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 EcologyPDF DOI

Larger mothers, larger offspring

Larger larvae from the colonial bryozoan species Bugula neritina had higher survival and growth relative to smaller larvae, but when amongst siblings, smaller larvae were positively advantaged and grew as large (or even larger) than their bigger counterparts.

There may be an adaptive explanation for these findings. Larger mothers may produce larger offspring to facilitate their dispersal to habitats where they perform best (that is, in isolation from siblings) and smaller mothers may produce smaller offspring that disperse less and therefore will end up in habitats with siblings where they perform best.

The results from this study by Hayley Cameron and colleagues from the Centre for Geometric Biology and the National Oceanic and Atmospheric Administration in the USA, contrast with classical theories that predict that larger offspring are produced by larger, more fecund, mothers to offset the competitive effects of more siblings.

Bryozoan colonies growing on plates at different densities. Survival and growth was recorded after four weeks.
Bryozoan colonies growing on plates at different densities. Survival and growth was recorded after four weeks.

Identifying the mechanisms for these underlying correlations between maternal size and offspring size has important ecological implications. Average body size of individuals has been reduced in many systems, and this research suggests – for some species at least – the smaller offspring that will result from smaller mothers may still be able to perform well in the maternal habitats, but that dispersal to new habitats may be constrained.

This is one of the few field studies testing these theories. Hayley and her colleagues were able to experimentally manipulate both sibling density and offspring size of the arborescent bryozoan, Bugula neritina, and monitor the survival and growth of different-sized individual larvae following deployment in the field. Further studies will be needed to increase our understanding of why offspring size co-varies with maternal size across a range of taxa.

This research was published in the journal Ecology.

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 DOI