Ecology Pages

Marietta College

Biology  Department


The word symbiosis literally means "together life".  It refers to organisms that live in close approximation; often one cannot live without the other.  In extreme cases, one organism actually lives inside the other organism.


There are 3 types of symbiosis:

1.                  Parasitism: parasite benefits, host is hurt.

     Parasitism is a relationship in which one organism - the host - is the source of food and/or shelter for another organism, the parasite.  In this relationship, all of the benefits go to the parasite; the host is harmed by the relationship.  An example is a human and a tapeworm living in the intestines.  The tapeworm derives food (and shelter) from the human host; the human is denied the nutrition that is consumed by the tapeworm.

2.                  Commensalism: one species benefits, the other is neither hurt nor helped.

  In commensalism, one organism benefits from the relationship while the other is neither helped nor hurt.  Example: a few orchids growing epiphytically on a tree.  If there are a lot of commensals on a single "host" then it stands to reason that the host will be hurt and the relationship will slide towards the parasitic (Note: some ecologists define parasitism functionally, that is the parasite must be adapted to feed on the host; other ecologists define the terms logically, that is if the host is hurt and the parasite benefits then the relationship is parasitic, even if feeding does not take place.  This latter definition makes many commensal relationships appear parasitic. )

3.                  Mutualism: both species benefit

  Finally we come to mutualistic relationships where both parties benefit.  In fact, many people use the term symbiosis a bit too casually, using the term symbiosis to refer to mutualistic relationships (they should really call a relationship where both species benefit  mutualistic instead of symbiotic, as the latter term leaves open the possibility that the relationship could be mutualistic, parasitic or commensal in nature).  Example: corals and zooxanthellae (zooxanthellae are algae that take up residence in a coral animal.  The photosynthetic zooxanthellae provide the coral with sugars in return for nitrogen and other nutrients from the coral).

Tightness and Looseness:  All relationships between organisms range over a continuum from obligate (where one or both organisms would die without the other) to facultative (where the presence/absence of the other isn't really necessary).  Obligate relationships - such as a human tapeworm in our gut - are considered "tight", while facultative ones - a squirrel living in a tree - are considered "loose".  Some ecologists place the 3 types of relationships first, that is there are parasitic, commensalistic, and mutualistic relationships, and only the obligate ones in any of these 3 categories are called symbioses.

Catalpa worm  (Ceratomia catalpae) with parasitoids (Cotesia congregatus)

Catalpa worm  (Ceratomia catalpae) with parasitoids (Cotesia congregatus)

Parasitism tight and loose:  The Catalpa Worm (above) is being parasitized by tiny wasp larvae.  The adult wasps (right) sting the caterpillar, injecting their eggs.  The eggs hatch and devour the caterpillar from the inside, being careful not to disrupt any vital functions.  Eventually they emerge and spin cocoons of silk in which they transition from larvae to adult.  Technically, these insects are parasitoids, since, unlike true parasites, they kill their hosts.  This is a failry tight relationship; the wasps could not survive without  caterpillars in the same family (although the caterpillar would do just fine without the wasps).  Much looser parasitism is shown by ectoparasites, which feed from the outside.  Mosquitoes (below right) of course suck blood (only the females at that; they need the protein to make eggs).  Oak Treehoppers (below) suck sugar-rich juices from the phloem of trees).  Often these relationships are not species-specific; the mosquitoes would probably go after any other warm-blooded prey and the oak treehoppers pictured were in fact on a sycamore tree.

Catalpa worm  (Ceratomia catalpae) parasitoids (Cotesia congregatus)

Oak Treehopper - Platycotis vittata (Fabricius)

Opossum (Didelphis virginiana) with mosquitoes


The squawroot (left) may look like a fungus, but it is actually a flowering plant.  It is parasitic on trees, usually oaks, and gets its energy by tapping into the oak's roots.  It betrays its true nature when it comes time to reproduce, however. I'm not sure about squawroot in particular, but other related plants are often self-pollinating and thus don't even need showy flowers to attract pollinators.  The squawroot is a distant relative of the magnolia.

