BIOL 4120

Principles of Ecology

Phil Ganter

320 Harned Hall


The flowers on this cactus are not cactus flowers.  They are the flowers of a holoparasite, a mistletoe, that lives inside of the cactus.

Lecture 15 Symbioses:  Mutualism, Commensalism & Parasitism

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Overview - Link to Course Objectives

Symbiosis, Parasitism and Mutualism


  • Parasitism is one of the +,- species interactions
  • There is no easy definition that will separate everything biologists consider parasites from herbivores or predators
    • almost never kill the host directly (although some diseases do this, of course)
    • usually live in intimate contact with their hosts (although some insects commonly considered parasites, like ticks and mosquitoes, spend much of their life span not in contact with a host)
    • tend to be  much smaller than their hosts so that one host often supports many parasites
  • Host is the organism from which the parasite or parasitoid derives its sustenance
    • some parasites have only one species as a host
      • many diseases infect only one or a couple of host species
    • some parasites can have many (usually related) species as hosts
      • ticks and mosquitoes will bite any warm-blooded animal they find
    • Host Range is the number of host species one parasite species will attack (thought to be narrower for parasites that are totally dependent on the host for all feeding)
    • some parasites require more than one host species before they can complete their life cycle
      • these parasites (many parasitic worms [Platyhelminthes and Nematoda]) have complex life cycles
      • often two different species as hosts (rarely three)
        • often the hosts are not closely related for parasites with complex life cycles
        • schistosomiasis nematode must infect both a freshwater mollusk and a vertebrate to complete the life cycle
      • host in which meiosis takes place is called the Definitive Host
      • other hosts are referred to as Intermediate Hosts
          • parasite reproduces asexually in intermediate host
    • Vectors are organisms that are necessary to transmit the disease
      • some vectors are not affected by parasite and can't be considered hosts, just vehicles to transport the parasite
      • some vectors are also hosts
    • Reservoirs are alternative hosts where the infection may remain if it is eliminated from another population (deer are reservoirs of eastern equine encephalitis for human populations)
  • Parasites can be divided into:
    • Ectoparasites that remain outside of the host's body
    • Endoparasites that enter the host's body
    • Holoparasites (used for plants only) plants that parasitize other plants and no longer photosynthesize but get all water and food from the host (ex: Dodder, Dutchman's Pipes)
    • Hemiparasites (used for plants only) plants that parasitize other plants for water and minerals, but photosynthesize to make their own food (ex: Mistletoe)
        • Don't confuse hemiparasites with epiphytes, plants that grow on other plants but do not invade their tissues to steal water and nutrients (ex. many orchids
  • Effect of Parasite on host
    • Parasites may kill (as when a disease kills its host)
    • Parasites may reduce host fitness through lost growth or lost reproduction due to stress from harboring parasite
    • Parasites may sterilize the host
    • Parasites may alter the hosts phenotype
        • some parasites change the sex of the host
        • some parasites alter behavior of the host so that the host acts to benefit the parasite (at its own expense)
  • Parasites
    • Come from almost all taxonomic groups
      • parasitic bacteria. plants, fungi, protists, and animals
    • Attack all kingdoms, including bacteria (which have viruses)


  • 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
  • Mutualisms can be:
    • Obligatory - organisms cannot survive in the absence of the other partner
    • Facultative - organism can lead an independent existence
  • 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
    • Example of asymmetry -
        • 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
  • Importance of Mutualisms
    • Mutualism once thought to be among the important interactions among species
      • Allee and the terrestrial 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 that conditions that favored mutualisms were narrow and unlikely to be found in nature.
    • Current view has restored mutualisms as important in natural systems
      • important mutualisms identified (mycorrhizae, nodulation, arthropod-bacterial nutritional mutualisms, etc.)
      • theory modified to widen the conditions under which mutualism is favored by selection.

