Bugula neritina

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Avoiding low-oxygen environments: oxytaxis as a mechanism of habitat selection in a marine invertebrate

Authors: Marcelo E Lagos, Craig R White and Dustin J Marshall

Published in: Marine Ecology Progress Series, volume 540 (November 2015)

Abstract

Oxygen-poor habitats are increasingly common in aquatic environments. Human activities are accelerating the spread of oxygen-poor environments, yet the way in which larvae avoid low-oxygen conditions remains poorly resolved.

For organisms with a sessile or sedentary adult phase, habitat selection is crucial, and many organisms show sophisticated responses to various habitat cues during colonization.

Whether oxygen availability serves as such a cue is unknown, yet increasingly, it seems that oxygen is an essential limiting resource in some systems.

In a series of experiments, we manipulated oxygen levels during dispersal and colonization in larvae of the model marine invertebrate Bugula neritina in the laboratory.

We found that, in the presence of lower oxygen levels, larvae reduce the time spent in habitat exploration and that they delay settlement. We also found that larvae avoid hypoxic water (positive oxytaxis) — the first such demonstration for marine larvae. All of these behaviors may decrease the likelihood of colonizing low-oxygen habitats in nature.

Our results suggest that marine invertebrate larvae, in this species at least, can use oxygen availability as an initial cue for habitat selection but that additional factors (e.g. biofilms) determine settlement patterns.

Citation

Lagos ME, White CR, Marshall DJ (2015) voiding low-oxygen environments: oxytaxis as a mechanism of habitat selection in a marine invertebrate, Marine Ecology Progress Series, 540 99–107 PDF (332 KB) doi:10.3354/meps11509

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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)

Abstract

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.

Citation

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

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Environmental stress, facilitation, competition, and coexistence

Authors: Simon P Hart and Dustin J Marshall

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

Abstract

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

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How do dispersal costs and habitat selection influence realized population connectivity?

Authors: Scott C Burgess, Eric A Treml and Dustin J Marshall

Published in: Ecology, volume 93, issue 6, doi: 10.1890/11-1656.1

Abstract

Despite the importance of dispersal for population connectivity, dispersal is often costly to the individual.

A major impediment to understanding connectivity has been a lack of data combining the movement of individuals and their survival to reproduction in the new habitat (realized connectivity).

Although mortality often occurs during dispersal (an immediate cost), in many organisms costs are paid after dispersal (deferred costs). It is unclear how such deferred costs influence the mismatch between dispersal and realized connectivity.

Through a series of experiments in the field and laboratory, we estimated both direct and indirect deferred costs in a marine bryozoan (Bugula neritina). We then used the empirical data to parameterize a theoretical model in order to formalize predictions about how dispersal costs influence realized connectivity.

Individuals were more likely to colonize poor-quality habitat after prolonged dispersal durations. Individuals that colonized poor-quality habitat performed poorly after colonization because of some property of the habitat (an indirect deferred cost) rather than from prolonged dispersal per se (a direct deferred cost).

Our theoretical model predicted that indirect deferred costs could result in nonlinear mismatches between spatial patterns of potential and realized connectivity.

The deferred costs of dispersal are likely to be crucial for determining how well patterns of dispersal reflect realized connectivity. Ignoring these deferred costs could lead to inaccurate predictions of spatial population dynamics.

Full paper

Burgess SC, Treml EA, Marshall DJ (2012) How do dispersal costs and habitat selection influence realized population connectivity? Ecology 93: 1378–1387 PDFPDF 622 KB doi: 10.1890/11-1656.1