Commensalism?  Many would argue that a flicker making its home in a cactus (below left) is a good example of commensalism.  In a forest, such a relationship usually is commensalistic; the flicker below has excavated its nest in the dead wood of a living sycamore tree.  To my eye, the desert bird has gone through some living tissue to make its nest.  Still, the overall damage to the cactus is small.

Flicker on cactus

Flicker on sycamore

White-winged Dove (Zenaida asiatica) on cactus

The white-winged dove (left) has a mutualistic relationship with the Saguaro Cactus.  The cactus provides food for the bird in the form of a large fruit.  The bird consumes the fruit, also ingesting the cactus' seeds.  The bird then flies off, and later deposits the seeds in a new location (with a nice dose of fertilizer to boot!).  In this way, the cactus gets its seeds transported away from the parent plant, allowing it to potentially colonize new places.  This type of mutualism is known as a dispersive mutualism.

The Cattle Egret (below left) is often seen in the company of grazing animals.  The grazers stir up insects, which the egret then eats.  This is probably a loose sort of commensalism; there is no apparent benefit to the cattle.  The commensalism is loose because the egrets will follow any cattle; in Florida, in fact, I have seen them following mowers.

On the other hand, the oxpecker (not pictured) is a bird that rides around on the backs of cattle and other large animals such as rhinos.  The oxpecker feeds on ectoparasites of the cattle such as ticks and warns the animals of approaching predators; thus both organisms benefit in a loose mutualism.  On the other hand, the oxpeckers also pick at scabs and wounds on the animals and may ingest bits of flesh and blood (thus making them more like parasites).  The natural world is complicated!


Cattle Egret (Bubulcus ibis) Tick White Rhinoceros (Ceratotherium simum)

Elkhorn Coral (Acropora palmata)

Brain coral (Diploria sp.

Symbiosis in the seas:  Some of the best examples of symbiosis are found in the oceans - not surprising since life has had longer to evolve and form close associations in the oceans.  Above, the corals are perhaps the best example of a mutualistic symbiosis.  Tiny coral animals (which individually resemble this freshwater hydra)Freshwater Hydra form huge colonies, with each hydroid encased in stone secreted by the animals.  Collectively, these colonies can grow very large.  Brain coral (above right) typically forms huge colonies; the dark "boulder" to the left of the picture immediately right is actually a colony of brain coral that may be thousands of years old (the fish is 5 feet long).  Each hydroid in turn may harbor cells of photosynthetic algae (usually dinoflagellates); these algal endosymbionts are called zooxanthellae and give the coral its brown or green appearance. As mentioned above, the zooxanthellae "trade" sugars for nutrients; it's convenient that the wastes of the coral (CO2, ammonia, etc.) are the very things needed by the algae for photosynthesis.  Interestingly, both the corals and the zooxanthellae can survive without the other (at least for a while); under conditions of stress the corals are known to expel the endosymbionts in a phenomenon known as coral bleaching.  Under happier times, the corals direct their growth to maximize sun exposure for their algal guests; you can see this clearly in the photo of the Elkhorn Coral above.

Another Cnidarian with endosymbionts is the Cassiopeia Jellyfish (AKA the Upside-Down Jellyfish) (below left).  This jellyfish spends its time upside down in the shallows of mangrove swamps exposing its algal endosymbionts to the sun.

Two other mutualistic symbioses found on the coral reef are pictured to the right, although they are not as tight as the endosymbioses of coral and zooxanthellae.  In the photo to the right, a barracuda takes an unusual heads-up posture.  He has arrived at the large brain coral, which makes a conspicuous landmark (seamark?) for tiny Cleaning Fish to set up shop.  When the barracuda takes this pose, the Cleaning Fish know it is safe for them to approach - the 'cuda is looking for a cleaning, not a meal.  The tiny fish will scour the skin, mouth and gills of the Barracuda, removing any ectoparasites they find (and getting a good meal out of it).  There was a line of about 6 barracuda waiting to get cleaned here; the others were behind me in the line.

