Field manipulations of resources mediate the transition from intraspecific competition to facilitation

Authors: Karin Svanfeldt, Keyne Monro, and Dustin J Marshall

Published in: Journal of Animal Ecology, volume 86, issue 3 (May 2017)


Population density affects individual performance, though its effects are often mixed. For sessile species, increases in population density typically reduce performance. Still, cases of positive density-dependence do occur in sessile systems and demand explanation. The stress gradient hypothesis (SGH) predicts that under stressful conditions, positive effects of facilitation may outweigh the negative effects of competition.

While some elements of the SGH are well studied, its potential to explain intraspecific facilitation has received little attention. Further, there have been questions regarding whether the SGH holds if the stressor is a resource. Most studies of interactions between the environment and intraspecific facilitation have relied on natural environmental gradients; manipulative studies are much rarer.

To test the effects of intraspecific density and resources, we manipulated resource availability over natural population densities for the marine bryozoan Watersipora subtorquata.

We found negative effects of density on colony performance in low resource environments, but mainly positive density-dependence in high resource environments. By adding resources, competition effects were reduced and the positive effects of facilitation were revealed.

Our results suggest that resource availability mediates the relative strength of competition and facilitation in our system. We also suggest that intraspecific facilitation is more common than may be appreciated and that environmental variation may mediate the balance between negative and positive density-dependence.


Svanfeldt K, Monro K, Marshall DJ (2017) Field manipulations of resources mediate the transition from intraspecific competition to facilitation. Journal of Animal Ecology, PDF 233 KB doi:10.1111/1365-2656.12644

Why do larger mothers produce larger offspring? A test of classic theory

Authors: Hayley Cameron, Keyne Monro, Martino Malerba, Stephan Munch and Dustin Marshall

Published in: Ecology, volume 97, issue 12 (December 2016)


Across a wide range of taxa, larger mothers produce larger offspring.

Theory assumes that larger, more fecund mothers create higher local densities of siblings, and so larger mothers produce larger offspring to offset sibling competition. This assumption has been debated for over 30 years, but direct empirical tests are surprisingly rare.

Here, we test two key assumptions of classic theories that predict sibling competition drives maternal-size–offspring-size (MSOS) correlations:

  1. independent effects of offspring size and sibling density on offspring performance or
  2. as a product of an interaction between these two factors.

To simultaneously test these alternative assumptions, we manipulate offspring size and sibling density in the marine invertebrate, Bugula neritina, and monitor offspring performance in the field.

We found that, depending on the fitness metric being considered, offspring size and sibling density can either independently or interactively affect offspring performance. Yet sibling density did not affect offspring performance in the ways that classic theories assume.

Given our results, it is unlikely that sibling competition drives the positive MSOS correlation observed in this species. Empirical support for these classic theories remains lacking, suggesting alternative explanations are necessary.


Cameron H, Monro K, Malerba M, Munch S, Marshall DJ (2016) Why do larger mothers produce larger offspring? A test of classic theory. Ecology, 97: 3452–3459. PDF 415 KB doi:10.1002/ecy.1590