Month: March 2013

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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

Abstract

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

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Costs of dispersal alter optimal offspring size in patchy habitats: combining theory and data for a marine invertebrate

Authors: Scott C Burgess, Michael Bode and Dustin J Marshall

Published in: Functional Ecology doi: 10.1111/1365-2435.12080

Abstract

Much of the theory on offspring size focuses on the effects of habitat quality on the relationship between offspring size and fitness. Habitat spacing may be another important factor that affects selection on offspring size when offspring disperse prior to colonization and accrue deferred costs that are mediated by offspring size.

We developed a theoretical model, based on a well-known optimality model, of how selection on offspring size changes with dispersal distance. The model assumes that offspring fitness depends on both offspring size and dispersal duration and that dispersal time and distance are positively related. Such assumptions are based on thousands of marine invertebrate species with non-feeding larvae, but our model also applies more generally to any organism where offspring size modifies the energetic costs of dispersal, and there is a positive relationship between dispersal duration and distance.

Our model predicts that, even when habitat quality does not vary, more isolated habitats may favour the production of fewer, larger offspring if smaller offspring incur greater deferred costs of dispersal. We then empirically demonstrate that offspring size and dispersal duration have interactive effects on post-settlement survival in a marine invertebrate (Bugula neritina), and such size-dependent deferred costs of dispersal are of a magnitude sufficient enough to potentially favour larger offspring in isolated habitats.

Together, our results indicate that the spatial pattern of suitable habitat could impose very different selective regimes on offspring size compared with the effects of habitat quality. Furthermore, our predictions contrast to those predicted for seed size and dispersal in plants, where the production of smaller, more numerous seeds is often a more efficient way for mothers to access distant, suitable habitat.

Full paper

Burgess SC, Bode M, Marshall DJ (2013) Costs of dispersal alter optimal offspring size in patchy habitats: combining theory and data for a marine invertebrate. Functional Ecology PDFPDF 329 KB doi: 10.1111/1365-2435.12080