The Conversation: Daylight robbery: how human-built structures leave coastal ecosystems in the shadows

Human-built structures are home to a wide variety of creatures.

By Martino Malerba, Craig White, Dustin Marshall, and Liz Morris, Monash University.

This article is republished from The Conversation under a Creative Commons license.

About half of the coastline of Europe, the United States and Australasia is modified by artificial structures. In newly published research, we identified a new effect of marine urbanisation that has so far gone unrecognised.

When we build marinas, ports, jetties and coastal defences, we introduce hard structures that weren’t there before and which reduce the amount of sunlight hitting the water. This means energy producers such as seaweed and algae, which use light energy to transform carbon dioxide into sugars, are replaced by energy consumers such as filter-feeding invertebrates. These latter species are often not native to the area, and can profoundly alter marine habitats by displacing local species, reducing biodiversity, and decreasing the overall productivity of ecosystems.

Incorporating simple designs in our marine infrastructure to allow more light penetration, improve water flow, and maintain water quality, will go a long way towards curbing these negative consequences.

Pier life

We are used to thinking about the effects of urbanisation in our cities – but it is time to pay more attention to urban sprawl in the sea. We need to better understand the effects on the food web in a local context.

Most animals that establish themselves on these shaded hard structures are “sessile” invertebrates, which can’t move around. They come in a variety of forms, from encrusting species such as barnacles, to tree-shaped or vase-like forms such as bryozoans or sponges. But what they all have in common is that they can filter out algae from the water.

In Australian waters, we commonly see animals from a range of different groups including sea squirts, sponges, bryozoans, mussels and worms. They can grow in dense communities and often reproduce and grow quickly in new environments.

The sheltered and shaded nature of marine urbanisation disproportionately favours the development of dense invertebrate communities, as shown here in Port Phillip Bay.

How much energy do they use?

In our new research, published in the journal Frontiers in Ecology and the Environment, we analysed the total energy usage of invertebrate communities on artificial structures in two Australian bays: Moreton Bay, Queensland, and Port Phillip Bay, Victoria. We did so by combining data from field surveys, laboratory studies, and satellite data.

We also compiled data from other studies and assessed how much algae is required to support the energy demands of the filter-feeding species in commercial ports worldwide.

In Port Phillip Bay, 0.003% of the total area is taken up by artificial structures. While this doesn’t sound like much, it is equivalent to almost 50 soccer fields of human-built structures.

We found that the invertebrate community living on a single square metre of artificial structure consumes the algal biomass produced by 16 square metres of ocean. Hence, the total invertebrate community living on these structures in the bay consumes the algal biomass produced by 800 football pitches of ocean!

Similarly, Moreton Bay has 0.005% of its total area occupied by artificial structures, but each square metre of artificial structure requires around 5 square metres of algal production – a total of 115 football pitches. Our models account for various biological and physical variables such as temperature, light, and species composition, all of which contribute to generate differences among regions.

Overall, the invertebrates growing on artificial structures in these two Australian bays weigh as much as 3,200 three-tonne African elephants. This biomass would not exist were it not for marine urbanisation.

Colonies of mussels and polychaetes near Melbourne.

How does Australia compare to the rest of the world?

We found stark differences among ports in different parts of the world. For example, one square metre of artificial structure in cold, highly productive regions (such as St Petersburg, Russia) can require as little as 0.9 square metres of sea surface area to provide enough algal food to sustain the invertebrate populations. Cold regions can require less area because they are often richer in nutrients and better mixed than warmer waters.

In contrast, a square metre of structure in the nutrient-poor tropical waters of Hawaii can deplete all the algae produced in the surrounding 120 square metres.

All major commercial ports worldwide with associated area of the underwater artificial structures (size of grey dots) and trophic footprint (size of red borders). Trophic footprints indicate how much ocean surface is required to supply the energy demand of the sessile invertebrate community growing on all artificial structures of the port, averaged over the year. This depends on local conditions of ocean primary productivity and temperature. Ports located in cold, nutrient-rich waters (dark blue) have a lower footprint than ports in warmer waters (light blue).

