Bigger is better when it comes to female fish and feeding the planet

An international study led by the Centre for Geometric Biology has found that larger fish are much more important to feeding the planet than previously thought.

The research confirmed what field biologists have long suggested: that larger mothers reproduced disproportionately much more than smaller ones. Furthermore, larger mothers may produce offspring that perform better and are more likely to survive to adulthood.

The findings clash with current theories. And the results have major implications for fisheries, the value placed on marine protected areas, the impacts of climate change and the 20% of people globally who rely on fish for protein.

The Centre’s Diego Barneche, Craig White, and Dustin Marshall, with Ross Robertson from The Smithsonian Tropical Research Institute, collated and analysed data from 342 species of fish across 14 orders gathered from studies undertaken over a 100-year time span. The team were particularly interested in understanding the relationships between female size and the number of eggs produced, egg volume and egg energy content.

Most life-history theories assume that reproductive output increases proportionately with female size; for every unit increase in female size, there is a proportional increase in reproductive output.  That is, the combined reproductive output of two one-kilogram fish is assumed to be the same as a single two-kilogram fish. But for the overwhelming majority of species, the research team found that overall reproductive output increased disproportionately with female body size. Bigger is much, much better.

The consequences for fisheries cannot be understated. Reproductive output drives population replenishment, and larger fish are much more important for the replenishment of marine fish populations than previously assumed. Outdated models for sustainable harvesting of fish populations are fundamentally flawed.

Our models of how organisms grow and reproduce are based on the wrong assumptions, and as a consequence we are overharvesting wild populations with calamitous consequences.Dustin Marshall

The costs of global change make the study findings even more stark. Climate change is predicted to cause fish body sizes to decrease. Warmer oceans will likely have fewer (and smaller) fish, and drastically reproductive output.

But the research also points to some good news, suggesting that current conservation strategies are more potent than previously thought.  Marine protected areas have been shown to increase fish size by 28% on average. That means that the per-capita reproductive output of fish inside these areas will be much higher than is generally appreciated.

Our discovery means that the benefits of marine protected areas have been massively underestimated, they produce far more new fish than unprotected areas of the same size.Dustin Marshall

This research is published in the Journal Science.

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Research fellow position: Life History Empiricist

  • Level A, research-only academic
  • $64,450 to $87,471 pa + 9.5% superannuation
  • Full-time, starting late 2017
  • Two-year, fixed-term
  • Monash University Clayton campus

The Centre for Geometric Biology is currently seeking to recruit an experienced zooplankton biologist.

You will further be expected to maintain consistently high research output in the form of quality publications, supervision of students, development and submission of grant proposals to external funding agencies, contribute more generally to communicating the research activities of the group, and participation in appropriate career development activities.


Key selection criteria

  1. A PhD on zooplankton or a relevant area, from a recognised university with subsequent relevant work experience, or an equivalent combination of experience and training.
  2. Demonstrated experience in developing theoretical models in fundamental ecology or empirical research using cutting-edge quantitative approaches.
  3. Demonstrated ability to undertake outstanding research; with a high quality research publication record in recognised journals.
  4. Ability to solve problems by using discretion, innovation and the exercise of high level diagnostic skills within areas of functional responsibility or professional expertise.
  5. Excellent written communication and verbal communication skills with proven ability to effectively analyse information and produce clear, succinct reports and documents which requires interaction with others.
  6. Demonstrated planning and organisational skills, with the ability to prioritise multiple tasks and set and meet deadlines.
  7. Demonstrated awareness of the principles of confidentiality, privacy and information handling.
  8. Demonstrated ability to effectively work independently and in a multidisciplinary team to make a contribution to research and scholarship.
  9. Experience of, or willingness to work on, marine systems.
  10. A demonstrated understanding of questions in fundamental ecology and/or evolution.

Enquiries to Professor Dustin Marshall on +61 3 9902 4449

For more information, or to apply, refer to the Monash University website

Conference updates 2018

Many conferences seem to happen during the Melbourne winter providing a welcome opportunity for researchers to travel to warmer parts of the world to share their research findings.