Relationship between parasitism and mutualism

  • Many parasitic relationships evolve to reduce the cost for the host
    • benefits the host in that the parasites do less damage
    • benefits the parasite in that there will be more hosts to parasitize if they are not excessively harmed by the parasite
  • If the process crosses the line where the benefits outweigh the costs, a parasitism can become a mutualism
  • This model of the origin of mutualisms solves the "origin problem."
    • How do mutualisms begin if the benefits only arise when the partners come together?  Were the adaptations serendipitously already there? 
      • This seems not likely due to the prevalence of mutualisms.
    • Did the first insects to visit plants intend to be pollinators?  Probably not, they were probably just foraging.
      • Plants that attracted the insects to their reproductive structures benefited from the accidental pollinations
        • From the accidental beginning, where the plants were mostly harmed by the insects, the elaborate pollination systems (including flowers, floral rewards, specialized animal behaviors, etc.) developed over time to increase the benefits of the interaction to both partners.
  • Probably not the origin of all mutualisms


  • a relationship between individuals of two different species in which individuals of one species lives on or in individuals of the other species
  • 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
  • parasitic interactions may or may not be symbiotic
    • parasitic tapeworms can only grow and reproduce in the gut of a vertebrate and only leave one host to get to another - symbiotic
    • mosquitoes spend as little time on their hosts as possible (for obvious reasons) - 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.



  • 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.


    • 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.

Modeling Parasitism

Epidemiology is the science that studies disease

  • Infectious diseases are caused by parasites and are the most intensively studies parasite systems (due to their importance to our health)
  • parasites are referred to as pathogens in epidemiology

Factors in the spread of disease

  • Note that the parasite population is not usually studied, but the number of infected hosts is studied
    • If all parasites are in one host, and it dies, the pathogen population dies, no matter how large it is
    • If a smaller number of parasites are spread into many hosts, the death of a single host will not eliminate the pathogen
  • The variables below are needed to model the spread of a disease
    1. S = the density of susceptible hosts (notice that density is usually used here, not total population size)
    2. B = transmission rate (depends on virulence of disease, mode of transmission, and host behavior)
    3. L = the average period during which a host will be infectious
    4. Rp = the replacement rate of infected hosts (note that it is not the R0 of the parasite, as there may be lots of parasites and lots of parasite reproduction in each host). However, a bit like R0, if Rp is less than 1, the rate of infection is such that there are fewer and fewer infected hosts as time goes on, if Rp is equal to 1 then the number of cases is stable, and if Rp is over 1, then the disease is increasing in incidence

    We can relate these factors with the following equation:

Rp = SBL

Some consequences:

  • The longer the host is infective, the greater the replacement rate of parasitized hosts, so there is pressure on the parasites to keep the host alive (increase L, increase Rp)
  • High transmission rates (large B) leads to greater replacement rate of parasitized hosts, so there is pressure on the parasites to evolve greater rates of transmission (increase L, increase Rp)
  • Given the limited resources of the host, it may not be possible to do both of the above

An important consideration (for the parasite) is NT

NT = threshold population of susceptible hosts at which Rp = 1 and below which Rp <1

If Rp in the first equation is set to 1, then

Some consequences:

  • If NT is not constant, an increase in either transmission rate or infectious period will reduce the size of the host population needed to maintain the parasite
  • If NT is constant, then an increase in one parameter (either transmission rate or infectious period) will lead to a decrease in the other parameter (in other words, an increase in transmission rate will reduce the infectious period and vice versa)

Epidemiologists often try to define NT so that they can predict the critical density of a susceptible population

Evolution and Mutualism/Parasitism

  • Evolution of parasitism and mutualism are excellent examples of the process of Coevolution
    • 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 of an organisms that is involved in the mutualism/parasitism by interacting with some feature of the other partner
      • an 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)
  • Parasites and Hosts are also coadapted
    • Coadaptation often due to arms race type of coevolutionary changes in host and parasite
    • Parasites differ with respect to their host specialization
      • Monophagous parasites attach a single species of host
      • Polyphagous parasites attack several species of hosts (usually they are related)
    • Endoparasites are, in general, more often monophagous than are ectoparasites, although there are many exceptions to this observation.