Does it matter?

Should we be worried about all of this? To some extent, it depends on context.

These dense filter-feeding communities are removing algae that normally enters food webs and supports coastal fisheries. As human populations in coastal areas continue to increase, so will demand on these fisheries, which are already under pressure from climate change. These effects will be greatest in warmer, nutrient-poor waters.

But there is a flip side. Ports and urban coastlines are often polluted with increased nutrient inputs, such as sewage effluents or agricultural fertilisers. The dense populations of filter-feeders on the structures near these areas may help prevent this nutrient runoff from triggering problematic algal blooms, which can cause fish kills and impact human health. But we still need to know what types of algae these filter-feeding communities are predominantly consuming.

Our analysis provides an important first step in understanding how these communities might affect coastal production and food webs.

In places like Southeast Asia, marine managers should consider how artificial structures might affect essential coastal fisheries. Meanwhile, in places like Port Phillip Bay, we need to know whether and how these communities might affect the chances of harmful algal blooms.The Conversation

Mussels in the port of Hobart.

The Conversation

The Conversation: No-take marine areas help fishers (and fish) far more than we thought

A juvenile Plectropomus leopardus from the Whitsundays. Image credit: David Williamson, James Cook University.

By Dustin Marshall and Liz Morris, Monash University

This article is republished from The Conversation under a Creative Commons license. 

One hectare of ocean in which fishing is not allowed (a marine protected area) produces at least five times the amount of fish as an equivalent unprotected hectare, according to new research published today.

This outsized effect means marine protected areas, or MPAs, are more valuable than we previously thought for conservation and increasing fishing catches in nearby areas.

Previous research has found the number of offspring from a fish increases exponentially as they grow larger, a disparity that had not been taken into account in earlier modelling of fish populations. By revising this basic assumption, the true value of MPAs is clearer.

Marine Protected Areas

Marine protected areas are ocean areas where human activity is restricted and at their best are “no take” zones, where removing animals and plants is banned. Fish populations within these areas can grow with limited human interference and potentially “spill-over” to replenish fished populations outside.

Obviously MPAs are designed to protect ecological communities, but scientists have long hoped they can play another role: contributing to the replenishment and maintenance of species that are targeted by fisheries.

Wild fisheries globally are under intense pressure and the size fish catches have levelled off or declined despite an ever-increasing fishing effort.

Yet fishers remain sceptical that any spillover will offset the loss of fishing grounds, and the role of MPAs in fisheries remains contentious. A key issue is the number of offspring that fish inside MPAs produce. If their fecundity is similar to that of fish outside the MPA, then obviously there will be no benefit and only costs to fishers.

Big fish have far more babies

Traditional models assume that fish reproductive output is proportional to mass, that is, doubling the mass of a fish doubles its reproductive output. Thus, the size of fish within a population is assumed to be less important than the total biomass when calculating population growth.

But a paper recently published in Science demonstrated this assumption is incorrect for 95% of fish species: larger fish actually have disproportionately higher reproductive outputs. That means doubling a fish’s mass more than doubles its reproductive output.

When we feed this newly revised assumption into models of fish reproduction, predictions about the value of MPAs change dramatically.

Fish are, on average, 25% longer inside protected areas than outside. This doesn’t sound like much, but it translates into a big difference in reproductive output – an MPA fish produces almost 3 times more offspring on average. This, coupled with higher fish populations because of the no-take rule means MPAs produce between 5 and 200 times (depending on the species) more offspring per unit area than unprotected areas.

Put another way, one hectare of MPA is worth at least 5 hectares of unprotected area in terms of the number of offspring produced.

We have to remember though, just because MPAs produce disproportionately more offspring it doesn’t necessarily mean they enhance fisheries yields.

