Can nature compensate for human impacts?

Algal turfs dominating under acidified conditions at cold-water (temperate) CO2 vents, which we use at “natural experiments” to try and understand the effects of carbon emissions on our oceans. You can just see the fronds of a solitary kelp plant in the right of the photo, otherwise they are rare at the site (when they should be 8 – 10 plants per metre!). This is a system that has been pushed past its ability to resist or compensate for human activities.

One thing that humans are really good at is having an impact on the environment through their activities. The problem is that we generally don’t realise that we’re having an impact until something changes in a drastic way. We talk about things called phase-shifts, where the environment changes from one “phase” to another. Good (and unfortunately common) examples are the loss of kelp forests for bare reef, seagrass meadows for bare sand, or coral reefs for algal habitats. In all of these cases, the environment has been degraded to the point where it no longer functions as it should, meaning that biodiversity and productivity are massively reduced.

There are two questions to ask here, (1) why don’t we see these phase-shifts coming, and (2) does nature have any resistance to them? A new paper by one of my PhD students, Giulia Ghedini, shows that nature may actually try to resist human-caused stressors (such as increased nutrient pollution, ocean acidification, warming) by increasing the strength of compensation. In this case, Giulia found that the compounding effects of multiple disturbances increasingly promoted the expansion of weedy algal turfs (which replace kelp forests), but that this response was countered by a proportional increase in grazing of those same turfs by gastropods. This is a natural compensatory mechanism, but it has limits.

What does this mean for our understanding of phase-shifts? First, it means that nature is stronger at resisting than we realised. BUT, because it is extremely difficult to either see or quantify this resistance we generally don’t realise it is happening…. until it stops. Then, once we push the systems past their ability to compensate for the increased pressure we place on them we see a sudden shift. It’s like watching a duck on a river – it may look extremely calm on the surface, seemingly stationary, but underneath it is paddling extremely hard. At some point the current strengthens too much and it can’t paddle harder and so, seemingly suddenly, the duck begins to float down the river.

Unfortunately, when put together, this means that more systems may be more stressed than we realise, and the only way to stop detrimental phase-shifts is to take the conservative approach and start to reduce our impacts on these systems. For example, we know that nutrient pollution, carbon emissions, overfishing and many other activities have damaged marine ecosystems, why not begin to reduce our impacts before we add more systems to the list of those we didn’t realise were at breaking point?

Overfishing: a problem for everyone

I am currently writing a series of articles for a Chinese produced magazine which targets wildlife conservation. As you may guess, they started with Panda conservation, so the magazine is called Giant Panda, but they are running a series on over exploitation of natural resources. Which is where I come in, contributing a series on fisheries. Over the next few months I will post abridged versions of these articles here. The first, as the title suggests, is about overfishing.

 

It shouldn’t be a surprise to most people that many of the world’s fisheries are overexploited. Most of the world’s population eats seafood. In fact, the amount of seafood that each person eats, on average, has risen to 19.2 kg per person per year, with over 1 billion people relying on seafood for their primary source of protein. This means that seafood is an extremely important part of our lives. The problem is that over 90% of the world’s fisheries are either fully exploited or overfished (FAO 2014 report on the global fisheries), meaning that if we take any more from those fish stocks they will collapse, perhaps forever.

 

Over fishing and fisheries collapse

Overfishing has a long history. One of the best documented cases of a fish stock collapse is that of the Atlantic Cod. When the fishery was discovered in the late 1400’s the cod were so plentiful that it was assumed that the stock was unending. There are stories of people dipping a basket into the ocean and pulling it out full of cod! Catches of cod steadily increased from the early 1500’s, supplying a major proportion of the world’s protein, but were relatively small until industrialisation meant that catches increased dramatically. In the late 1960’s the annual catch peaked at over 1.5 million tonnes, an unsustainable catch. Years of overfishing caused the stock to collapse, and despite ever-improving fishing technology and manpower, the catches continued to decline until the fishery was forced closed in 1992. By that time, the total biomass of cod remaining in the Atlantic was estimated to be less than 1% of the original stock, and still has not and may never recover.  (for a great read on this topic pick up the book “Cod: A biography of the fish that changed the world” by Mark Kurlansky).

