ADVERTISEMENT
  About the SA Blog Network













Culturing Science

Culturing Science


Biology as relevant to us earthly beings
Culturing Science Home

Don’t forget the parasites! Reevaluating the pyramid of numbers

The views expressed are those of the author and are not necessarily those of Scientific American.


Email   PrintPrint



Just like astrophysicists seek underlying patterns in space/time, ecologists seek similar patterns in life on earth. And there’s one they thought they had pegged: the pyramid of numbers.

A pyramid of numbers by W. E. Pequegnat from the January 1958 issue of Scientific American

The first known pyramidal of numbers was drawn by Charles Elton in 1927 to explain the flow of energy through ecosystems. Plants convert carbon in the air into sugars, and this bulk carbon forms the base of his pyramid. (The phytoplanktonic base in the image above.) When herbivores consume these plants, because of waste and digestive inefficiencies, the starting carbon pool shrinks, with the herbivores absorbing less carbon than they eat. And then when the herbivores are, in turn, eaten, the resulting pool of available carbon shrinks even more.

Thus, the pyramid illustrates the change in absorbed carbon with each step up from the starting pool. But they are also used to explain other patterns of energy distribution in ecosystems. Typically there are more plants than rabbits than foxes than hawks — or there are fewer individual organisms representing higher trophic levels. Similarly, there is typically more available biomass total in lower trophic levels than higher ones.

The metabolic theory of ecology combines these pyramids of numbers with temperature to describe the fundamental biological growth rate. I’ve never been good with formulas, but essentially the idea is that small animals are more abundant and grow more quickly, while larger animals are less common and grow more slowly.

And this rule is pretty widely followed — but only if we leave out those grand blood suckers and freeriders, the parasites. Parasites make up at least half of species diversity, but we tend to forget them when devising grand theories branching across life on earth.

Think about it: where would you stick a parasite in a pyramid of numbers? Feather lice, for example, live in the feathers of birds. If they are sucking the blood of a hawk, does that make them higher on the food chain than the hawk? Are they at a greater trophic level, despite being as small as they are? Does parasitism count as predation?

Basically: They mess up our theories. They don’t fit in neatly. And they’re so small that it’s easy to forget about them.

A group of researchers published a paper in the journal Science last week trying to integrate parasites into trophic ecological theory. They collected data about organisms living in three Californian estuaries, and plugged them into equations representing the metabolic theory. Unsurprisingly, the parasite data — those quickly-reproducing, infrequent and small buggers — did not fit with the accepted generalizations of the theory. But when the researchers reformulated the equations slightly to incorporate the trophic level of the organisms, the data followed a consistent pattern, suggesting a broader ecological theory.

Trophic level and primary production with body size, redrawn from the paper for clarity.

Of all the figures in the paper, one pair stood out, which I’ve redrawn below: those showing the relationships between body size with trophic level and population production. The top graph overturns a standard “pyramid of numbers” trope that trophic level increases with body size. As you can see here, the little bitty parasites at a high trophic level flip the line up into a U shape, rejected the linearity of non-parasite models.

The bottom figure goes after the “lower trophic levels produce more biomass” trope. Keep in mind that population production measures the biomass produced for the average size of an organism, not total biomass, and the entire figure is normalized for trophic level. What that straight line means is that, at each trophic level, organisms of a particular size will produce the same amount of biomass regardless of whether they’re free-living or parasitic.

The paper is titled “A Common Scaling Rule for Abundance, Energetics and Production of Parasitic and Free-living Species,” and this is the rule the authors speak of.

What does it mean? If the rule holds up, it could help those who measure land to figure out how much biomass is available in the system, based on the size of the organisms alone. They wouldn’t need to think about trophic level, period; just size. And that would simplify things a lot.

ResearchBlogging.orgHechinger, R., Lafferty, K., Dobson, A., Brown, J., & Kuris, A. (2011). A Common Scaling Rule for Abundance, Energetics, and Production of Parasitic and Free-Living Species Science, 333 (6041), 445-448 DOI: 10.1126/science.1204337

Hannah Waters About the Author: Hannah Waters writes about natural history and the way people think about nature. She lives and works in Philadelphia, PA, but really on the internet. Follow on Twitter @hannahjwaters.

The views expressed are those of the author and are not necessarily those of Scientific American.





Rights & Permissions

Comments 2 Comments

Add Comment
  1. 1. Torbjörn Larsson, OM 5:32 pm 07/26/2011

    Thank you _so_ much. I came away from another blog with the conclusion that the old theory was fine. Reading the abstract and following your hint, one can indeed surmise a simpler hypothesis. (But I am sure the strengthened connection to trophic levels can come in handy too.)

    Link to this
  2. 2. hanjeanwat 1:43 pm 08/2/2011

    Glad you found it useful. Thanks for stopping by!

    Link to this

Add a Comment
You must sign in or register as a ScientificAmerican.com member to submit a comment.

More from Scientific American

Scientific American Back To School

Back to School Sale!

12 Digital Issues + 4 Years of Archive Access just $19.99

Order Now >

X

Email this Article



This function is currently unavailable

X