Pyramids of species richness

This post is written by PhD student Shaun Turney, and highlights a recent publication from the lab.

Two years ago, I was finishing my MSc and considering whether I’d like to do a PhD, and if so, with whom. I met with Chris and we threw around a few ideas for PhD projects. It was when he brought up a certain mystery that my decision to do a PhD in his lab was cemented. The mystery? Chris and his former PhD student Crystal Ernst were puzzled why there seem to be so many carnivores on the Arctic tundra, and relatively few herbivores to feed them.

How could it be possible? Is there a high level of cannibalism? (But then it would be like pulling oneself up by ones bootstraps — how does the energy and biomass enter the carnivore population in the first place?) Are the carnivores really omnivores? Is our methodology for sampling the tundra biota biased towards carnivores? Is the transfer of energy from herbivores to carnivores somehow more efficient (less energy loss) than in other ecosystems? These sorts of questions touch on some fundamental questions in ecology and I was hooked.

Shaun Turney, vacuuming the Tundra.

Shaun Turney, vacuuming the Tundra.

It seemed to me the logical first step would be to find out what is a typical predator-prey ratio. In what proportions are the organisms in an ecosystem divided up from plant (lowest trophic level) to top predator (highest trophic level)? The answer to that questions has already been very much explored when it comes to biomass and abundance. Charles Elton explained about 80 years ago that typically the mass and number of organisms form “pyramids”: They decrease with trophic level because energy is lost with each transfer from resource to consumer. But what about diversity? How does the number of species change with trophic level?

I decided to look at the food webs in the data base GlobalWeb to answer this question, and we just published a paper in Oikos on this topic. I found that typically ecosystems form “pyramids of species richness”, just like the pyramid of numbers and pyramid of biomass described by Elton. But some types of ecosystems, notably in terrestrial ecosystems, we can consistently observe a uniform distribution or even an “upside-down pyramid” rather than a pyramid like Elton described. That is, there are consistently cases where there more carnivore species than herbivore species in an ecosystem.

An example of aquatic compared to terrestrial food-web structure (from Turney and Buddle)

An example of aquatic compared to terrestrial food-web structure (from Turney and Buddle)

So evidently, at least when it comes to diversity, the pattern that Chris has observed in the tundra is not so unusual! The next step for me is to try to figure out why. Stay tuned!

Reference:

Turney S and CM Buddle. Pyramids of species richness: the determinants and distribution of species diversity across trophic levels. Oikos. DOI: 10.1111/oik.03404

 

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One thought on “Pyramids of species richness

  1. Pingback: Morsels For The Mind – 01/04/2016 › Six Incredible Things Before Breakfast

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