You don’t need an island, you ARE one
Did you know, you are an island? And that I am an island, too, and that we all are “islands”? Simon and Garfunkel must have known, for they sang “I am a rock, I am an island” and Kenny Rogers and Dolly Parton also knew that we are “islands in the stream”.
For organisms living in or on a person, that person does indeed represent an island. From the moment of our births, humans like you and me, are getting colonized by a variety of organisms, which is why I had intended to produce a film titled “The colonization of man” when working as a scientific advisor with a Television company in my first job after graduation from university. I never managed to make that film and produced another instead (“Plants that prey on animals”). Yet, humans as islands have remained an interest of mine. The colonization of a person by parasites and commensals (the latter harmless occupants of humans, not causing sickness or discomfort) obeys rules that have been worked out for proper geographic islands.
What determines the number of species co-inhabiting an island depends on the size of the island and the number of niches available on it; it further depends on the island’s distance to the nearest place from which immigrant species can arrive and on the number of species emigrating from the island or becoming extinct on it. Obviously, a large, hairy body can be expected to harbour more species than a small hairless one. Take for instance the case of a sloth. These slow, dog-sized phytophagous animals of the South American jungle grow green algae in their shaggy fur and because of that the fur of a sloth is an ecosystem of its own. Not only does one find the usual ectoparasites like ticks and fleas, biting flies and lice in them; no, these furs also host large numbers of non-parasitic species. More than 900 species of arthropods, including maggots, beetles, moths, mites, flies and spiders have been reported from the sloth.
One reason for this popularity of the sloth is the fact that many of the sloth’s inhabitants are coprophages, i.e., dung-eaters. Since sloth’s defaecate infrequently it makes perfect sense to live on the supplier of the dung, which happens to be the animal itself. Organisms colonizing human bodies have other goals. Of the 300 or so species of microorganisms associated with humans (i.e., bacteria, protozoans, fungi), 80 species alone live in the mouth. A further 100 call the gut their home and the rest inhabit the skin. There can be larger animals as well: pin-, hook-, and tape worms love our intestines, ticks, fleas, and lice are equally fond of our bodies, and we could mention a whole army of disease-causing species invading the brain, lungs, blood, lymphatic, urinary, and genital systems. Luckily, in most cases we know how to combat these unwanted colonizers.
People usually don’t like to hear or know that they are hosting a multitude of organisms, which is why I used a trick when I taught the parasite course at my university in New Zealand. I promised to give a bar of chocolate to the first student who could squeeze a follicle mite (Demodex folliculorum) out of the pores of his or her forehead or cheek. That trick usually worked and we regularly obtained specimens of these wormlike, ubiquitous and harmless occupants of our face to examine them alive under the microscope. But what puzzled me in all those years I conducted these parasite exercises: only boys and not a single female student ever won the bar of chocolate. Don’t girls like chocolate? Or did they perhaps have other reason for not finding any follicle mites in their faces?
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2016.
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