At the moment Giulia Ghedini is heading towards the University of New England (Maine, USA) to attend a Gordon Research Seminar on unifying ecology across scales. Giulia will be meeting up again with Diego Barneche who has moved on from the Centre for Geometric Biology and is pursuing his interests in ecological theory at The University of Sydney.

Giulia will be talking about her work that tests the idea that individual metabolic rates can predict energy use of populations and communities; a major goal of metabolic approaches to understanding energy flow in ecology. In contrast to predictions Giulia has found that population density reduced individual energy use and that these effects overwhelmed the constraints of body size on metabolism. But she also found that the sum of species-specific individual rates at any point in time was a good predictor of overall community metabolism.

Giulia will be joined in New England by Dustin Marshall who is presenting at the related meeting that is dealing with the “synthesis of information, energy and matter to understand the ecology of a changing world”. Dustin will be talking about offspring size and how developmental costs are affected by temperature.

In the meantime Louise Solveig Nørgaard and Martino Malerba are heading to Montpellier in France for the Evolution 2018 conference. Louise will be talking about her PhD research, supervised by Matt Hall, which suggests that the invasion success and the subsequent intensity of infectious disease can be influenced by the population dynamics of the host population. Louise is interested in exploring how the dynamics of expanding host populations, and the capability of different pathogen genotypes to disperse into new host populations, will likely affect the epidemiology and evolution of infectious disease.

Martino will be presenting his latest work that combines the experimental data from his artificial selection experiments with energy based models. Martino has used this combination of approaches to provide direct evidence on the costs and benefits of different cell sizes. His findings suggest that the current size of a species is the product of context dependent selection pressures in nature.

We wish them all happy and productive travels.

Causes and consequences of variation in offspring size

Offspring size affects all aspects of an organism’s life, from birth through to reproduction, and  studies show that larger offspring do better overall. 

Despite the long standing interest in the drivers of differences in offspring size, most studies focus only on one particular taxon or system.

Dustin Marshall, Amanda Pettersen and Hayley Cameron were interested in looking at offspring size across all taxa and at different levels of organisation – within a brood, between individuals and across different species and environments – to see if this wider scope could help them better understand the causes and consequences of variation in offspring size.

They started by looking at a pattern that will be familiar to many ecologists and bio-geographers; offspring size tends to get bigger as you move from the tropics to the poles. They found that this was true for practically all species they compiled data for, with the notable exception of turtles and plants. 

Dustin, Amanda and Hayley suspect that the difference in the patterns they recorded relates to the way offspring size and temperature affects development.  Small increases in temperature are known to yield large increases in development rate. The lower number of warmer days in higher latitudes might just mean that there just isn’t time for larger seeds or turtle eggs to complete development. Importantly, turtles don’t incubate their eggs and so will be more susceptible to environmental temperature than taxa that do (birds for example). Collecting data on egg size variation in other reptiles would help to test this theory.  

For taxa such as fish, amphibians and invertebrates the overall smaller egg size in comparison to seeds and other vertebrates might preclude development time as a limitation on size.

Offspring size also varies across populations and within broods from the same females.  Dustin and colleagues highlight that sources of variation might be external whereby mothers buffer their offspring from harsher environments by making them bigger, or choose to maximise numbers in more benign environments, meaning that offspring are smaller. Mothers might also provision offspring unequally within a brood to ensure that whatever environment the offspring find themselves in, some at least, will do well.  

Hayley’s PhD work however, suggests that variation in size within a brood reduces competition between siblings and all offspring, regardless of their size, do better.

Finally the team considered the question as to why larger offspring generally tend to perform better than smaller offspring. They were interested in understanding the costs and benefits of a larger size to the energy available for fitness-enhancing functions such as growth and reproduction.  

It seems that larger offspring often access more energy resources than smaller offspring. In plants, seed size likely affects photosynthetic capacity, in certain fish and snakes, a larger gape size at birth allows for more efficient energy acquisition and, in filter feeding invertebrates, larger offspring initially produce more or larger feeding structures. In addition, larger offspring should expend relatively less energy than smaller offspring and complete energetically costly developmental stages with more energy reserves intact.