For protected areas to increase catch sizes, offspring need to move to fished areas. To calculate fisheries yields, we need to model – among other things – larval dispersal between protected and unprotected areas. This information is only available for a few species.

We explored the consequences of disproportionate reproduction for fisheries yields with and without MPAs for one iconic fish, the coral trout on the Great Barrier Reef. This is one of the few species for which we had data for most of the key parameters, including decent estimates of larval dispersal and how connected different populations are.

No-take protected areas increased the amount of common coral trout caught in nearby areas by 12%. Image credit: Paul Asman and Jill Lenoble via Flickr.

We found MPAs do in fact enhance yields to fisheries when disproportionate reproduction is included in relatively realistic models of fish populations. For the coral trout, we saw a roughly 12% increase in tonnes of caught fish.

There are two lessons here. First, a fivefold increase in the production of eggs inside MPAs results in only modest increases in yield. This is because limited dispersal and higher death rates in the protected areas dampen the benefits.

However the exciting second lesson is these results suggest MPAs are not in conflict with the interests of fishers, as is often argued.

While MPAs restrict access to an entire population of fish, fishers still benefit from from their disproportionate affect on fish numbers. MPAs are a rare win-win strategy.

It’s unclear whether our results will hold for all species. What’s more, these effects rely on strict no-take rules being well-enforced, otherwise the essential differences in the sizes of fish will never be established.

We think that the value of MPAs as a fisheries management tool has been systematically underestimated. Including disproportionate reproduction in our assessments of MPAs should correct this view and partly resolve the debate about their value. Well-designed networks of MPAs could increase much-needed yields from wild-caught fish.
The Conversation

How well do we understand the way body size affects populations?

We have all heard the saying “live fast, die young” but it doesn’t only apply to film stars; smaller life forms also abide by this rule. Microscopic phytoplankton cells can double in numbers every few days, while the much larger elephant lives almost 100 years and reproduces slowly. This relationship between body mass and the ‘pace of life’ is well known. But the underlying mechanisms are far from resolved.

A number of years ago, scientists proposed a theory to link the ‘pace of life’ of individuals to the ecology of populations, communities, and ecosystems. The Metabolic Theory of Ecology predicts how energy use (metabolism) is affected by body size and how growth rate, development time and death rates change with energy use. These changes in individual rates will, in turn, affect rates of change in populations.

Metabolic Theory is very appealing because it explains many patterns we see in nature and is based on a fundamental rate which applies to all levels of organisation. But it is difficult to test. Detecting differences in energy use accurately requires having consistent differences in body sizes among organisms, while controlling for other variables (e.g. age, nutrition, health). This usually means comparing across species, which differ in more ways than just size.

The artificially evolved phytoplankton were almost 400 generations old when this work was done. The graph shows the considerable differences in size between small and large algae which allowed Martino and Dustin to ask their questions about body size, metabolism and populations within a species.

Few tests of these predictions have been able to control for these variables but recently Martino Malerba and Dustin Marshall were able to do this and they found that the body size-metabolism relationship did not predict population change as expected.

Martino and Dustin were in the enviable position of having access to artificially-evolved large and small phytoplankton cells of the same species that differed in size enough for them to ask “what happens to the population of a species if the average body size of individuals change?” They could see how body size affected energy use, growth rates, density and biomass.

It turns out that the story was far more complex than expected. The effect of body size on how organisms use energy and grow was very strong but also varied during the course of evolution.

So why didn’t body size explain trends in growth and energy use among size-evolved organisms? The answer may lie with previous work from the Centre for Geometric Biology. Martino and his colleagues found that large-evolved plankton cells optimized their photosynthetic pigments and produced more energy overall than smaller cells. This suggests larger cells have greater access to resources than smaller cells and so the way in which body size and metabolic rate influence the demography of a species is not as predictable as we once thought.

Instead of the classical view, where body size determines the rate at which organisms use energy, which then determines demographics, Martino and Dustin suggest that body size can affect metabolism and populations at the same time.

