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

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