How much an individual organism can grow depends on both the amount of energy acquired (from food consumption) and the energy expended on respiration (metabolic rate). Increasing population densities can affect the availability of energy by increasing competition for resources (such as food) which can, in turn, affect the rates at which individuals acquire and expend energy.
Post-doctoral fellow, Giulia Ghedini, and colleagues Craig White and Dustin Marshall are interested in whether feeding rates and metabolic rates change in the same way when population densities increase. They wanted to explore why individuals typically reach smaller sizes in denser populations while knowing that growth ultimately depends on how rates of energy intake change relative to rates of energy expenditure.
The research team considered four possible scenarios where the rates of feeding and metabolism varied and which would have different outcomes in terms of the energy available for growth (see figure). Few studies have considered both these rates at the same time and so the ways in which both intake (feeding) and expenditure (metabolism) are affected by density remains unclear making it difficult to predict which scenario would apply.
Giulia and colleagues used the colonial bryozoan Bugula neritina as a study organism and experimentally created populations of different densities by settling increasing numbers of larvae on experimental plates. After metamorphosis these larvae developed into adult colonies that were grown in the field at the different population densities ranging from 1 to 30 individual colonies per plate.
The different populations of Bugula were supplied with equal amounts of food (a single celled alga) and feeding rates were calculated after three hours. Metabolic rates were calculated by measuring changes in percentage oxygen over the same period of time (three hours) and the ‘scope for growth’ of each individual colony was calculated by subtracting energy expenditure (metabolism) from energy intake (feeding).
Giulia and her colleagues found that while both feeding rates and metabolic rates decreased with increasing population densities, energy gains from food intake decreased faster than the energy expended on metabolism, reducing the amount of energy available for growth (scenario 3 on figure). This explains why individual growth and reproductive output decrease in denser populations.
What is more, the researchers have also demonstrated that the difference between energy intake and expenditure at the individual colony level could predict the average body size that colonies reached in populations of varying densities.