On Earth Day, Remember Parasites

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When the mammoth became extinct, its specialist parasites became extinct as well. Adapted from image courtesy of Tracy O: CC BY-SA 2.0

On Earth Day 2013, people will walk, run, and cycle. They’ll plant trees, install rain barrels, and attend events featuring the natural world. The chances are that none of those events will feature a parasite, but some of them probably should, because ecosystems everywhere depend on parasites, and we are losing them.

In general, people don’t love invertebrates: bugs, worms, spiders, slugs, and jellyfish, to name a few. And most people have a special loathing for invertebrate parasites that live in or on other things.

We have good reason to dislike parasites: even those that speak up for them (Gomez et al) describe them as “a diverse suite of organisms that threaten the health of individuals and populations.” These same scientists argue, however, that if we consider the positive things these creatures do, and the threats they face, we “severely overlook the former and underestimate the latter.”

 Why the Earth Needs Parasites

It’s hard to intuitively see why we need parasites. Even scientists tend to see them as bad. But in recent years, many areas of scientific research have revealed just how important parasites are. Estimates of parasite diversity reveal a startling fact: there are more species of parasites than there are species of free living organisms, and there are many not yet known to science. We are outnumbered. Imagine if we eradicated all parasites: more than half of the life on Earth would be gone.

The remaining species would likely follow soon after, because those parasites are a vital part of food webs, where every species is being fed, one way or another. Gomez and colleagues write that there are “more parasite–host links on average than predator–prey links…. empirical evidence demonstrates a positive influence of parasites on food web[s].” Parasites also keep populations in balance. They directly kill some organisms of course, but they can also make infected hosts more likely to be eaten by a predator, or interfere with the host’s ability to reproduce.

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