Temperature effects on mass-scaling exponents in colonial animals: a manipulative test

Authors: Diego R Barneche, Craig R White and Dustin J Marshall

Published in: Ecology, volume 98, issue 1 (January 2017)

Abstract

Body size and temperature are fundamental drivers of ecological processes because they determine metabolic rates at the individual level. Whether these drivers act independently on individual-level metabolic rates remains uncertain.

Most studies of intraspecic scaling of unitary organisms must rely on pre-existing differences in size to examine its relationship with metabolic rate, thereby potentially confounding size-correlated traits (e.g., age, nutrition) with size, which can affect metabolic rate.

Here, we use a size manipulation approach to test whether metabolic mass scaling and temperature dependence interact in four species (two phyla) of colonial marine invertebrates. Size manipulation in colonial organisms allows tests of how ecological processes (e.g., predation) affect individual physiology and consequently population- and community-level energy flux.

Body mass and temperature interacted in two species, with one species exhibiting decreased and the other increased mass-scaling exponents with increasing temperature.

The allometric scaling of metabolic rate that we observe in three species contrasts with the isometric scaling of ingestion rates observed in some colonial marine invertebrates.

Thus, we suggest that the often observed competitive superiority of colonial over unitary organisms may arise because the difference between energy intake and expenditure increases more strongly with size in colonial organisms.

Citation

Barneche DR, White CR, Marshall DJ (2016) Temperature effects on mass-scaling exponents in colonial animals: a manipulative test, Ecology, 98(1), 2017, pp. 103–111 PDF 534 KB doi:10.1002/ecy.1624

The other 96%: Can neglected sources of fitness variation offer new insights into adaptation to global change?

Authors: Evatt Chirgwin, Dustin J  Marshall, Carla M Sgrò, and Keyne Monro

Published in: Evolutionary Applications, volume 10, issue 3 (March 2017)

Abstract

Mounting research considers whether populations may adapt to global change based on additive genetic variance in fitness. Yet selection acts on phenotypes, not additive genetic variance alone, meaning that persistence and evolutionary potential in the near term, at least, may be influenced by other sources of fitness variation, including nonadditive genetic and maternal environmental effects. The fitness consequences of these effects, and their environmental sensitivity, are largely unknown.

Here, applying a quantitative genetic breeding design to an ecologically important marine tubeworm, we examined nonadditive genetic and maternal environmental effects on fitness (larval survival) across three thermal environments.

We found that these effects are nontrivial and environment dependent, explaining at least 44% of all parentally derived effects on survival at any temperature and 96% of parental effects at the most stressful temperature. Unlike maternal environmental effects, which manifested at the latter temperature only, nonadditive genetic effects were consistently significant and covaried positively across temperatures (i.e., parental combinations that enhanced survival at one temperature also enhanced survival at elevated temperatures).

Thus, while nonadditive genetic and maternal environmental effects have long been neglected because their evolutionary consequences are complex, unpredictable, or seen as transient, we argue that they warrant further attention in a rapidly warming world.

Citation

Chirgwin E, Marshall DJ, Sgrò CM, Monro K (2016) The other 96%: Can neglected sources of fitness variation offer new insights into adaptation to global change? Evolutionary Applications, 10(3) PDF 445 KB doi: 10.1111/eva.12447

The evolution of reproductive phenology in broadcast spawners and the maintenance of sexually antagonistic polymorphism

Authors: , and Tim Connallon

Published in: The American Naturalist, volume 189, number 2 (February 2017)

Abstract

Reproductive phenology is a crucial life-history trait that evolves in response to external environmental conditions and frequency- and density-dependent interactions within species.

Broadcast spawners — which represent a large fraction of aquatic biodiversity — evolve phenologies that balance strong density-dependent fertilization success against abiotic environmental conditions that are required for successful reproduction. The overall balance between these processes may be particularly complex in dioecious species, where selection on reproductive timing potentially differs between the sexes.

Here, we develop a population genetic model of reproductive phenology in a dioecious broadcast spawning species and show that environmental variability and density-dependent fertilization dynamics naturally give rise to profound sex differences in selection on gamete release strategies.

The frequency-dependent nature of sperm competition generates sexually antagonistic selection on reproductive timing and facilitates the maintenance of genetic variation in phenological traits. Selection in females favors monomorphic spawning phenologies that maximize net fertilization success and offspring survival across environmental conditions, whereas selection in males often favors polymorphic phenologies that are primarily shaped by sperm competition.

Our model helps explain several well-documented empirical observations in aquatic species, including high intraspecific variance of reproductive phenologies, sex-specific spawning phenologies, and spawning during environmentally suboptimal times.