The Impact of Parasites:

  • Host defenses
    • Cellular Defense Reactions
      • Encapsulation of parasite's cells (often reproductive cells) by the host so that they are non-functional
      • Cell surface changes
        • Change the marker molecule and the parasite may not recognize the host
      • Immune response
    • Grooming and preening to remove ectoparasites
  • Epidemics
    • Often can see the effect of an addition of the parasite to the host population as an epidemic (outbreak) of a disease
    • Difficult to remove the parasite from a natural population and so it can be difficult to do field experiments with parasitic systems
        • If this were not so, we would have performed many such removals in trying to cure us and our crops and livestock of disease
  • Disease may show cycles similar to predator-prey cycles (in humans, whooping cough and measles show this cycling)
    • basis is the proportion of susceptible hosts
      • susceptible hosts become non-susceptibles after infection, as immunity's memory system makes a second infection unlikely
      • After an outbreak, enough hosts become immune to drop Rp below 1, so the disease declines in the population
      • As disease prevalence falls, new individuals entering the population (births and migration from populations without the parasite) boost the proportion of susceptibles
      • When this proportion is high enough to boost Rp over 1, another outbreak begins, starting the cycle over again
        • Can you see why this cycling is most apparent in diseased that affect children?
  • Disease can set limits to the population size or the distribution of a host or hosts
    • Rinderpest in Southern Africa - virus with wide host range (large, grazing mammals)
      • Buildup of host (cattle) after establishment of European-style ranching
      • Outbreak of parasite after introduction of diseased cattle from Southeast Asia caused decline in cattle
        • also led to loss of natural populations of other hosts
          • Decline in wild populations of large grazing animals (antelopes, gnu, etc.) lead to:
              • change in vegetation over wide areas
              • reduction of tsetse fly population, which feed on large mammals
    • Decline in tsetse fly population lead to decline in cases of sleeping sickness caused by a trypanosome transmitted by the tsetse fly from human to human and from other large mammals to humans
  • Competition can be mediated through parasite  - called Apparent Competition
    • White-tail deer and Parelaphostrongylus tenuis
      • White-tail deer are tolerant
      • Other cervids (moose, other deer like the mule deer, pronghorn) are harmed
    • Where white-tail act as a reservoir, other cervids do not occur
  • Evolution may change the character of host and parasite
    • Evolution of resistance to antibiotics an example of the evolutionary potential of parasites
    • Virulence (transmission rate and infectious period) may vary through time
      • Rabbits and Myxomatosis
        • Less virulent strain of virus evolved
      • When both are present in a rabbit, virulent strain grows faster, overgrowing the less virulent strain, and wins by being the strain transmitted to the next host
        • When alone, less virulent strain meant that rabbits would live longer, infect more bloodsucking insects (vector)
      • More vector meant the less virulent strains had a higher rate of transmission as the rabbits lived longer to be bitten and the less virulent strains would win at the global level, although it loses at the individual rabbit level to more virulent strains
        • after time, Myxomatosis became a non-lethal disease and now a second virus, Calcivirus, is being used
  • Biological control through the use of parasites and parasitoids:
    • Attempt to reduce the population of a pest to an acceptable level through manipulation of the population ecology of that pest
      • Note that it says reduction and not elimination of the pest
      • elimination may sometimes occur buy the usual outcome is the reduction of host to lower population levels than without the parasite
    • Not all biological control involves parasitism
      • Herbivores and predators are also used
      • sterile male release also used (screw worm)
    • Strategies can attack either death rates or birth rate (or both)
      • Death rate strategies
        • Rabbits and Myxomatosis
        • Poses potential problems as the disease might jump to new hosts in the new environment and kill non-target species
      • Birth rate programs
        • Sterile male programs
        • Med fly and Screw worm programs

Examples of Mutualisms


  • 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 (Pseudomyrmex) to live in swollen base of acacia 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)
  • Some defensive mutualisms involve plants and fungi
    • Some grasses are infected with fungi (Clavicepts and other Ascomycetes) - long though to be parasitic but the fungi are the source of alkaloids
    • the alkaloids are protection from herbivory as they are toxic and bitter
    • some evidence that infected plants grow faster and produce more seed.

