[img]2012q2/ct_etoefm_etoeflistz_0870_20124[/img] [br] According to the lecture,

P Alright, let’s get going. Ah, people usually think of symbiosis as a mutually beneficial relationship between members of two species. Bees and flowers, for example, both benefit from interaction. The flowers give nourishing nectar to the bees, and the bees in turn help pollinate the flowers by carrying pollen on their bodies and fertilizing other plants. This kind of relationship is called mutualism--obviously because both species benefit mutually-- and actually is just one of many types of symbiosis. You see, symbiosis, is defined rather broadly ... to include all kinds of relationships, involving benefit, harm, and no effect at all. Let’s turn to the other, ah, widely accepted types first. OK, next we have parasitism. It’s easy to guess that this kind of relationship involves benefit to one species and harm to the other. Can anyone think of a parasitic relationship?
S Um, a mosquito biting a person?
P Uh, OK, not quite right, but you do point out a very important distinction. Parasitism involves a long-term ... and intimate relationship. Your example of a mosquito is similar to a parasite, but after it has its fill of blood, it leaves. This behavioral pattern is closer to predation, for example, say ... a wolf taking a bite out of a deer. Compare the mosquito to a tick, and I think the difference will be clearer. See, the tick takes nourishment from the host and remains, literally making a living from the host. And there are other problems with this example that I think we can get into later.
Now, parasitism is different from commensalism because in the former you have some harm to the host species. But with commensalism, one species benefits, and one is not affected either way. The beneficiary organism may gain protection or nourishment. For example, various fish species dwell among the poisonous tentacles of jellyfish and anemones. The fish species are immune to the poison, and so are protected from predator species that are not immune. The hosts in these cases receive neither benefit nor harm.
Every now and then you’ll run into a relationship that is not clearly mutualistic, parasitic or commensalistic, because observers simply can’t clearly identify benefit or harm. For example, many of you have probably seen remoras, those fish attached to sharks or whales. They appear to be parasites, but it is widely accepted that they pose no harm to their hosts. Now, the remoras clearly derive benefit, in the form of transportation, protection, and nourishment. The question here is whether they provide any benefit. Remora species eat leftover scraps, droppings, or bacteria. If the remora species eats bacteria and other parasites that threaten the host, it is a mutualistic relationship because the host is receiving benefit.
OK, we’ve covered the major types of symbiosis. Now let’s move on to some ... um, more complicated types. Neutralism is a relationship in which neither species is affected, positively or negatively. Amensalism is when one species is harmed and the other is unaffected. Here again we see some difficulty with the inability to identify harm or benefit. While it can be hard to see harm or benefit sometimes, well, it’s possible that there are indirect effects. Um, for example, a plant producing a chemical that kills off other plant species could be called a mensalistic relationship, but some would argue that by eliminating surrounding plants there is some benefit in exclusive access to nutrients in the surrounding soil, reduced competition for sunlight, et cetera. Similarly, it would be difficult to say that two interacting species have no effect on each other in a so-called neutralistic relationship.
S Professor, then how would you classify the relationship between mosquitoes and humans?
P Right, I was trying to get back to that ... well, that’s a good question, and it kind of leads me to my next point. Another problematic type of symbiosis is competition. Now, I think everyone can understand that competition is a relationship where both involved species are harmed in some way, and I think most people would say that mosquitoes and humans clearly do harm each other. However, humans help mosquitoes in some ways as well. For example, humans actually create safe mosquito breeding habitats in open containers, discarded tires, bad pavement and sewers _. anywhere standing water can collect. We see in this example that there can be a lot of overlap in classification. So symbiotic relationships are not so neatly defined, and it’s helpful to view symbiosis with a flexible mind.
P Parasitism involves a long-term ... and intimate relationship. Your example of a mosquito is similar to a parasite, but after it has its fill of blood, it leaves. This behavioral pattern is closer to predation, for example, say ... a wolf taking a bite out of a deer. Compare the mosquito to a tick; and I think the difference will be clearer. See, the tick takes nourishment from the host and remains, literally making a living from the host.
Why does the professor mention a tick?