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

Two sexes, one body: intra- and intersex covariation of gamete phenotypes in simultaneous hermaphrodites

Authors: Keyne Monro and Dustin J Marshall

Published in: Ecology and Evolution doi:10.1002/ece3.1035


By harboring male and female functions in the same genome and expressing them in every individual, simultaneous hermaphrodites may incur sexual conflict unless both sex functions can evolve phenotypic optima independently of each other.

The first step toward understanding their capacity to do so lies in understanding whether sex functions are phenotypically correlated within individuals, but remarkably few data address this issue.

We tested the potential for intra- and intersex covariation of gamete phenotypes to mediate sexual conflict in broadcast-spawning hermaphrodites (the ascidians Ciona intestinalis and Pyura praeputialis), for which sex-specific selection acts predominantly on sperm–egg interactions in the water column.

In both species, gamete phenotypes covaried within and across sex functions, implying that selection may be unable to target them independently because its direct effects on male gametes translate into correlated effects on female gametes and vice versa. This alone does not preclude the evolution of a different phenotypic optimum for each sex function, but imposes the more restrictive requirement that selection – which ultimately sorts among whole individuals, not sex functions – aligns with the direction in which gamete phenotypes covary at this level.

Full paper

Monro K, Marshall DJ (2014) Two sexes, one body: intra- and intersex covariation of gamete phenotypes in simultaneous hermaphrodites. Ecology and Evolution PDFPDF 274 KB doi:10.1002/ece3.1035

Environmental stress, facilitation, competition, and coexistence

Authors: Simon P Hart and Dustin J Marshall

Published in: Ecology, volume 94, issue 12 (December 2013)


The major theories regarding the combined influence of the environment and species interactions on population and community dynamics appear to conflict.

Stress/disturbance gradient models of community organization, such as the stress gradient hypothesis, emphasize a diminished role for competition in harsh environments whereas modern coexistence theory does not.

Confusion about the role of species interactions in harsh environments is perpetuated by a disconnect between population dynamics theory and data.

We linked theory and data using response surface experiments done in the field to parameterize mathematical, population-dynamic competition models. We replicated our experiment across two environments that spanned a common and important environmental stress gradient for determining community structure in benthic marine systems. We generated quantitative estimates of the effects of environmental stress on population growth rates and the direction and strength of intra- and interspecific interactions within each environment.

Our approach directly addressed a perpetual blind spot in this field by showing how the effects of competition can be intensified in stressful environments even though the apparent strength of competition remains unchanged.

Furthermore, we showed how simultaneous, reciprocal competitive and facilitative effects can stabilize population dynamics in multispecies communities in stressful environments.

Full paper

Hart SP, Marshall DJ (2013) Environmental stress, facilitation, competition and coexistence. Ecology, 94(12): 2719–2731 PDFPDF 1.7 MB doi:10.1890/12-0804.1

PhD positions available: the evolutionary ecology and/or quantative genetics of marine invertebrates

Two fully-funded PhD stipends are available to students interested in working on the evolutionary ecology of marine invertebrates in Dr Dustin Marshall’s group.

The stipends include all course fees plus ~$25,000 AUD per annum tax-free (the equivalent of  approx. $33,000 before tax) with no teaching requirements.

I can guarantee funding of project costs including the costs of attending at least one conference per year.

Project start dates can be any time in 2012.

Interested students should send their CVs, a brief statement of their interests and the contact details of two referees.

To be eligible, applicants must have completed at least one year of post-graduate research and ideally a masters degree.

Preference will be given to those with strong quantitative skills.