This research was published in the journal Functional Ecology.

Time to go back to school? Geometry helps predict change in ecosystem function

Humans are continually modifying the marine environment either directly, with activities such as fishing, or indirectly as with climate change or the introduction of invasive species. A common consequence of these activities is a change in the body size of individuals that make up an ecological community. 

Understanding the impacts of such changes on the way in which communities gain and use energy is of particular interest to Giulia Ghedini, a post-doc in the Centre for Geometric Biology.

“We know that human impacts can change the size of organisms and we also know that the size of an organism determines the speed at which it uses resources and contributes to the flow of energy within a system” explains Dr Ghedini.

“Understanding how changes in the ‘geometry’ of a whole community might affect ecosystem functioning through changes in metabolic rates is not only theoretically interesting but of practical significance as well” she said. 

Metabolism measurements indicate how much oxygen and food an individual, or an entire community, consumes. Understanding how changes in individual body size affect the energy use of whole communities provides direct information on the amount of resources required for these communities to live. 

Researchers from the Centre for Geometric Biology at Monash University were able to test predictions that older communities, made up of larger organisms, would have lower metabolic rates per unit mass than younger communities of smaller individuals. 

“We know that increases in metabolic rates slow down as organisms get larger – and we wanted to know if this same pattern occurs at the level of whole communities” said Dr Ghedini.

To their surprise, the research team found that the community metabolic rates remained directly proportional to total community mass as communities got older and larger, which contrasted with the way metabolic rate scaled with changes in size of the dominant species. 

“But,” said Dr Ghedini “when we deconstructed the community into individuals and calculated their individual metabolic rates based on their size and species-specific metabolism, we found that community rates were largely the sum of their parts with respect to metabolism.” 

Measuring metabolism of a whole community can be hard, and so studies frequently estimate community metabolism from the dominant species in that community; we now know that for these estimates to be accurate we need to know the sizes of the individuals that make up the community.

“We also found that as communities got older, the same area was able to support much higher biomasses and energy use – three times as much as the younger communities. We attributed this to changes in the shape of the community; that is, a more 3D structure allowed certain individuals greater access to food in the water column and increased oxygen delivery via increased water flow.”

Changes in rates of energy use have long been used as an indicator of change in ecosystem function. 

By unravelling the relationship between the size of individuals and the energy use of whole communities, this study will help us predict how changes in the geometry of communities will impact on the use of resources; a measure of ecosystem function.

This research was published in a special issue of the journal Functional Ecology.

A global synthesis of offspring size variation, its eco‐evolutionary causes and consequences

Authors: Dustin J Marshall, Amanda K Pettersen, and Hayley Cameron

Published in: Functional Ecology, volume 32, issue 6 (June 2018)


Offspring size is a key functional trait that can affect all phases of the life history, from birth to reproduction, and is common to all the Metazoa. Despite its ubiquity, reviews of this trait tend to be taxon‐specific. We explored the causes and consequences of offspring size variation across plants, invertebrates and vertebrates.

We find that offspring size shows clear latitudinal patterns among species: fish, amphibians, invertebrates and birds show a positive covariation in offspring size with latitude; plants and turtles show a negative covariation with latitude. We highlight the developmental window hypothesis as an explanation for why plants and turtles show negative covariance with latitude. Meanwhile, we find evidence for stronger, positive selection on offspring size at higher latitudes for most animals.

Offspring size also varies at all scales of organization, from populations through to broods from the same female. We explore the reasons for this variation and suspect that much of this variation is adaptive, but in many cases, there are too few tests to generalize.

We show that larger offspring lose relatively less energy during development to independence such that larger offspring may have greater net energy budgets than smaller offspring. Larger offspring therefore enter the independent phase with relatively more energy reserves than smaller offspring. This may explain why larger offspring tend to outperform smaller offspring but more work on how offspring size affects energy acquisition is needed.

While life‐history theorists have been fascinated by offspring size for over a century, key knowledge gaps remain. One important next step is to estimate the true energy costs of producing offspring of different sizes and numbers.