Finally, everyone who has seen "Finding Nemo" knows about the association between Clownfish and Anemones.   By working its way carefully into the anemone, the clownfish gradually accustoms the anemone to the chemical makeup of the fish's skin; this gradual acclimatization prevents the anemone from stinging the clownfish (while fish with a different "taste" will be stung and eaten).  The fish gets a safe house and some tidbits; the anemone gets cleaned and has the clownfish working as lures to bring in potential prey, or chasing away fish that would harm the anemone.  Some scientists do not see any benefit for the anemone and classify this as a commensalism.

Great barracuda, Sphyraena barracuda

Upside-down or Mangrove Jellyfish (Cassiopea sp.)

Clown Fish

Pacific Lamprey (Lampetra tridentata)

Spider Wasp

Brown-throated Three-toed sloth (Bradypus variegatus)

The Sea Lamprey, above left, is a sort of temporary parasite.  It latches onto a fish and uses the teeth to hold on and rasp away the skin, leaving an open wound for the lamprey to feed on.  It drops off, usually without killing the "host".  Sea Lampreys are not specific on any species of fish; they will latch onto any living thing and try to feed. 

The wasp above has stung and paralyzed a spider.  It will take the spider to a nest and lay an egg on it.  The larvae will consume the still-living spider; often from the inside.  This is usually considered to be a parasitoid relationship.

Two more mutualistic relationships from the Costa Rican forests.  The Tree Sloth (left) has  algae growing in its fur.  These algae help to camouflage the sloth against the lichen-covered tree (note the brown fur of the baby, not yet covered with algae).  There is even a moth that lives only in the sloth's fur and consumes the algae; this is a commensal relationship between the moth and the sloth.

Below, a mutualistic relationship.  The Acacia Tree is partially protected by large thorns, but it gets extra protection from Acacia Ants.  The plant does 3 things to lure in the ants.  First, the large thorns are hollow and provide a place for the ants to live. Second, the plants have swollen glands, nectaries, which produce a sugary solution the ants drink.  The nectaries are obvious in the photo below.  The third thing the plant does is to produce Beltian  bodies, small structures which the ants bite off and eat; the Beltian bodies are rich in protein and supplement the sugars provided by the nectaries.  In return for the room and board the ants chase off herbivores, kill and eat herbivorous insects, and destroy and plants that try to compete with the acacia.

More on Acacia Ants     See Also Cecropia Ants

Acacia Ant (Pseudomyrmex sp.)

Acacia Ant (Pseudomyrmex sp.) at nectary

Right  - a really strange parasite.  The horsehair worm starts life as an egg laid in a puddle.  The puddle dries out and a grasshopper or similar insect comes along and eats the egg, which promptly hatches and burrows through the gut of the insect into its body cavity or hemolymph.  Here, surrounded by the nutritious blood of the insect it grows until it reaches adulthood. At that point it starts producing chemicals which take over the brain of the insect and cause the insect to seek out water, which it jumps into.  The worm then exits the hopper and lives in the puddle, mating and laying more eggs.  The grasshopper, if it doesn't drown, may survive the ordeal.  

Below, a social parasite.  This cricket lives in an ant nest.  It disguises itself with a chemical signature that fools the ants into thinking it is just another ant.  It is free to roam the nest and it even gets the ants to feed it.

Horsehair Worm - Nematopmorpha




Brown-headed Cowbird


Parasites and Commensals:  The Brown-Headed Cowbirds (above) are nest parasites.  They originally followed the bison on the Great Plains, feeding on insects kicked up by the large herds.  Since the bison themselves migrated, following the melting snows and eating the fresh spring grass, the cowbirds had to move as well.  This presented a problem, as it's hard to incubate eggs on the move.  The solution?  Lay the eggs in other birds' nests, and trick the other birds into raising your young.  The cowbirds hatch out first, push the other eggs out of the nest, and the nest-builders (often much smaller than the rapidly growing cowbird) end up feeding it instead of their own young.  Even though the other birds may pattern their eggs the cowbirds are up to the challenge.  Cowbirds hesitate entering forests, but roads, farms, powerlines and other human intrusions give them a pathway deep into the woods where they are free to parasitize the nests of birds which until the arrival of humans didn't have to worry about the cowbirds.  Some of these bird species are on the verge of extinction as a result.