IIf these laboratory cultures are representative of natural populations, we would predict that current trends of reduced body size (from global warming) could lead to lower rates of population increase, biomass productivity and maximum biomass. This is the opposite of what current theory would predict. This is particularly important when we consider the role of phytoplankton in fixing carbon and supporting food chains.

This research was published in Ecology Letters.

Why do cooler mothers produce larger offspring?

In a recently published letter, Amanda Pettersen, Craig White, Rob Bryson-Richardson and Dustin Marshall propose a simple model to explain a pervasive conundrum – why do cooler mothers produce larger offspring?

Life history theory maintains that mothers balance the costs and benefits of making a few larger and better performing offspring against making many smaller and poorer performing offspring.

A major challenge to the theory is the fact that temperature seems to alter the optimisation of this trade off. Observations indicate that across a wide range of taxa and systems, mothers in warmer conditions produce smaller offspring. What is more, experimental studies have also shown that increasing temperatures decrease offspring size.

Amanda and her PhD supervisors are proposing that linking life history theory and metabolic theory, which relates to energy use, can provide a widely applicable explanation to the offspring size / temperature relationship.

Their model is centred on the cost of development. Mothers must provision their offspring until they are able to feed for themselves, that is, attain nutritional independence. The time spent in this developmental phase coupled with the energy expended will comprise the ‘cost’ of development. The minimum offspring size that allows individuals to reach nutritional independence must, then, increase with increasing cost of development.

As temperatures increase, developmental rate is expected to increase so that less time is spent in the developmental phase and metabolic rates (rates of energy use) are also expected to increase. The research team are suggesting that we consider how sensitive these two components are to temperature. If developmental rate is more sensitive to changes in temperature than metabolic rate, then the cost associated with provisioning offspring to achieve nutritional independence will decrease with increasing temperatures.

Or to put it another way, if developmental rate increases more than metabolic rate as temperatures rise, so that the developmental time is shorter in relation to metabolic rate, then the developmental cost is lower and offspring are smaller at higher temperatures. If, however, metabolic rate is more sensitive to changing temperatures than developmental rate then the converse is true; the developmental cost will increase with increasing temperatures and offspring are predicted to be larger at higher temperatures.

In order to develop and test these ideas the team needed to generate measures of temperature dependence of metabolic rate and developmental rate simultaneously, something that hadn’t been done before in a systematic fashion.

They started by methodically searching published literature to determine the relationship between the temperature that mothers experience and the size of their offspring. They then experimentally manipulated temperature to examine how developmental rate and metabolic rates changed in two very different species – the bryozoan Bugula neritinaand the zebrafish Danio rerio. They used the data from these experiments to develop the mathematical functions for their model to determine how the costs of development change with temperature. Finally they searched the literature again to get data on the temperature dependence of developmental and metabolic rates for a wide range of species because they wanted to test whether their model could apply more generally.

Amanda and her colleagues found that the offspring size / temperature relationship is widespread. Also in the two species they collected experimental data for, they found that development time is more sensitive to temperature than metabolic rates. This means that the overall costs of development decrease with temperature. What is more, they found that this pattern applies more broadly – for 72 species across five phyla the costs of development are higher at cooler temperatures.

Combining life history theory and metabolic theory has allowed the research team to provide a general explanation for offspring size / temperature relationships. In colder temperatures mothers show an adaptive response whereby they offset the increased costs of development by making larger offspring that possess greater energy reserves.

This research is published in the journal Ecology Letters.

This figure shows the relationship between two biological rates; development time (D) and metabolic rate (MR). In the left-hand graph we can see that as temperature increases from T4 to T1, development time is expected to decrease and metabolic rate is expected to increase. The right-hand panels demonstrate what is predicted to happen when b) the developmental rate is more sensitive to temperature than metabolic rate and so c) the total costs of development (and therefore offspring size) should decrease with increasing temperature. In d) the converse is true – metabolic rate is more sensitive to temperature than developmental rate and so e) total costs of development (and offspring size) will increase with increasing temperature.