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
  • All species of aphid and leaf hopper have a nutritional mutualism, usually with bacteria but a few species of each have fungal mutualist partners instead of bacteria
    • In one case, the leaf hopper does not excrete the uric acid it produces as its nitrogenous waste but recycles it to the yeast, which use it in amino acid synthesis
    • In the same insect, the yeast synthesize proteins that are stored in the eggs and, without these proteins, the eggs will not produce viable zygotes when fertilized
  • This sort of mutualism, in which specialized cells harbor microbial mutualists that are needed to improve the quality of the animal's food, are not confined to aphids and plant-hoppers
    • they also occur in some beetles (Anobiid Beetles) that live in dead wood (called powder-post beetles because of the wood dust from their boring activities)
    • Some insects called Lacewings (Neuroptera) are important in biological control because their larvae eat aphids and planthoppers but the adults of some species feed on plant sap and have yeast in their guts that provide them with required amino acids and lipids


  • 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

Plant - 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) are free-living microbes that can fix nitrogen
      • the bacteria are all anaerobes and live in regions of the soil where oxygen has been depleted (see section on coadaptation and leghemoglobin below for why this is so)
  • 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
  • Flower Beetles and Yeast
    • Flower Beetles (Nitulid beetles) and yeast have a mutualism very similar to Drosophila and yeast
      • the beetles carry yeast from flower to flower
        • the yeast use the flower for food
        • the beetles eat the yeast

Coral-Crab Mutualism

  • Hard corals need sunlight (see the coral-algae mutualism above)
    • Overgrowth of corals by seaweeds (macroalgae) can shade them and kill them.
      • Some corals (Oculina arbuscula is an example) avoid overgrowth because an herbivorous crab (Mithrax forceps) forages on the algae.
    • When the crab is present, no overgrowth occurs, the coral grows faster and survival is greatly increased.
      • The crab gets not only the algae as food.  It lives in the coral and avoids predation as a result.  So the coral is not just food, it is a protector.

Agricultural Mutualisms

  • These are mutualisms in which an animal cultivates a fungus by providing a place and the plant material that is the fungus' food and eats the fungus
  • The advantage for the animal is that the fungus is higher quality food (more protein, less indigestible carbohydrate, fewer secondary chemicals, more vitamins) than the plant material
  • the advantage for the fungus is that it is provided food and, in some case, a controlled environment in which to grow

    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

    Termite - Fungus Mutualism

    • Termites are famous for their mutualistic association with protists that reside in their gut
      • The termites cannot digest the wood they eat but the protists can, so the beetles eat the wood but the protists digest it and the beetles digest the protistans
    • However, this mutualism is not characteristic of all termites
      • 75% of all termites have no protistan mutualists and some of these can produce cellulase, the enzyme needed to digest wood, themselves
      • Not all termites feed directly on wood
        • Some termites farm fungi in their nests, where the fungi digest the wood and the termites eat the fungi
        • The termites carry the fungi to new nests and will die if the fungal mutualist is lost (which can happen if another fungus or a bacterium contaminates the nest

