Humans are continually modifying the marine environment either directly, with activities such as fishing, or indirectly as with climate change or the introduction of invasive species. A common consequence of these activities is a change in the body size of individuals that make up an ecological community. 

Understanding the impacts of such changes on the way in which communities gain and use energy is of particular interest to Giulia Ghedini, a post-doc in the Centre for Geometric Biology.

“We know that human impacts can change the size of organisms and we also know that the size of an organism determines the speed at which it uses resources and contributes to the flow of energy within a system” explains Dr Ghedini.

“Understanding how changes in the ‘geometry’ of a whole community might affect ecosystem functioning through changes in metabolic rates is not only theoretically interesting but of practical significance as well” she said. 

Metabolism measurements indicate how much oxygen and food an individual, or an entire community, consumes. Understanding how changes in individual body size affect the energy use of whole communities provides direct information on the amount of resources required for these communities to live. 

Researchers from the Centre for Geometric Biology at Monash University were able to test predictions that older communities, made up of larger organisms, would have lower metabolic rates per unit mass than younger communities of smaller individuals. 

“We know that increases in metabolic rates slow down as organisms get larger – and we wanted to know if this same pattern occurs at the level of whole communities” said Dr Ghedini.

To their surprise, the research team found that the community metabolic rates remained directly proportional to total community mass as communities got older and larger, which contrasted with the way metabolic rate scaled with changes in size of the dominant species. 

“But,” said Dr Ghedini “when we deconstructed the community into individuals and calculated their individual metabolic rates based on their size and species-specific metabolism, we found that community rates were largely the sum of their parts with respect to metabolism.” 

Measuring metabolism of a whole community can be hard, and so studies frequently estimate community metabolism from the dominant species in that community; we now know that for these estimates to be accurate we need to know the sizes of the individuals that make up the community.

“We also found that as communities got older, the same area was able to support much higher biomasses and energy use – three times as much as the younger communities. We attributed this to changes in the shape of the community; that is, a more 3D structure allowed certain individuals greater access to food in the water column and increased oxygen delivery via increased water flow.”

Changes in rates of energy use have long been used as an indicator of change in ecosystem function. 

By unravelling the relationship between the size of individuals and the energy use of whole communities, this study will help us predict how changes in the geometry of communities will impact on the use of resources; a measure of ecosystem function.

This research was published in a special issue of the journal Functional Ecology.