The most important lesson to learn from the Atlantic Cod fishery is that any fishery which is overfished can and will collapse. In the last decade alone, many important fisheries have been listed as overfished, including the Largehead hairtail (Trichiurus lepturus), of which over 1 million tons is caught in Asian waters annually, the Mediterranean hake (Merluccius merluccius) and red mullet (Mullus barbatus), Cunene Horse Mackerel (Trachurus trecae), White Grouper (Epinephelus aenus), a number of shrimp species, the list is extensive, and most countries in the world feature at least one fishery. As mentioned above, over 90% of the world’s fisheries are already heading in the direction of being overfished and without good management they too will collapse. Unfortunately, the true frequency with which fisheries collapse can be masked by catch statistics. Global annual fisheries production has been relatively stable since the 1990’s. On the surface, it would appear that fisheries are well-managed and sustainable. What happens in reality, however, is that as we overfish one stock and it becomes unviable, either economically or biologically, so it is replaced by another, new fishery. So, the overall global catch stays the same but we have simply shifted the damage. Usually this means that we are doing something known as “fishing down food webs”, whereby we overfish one stock and then move on to fish a different species further down the food web, often the food of the species that is now over fished! This leads to a situation where the productivity of the oceans as a whole has reduced because the catch is now coming from a previously unfished source. Over time, this continual overfishing causes not only a decline in fish abundance but also massive damage to the ocean ecosystems (which will be topics of future articles in this series).

Ecosystem Impacts

Overfishing doesn’t only impact the particular fish species that is over exploited; it is not simply a matter of thinking “it is only one fish species, we will do better next time”. Removing a species from an ecosystem is like removing one cog from a finely tuned machine – it stops working properly. This is especially the case because many of the species that we prize play critical roles in regulating the function of ecosystems. When these species are removed from the ecosystem it begins to become unwell, not providing all of the ecosystem services that we take for granted. Then, as we fish down the food web and remove more species, the ecosystem degrades further.  A very good example of this is shark fishing. Sharks are usually the top predators in ecosystems and control how it functions. To be healthy and function properly, marine ecosystems need these top predators. However, nearly all shark fisheries in the world are over fished, with some species of shark becoming extremely rare. As most species of shark are long-lived they tend to be particularly susceptible to over fishing, and the only way that their populations will recover is by not fishing them.

Indeed, some of the most dramatic changes we see in ecosystems are because of over fishing. A good example from colder oceans would be the overfishing of large predatory fish such as snapper, which are prized by humans to eat, allowing species like sea urchins to become overly abundant because normally the predatory fish would keep their numbers in balance. While sea urchins are a natural part of the ecosystem, in large numbers they completely consume kelp forests, which are the base of the food chain and removing them causes the loss of hundreds of species. Unfortunately, these are not isolated examples, and every country in the world has examples of ecosystems which are degraded by overfishing.

 

Why aren’t fisheries sustainable?

The answer to this question is that they actually are sustainable, as long as we do not take too much. In fact, the goal of fisheries managers is to maintain catches at the Maximum Sustainable Yield (MSY), or the catch that you can take from a particular fishery forever. In its simplest form, the MSY is an easy concept – you just need to harvest slightly less than the total number of fish which recruit to the fishery each year. It is, however, exceptionally hard to calculate the MSY for a fishery for a number of reasons, in particular that (1) we cannot know how many fish there actually are because we cannot actually count them all, (2) the number of fish which recruit into a fishery, the number we need to know so that we can set catch limits, is dependent not only on how many fish are in the stock, but also a myriad of environmental factors, and (3) we don’t really know how many fish are being taken from a stock because of unmonitored recreational and illegal fishing. This third pressure can be very problematic as people often take fish that are too small, and taking fish before they are able to reproduce (that is, they are immature) means that they cannot contribute young to the next generation before they are caught. In addition to these factors, governments, businesses and the public in many countries often place immense pressure on fisheries managers and fishermen to take more fish to keep supply high. Ultimately, this proves to be counterproductive as when a fishery becomes fully exploited, catches begin to decline and prices rise. Increasing fishing effort at this point leads to overfishing and extremely high prices, making that particular species unavailable to everyone, from the consumer who can’t afford to buy it to the fisherman who can no longer make a living and also the forgotten victim – the ecosystem itself.

 

What’s the solution?