Marshall DJ, Pettersen AK, Cameron H (2018) A global synthesis of offspring size variation, its eco-evolutionary causes and consequences, Functional Ecology, PDF DOI 

School visit

On Friday 8 June, Belinda Comerford and Liz Morris visited the grade 5/6 students at Windsor Primary School. The students were given a brief introduction to the research going on within the Centre for Geometric Biology before splitting up to examine communities of sessile (non-moving) invertebrate animals.

The students used identification cards and jars of a single celled algae to use as a food source to investigate the following questions:

  • How many different types of animals can you see?
  • Are there more or less than you thought there might be?
  • What do you notice about their size and shape?
  • How do they get their food?
  • How are they all able to live together?

Thanks to the 5/6 teachers Laura O’Meara and Tom Gosling for inviting us into the classroom.

Focus on PhD research

May has been a busy month for the postgraduate students within the Centre for Geometric Biology. Not least with Amanda Pettersen graduating just before flying out to Sweden to begin a postdoc position.

Alex Gangur, who arrived from the UK in February of this year is immersed in pilot studies to help him design an experiment that will be central to his PhD research.  

Alex is interested in how natural selection will play out in areas that differ in productivity. He is planning a large, long term laboratory experiment where he will manipulate food densities of a marine, harpacticoid copepod (Tisbe sp.) to provide environments that are able to support different numbers of copepod, that is, have different carrying capacities. Alex will then be able to track numerous life history characteristics, such as size, reproductive effort, age at reproduction etc, in the copepods over multiple generations.  

In order to be sure that his proposed food densities create environments where population growth is limited by food and not some other factor, Alex is testing his experimental food densities in a pilot study. If populations stop increasing as food is increased then the population size is limited by something other than food and Alex will need to use a lower food concentration.

To further fine-tune his experimental protocols, Alex wants to know if he can use chemostats to grow up his copepod populations rather than glass bottles.  

The chemostats work by bubbling air up through the bottom of the chamber which, while an advantage in keeping waste build-up to a minimum, can have a downside as airflow can kill copepods if it gets in under the carapace.  Alex will monitor the growth of a population split between a glass bottle and a chemostat to see if they are the same and, if they are, he can go ahead and use the chemostat. 

Lukas Schuster is approximately half way through his PhD candidature. He is currently running two experiments using one of the lab’s favourite animals; the colonial bryozoan Bugula neritina. Lukas is interested in how metabolic rate (a measure of energy use) co-varies with measures of survival, growth and reproductive rate. By directly linking metabolic rate to measures of fitness Lukas can measure how selection acts on metabolic rate.

In his first experiment for this field season Lukas is deploying newly settled Bugula on panels that are either hung horizontally – a benign environment for Bugula – or vertically; a harsher environment due to increased exposure to UV and sediments. 

We know that the harsher environment tends to reduce growth rates, reproductive output and survival in this species, but what we don’t know is if, or how, selection on metabolic rates differs between these two environments.  Every 2 weeks Lukas collects his panels from the two environments and brings them back to the lab where he measures colony size, growth rate, survival, and reproductive output (number of ovicells) as well as metabolic rate for each of approximately 400 individual colonies. 

Lukas has another 1,000 colonies in the field that he is monitoring fortnightly for a separate experiment. In this one, Lukas has taken a field populations of Bugula and measured metabolic rate for every individual colony. He then selected colonies with low, high or intermediate metabolic rates so that he has effectively created new populations that have different mean metabolic rates. At the same time, he is manipulating densities so that he will be able to tell if it is population density or metabolic rate that is having an effect on the growth rate, survival and reproductive output of the Bugula colonies.

Hayley Cameron is in her final year of her PhD and is following up on earlier experiments. Hayley is also using Bugula neritina as a model species to test some fundamental theoretical ideas. Hayley is interested in the outcomes of maternal investment. She is spawning Bugula in the lab and then choosing big, small or intermediate larvae to settle and create populations of siblings that differ in mean size. Hayley wants to know if it is better to produce a few, larger, offspring, or more, smaller, offspring when they have to compete with their siblings (which many marine invertebrates do).