New projects 2019

A number of large new projects will be getting underway in 2019 as a result of ARC funding schemes. Dustin Marshall and Matt Hall are now Future Fellows and Giulia Ghedini has received a Discovery Early Career Researcher Award. Dustin and Giulia will be using marine invertebrates to look into impacts of global warming whilst Matt is tackling the importance of sex in the evolution of infectious disease.

Within a given species, often the greatest heterogeneity that a pathogen will encounter will be due to differences between males and females. Yet, up until recently, insight into this crucial topic was driven by research into one sex, typically males.


Matt’s recent work has shown that, in the water-flea Daphnia magna, not only is pathogen fitness lower in males, but so is a pathogen’s evolutionary potential. What is more, the relative proportion of males in a population can fundamentally alter the overall transmission potential of a pathogen.

A. The graph on the left demonstrates that pathogen fitness is sex specific; in this case pathogen fitness is greater in females. B. The graph on the right indicates how changes in the relative proportion of males can increase the burden of disease for every individual.

This project was stimulated by Matt’s recognition that there is an absence of theory that explicitly considers how males and females can impact on the evolution and epidemiology of infectious disease.  Matt is seeking to address this imbalance and integrate sex-specific effects into a general framework for disease evolution and epidemiology.

Matt will be using the water-flea Daphnia magnaand its associated pathogens to provide an experimental system in which he can manipulate infections in males and females, characterise the degree of differentiation, and generate predictive models.


Dustin will be investigating how temperature affects the life-history stages of feeding and non-feeding larvae. Marine life histories show strong biogeographic patterns: warmer waters favour species with feeding larvae and cooler waters favour species with non-feeding larvae. Warming could be particularly problematic for Australian species because in 2012, Dustin discovered that Australian coastal species predominantly have non-feeding larvae. This means that future temperatures increases could affect native Australian invertebrates disproportionately relative to other regions of the world. (Put in schematic from application here)

Schematic of the data pipeline to estimate developmental energetics across temperature regimes. For each species, parents will be exposed to a range of temperatures, after which the size and number of offspring that are produced will be measured. These offspring will then have every phase of their energy usage and acquisition, from fertilisation through to metamorphosis estimated across an orthogonal temperature range. Dustin will then integrate these estimates to create a thermal energy performance curve for each species and use these data to parameterise models of connectivity, viability and life history evolution.

At the end of an intensive experimental period, Dustin will have quantitative estimates of how temperature alters the success of a range of species from the gamete to the juvenile. At this stage Dustin will work with collaborators to generate predictive models to determine

  1. how does temperature alter the relative advantages for each of the two developmental modes?
  2. how does temperature affect dispersal and connectivity among populations for each developmental mode? and finally
  3. how does temperature affect the distribution of marine organisms with feeding or non-feeding larvae?


Giulia will be investigating how global warming will affect entire ecological communities.

We already know that warming can affect individuals by reducing their body size and speeding up energy use, as well as reducing water viscosity.  But what we don’t know is how these changes at the individual level might play out at the population and community level and affect the energy intake or expenditure of whole communities.

Giulia will be looking at how changes in body size at the individual level interact with population and community level ecosystem functions.

Giulia is particularly interested in this knowledge gap and will be investigating the implications of warming sea temperatures for important ecosystem functions such as productivity, food web stability or resistance to invasion.

Giulia has planned a series of experiments, using communities of easily manipulated, sessile, marine invertebrates, to explore 4 main questions.

  1. How do changes in community size-structure and composition under warming alter the energy intake (phytoplankton) and expenditure (oxygen) of marine invertebrate communities?
  2. Since the availability of energy can mediate biological invasions, does warming alter the energy usage of communities so that they are more susceptible to invasive species?
  3. Are the responses of invertebrate communities to warming mediated by changes in their food (phytoplankton)?
  4. Given that warming reduces water viscosity, how does this mechanical effect alter food consumption and metabolic expenditure in marine communities of different size-structure?