Contrary to what we used to believe, the oceans are not an endless supply of resources; the ocean has a limited productivity budget. But, this doesn’t mean that we cannot sustainably harvest seafood from the oceans, we just need to ensure we don’t take too much.

What does this mean for the future? At a time when the consumption of seafood is increasing, 90% of the world’s fisheries cannot produce any more, meaning that we need to look to other ways to produce our seafood and reduce consumption. The logical way to do this is through environmentally sustainable aquaculture, or farming of seafood. Aquaculture is already common around the world, making up over 40% of total seafood production, but there is still a lot of room for sustainable expansion.

How can you help? The best way to help is to be a discerning consumer. Rather than not eating seafood, ask where it comes from. Is it from a wild fishery? If so, is it sustainably managed? Is the fish you’re eating grown in aquaculture in a sustainable manner? While it may be hard to get the answers to these questions, if you ask at restaurants or where you buy your seafood you will then force the suppliers to ask the same questions. This will then force industries to become more responsible and manage fisheries in a sustainable manner. In some countries, this public pressure has shown to be an effective way to change fishing practices.

 

Next time

In the next article I will discuss two different types of fishing, trawling and long-line fishing, and the damage that they cause to marine ecosystems.

What’s in a little noise?

Different sources of noise in the marine environment. Image source: http://www.marineinsight.com/marine/environment/effects-of-noise-pollution-from-ships-on-marine-life/

Everyone has seen some sort of human impact in the ocean, from plastic washed up on the beach, to a plankton bloom driven by nutrient pollution, possibly even something as confronting as a fish kill (or even dolphins!). But what about the things you can’t see, say some noise?

Marine noise pollution has again become topical in South Australia, with the announcement that seismic surveys in the waters south and west of Kangaroo Island will begin in 2015. But this raises the question, what do we know about the effects of seismic surveys? The answer is…. not much. There is obviously immense community concern, and I was lucky enough to talk about it on ABC radio today.

For those of you who don’t know, the most common method of seismic surveys in marine waters is to use an array of air guns that are towed below the surface (at say, 8 m depth) behind a ship, firing in a sequence at intervals from seconds to minutes. The sound that is reflected back is then analysed to tell you what is on and under the sea floor, important information if you’re looking to extract resources. These surveys can span hundreds of square kilometres and run for months.

There is some literature on the effect of these surveys, but woefully little, and none in this region. The little information that we do have suggests that the effects will be variable, depending on taxa. Whales and dolphins seem to alter the way they communicate and potentially migration routes or residency patterns, at least in the short term, which is concerning because of the seasonal Blue Whale and Southern Right Whale populations in this region. Fish may become stressed and migrate away from the testing area, which includes important fisheries for species such as the Southern Bluefin Tuna. In contrast, it seems that at least some invertebrates may not be affected. I would reiterate, however, that the evidence in either direction is extremely sparse, which concerns me because this region (South Australia) is a global hotspot for species diversity and endemism.

This is where the discussion collides with another topical issue in Australia – how much information do we need to properly assess applications to develop marine resources, and which activities should we allow in our marine (and terrestrial) environments in the name of “progress”? Although some development and an increase in productivity is good, there is more and more support from the scientific community to make sure we don’t damage our environment beyond repair. I won’t go into detail on this, however, as others have written about this topic in much more depth. But, I note that other countries are taking the issue of marine noise seriously, and discussing it, so why aren’t we?

Declining productivity

We’ve all heard about productivity, but I suspect that the only context most people have heard the term used in is about the productivity of the workplace, or perhaps the economy.

Phytoplankton may be tiny but they are the base for much of what we see and use in the ocean!

