Can communities minimise energy wastage over time?

Understanding how energy is used (or not used) in individuals, populations and communities has fascinated biologists for many years. Giulia Ghedini, an Australian Research Council research fellow in the Centre for Geometric Biology, is particularly interested in how communities use energy. Giulia and her colleagues have found that older communities of marine phytoplankton waste less energy than new or early successional communities.

In a previous post, we described how Giulia and her colleagues used marine invertebrates to test whether ‘older’ communities minimised energy wastage as predicted by Robert MacArthur in 1969. MacArthur basically said that, due to competition for resources, a community of species will, over time, maximise the efficiency with which it consumes resources meaning that older communities have very little energy (resource) wastage.

If MacArthur was right, then we can better understand and predict how disturbances that change the age or successional stage of a community might impact community function and invasion risk.  An older community might be able to sustain a greater biomass with the same resource requirements of a younger community. Not only that, but a community that efficiently uses all the energy or resources available makes it difficult for a new species to gain a foothold.

While the team found some support for MacArthur’s theory in their initial study, it was far from clear-cut and they thought that this might be because their experimental setup allowed for predation and immigration; MacArthur’s theory relates to a closed system where competition between species is the driving force for community structure and function.

To tackle this problem Giulia and her colleagues designed a new experimental setup using a completely different system: marine phytoplankton. They were able to create ‘starter’ assemblages of phytoplankton in the lab. They used six species and then followed these assemblages through time measuring energy inputs (light and nutrients) and energy use (photosynthesis, metabolism and overall production). This allowed them to work out how much energy from resources was not used and how much was lost to maintenance  – metabolism and mortality – the two components of MacArthur’s ‘wastage’. They also tracked changes in species composition and the size structure of the communities to understand their links with energy wastage.

The team found some marked changes in the phytoplankton communities. Larger species became more dominant over time and this was consistent in all 20 of the experimental communities. Surprisingly, neither changes in dominant species nor the homogenisation of communities were driving changes in energy flux.

So, while the composition of the species changed in a consistent way as the communities ‘aged’, the functioning of communities tracked less consistently, meaning that the oldest communities were not necessarily the most energy efficient. But there was a general pattern that fitted with MacArthur’s prediction that older more established communities would minimise energy wastage. And, as predicted, the changes were due to the two components of energy use changing in opposite directions.

The combined results from the team’s work suggest that overall patterns in community function might be predictable over time and that MacArthur’s minimisation principle might apply across very different systems.

Tracking species composition and energy use through time enabled Giulia and her colleagues to see that as the community was better able to harvest and use the available resources so that resource waste decreased over time (green), more energy was lost to metabolism and death (magenta). The black line represents the community trade off between the ability to uptake and use resources and the costs of maintenance that this competitive ability requires.

This research was published in the journal Ecology.