How well do we understand the way body size affects populations?

We have all heard the saying “live fast, die young” but it doesn’t only apply to film stars; smaller life forms also abide by this rule. Microscopic phytoplankton cells can double in numbers every few days, while the much larger elephant lives almost 100 years and reproduces slowly. This relationship between body mass and the ‘pace of life’ is well known. But the underlying mechanisms are far from resolved.

A number of years ago, scientists proposed a theory to link the ‘pace of life’ of individuals to the ecology of populations, communities, and ecosystems. The Metabolic Theory of Ecology predicts how energy use (metabolism) is affected by body size and how growth rate, development time and death rates change with energy use. These changes in individual rates will, in turn, affect rates of change in populations.

Metabolic Theory is very appealing because it explains many patterns we see in nature and is based on a fundamental rate which applies to all levels of organisation. But it is difficult to test. Detecting differences in energy use accurately requires having consistent differences in body sizes among organisms, while controlling for other variables (e.g. age, nutrition, health). This usually means comparing across species, which differ in more ways than just size.

The artificially evolved phytoplankton were almost 400 generations old when this work was done. The graph shows the considerable differences in size between small and large algae which allowed Martino and Dustin to ask their questions about body size, metabolism and populations within a species.

Few tests of these predictions have been able to control for these variables but recently Martino Malerba and Dustin Marshall were able to do this and they found that the body size-metabolism relationship did not predict population change as expected.

Martino and Dustin were in the enviable position of having access to artificially-evolved large and small phytoplankton cells of the same species that differed in size enough for them to ask “what happens to the population of a species if the average body size of individuals change?” They could see how body size affected energy use, growth rates, density and biomass.

It turns out that the story was far more complex than expected. The effect of body size on how organisms use energy and grow was very strong but also varied during the course of evolution.

So why didn’t body size explain trends in growth and energy use among size-evolved organisms? The answer may lie with previous work from the Centre for Geometric Biology. Martino and his colleagues found that large-evolved plankton cells optimized their photosynthetic pigments and produced more energy overall than smaller cells. This suggests larger cells have greater access to resources than smaller cells and so the way in which body size and metabolic rate influence the demography of a species is not as predictable as we once thought.

Instead of the classical view, where body size determines the rate at which organisms use energy, which then determines demographics, Martino and Dustin suggest that body size can affect metabolism and populations at the same time.

IIf these laboratory cultures are representative of natural populations, we would predict that current trends of reduced body size (from global warming) could lead to lower rates of population increase, biomass productivity and maximum biomass. This is the opposite of what current theory would predict. This is particularly important when we consider the role of phytoplankton in fixing carbon and supporting food chains.

This research was published in Ecology Letters.