Citation

Olito C, Marshall DJ, Connallon T (2017) The evolution of reproductive phenology in broadcast spawners and the maintenance of sexually antagonistic polymorphism, The American Naturalist, 189(2) PDF 717 KB, doi:10.1086/690010

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)

Abstract

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.

Citation

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 415 KB doi:10.1002/ecy.1590

Spatial pattern of distribution of marine invertebrates within a subtidal community: do communities vary more among patches or plots?

Authors: Chun‐Yi Chang and Dustin J Marshall

Published in: Ecology and Evolution, volume 6, issue 22 (November 2016)

Abstract

Making links between ecological processes and the scales at which they operate is an enduring challenge of community ecology.

Our understanding of ecological communities cannot advance if we do not distinguish larger scale processes from smaller ones.

Variability at small spatial scales can be important because it carries information about biological interactions, which cannot be explained by environmental heterogeneity alone.

Marine fouling communities are shaped by both the supply of larvae and competition for resources among colonizers—these two processes operate on distinctly different scales.

Here, we demonstrate how fouling community structure varies with spatial scale in a temperate Australian environment, and we identify the spatial scale that captures the most variability. Community structure was quantified with both univariate (species richness and diversity) and multivariate (similarity in species composition) indices.

Variation in community structure was unevenly distributed between the spatial scales that we examined. While variation in community structure within patch was usually greater than among patch, variation among patch was always significant.

Opportunistic taxa that rely heavily on rapid colonization of free space spread more evenly among patches during early succession. In contrast, taxa that are strong adult competitors but slow colonizers spread more evenly among patches only during late succession.

Our findings show significant patchiness can develop in a habitat showing no systematic environmental spatial variation, and this patchiness can be mediated through different biological factors at different spatial scales.

Citation

Chang CY, Marshall DJ (2016) Spatial pattern of distribution of marine invertebrates within a subtidal community: do communities vary more among patches or plots? Ecology and Evolution 6(22)83308337 PDF 522 KB doi: 10.1002/ece3.2462

Environment-dependent variation in selection on life history across small spatial scales

Authors: Rolanda Lange, Keyne Monro and Dustin J Marshall

Published in: Evolution, volume 70, issue 10 (October 2016)

Abstract

Variation in life-history traits is ubiquitous, even though genetic variation is thought to be depleted by selection.

One potential mechanism for the maintenance of trait variation is spatially variable selection.

We explored spatial variation in selection in the field for a colonial marine invertebrate that shows phenotypic differences across a depth gradient of only 3 m. Our analysis included life-history traits relating to module size, colony growth, and phenology.

Directional selection on colony growth varied in strength across depths, while module size was under directional selection at one depth but not the other. Differences in selection may explain some of the observed phenotypic differentiation among depths for one trait but not another: instead, selection should actually erode the differences observed for this trait.

Our results suggest selection is not acting alone to maintain trait variation within and across environments in this system.

Citation

Lange R, Monro K, Marshall DJ (2016) Environment-dependent variation in selection on life history across small spatial scales, Evolution 70(10): 2404–2410 PDF 497 KB doi:10.1111/evo.13033

Dispersal duration mediates selection on offspring size

Authors: Karin Svanfeldt, Keyne Monro, and Dustin J Marshall

Published in: Oikos, volume 126, issue 4 (April 2017)

Abstract

Offspring size varies at all levels of organisation, among species, mothers and clutches. This variation is thought to be the result of a tradeoff between offspring quality and quantity, where larger offspring perform better but are more costly to produce. Local environmental conditions alter the benefits of increased offspring size and thereby mediate selection on this trait.

For sessile organisms, dispersal is a crucial part of the offspring phase, and in animals, bigger offspring tend to better endure longer dispersal distances than smaller offspring because they have more energy. Theory predicts that increasing distances between suitable habitats strengthens selection for larger offspring.

We manipulated the dispersal duration of offspring of different sizes in the bryozoan Watersipora subtorquata and then examined the relationship between offspring size and post-metamorphic performance in the field.

We found that selection on offspring size is altered by larval experience. Larger offspring had higher post-settlement performance if the larval period was short but, contrary to current theory, performed worse when the larval period was extended.

The reversal of the relationship between offspring size and performance by extending the larval phase in Watersipora may be due to the way in which offspring size affects growth in this species. Regardless of the mechanism, it appears that experiences in one life-history stage alter selection on offspring size in another stage, even when they occupy identical habitats as adults.

