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

Limiting resources in sessile systems: food enhances diversity and growth of suspension feeders despite available space

Authors: J Robin Svensson and Dustin J Marshall

Published in: Ecology, volume 96, issue 3 (March 2015)


Much of our understanding of competition comes from observations in sessile systems, such as rainforests and marine invertebrate communities.

In terrestrial systems, sessile species often compete for multiple limiting resources (i.e., space, light, and nutrients), but in marine systems, space is viewed as the primary or sole limiting resource. Competition theory, on the other hand, suggests that competition for a single limiting resource is unlikely to maintain high species diversity, but manipulative tests of competition for other resources in marine benthic systems are exceedingly rare.

Here, we manipulate the availability of food for a classic system, marine sessile invertebrate communities, and investigate the effects on species diversity, abundance, and composition during early succession as well as on the growth of bryozoan populations in the field.

We found the number of species to be greater, available space to be lower, and the community composition to be different in assemblages subjected to increased food availability compared to controls. Similarly, laboratory-settled bryozoans deployed into the field grew more in the presence of enhanced food.

Our results suggest that food can act as a limiting resource, affecting both diversity and abundance, even when bare space is still available in hard-substratum communities. Consequently, broadening the view of resource limitation beyond solely space may increase our understanding and predictability of marine sessile systems.


Svensson R, Marshall DJ (2015) Limiting resources in sessile systems: food enhances diversity and growth of suspension feeders despite available space, Ecology, 96(3) 819–827 PDF 836 KB doi:10.1890/14-0665.1

Competition in benthic marine invertebrates: the unrecognised role of exploitative competition for oxygen

Authors: Nick Ferguson, Craig R White and Dustin J Marshall

Published in: Ecology, volume 94, issue 1 (January 2013) doi: 10.1890/12-0795.1


Competition is a ubiquitous structuring force across systems, but different fields emphasize the role of different types of competition.

In benthic marine environments, where some of the classic examples of competition were described, there is a strong emphasis on interference competition: marine invertebrates are assumed to compete fiercely for the limiting resource of space. Much of our understanding of the dynamics of this system is based on this assumption, yet empirical studies often find that increases in density can reduce performance despite free space being available. Furthermore, the assumption that space is the exclusively limiting resource raises paradoxes regarding species coexistence in this system.

Here, we measure the availability of oxygen in the field and in the laboratory, as well as the tolerance of resident species to low-oxygen conditions.

We show that oxygen can be the primary limiting resource in some instances, and that exploitative competition for this resource is very likely among benthic marine invertebrates. Furthermore, growth form (and the associated risk of oxygen limitation) covaries with the ability to withstand oxygen-poor conditions across a wide range of taxa.

Oxygen availability at very small scales may influence the distribution and abundance of sessile marine invertebrates more than is currently appreciated. Furthermore, competition for multiple resources (space and oxygen) and trade-offs in competitive ability for each may promote coexistence in this system.

Full paper

Ferguson N, White CR, Marshall DJ (2013) Competition in benthic marine invertebrates: the unrecognised role of exploitative competition for oxygen. Ecology, 94(1) 126–135 PDFPDF 311 KB 311 doi: 10.1890/12-0795.1