I think that the reason tigers, eagles, and whales "come to mind" is becuase they are the ones that have become endangered on account of human actions. Just now are we starting to realize that we are not the only important creatures on the planet, and are trying to do something about it. The reason people probably never think about perserving nature in the form of insects and microbes is because we are so overrun by them. All the same, you are right, they too are creatures worthy of attention that they do not get. They too are a part of nature. I liked your essay. It had some good thoughts and nice imagery.