MacArthur or MacMartha? Mixed support for MacArthur’s minimisation principle

Robert MacArthur is a name familiar to many undergraduate ecology students. MacArthur’s niche theory made important contributions to the theory of ecology by describing a model whereby in a community of species that competes for a resource, the total energy wastage will be minimised over time.

Despite the importance of this theory to community ecology, it has received very little testing in real world situations. Giulia Ghedini and colleagues from the Centre for Geometric Biology along with Michel Loreau from France have used the well described, and easily manipulated, marine invertebrate, model system to test this theory.

Giulia and colleagues were able to create communities that were of different ages or ‘successional stages’, by manipulating the timing of deployment of bare Perspex plates hung upside down within Brighton marina.

Following MacArthur’s minimisation principle the team predicted that older communities (later successional stage) would have less unutilised food resources, higher maintenance costs due to metabolism and mortality, but with an overall net reduction of energy wastage. 

A real world application of this theory is understanding how susceptible a community is to invasive species. In MacArthur’s framework, late-stage communities (usually more diverse) have very little unutilised food resources, making invasion by an additional species very difficult.  However, while there are many studies that find more speciose communities are more resistant to invasion, some studies show the opposite.  But what if diversity is, in some cases, a poor predictor of resource use? Then MacArthur’s framework may help to better predict which communities are more susceptible to invasion.

In order to test all the theory, the researchers collected the plates, that hosted the different aged communities and measured how much space they occupied on the plate, how much food (phytoplankton) they consumed, their metabolic rates (change in oxygen concentrations), and also recorded mortality by mapping individuals on each plate through time.  Finally, they recorded the biomass of each community.

So, do communities minimise the wastage of energy over time as MacArthur predicted? Giulia and colleagues found mixed support for MacArthur’s minimisation principle.  While energy lost to maintenance increased in communities as they got older (due to these older communities having higher metabolic costs and higher mortality), the amount of unutilised food (energy wastage due to inefficient harvesting of available food resources) varied with successional stage and depended on the amount of phytoplankton that was available to start with.

When food was abundant, the mid-stage communities were more effective at capturing this food, but when food concentrations were low, all communities performed poorly although there was some evidence to suggest that late-stage communities were slightly better at capturing resources.

The team proposes several reasons to explain why their results do not completely correspond to MacArthur’s principle of minimisation. 

First, MacArthur’s principle holds true but acts over longer time periods and what Giulia and colleagues were measuring was an intermediate step where efficiency in food utilisation fluctuates with changes to the numbers and types of different species.  

Second, the principle might not hold true if competition is not the strongest driver of energy use in the community. Instead, other interaction types such as facilitation might be more important. In this example the high densities of barnacles in the mid-stage communities may have facilitated feeding by other members of the community by mixing the flow of water. 

Finally, it seems that more diverse communities are not necessarily better at capturing resources – which might explain why diversity can be both positively and negatively correlated to species invasions. However, we need more information before we can confidently predict how ecosystem processes and biological invasions change as communities grow older and how they will respond to changing environments. 

This research was published in the journal Ecology Letters.

Unutilised food resources depended on both successional stage and food concentration (a) with mid-stage communities performing best at medium and high food concentrations. Energy lost to maintenance increases during succession as predicted by MacArthur (b) and the pattern in graph (c) suggests that unutilised resources (graph a) are driving overall energy wastage. Energy waste per unit mass declines in later successional stages but mid-stages have lower wastage than late stages particularly under high food concentrations (d).