Bromeliads (left, above left) avoid the hassle of crating a trunk to lift their leaves above the forest floor and closer to the sun.  They simply grow on the branches of trees.  Since the bromeliads don't take any nutrients from the trees this is usually classified as a commensalism, but if there are a lot of bromeliads (left) the tree will need to add extra wood to support the weight (a bromeliad can trap up to 10 gallons (80 pounds) of water in its leaves). So, if there are a lot of bromeliads the relationship overall turns into a negative for the tree.  The bromeliads also host a number of organisms in the water they trap; the wastes from the animals living there undoubtedly fertilizes the bromeliad in a mutualistic relationship. The tree at lower left is absolutely covered with epiphytes.

Leeches (below left) are usually thought of as ectoparasites (although some are predators).  They attach to a vertebrate host and take a blood meal before dropping off.  Most aren't adapted to a single vertebrate host, but they are highly adapted to sucking blood; their saliva includes anesthetics to help keep the host from noticing the bite, as well as anticoagulants to keep the blood flowing.  

Below is a larval mussel (freshwater clam).  If there is any case of "good" parasitism, this may be it.  A gravid mussel  extends part of its body out of its shell to attract fish, which then get sprayed in the face with larval mussels like the one pictured.  The little mussels go into the mouth and pass over the gills.  Here, they clamp down by closing the shell and digging in with the little teeth pictured at the edge of the shell.  They ingest blood from the fish as it swims upstream; some time later they drop off from the fish and begin their long (60 + year) life of filter feeding.  The fish provides a meal and transport upstream (moving is not something mussels do well over long distances, particularly upstream).  



Mussel Glochidium

British Soldier

British Soldier


Lichens (above and left) are mutualistic associations between a fungus and an algae or cyanobacteria.  They are the terrestrial equivalents in some ways of corals. The fungus provides a tough, waterproof body able to withstand extreme environments on rocks and tree trunks.  It is good at obtaining water and secretes acids to dissolve minerals from the rocks.  It also produces carbon dioxide.  All of these materials are then funneled to the endosymbiotic algae or cyanobacteria, which use the materials in photosynthesis and produce sugars which are then shared with the fungus.  This web page has a lot more information about the British Soldier (Cladonia cristatella) lichens pictured above (so named for the red "caps").  Some studies have shown that the fungus benefits from this relationship more so than the algae; at least under good conditions algae grown without the fungus grow faster then they do with the fungus.

Below: another parasitoid; here a wasp burying its prey on the sand of a volleyball court.  This wasp has stung and paralyzed a stink bug and is dragging it to its underground lair.  Here it will deposit an egg and the larvae that hatches from the egg will eventually consume the bug.  Keeping the bug alive but paralyzed ensures it doesn't rot.

Bembix wasp with stink bug

Bembix wasp with stink bug

3 ecotparasites of mammals.  The two lice to the right parasitize humans.  The body louse (above) can attach to hairs of the body or head and then suck blood from the host.  While it is relatively easy to remove the adults (particularly if your hair is thin), the eggs are another story.  The eggs are called nits and are glued to the hairs, the careful search for these tiny eggs has given us the term "nitpicking".  The larger claws of the crab louse allow it to grasp the thicker pubic hairs.  Overall, lice aren't the biggest health concenr humans face; on their own they do relatively little damage.  The diseases they can transmit, however, can cause devastating epidemics and many deaths.

Fleas (below) are adapted to live in mammals with thicker hair.  The comb-like structures help them hang on.

human body louse (Pediculus humanus)

Flea (Siphonaptera)

crab louse (Phthirus pubis)

Silphid Beetle with phoretic mites

Mosquito biting human (me!)

Parasitic Mites on Bumblebee

The mosquito (above) is a very temporary exoparasite; it probably shouldn't be counted as a symbiont so much as a predator.