Linking life-history theory and metabolic theory explains the offspring size-temperature relationship

Authors: Amanda K Pettersen, Craig R White, Robert J Bryson‐Richardson, and Dustin J Marshall

Published in: Ecology Letters


Temperature often affects maternal investment in offspring. Across and within species, mothers in colder environments generally produce larger offspring than mothers in warmer environments, but the underlying drivers of this relationship remain unresolved.

We formally evaluated the ubiquity of the temperature–offspring size relationship and found strong support for a negative relationship across a wide variety of ectotherms. We then tested an explanation for this relationship that formally links life‐history and metabolic theories. We estimated the costs of development across temperatures using a series of laboratory experiments on model organisms, and a meta‐analysis across 72 species of ectotherms spanning five phyla.

We found that both metabolic and developmental rates increase with temperature, but developmental rate is more temperature sensitive than metabolic rate, such that the overall costs of development decrease with temperature. Hence, within a species’ natural temperature range, development at relatively cooler temperatures requires mothers to produce larger, better provisioned offspring.

Pettersen AK, White CR, Bryson-Richardson RJ, Marshall DJ (2019) Linking life-history theory and metabolic theory explains the offspring size-temperature relationship. Ecology Letters PDF DOI

Why release small amounts of sperm slowly?

Sperm competition theory has been central to our understanding of male reproductive biology for many years and is dominated by the idea that males compete strongly to fertilise female’s eggs. But in many species the external environment will also influence reproductive strategies and, in their new publication, Colin Olito and Dustin Marshall ask an obvious but neglected question “what would reproductive strategies look like in the absence of sperm competition?”

Their interest was sparked by the fact that broadcast spawning species (e.g. seaweeds, corals annelid worms, sea stars and many fish taxa) release sperm and eggs to be fertilised externally, which provides an increased opportunity for the environment to influence the evolution of spawning strategies when compared to internal fertilisers.

In addition, broadcast spawners also have spawning strategies that differ markedly from predictions arising from classic sperm competition theory. For example, many broadcast spawning species have very long spawning times characterised by slow individual gamete release rates and, what is more, large males do not necessarily release more sperm than small males despite a large investment in gonads; neither strategy is predicted by classic theory.

Colin and Dustin devised two experiments to consider how fertilisation success changes with the amount of sperm released (ejaculate size) and the rate at which it is released. They used a marine intertidal polychaete worm, Galeolaria caespitosa, that has separate sexes and releases gametes initially into its tube and then, through rhythmic whole-body contractions, out of the tube in slow steady pulses.

A female Galeolaria removed from its tube and releasing eggs.

By repeatedly injecting different volumes of sperm (at the same concentration) and at different speeds into a flume set up to have laminar flow, Colin and Dustin were able to measure the fertilisation success of eggs placed ‘downstream’ of the sperm injection point.

Experimental set-up using the flume. Laminar flow was achieved by using collinators (drinking straws).

They used an experimental design that ensured that there was no variation in the number of males contributing to the pooled ejaculate used for the different experimental treatments. So, strictly speaking the experiments were not done in the absence of sperm competition, but, instead, in the absence of variation in sperm competition.

Colin and Dustin found that the benefits of releasing sperm quickly or slowly depended on ejaculate size: when only a small amount of sperm was released, it was better to release it slowly but when ejaculate size was larger and released at a faster rate, fertilisation success was greater for eggs further away. However, there was a substantial ‘cost’ associated with this higher fertilisation success for distant eggs. The more sperm males release, the more is wasted during sperm dispersal.

Colin and Dustin’s study suggests that slow sperm release rates are expected to evolve whether or not males experience strong sperm competition, and highlight the importance of taking account of selection from the external environment when seeking adaptive explanations for male broadcast spawning strategies.

This work has been published in the Journal of Evolutionary Biology.