Faster is not always better: selection on growth rate fluctuates across life history and environments

Authors: Keyne Monro and Dustin J Marshall

Published in: The American Naturalist, volume 183, number 6 (June 2014)


Growth rate is increasingly recognized as a key life-history trait that may affect fitness directly rather than evolve as a by-product of selection on size or age.

An ongoing challenge is to explain the abundant levels of phenotypic and genetic variation in growth rates often seen in natural populations, despite what is expected to be consistently strong selection on this trait. Such a paradox suggests limits to how contemporary growth rates evolve.

We explored limits arising from variation in selection, based on selection differentials for age-specific growth rates expressed under different ecological conditions. We present results from a field experiment that measured growth rates and reproductive output in wild individuals of a colonial marine invertebrate (Hippopodina iririkiensis), replicated within and across the natural range of succession in its local community.

Colony growth rates varied phenotypically throughout this range, but not all such variation was available for selection, nor was it always targeted by selection as expected.

While the maintenance of both phenotypic and genetic variation in growth rate is often attributed to costs of growing rapidly, our study highlights the potential for fluctuating selection pressures throughout the life history and across environments to play an important role in this process.

Full paper

Monro K, Marshall DJ (2014) Faster isn’t always better: selection on growth rate fluctuates across the life history and environments. The American Naturalist, 183(6): 798–809 PDFPDF 393 KB doi:10.1086/676006

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

Evolutionary constraints and the maintenance of individual specialization throughout succession

Authors: Keyne Monro and Dustin J Marshall

Published in: Evolution, volume 67, issue 12 (December 2013)


Constraints on life-history traits, with their close links to fitness, are widely invoked as limits to niche expansion at most organiza- tional levels.

Theoretically, such constraints can maintain individual specialization by preventing adaptation to all niches available, but empirical evidence of them remains elusive for natural populations. This problem may be compounded by a tendency to seek constraints involving multiple traits, neglecting their added potential to manifest in trait expression across environments (i.e., within reaction norms).

By replicating genotypes of a colonial marine invertebrate across successional stages in its local community, and taking a holistic approach to the analysis of ensuing reaction norms for fitness, we show the potential for individual specialization to be maintained by genetic constraints associated with these norms, which limit the potential for fitness at one successional stage to improve without loss of fitness at others.

Our study provides new insight into the evolutionary maintenance of individual specialization in natural populations and reinforces the importance of reaction norms for studying this phenomenon.

Full paper

Monro K, Marshall DJ (2013) Evolutionary constraints and the maintenance of individual specialization throughout succession. Evolution 67(12): 3676–3644 PDFPDF 495 KB doi:10.1111/evo.12220

Revisiting competition in a classic model system using formal links between theory and data

Authors: Simon P Hart, Jacqueline R Burgin and Dustin J Marshall

Published in: Ecology, volume 93, issue 9, doi: 10.1890/11-2248.1


Formal links between theory and data are a critical goal for ecology. However, while our current understanding of competition provides the foundation for solving many derived ecological problems, this understanding is fractured because competition theory and data are rarely unified.

Conclusions from seminal studies in space-limited benthic marine systems, in particular, have been very influential for our general understanding of competition, but rely on traditional empirical methods with limited inferential power and compatibility with theory.

Here we explicitly link mathematical theory with experimental field data to provide a more sophisticated understanding of competition in this classic model system. In contrast to predictions from conceptual models, our estimates of competition coefficients show that a dominant space competitor can be equally affected by interspecific competition with a poor competitor (traditionally defined) as it is by intraspecific competition.

More generally, the often-invoked competitive hierarchies and intransitivities in this system might be usefully revisited using more sophisticated empirical and analytical approaches.

Full paper

Hart SP, Burgin JR, Marshall DJ (2012) Revisiting competition in a classic model system using formal links between theory and data. Ecology, 93(9) 2015–2022 PDFPDF 777 KB doi: 10.1890/11-2248.1