    Beetle - Fungus Mutualism

    • Bark beetles bore into tree trunks and excavate tunnels under the bark or into the woody portions of the trunk
      • Some attack live trees and some bore into fallen trees
      • Those that attack living trees can cause the death of the tree
      • These beetles have special pockets on the surface of the exoskeleton which carry fungus from tree to tree
    • The beetles get:
      • food, as fungi digest the tree and are, in turn, eaten by the beetles and their larvae
      • protection from toxic secondary chemicals found in the tree trunks
      •  Some trees have resins (think of pine trees) or latex in channels which, when the beetles tunnel into them, flood the beetle's tunnels (also called galleries) and kill them.  The fungi can seal off the tunnels and protect the beetles
    • The fungi get:
      • Transportation from tree to tree
      • The tunnels, which penetrate the trunk and make the tree available as food for the fungi
    •  The most extreme example of beetle fungal farmers are the Ambrosia Beetles (over 3,500 species)
      • Ambrosia beetles attack living tree, but do not kill them
      • Living in the tunnels has resulted in some unusual behaviors in these beetles
        • The adults tunnel out nurseries for their larvae and feed them on the fungus in their nursery chambers
      • Living in tunnels in a living tree is a habitat that can be exploited for many generations of beetles and this has led to:
        • inbreeding (sib-mating) as a normal means of reproduction
        • haplodiploidy in many of the ambrosia beetles (note that the evolution of this reproductive mode has occurred several times, indicating that it is not just an accident that it occurs in these tunneling beetles)
      • The fungal community within a tree become diverse over time
      • The beetles have very specialized pockets on their exoskeleton for transport of pure fungal cultures
        • The pockets supply secretions to feed the fungus and secretions that kill bacteria and mold (another type of fungus) to keep their fungal cultures from contamination

    Snail-Fungus Mutualism

    • Littorina is a  snail that crawls over the stem and leaves of Spartina, marsh cord grass.
    • Spartina is the dominant species in some areas within salt marshes worldwide and salt marshes are of great economic importance for their fish and for their ability to remove toxins from human wastes carried into estuaries by rivers
    • The snails do not feed on the Spartina but scrape the surface
    • After scraping the surface, they deposit fungal spores
    • The spores germinate and feed on the damaged plant tissue
    • The snails then eat the fungus

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
    • So, considering the Ant-Aphid mutualism and the Agricultural mutualism, it appears that we did not discover either farming or ranching, or, if we did, we did not discover it first.
  • Bees and Microbes
    • by the way, not only do insects farm and ranch, but they are bakers as well.
    • Some bee species feed their larvae on pollen, but not before it has been mixed with fungi and bacteria and allowed to ferment (like a baker allowing the bread to rise)
      • the fermented mixture is referred to as "bee bread"
    • the microbes provide nutrition and turn the pollen, which will not support the larvae alone, into a high quality diet
    • the bees transport the microbes to new nests


  • Situations in which one species benefits from the presence or activity of another species, but the other species gains no benefit nor suffers any harm
    • commensal organisms might evolve into either parasites or mutualists
  • 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

Literature Cited

Silliman, B. R. and Y. Newell.  2003.  Fungal farming in a snail.  Proceedings of the National Academy of Sciences of the USA 100:643-648

Silliman, B. R. and J. C. Zieman.  2001.  Top-down control on Spartina alterniflora production by periwinkle grazing in a Virginia salt marsh.  Ecology 82:2830-2845


Parasitism, Host, Host Range, Complex Life Cycle, Definitive Host, Intermediate Host, Vector, Reservoir, Ectoparasite, Endoparasite, Holoparasite, Hemiparasite, Epiphyte, Mutualism, Obligatory Mutualism, Facultative Mutualism, Allee effect, Symbiosis, Diffuse Mutualism, Epidemiology, Pathogen, S (Susceptible Host), B (Transmission rate), L (Infectious Period), Rp (Replacement Rate of Infected Hosts), NT (Threshold Population Size of Susceptible Hosts), Critical Density, Coevolution, Coadaptation, Serendipity, Arms Race, Monophagous Parasite, Polyphagous Parasite, Host defense, Cellular Defense Reaction, Immune Response, Epidemic, Rinderpest, Apparent Competition, Biological Control, Pollination, Cheating, Dispersal Mutualism, Cleaning Mutualism, Defense Mutualism, Beltsian Body, Bacterocyte, Lichen, Mycorrhizae, Ectomycorrhizae, Vesicular-Arbuscular Mycorrhizae, Orchid Mycorrhizae, Nitrogen-Fixing Bacteria, Leghemoglobin, Ambrosia Beetle, Commensalism, Phoresy

Last updated February 26, 2007