Not all situations are readily apparent.  The mites on the bumblebee at left are in fact sucking fluids from it; mites have been implicated in the decline of our commercial honeybees.  This is a clear case of ectoparasitism.  On the other hand, the mites in the image above left are merely hitching a ride on the Carrion Beetle.  This beetle locates dead animals and flies to the carcasses to lay its eggs, which hatch and feed on maggots on the carcass.  The mites are interesting.  Often, they feed on fly eggs and small maggots; this reduces competition for the carrion by the flies, and thus actually helps the beetles out a bit.  The mites do NOT suck fluids from the beetle; they merely hitch a ride and thus make a trip they would not be able to make on their own.   This hitchhiking is called phoresy, and as long as the phoretic animals are much smaller than their hosts - and there aren't too many of them - this would qualify as a commensal relationship.  If the mites help to reduce the maggot population and thus reduce competition for the beetle, they may actually be benefiting the beetle and thus move this relationship into mutualism.

Right - A leafcutter ant tending fungus in its underground nest.  The fungus is almost completely dependent on the ants.  The ants bring in nutrients (bits of plant leaves), prune the fungus back, transfer it to new bits of leaves (and even to new ant nests), remove competing fungi, bring in only leaf bits from trees without chemicals which would hurt the fungus, etc.  Perhaps most amazing is the fact that the ants enlist a second symbiont - bacteria of the genus Streptomyces that the ants grow in specially modified areas of their own exoskeletons.  The Streptomyces is then used to produce antibiotics that inhibit the growth of fungi which would compete with the fungi the ants are growing.  There is a lot more to this mutualistic interaction; try this page built with pictures from our Costa Rica trips :  Leafcutter Ants



B.                Keystone species: species that are critical to their ecosystems  

Sometimes organisms have effects on more than one species at a time - they can effect whole ecosystems.  We call these organisms Keystone species

1.                  Keystone predators may control key competitors at lower levels in the food chain, thus allowing other species to thrive.

2.                  Keystone mutualists may provide needed resources for a wide host of organisms (example: fruit trees provide food and shelter)

3.                  Keystone competitors, if removed, allow one competitor to dominate, reducing diversity

Gopher Tortoise (Gopherus polyphemus) in Burrow

Gopher Tortoise (Gopherus polyphemus)  Burrow

The Gopher Tortoise (above and right) is a classic example of a keystone mutualist.  It excavates large burrows which may extend 10 meters or more, and which are almost 1 foot in diameter (with some larger chambers as well, so the turtle can turn around.  A number of other species including burrowing owls, gopher frogs, indigo snakes, and a number of invertebrates are highly dependent on these burrows; they often live in the burrow alongside the tortoise (benefits to the tortoise of this arrangement are not clear).  

Studies of the Purple Sea Star (Pisaster ochraceus) have shown it to be a keystone predator - it preys preferentially on species - such as certain mussels - which would otherwise outcompete all the other species trying to gain a foothold on the rocks.  By reducing the number of mussels, the sea stars open up habitat for other species and thus increase the overall diversity of the ecosystem (note that the sea stars are a predator to the mussels, not a mutualist!).

The Red Mangrove, below, has long stilted roots that arch down to the water at the edge of tropical shores.  These roots stabilize the soil, protect coastal areas from erosion, and provide hiding places for many animals, including the young of many coral reef fish.  In this way the Red Mangrove is a keystone mutualist like the Gopher Tortoise.


Gopher Tortoise (Gopherus polyphemus)

Red Mangrove. (Rhizophora mangle) - roots on tidal flat

Purple Sea Star (Pisaster ochraceus) on mussels

American Alligator

The American Alligator, left, excavates depressions in its habitat that fill with water.  During dry times, these gator holes may be the only places with water.  Thus, to all the organisms whose survival depends on the water in those holes the alligator is a keystone mutualist.  Of course, the gator might eat a few of those things that come to live in its wallow.

Beaver are well-known for building dams.  These dams create relatively large areas of still water where there once was a small stream.  For organisms that live in such still waters the beaver is a wonderful keystone mutualist; for animals that like flowing water it's not such a good deal.


This article expands on the concept of keystone species.

Beaver (Castor canadensis)

Summary: Interactions between organisms:



Effect of Interaction On:

Type of Interaction

Species 1

Species 2





Predator (+)

Prey (-)


Parasite (+)

Host (-)


Herbivore (+)

Plant (-)


Detritivore (+)

Detritus (0)









Ecology Pages

Marietta College

Biology  Department