Parasite biodiversity – a missing dimension?

This post was originally published on BioDiverse Perspectives – a research blog aimed at fostering communication about biodiversity.

Here are a few statistics:

  • Forty-percent of all species are parasites, and more than 75% of links in natural food webs are likely to involve them.
  • As many as 10,000 parasitic helminth species are threatened with extinction.
  • Decreases in avian diversity due to habitat loss and climate change will contribute to even greater parasite species loss in the future.

Parasites are everywhere and outnumber what we can see by a huge margin. So what does this mean when we start losing all these parasite species?

In their 2008 paper in PNAS, Dobson et al. had three objectives: 1) show exactly how abundant parasites are compared to other organisms, 2) estimate how many parasites are threatened with extinction, and 3) evaluate the potential impacts of parasite extinction.

They started by looking at previous estimates of parasite diversity, concluding that there could be over 300,000 parasitic helminth species that use vertebrates as hosts. They then took an alternate approach – evaluating all organisms in single habitat – and asked, how many parasites are there here? Their conclusion: at least 40% of all species in marshes along the California Coast are parasites. Not only that, but the structure of food webs changes dramatically depending on whether or not you include parasites in it.

Next, they looked at expected host extinction rates, and asked at what rate are parasites going to go extinct? Then they used theory on host-parasite interactions to ask, what can we expect as consequences of parasite extinctions?

This paper is a really cool example of what a talented group of researchers* can do when they really dig in to three simple questions about biodiversity and biodiversity loss, but here’s the reason that this paper should be considered a frontier in biodiversity research:

The authors fundamentally and meaningfully argued for a change in the way scientists describe biodiversity by showing that including parasites dramatically changes our understanding of global patterns of diversity, food-web structure, and the consequences of environmental change.  And in doing so, they challenged my notions about generality in ecological research.

What does it mean that 90% of biodiversity research addresses about half of biodiversity? Maybe nothing – Hechinger et al. (2011) argue that parasites obey similar ecological rules as free-living organisms when it comes to abundance, energetics, and production. And if parasites obey the same ecological rules as their free-living counterparts, then maybe it’s not that big of a deal that most ecological research ignores them. However, a recent meta-analysis by Kamiya et al. from the University of Otago in New Zealand suggests that parasite biodiversity may be structured by entirely different processes than those controlling the diversity of free-living organisms. If this is the case, then maybe we do have a problem.

I am aware of ongoing debates on the value of model systems vs. purely empirical, system-specific work in gleaning ecologically relevant information. Generally, I’m of the opinion that there is value in all of these approaches. Yes, while many specific ecological systems can be context dependent, theoretical models and microcosm experiments can tell us a lot about generality despite context dependence. But what if it’s not the context-dependence that we’re getting wrong. What if general biodiversity research is only targeting half of biodiversity? How general are even the most general of theories then?

Ok, so parasites are a diverse group, and much of biodiversity research has historically failed to address them. Are there other systems that biodiversity research has failed to do justice to?

*And a courteous airline staff? A note from the acknowledgements of this paper: ”The first draft of the article was written in Kilimanjaro, Nairobi, and Heathrow Airports; A.D. thanks British Airways and Precision Air for the patience, care, and attention of their ground staff.”

6 Replies to “Parasite biodiversity – a missing dimension?”

  1. Thanks for the article Fletcher! I admit I don’t think about parasites a lot (ok, ever), but when has that ever stopped me? A few thoughts:

    -parasites by definition benefit at the expense of their hosts. Ecological theory is rife with examples of how diversity enhances resource use. So, at the risk of putting a value judgment on things, isn’t loss of parasite diversity beneficial? Or do ecosystems (and by extension, human beings) somehow benefit from parasites? Enlighten me!

    -are many parasites specialists, in the sense they only parasitize one species? Is it ever the case that diversity of parasites > diversity of hosts? So if there is an approximately linear relationship between parasite diversity and host diversity, then isn’t it entirely appropriate to investigate ecosystem processes using host diversity, because you’re largely capturing variance in parasite diversity as well? Or are the ecological expectations different so host diversity is an inappropriate proxy? You mention CA marshes where parasites changed the food web structure. I would be interested to know more!

    Cheers, Jon

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  2. Hey Jon,

    First, I think parasites provide pretty valuable ecosystem services. To draw from the Dobson et al article, parasites can play important roles in regulating host populations (one example is thru Janzen-Connell type processes, another might be thru apparent competition). The Dobson paper also makes an argument that parasites actually serve to regulate pollutant concentrations. In those ways, losing parasites could contribute to very different ecosystem services.

    Unfortunately, my answer to your second question is a lot more fuzzy. Yes, many species are specialists, but many species are generalists too… There’s a great quote in the Dobson paper (in the section titled “How many parasite species per host species?”): “In the best-studied taxa, an average mammalian host species appears to harbor two cestodes, two trematodes, and four nematodes, and an acanthocephalan is found in every fourth mammalian species examined.”

    Recently, there’s been some push to compare parasite ecology to metacommunity ecology. Some patches (hosts) harbor many parasites, while others harbor just a few, and there are a lot of processes that may contribute to patterns of parasite species richness within and between hosts, such as body size, host density, etc. As a result, a strictly linear relationship between parasite and host diversity isn’t necessarily expected. Also, if hosts vary in competence (the ability to transmit parasites) and susceptibility (the ability to become infected with parasites), then you might expect that the presence of specific hosts might control parasite richness. So processes that control specific host community structure might be important for controlling parasite richness. Finally, since (as I noted above) parasites can control the structure of host communities, there might be feedbacks between host and parasite species richness…

    There have been a lot of cool studies of food web structure with parasites. Two that come to mind are Lafferty et al Ecol Letters (linked in the image caption), and a recent article in PlosOne by Dunne et al (and a lot of the same authors as the Dobson et al paper)

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