Hayley has returned her populations of different sizes to the field and every week measures size, number of ovicells and survival for each of her 300 individuals. She has been doing this for 9 weeks so far, but will continue to track each individual’s performance across their entire life span (probably a matter of months). In addition Hayley is spawning her colonies in the lab when they are reproductive because she wants to know what size offspring they make.  Will the bigger offspring do better, even when in competition with their siblings and, in turn, make bigger offspring themselves? Watch this space.

MacArthur or MacMartha? Mixed support for MacArthur’s minimisation principle

Robert MacArthur is a name familiar to many undergraduate ecology students. MacArthur’s niche theory made important contributions to the theory of ecology by describing a model whereby in a community of species that competes for a resource, the total energy wastage will be minimised over time.

Despite the importance of this theory to community ecology, it has received very little testing in real world situations. Giulia Ghedini and colleagues from the Centre for Geometric Biology along with Michel Loreau from France have used the well described, and easily manipulated, marine invertebrate, model system to test this theory.

Giulia and colleagues were able to create communities that were of different ages or ‘successional stages’, by manipulating the timing of deployment of bare Perspex plates hung upside down within Brighton marina.

Following MacArthur’s minimisation principle the team predicted that older communities (later successional stage) would have less unutilised food resources, higher maintenance costs due to metabolism and mortality, but with an overall net reduction of energy wastage. 

A real world application of this theory is understanding how susceptible a community is to invasive species. In MacArthur’s framework, late-stage communities (usually more diverse) have very little unutilised food resources, making invasion by an additional species very difficult.  However, while there are many studies that find more speciose communities are more resistant to invasion, some studies show the opposite.  But what if diversity is, in some cases, a poor predictor of resource use? Then MacArthur’s framework may help to better predict which communities are more susceptible to invasion.

In order to test all the theory, the researchers collected the plates, that hosted the different aged communities and measured how much space they occupied on the plate, how much food (phytoplankton) they consumed, their metabolic rates (change in oxygen concentrations), and also recorded mortality by mapping individuals on each plate through time.  Finally, they recorded the biomass of each community.

So, do communities minimise the wastage of energy over time as MacArthur predicted? Giulia and colleagues found mixed support for MacArthur’s minimisation principle.  While energy lost to maintenance increased in communities as they got older (due to these older communities having higher metabolic costs and higher mortality), the amount of unutilised food (energy wastage due to inefficient harvesting of available food resources) varied with successional stage and depended on the amount of phytoplankton that was available to start with.

When food was abundant, the mid-stage communities were more effective at capturing this food, but when food concentrations were low, all communities performed poorly although there was some evidence to suggest that late-stage communities were slightly better at capturing resources.

The team proposes several reasons to explain why their results do not completely correspond to MacArthur’s principle of minimisation. 

First, MacArthur’s principle holds true but acts over longer time periods and what Giulia and colleagues were measuring was an intermediate step where efficiency in food utilisation fluctuates with changes to the numbers and types of different species.  

Second, the principle might not hold true if competition is not the strongest driver of energy use in the community. Instead, other interaction types such as facilitation might be more important. In this example the high densities of barnacles in the mid-stage communities may have facilitated feeding by other members of the community by mixing the flow of water. 

Finally, it seems that more diverse communities are not necessarily better at capturing resources – which might explain why diversity can be both positively and negatively correlated to species invasions. However, we need more information before we can confidently predict how ecosystem processes and biological invasions change as communities grow older and how they will respond to changing environments. 

This research was published in the journal Ecology Letters.

Unutilised food resources depended on both successional stage and food concentration (a) with mid-stage communities performing best at medium and high food concentrations. Energy lost to maintenance increases during succession as predicted by MacArthur (b) and the pattern in graph (c) suggests that unutilised resources (graph a) are driving overall energy wastage. Energy waste per unit mass declines in later successional stages but mid-stages have lower wastage than late stages particularly under high food concentrations (d).