In the balance: competition and facilitation affected by resource availability

Congratulations to Karin Svanfeldt who recently submitted her PhD and has published some of her research in The Journal of Animal Ecology, with her supervisors, Keyne Monro and Dustin Marshall.

Karin was interested in the balance between the negative effects of increased population density, (when interactions are dominated by competition for resources), versus the positive effects of increased densities which can arise from facilitation. In examples of the latter, increased densities can provide benefits by making the habitat more suitable for other individuals or through increasing the availability of basic resources. Karin particularly wanted to test how manipulating resources affected density-dependent performance in the field.

There have been few field studies that have directly manipulated environmental conditions to test these types of ideas and so Karin and her colleagues designed an experiment where they could influence both food availability and flow rates and measure the performance of an encrusting colonial bryozoan, Watersipora subtorquata.

Watersipora subtorquata, commonly known as the ‘red rust’ bryozoan.
Watersipora growing on artificial substrates.

Watersipora is sessile (non-moving) and filters resources such as food and oxygen from the surrounding water. Reducing flow rates will therefore impact resource availability. Karin was able to manipulate food availability by creating slow releasing food blocks (‘Reef Feed’ mixed with dental plaster) and flow, by boxing in the artificial substrates that were home to different densities of Watersipora colonies. Performance was measured by recording changes in size of Watersipora colonies over 13 weeks as well as the colony survival and the proportion of the colonies that had senesced.

Karin’s PhD research found evidence that resource availability altered the balance between facilitation and competition in the system they were studying. When resources were abundant, facilitation dominated and when resources were scarce, competition dominated. This work adds to existing evidence that both competition and facilitation can be important and explores how the balance between the two affects interactions within and between populations.