast, most found only in cacti
    • mutualism is diffuse but obligatory
  • Ambrosia Bark beetles
    • tunnel into bark of some species of trees
    • each beetle species carries an inoculum of 1 or 2 species of yeast as it goes from tree to tree each generation
    • yeast lines tunnels of beetles and grow on plant sap
    • beetles eat sap and yeast, with almost all protein in diet coming from yeast

Commensalism

Situations in which one species benefits from the presence or activity of another species, but the other species gains no benefit nor suffers any loss

Phoresy

  • when one organism attaches itself to another as a means of dispersal
  • common way to disperse seeds, animals not harmed
    • small animals hitchhike on larger animals
    • Bird - Pollen mite
      • when birds drink from a flower, pollen mites (feeding on the pollen in the flower) jump on their beaks and nestle into their nostrils
      • mites jump off at next flower without harming bird

Burrowing animals often have commensal organisms living in the burrows

  • Can happen after the burrow is abandoned
    • many vertebrates live in burrows made by other species
  • Can happen when host is still living in burrow
    • Clams in worm burrows on mudflats
      • Clams are found no where else, so this is obligate for them
      • No evidence that the host worm benefits or is harmed by presence of clam

 

Terms

Mutualism, Commensalism, Competition, Allelopathy, Herbivory, Predation, Parasitism, Ammensalism, symbiosis, Obligatory, Facultative, Diffuse Mutualisms, Coadaptation, coevolution, Allee effect, Pollination, Orchids and insect pollinators, Fig - Agaonid wasp, Yucca - Yucca Moth, cheating, Dispersal Mutualisms, Cleaning Mutualisms, Defense Mutualisms, Ant - Acacia, Beltsian bodies, Ant - Aphid, Ant - Fungus, Lichens, Mycorrhizae, Ectomycorrhizae, Vesicular-Arbuscular Mycorrhizae, Nitrogen-Fixing Bacteria, Hard Corals - Algae, Giant Clam - algae, Cactophilic yeast- Drosophila, Ambrosia Bark beetles, Phoresy, Bird - Pollen mite, Clams - worm

Last updated September 27, 2006