What Types of Symbiosis Exist?

Hermit crabs are crustaceans who have a soft abdomen and live in the ocean. To compensate for the lack of a hard shell, these crabs burrow under a shell. The shell serves as their home until they grow too large to fit inside and they seek a new shell. Sea anemones , which possess long fingerlike tendrils, often attach to the top of the hermit crab's shell. (Any time you see the lips symbol, you can click on the word preceding the lips symbol and hear that word pronounced.) These tendrils contain a poison that is lethal to many fish.

When the hermit crab walks along the ocean floor, it carries the shell and sea anemones with it. This increases the feeding area for the sea anemones. Most predators of the hermit crab refuse to come near sea anemones.

1. The relationship between sea anemones and hermit crabs is best classified as

a. commensalism, a type of symbiosis.

b. mutualism, a type of symbiosis.

c. parasitism, a type of symbiosis.

d. interspecific competition.

e. predation.

Lichens, which are composed of a fungus and an algae living together, are another example of mutualistic symbiosis. Lichens can be found in habitats as diverse as deserts and arctic tundra. Two photographs of lichens found in Arizona are shown here. Click on the small image to see a larger photograph.

In the tundra lichens serve as a major food source for caribou (reindeer). The algae in the lichen performs photosynthesis, producing food that is shared with the fungus. The fungus in the lichen provides the supporting structure for the algae and helps retain moisture. If you separate the algae from the fungus in a lichen and try to grow them individually, they both die.

Symbiotic relationships may be commensalism, parasitism, or mutualism. Commensalism occurs when one organism benefits from the symbiotic relationship but the other organism is relatively unaffected. Parasitism occurs when one organism in a symbiotic relationship benefits at the expense of the other organism.

2. Pilot fish are small fish that swim alongside a shark. They feed on the morsels the shark drops from its mouth while eating.

a. commensalism, a type of symbiosis.

b. mutualism, a type of symbiosis.

c. parasitism, a type of symbiosis.

d. interspecific competition.

e. predation.

Look at Figure 46.6 and Figure 47.14 in your textbook. Figure 46.6 shows a newly-hatched baby cuckoo is the nest of a warbler bird. A mother cuckoo bird laid her egg in the warbler's nest, which contained a warbler egg. This photograph shows the newly-hatched cuckoo bird next to the warbler egg that has a longer incubation time. Instinctively the baby cuckoo will push the warbler egg out of the nest. The egg will smash on the ground, preventing the baby warbler from hatching and competing for precious food.

In Figure 47.14 the small mama warbler is shown feeding her big baby, the cuckoo. The cuckoo is the bird with its mouth wide open. Mama warbler thinks the cuckoo is her baby and tends it faithfully while Mama cuckoo is out seeing the town. Although many parasites live inside their host (e.g. tapeworms), this is not a requirement. Parasitism occurs in any symbiotic relationship where one organism benefits at the expense of another.

3. The cuckoo-warbler relationship is an example of ________, a type of symbiosis.

a. commensalism

b. mutualism

c. parasitism

 

In addition to symbiosis, species in a community may interact with one another through a predator-prey relationship. Predators feed on other living organisms - their prey. The records of the Hudson Bay Company provide a classic study of the interaction between predator and prey. During the 1800 's and early 1900's, Hudson Bay Company traded furs in the U.S. and Canada. (Hudson Bay Company continues to be an active company. The first photograph shows the store they operate in Banff, Canada. The second photograph shows a closeup of the regal plaque on the store.)

Hudson Bay Company operated when furs were a major source of clothing, paying trappers for their pelts. Of interest to biologists are the extensive records the Hudson Bay Company kept from 1845 to 1925. These records meticulously detail the number and type of pelts purchased (lynx, hare, fox) for these years.

Observe the graph associated with Figure 47.8. It reflects the number of Canadian lynx (dashed line) and snowshoe hares (solid line) that Hudson Bay Company purchased. Biologists analyzing this data make the assumption that the number of furs traded was proportional to the population of animals at that time. For example, if Canadian lynx were more numerous in some years than others, more lynx would be trapped and sold to Hudson Bay Company. What pattern(s) do you see in the data?

4. In the Hudson Bay Company data, what is the relationship between number of lynx (predator) and number of hares (prey)?

a. The number of lynx and hares are pretty much equal.

b. The number of lynx tends to always be higher than the number of hares.

c. The number of hares tends to always be higher than the number of lynx.

d. An increase in the number of hares is often followed by an increase in the number of lynx.

e. An increase in the number of lynx is often followed by an increase in the number of hares.

f. both c and d are true

g. both b and e are true

Remember the "Rule of Ten" from Module 1? We learned that only about 10% of the energy available at one trophic level is available to the next trophic level. Since lynx eat hares, there must be many more hares if lynx are to have sufficient food to survive. Thus, we see the number of hares is always higher than the number of lynx.

Imagine there is a spring with lots of rain that follows a mild winter. These ideal conditions support abundant growth of the grass and shrubs that hares use for food. The population of hares skyrockets as a result. What impact would we expect this to have on lynx?

Because the lynx' food supply is large, most of their offspring survive and the population soars. As this large population of lynx feeds on the hares, the number of hares drops dramatically. What effect will this have on the number of lynx? The population will drop as the food supply becomes scarce. This is why we see the number of hares increase first, then the number of lynx increase, then the number of hares decrease, and finally, the number of lynx decrease.

