Re: What might you infer about the relationship between crabs and seagulls?

Hello Brooke!

The relationship between crabs and gulls is one of the most intimate relationships that can occur in animals -- the predator-prey relationship. Gulls, and other shore birds, eat lots of crabs. If you were to go to the beach, you'd find empty shells and other crab bits that are the remnants of the gulls' lunches.

Now, to answer your questions. Predation doesn't really fall into one of the categories that you listed, but it is an example of symbiosis. Let me describe what biologists mean when we talk about symbiosis, parasitism, commensalism, and mutualism.

Parasitism, commensalism, and mutualism are all cases of symbiosis, which is what we call a very close association between two different organisms. Some people use the term "symbiosis" to infer a relationship where both partners benefit from the association, but I prefer to keep "symbiosis" as a general term that includes the more specific cases that I'll describe below.

A mutualistic relationship is one in which both partners benefit (+/+ ). One of my favorite examples of mutualism is the relationship between flowering plants and insects, particularly bees. I think bees are extremely cool, and want to keep a beehive some day when I grow up. Anyway, plants have evolved very elaborate flowers and sweet nectar to attract insects. When the insects go from flower to flower, feeding on the nectar, they tranport on their bodies the pollen of their plant hosts, and pollinate other flowers.

In a commensal relationship, one partner benefits and the other is unaffected (+/0). Examples of commensalism are difficult to document, because in biological systems it is unlikely that one partner of any relationship can be entirely unaffected by its intimate association with another organism.

Parasitism and predation are cases of symbiosis where one partner benefits from the association and the other is harmed (+/-). In predation, one organism eats the other (this definition includes herbivory, or the eating of plants by animals), and the eaten organism dies. In parasitism, one organism feeds from the other, but usually doesn't eat the entire host. Some examples of parasites that you might be familar with if you have a cat or dog are fleas, ticks, and tapeworms. Many animals carry parasites on and within their bodies as a normal part of life. Most parasites don't want to kill their host (because then where would they live?), but occasionally an animal's parasite load can be too high to be sustained, and the animal will die.

This probably seems like a long-winded answer to a short question, but I wanted to give you enough information to answer your question completely. I hope it helps you.

Allison J. Gong
Mad Scientist

Reference: Campbell, Reece, and Mitchell. 1999. Biology, fifth edition.