Lecture #19—Selection: Parasitism and Predation
Predators and parasites both have negative selection affect upon their hosts—they select out the most vulnerable hosts to attack. There is a strong selective pressure. In turn the hosts positively affect their predators and parasites, providing nourishment and a safe living environment in the case of parasite which live inside (endoparasites) or upon (ectoparasites) their hosts. Thus, we are dealing with a co-evolutionary war. If the parasite or predator becomes too effective, they will eliminate the host. It the host becomes too successful at preventing attack the parasite or host can be eliminated.
Parasites negatively affect their hosts by consuming energy of the host tissue. They much smaller than their hosts and include viruses, bacteria, fungi, worms and various insects. They are the most numerous organisms in the world. They damage cells and organs and produce toxins. They gradually weaken their host over time and can kill them. They are often larger than their prey and kill them immediately. All organisms have a large number of parasites and predators.
There are a wide range of attack and defense strategies used by host and predators and parasites.
Results of Parasite-Host Interactions
1. Host becomes extinct/parasite becomes extinct—the parasite is so successful that it eliminates the host. If the parasite exclusive to the host. Then the parasite will become extinct at least locally, e.g. lamprey and lake trout. In fact, when a person dies of AIDS, the HIV virus in that person becomes extinct.
2. Host becomes extinct but parasite continues to survive because of alternative hosts exist.
3. Co-existence: Host develops immunity but parasite switches its identity so that it is no longer detected. E.g. influenza virus in humans reappears each year.
4. Co-existence--Co-evolution: Both parasite and host evolve countermeasures to the attack and defense so they both survive but are constantly evolving. This is often called an arms race. E.g. myxoma virus and Australian rabbits.
Results of Predator-Prey Interactions
1. Prey becomes extinct/predator becomes extinct. Predator becomes so successful it eliminates its food source and then it becomes extinct at least locally. This only occurs if there are no alternative food sources.
2. Predator becomes extinct and prey survives. If the prey can successfully hide in refuges then the predator can die before all the prey are eliminated.
3. Co-existence—unstable equilibrium. This can occur if the prey have hiding places and can even if they are extinct locally others will migrate into the area periodically. The same is true with predators; migration of new individuals into or out of the area as the will replace losses.
4. Co-existence—prey switching. This can occur if the predator has alternative food sources when the original prey populations become scarce. This is possible with generalized predators who eat many kinds of foods. Prey switching isn’t possible with specialized predators (e.g. pandas).
5. Co-existence—predator/prey cycles like those seen with artic lemmings and their predators (~3.5 year cycles) occur. This can occur if predators can switch to alternative prey and then the prey population can recover. This also seems to depend upon if local extinction occurs then migration of predators and or prey move back into a vacant habitat.
6. Co-existence—predator kept at low levels by disease or by its own predators so it does not eliminate the prey.
7. Co-existence--Co-evolution—predator and prey evolve counter measures to attack and defense so they both survive. This is often called an arms race.
Avoiding predation strategies
1. Camouflage by mimicry of objects e.g. praying mantis mimicking flowers.
2. Camouflage by mimicking surroundings e.g. flounder fish
3. Camouflage by disruptive patterns e.g. Ptarmigan birds in Alaska
4. Camouflage by behavior e.g. fish that look and act like seaweed.
5. Camouflage by counter shading e.g. penguin color pattern.
6. Dazzle patterning e.g. zebra stipes
7. Batesian Mimicry—mimicking dangerous or unpalatable species e.g. flies mimicking bees
8. Mullarian Mimicry—several unpalatable of dangerous species adapting the same color pattern to avoid confusing predators
Co-existence example—guppies in Trinidad shift color pattern depending upon predation.
Terms/Concepts to define
Chestnut Blight Fungus
Lamprey
Myxoma
Gause
Buffer species
Co-evolution
Disruptive patterning
Counter shading
Dazzle patterning
Batesian Mimicry
Mullarian Mimicry
Can you answer these questions?
1. Why do we say that parasite/host and predator/prey relationships are co-evolutionary?
2. Give two different types of situations where there can be a co-existence of parasite and host.
3. Give three different types of situations where there can be co-existence of predator and its prey.
4. Why don’t predators eat up all of their prey?
5. How come we keep coming down with colds each year when our immune system keeps wiping out the virus each time?
6. Describe what happened when the Australians tried to get rid of rabbits by infecting them with the lethal myxoma virus.
7. Describe Gause’s experiments with the ciliated protozoans Paramecium and Didinium.
8. Compare the advantages of being a generalist predator vs. a specialist.
9. Give three examples of camouflage mimicry.
10. What is the difference between Batesian and Mullarian mimicry?
11. Describe the predator-prey relationship for guppies living in Trinidad.