Is the number of hares and lynx determined solely by their relationship with one another? What other biotic and abiotic factors could have caused the fluctuations we see in the Hudson Bay data?

Perhaps the vegetation in the area was overbrowsed. This would cause the number of hares to decrease without any influence from the lynx. Owls, martens and foxes also prey on hares. If the number of these predators increased, the number of hares would decrease and the lynx would not be the cause of this fluctuation. Perhaps a virus that infects lynx but not hares was widespread during certain years. The lynx would become infected and many might die, but the hares would be unaffected. The commentary beside Figure 48.8 identifies other complicating factors.

Thus, the predator-prey cycling we see in Hudson Bay's data may not be an accurate reflection of how predator and prey impact one another. This was not a controlled experiment - there is more than one variable that may be changing from year to year. It is hard to attribute the cause and effect in this data. In order to see if the predator-prey cycling is a valid phenomenon, a controlled experiment needs to be done. Since that is very difficult to accomplish in nature, we shift to the laboratory where we can control variables much more easily.

Looking at the Predator-Prey Relationship in a Controlled Experiment

In the 1930's a Russian biologist named Gause used Paramecium and Didinium to examine the predator-prey relationship. Both are microscopic organisms. Figure 22.7, photograph a, in your text shows Paramecium, the prey. Figure 22.1, photograph d, shows Didinium, the predator, consuming a Paramecium.

Gause conducted an experiment using two flasks. In flask 1 he put some Paramecium and their food. In flask 2 Gause put Paramecium, their food, and some Didinium. Didinium is a predator of Paramecium (its prey).

Gause observed the growth of the Paramecium in these two flasks for several weeks. He took daily samples to see how the population was changing. The results he obtained are shown in these two graphs:

5.What statement describes what Gause saw in his two flasks?

a. The prey, Paramecium, was unaffected by the presence of the predator, Didinium.

b. The number of prey, Paramecium, was limited by the presence of the predator, Didinium.

c. The number of predators, Didinium, was limited by the number of the prey, Paramecium.

d. both b and c are true

e. none of the above are true

Another difference between the flask and the natural world is that the prey has places to hide - refuges. Hares may hide in their burrows or camouflage themselves against the landscape. Snowshoe hares are particularly good at camouflage. Their fur becomes white in the winter, blending in with the snow. In spring their fur is beige, blending with the brush. In contrast, the Paramecium has no place to hide in the flask.

Thus Gause' results indicate that predator and prey limit one another. These results, developed in an environment in which only one factor differed between the experiment and control, give us more confidence that the predator-prey cycling seen in the Hudson Bay data represents a real phenomenon.

Reintroducing Lynx to Colorado

An interesting experiment is being tried in Colorado. In February, 1999, four Canadian lynx were released in the San Juan National Forest, a remote region of southwestern Colorado. Colorado's Division of Wildlife hopes to reestablish the lynx in regions where they used to thrive before humans intervened. Lynx have been absent from Colorado for twenty years.

Unfortunately by March, 1999, three of the elusive cats had died of starvation. The fourth cat, who was recaptured for examination, was found to have lost weight. However, she had a full stomach, which provided evidence that she had eaten recently. Critics question whether there are enough snowshoe hares left in the region to support a healthy, reproducing lynx population. They feel it is inhumane to reintroduce animals if they are not going to survive.

Supporters cite the successful reintroduction of American bald eagles, California condors, and red wolves in North Carolina as evidence such projects can work. Despite early casulties in these reintroduction programs, they ultimately were successful. It remains to be seen if the lynx reintroduction project will join the ranks of successful projects or end up a failure.

6. True or false: Gause's experiment with Paramecium and Didinium provides evidence that supports the competitive exclusion principle.

a.true

b. false

7. An area is covered with a glacier for 2000 years. Due to global warming, the glacier melts. Plants begin to grow where once it was barren. This is an example of

a. primary succession

b. secondary succession

What Are the Effects of Predation on Competing Prey Species?

The area where the ocean meets the land is called the intertidal zone. Because it is underwater at high tide but exposed to air during low tide, this area provides ideal conditions for many types of marine life: barnacles, clams, mussels, algae (seaweed), and snails. However, the space in this rocky intertidal zone is limited and competition among these species is fierce.

The starfish is a "monster" predator that moves over the intertidal zone, eating anything it contacts except for the algae. (Snails feed on algae.) Robert Paine, a professor at the University of Washington, asked what would happen if he took the starfish out of the intertidal zone. He isolated one area of the intertidal zone by putting up barriers. He removed all the starfish from this area and kept them out for a period of six years. For a control, Paine put a barrier around a similar intertidal region, but did not remove the starfish. Paine monitored the number of each species present in the control and experimental areas for six years.

Very quickly he observed that only one of the two barnacle species remained. Within one to two years, there were no barnacles at all. The mussels had outcompeted them. The number of algae had declined also. By the end of the six years, only one species of mussels was left. There were no algae, clams, or snails left in his experiment. However, all the original species were present in the control region. What had caused this result?

As one species began to outcompete another species in the control region, the starfish would consume more of the species that had become most abundant. In effect, the starfish was helping the underdog species by eating the competition. So the starfish actually promoted coexistence - it kept one species, the strongest competitor, from taking over. The starfish kept diversity in the intertidal zone.

8.The starfish is an example of a

a. prey

b. climax community

c. parasite

d. keystone predator

e. niche