July 26, 2011 | 2
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.
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.
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.
Hechinger, 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