Citation

Svanfeldt K, Monro K, Marshall DJ (2017) Dispersal duration mediates selection on offspring size, Oikos, PDF 1 MB doi:10.1111/oik.03604

Unravelling anisogamy: egg size and ejaculate size mediate selection on morphology in free-swimming sperm

Authors: Keyne Monro and Dustin J Marshall

Published in: Proceedings of the Royal Society B, volume 283, issue 1834 (July 2016)

Abstract

Gamete dimorphism (anisogamy) defines the sexes in most multicellular organisms.

Theoretical explanations for its maintenance usually emphasize the size-related selection pressures of sperm competition and zygote survival, assuming that fertilization of all eggs precludes selection for phenotypes that enhance fertility. In external fertilizers, however, fertilization is often incomplete due to sperm limitation, and the risk of polyspermy weakens theadvantage of high sperm numbers that is predicted to limit sperm size, allowing alternative selection pressures to target free-swimming sperm.

We asked whether egg size and ejaculate size mediate selection on the free-swimming sperm of Galeolaria caespitosa, a marine tubeworm with external fertilization, by comparing relationships between sperm morphology and male fertility across manipulations of egg size and sperm density.

Our results suggest that selection pressures exerted by these factors may aid the maintenance of anisogamy in external fertilizers by limiting the adaptive value of larger sperm in the absence of competition. In doing so, our study offers a more complete explanation for the stability of anisogamy across the range of sperm environments typical of this mating system and identifies new potential for the sexes to coevolve via mutual selection pressures exerted by gametes at fertilization.

Citation

Monro K, Marshall DJ (2016) Unravelling anisogamy: egg size and ejaculate size mediate selection on morphology in free-swimming sperm, Proceedings of the Royal Society B, 283:1834 PDF 2.7 MB doi:10.1098/rspb.2016.0671

Global change, life-history complexity and the potential for evolutionary rescue

Authors: Dustin J Marshall, Scott C Burgess and Tim Connallon

Published in: Evolutionary Applications, May 2016

Abstract

Most organisms have complex life cycles, and in marine taxa, larval life-history stages tend to be more sensitive to environmental stress than adult (reproductive) life-history stages.

While there are several models of stage-specific adaptation across the life history, the extent to which differential sensitivity to environmental stress (defined here as reductions in absolute fitness across the life history) affects the tempo of adaptive evolution to change remains unclear.

We used a heuristic model to explore how commonly observed features associated with marine complex life histories alter a population’s capacity to cope with environmental change.

We found that increasing the complexity of the life history generally reduces the evolutionary potential of taxa to cope with environmental change. Our model also predicted that genetic correlations in stress tolerance between stages, levels of genetic variance in each stage, and the relative plasticity of different stages, all interact to affect the maximum rate of environmental change that will permit species persistence.

Our results suggest that marine organisms with complex life cycles are particularly vulnerable to anthropogenic global change, but we lack empirical estimates of key parameters for most species.

Citation

Marshall DJ, Burgess SC, Connallon T (2016) Global change, life-history complexity and the potential for evolutionary rescue, Evolutionary ApplicationsPDF 434 KB doi:10.1111/eva.12396

Quantifying the role of colonization history and biotic interactions in shaping communities – a community transplant approach

Authors: Chun-Yi Chang and Dustin J Marshall

Published in: Oikos, volume 126, issue 2 (February 2017)

Abstract

The role of colonization history and subsequent biotic interactions in determining the species composition in communities has long been the subject of debate in ecology. While one narrative has emphasized deterministic assembly rules, another has emphasized historical contingency.

One problem lies in approach: community studies are typically either manipulative but somewhat unnatural, or observational but lacking manipulation. Furthermore, while most ecologists now recognize that both historical and biotic factors shape communities, too few studies have moved beyond qualitative descriptions of their roles.

Here we use a manipulative approach that leverages natural variation to provide quantitative estimates of the relative contributions of colonization history and the subsequent biotic interactions. 384 communities were developed on artificial substrata in a homogeneous environment before undergoing reciprocal transplantation. We then compare community structure before and after transplantation as proxies for colonization history and biotic interactions.

We found that the importance of history and the ensuing biotic environment differed at different times in community development. Early transplantations resulted in the local environment modifying community history faster compared to postponed transplantations. With a four-week difference in age, colonization history explained 20% more of the variation in older communities than in younger communities. Biotic interactions were able to modify colonization history at the age of 16 weeks, but older communities showed more resistance to the changing biotic environment.

Our method provides a manipulative and quantitative approach for understanding the relative contributions of colonization history and biotic interactions to community in natural systems.

Citation

Chang CY, Marshall DJ (2017) Quantifying the role of colonization history and biotic interactions in shaping communities – a community transplant approach, Oikos, 126: 204–211 PDF 550 KB doi:10.1111/oik.03478