Estimating energy use in communities

As ecologists, we often want to get an idea of how a community functions. For example, how much food does a community of animals consume every day? Or how much oxygen do plants produce every day? We can get an idea of these functions by measuring the energy use (or metabolism) of a community. But such functional measures can be difficult to collect, especially for an entire community. 

Since a community is made up of many species, estimating the total metabolism from the metabolism of each species separately is a way around this problem. But the methods we use, and the data required, vary and may not be validated against actual data because these are rarely available. 

Giulia Ghedini, Martino Malerba and Dustin Marshall have set out to test six different ways of estimating the energy use of a community. The authors measured actual metabolic rates in communities of phytoplankton (tiny marine microalgae) seeded with six different species of various sizes and left to change through time.

Previous work from the CGB meant the team had all the information they needed to estimate community energy use for the six methods. The different methods are summarised below.

There are a number of different ways we can estimate community metabolism that vary in complexity.

It turns out that when we are interested in whole community metabolism, size doesn’t matter. Why? Because larger species tend to be less abundant within communities, so even if each of them consumes more energy, the total energy use remains the same because there are fewer of them. While we knew this was the case for well-established communities, this new work shows that it is also true for communities as they change through time.

What is more, the team found that, in communities, the usual relationship between size and metabolism changed. Metabolism usually increases with size, but to a lesser extent for larger organisms (this is called an allometric relationship). This study showed that when measured across all species in the community, this relationship changed and the average energy use of species increased in direct proportion to their average size (this is an isometric relationship). This is why we can predict the total metabolism from the energy use per unit biomass. 

But towards the end of the experiment, when communities were dominated by larger cells, this ‘perfectly proportional’ relationship broke down. Giulia and her colleagues think that, when large species are very abundant, they suffer more from competition and reduce their metabolism more than smaller species. 

So, the good news is that we may be able to estimate community energy from easy to collect biomass data, but first, we need to see if this result applies to different communities. We also need more studies on how competition affects energy use, as the results from this study suggest they are important drivers of energy flow.

All of the methods are based on underlying theory so testing which methods are best at estimating actual metabolism can help us understand how energy use works in communities. This figure summarises what theory predicts we should have found compared to what we actually found.

This research was published in Proceedings of the Royal Society B: Biological Sciences.