Economists and governments are certainly concerned with productivity. But, we should all be concerned with productivity – of the oceans.
As we burn more fossil fuels and pump carbon dioxide into the atmosphere we are making astonishing changes to the global climate systems. Not the least of these is the addition of billions of tons of CO2 to the surface waters of the ocean. What does this mean for productivity of the oceans? A cursory analysis would lead you to believe that because many photosynthetic plants and algae can use in photosynthesis that productivity would increase. As the oceans produce about 50% of the oxygen we breathe and provide us with a substantial amount of food and other resources you may think that this would be a good thing. Unfortunately, the evidence is stacking up that productivity won’t increase, and in fact it is likely to decrease.
I have previously posted on work by my research group where we experimentally project that ecosystem productivity in temperate waters is likely to decrease because of an indirect effect whereby highly productive kelp forests will be replaced by lower productivity systems dominated by algal mats. Of potentially greater concern, however, is the emerging data from open-ocean pelagic systems. Recent work by Professor Kunshan Gao from the State Key Laboratory of Marine Environmental Science, Xiamen University, has demonstrated that the projected concentrations of CO2 in our oceans by 2050 (assuming we don’t suddenly decide to stop burning carbon!) will actually cause a decrease in the productivity of phytoplankton. And, the situation was even worse when the phytoplankton were exposed to increased light intensity, which will happen as the upper ocean that they live in shoals towards the surface. This result was initially surprising given that both light and CO2 are required for photosynthesis. In combination and high enough concentrations, however, they inhibit photosynthesis, leading to a decline in productivity.

What does all this mean? The changes that are happening in the ocean because of changes to our climatic systems, including (but not limited to) increased availability of CO2, ocean acidification and warming are going to be with for a very long time. The resources that we currently expect from the oceans will change, many declining. How do we stop this? By being a little smart – let’s stop burning carbon for fuel!

Relevant experimental scales for Ocean Acidification

Degraded reef where kelp have been replaced by algal turfs

In a few of my posts have discussed the potential effects of ocean acidification (OA), caused by the dissolution of CO₂ into seawater, on marine ecosystems. What I haven’t really discussed yet is how we make these predictions, because quite frankly attempting to predict the effects of OA is a difficult prospect. There are a couple of different ways that you can make such predictions, but for me one of the most obvious and effective ways is to identify the key species’ in a particular marine ecosystem and then experimentally expose them to elevated CO₂ based on the various emissions scenarios. On the surface that sounds simple…… but it turns out to be quite hard. The most simple way to do it is to bring organisms back into the lab and do the experiments there. True, it’s easier to manipulate the CO₂ by bubbling mixed air with elevated concentrations of CO₂ under lab conditions, but invariably you end up with a situation where you’re looking at the physiological responses of organisms. This is a very valid thing to do, but you can also be in for some surprises when you try to scale up to identify ecological effects. For example, based on

laboratory based experiments we have predicted that algal turfs will replace kelp forests and corals under

Healthy forest of the kelp Ecklonia radiata

future OA conditions (picture to the right; link to the kelp study) because these algal turfs use the extra CO₂ as a resource and grow faster. This conclusion, based on physiological changes, was and still is quite valid. HOWEVER, when we scaled up our experiments to mesocosms (literally “medium” experimental environment or ecosystem) and included the kelp we discovered that the kelp were able to resist a lot of this effect by suppressing the growth of the turfs. But, realising that this mesocosm study was also limited because it only occurred over one generation of kelp, and you may need to study multiple generations because the adults may not be the “weak point”, we took this work up to the next scale, field experiments at naturally occurring CO₂ vents – currently our best “ecosystem” approach to understanding OA.

But we were interested in not only the larger, system response, but also how well our other experiments may predict ecosystem outcomes. We tested this thought by combining laboratory and field CO₂ experiments (which is difficult but possible) and data from ‘natural’ volcanic CO₂ vents. Interestingly, and to our great

Coral reefs are structurally complex and “cemented” together by Crustose Coralline Algae.

relief, we found that algal mats showed the same direction of response to elevated CO2 (i.e. they grew more) across all scales of experiments but that the strength of response was modified by the ecosystem complexity. Basically, the things that either eat or suppress the growth of algal turfs slow the rate at which they will come to dominate the systems. BUT, we did find that these turfs have enhanced productivity and more expansive covers in situ under projected near-future CO₂ conditions both in temperate and tropical conditions.; that is, our original predictions from the laboratory experiments that these weedy turfs could come to replace kelps and corals seems to hold up, it’s just that the rate of change will be a bit slower.

 

Digital library links for:
Lab based kelp study (Russell et al. 2009)
Kelp resisting turfs (Falkenberg et al. 2012)
Need to study multiple life stages (Russell et al. 2012)
Field manipulations of CO₂ (Kline et al. 2012)
Ecological outcomes across different experimental scales (Connell et al. 2013)

Don’t forget to remember the past

I have recently returned from the 10^th International Temperate Reefs Symposium in Perth. It was great to spend a week talking good science

Seagrass may increase their productivity in the future as they use CO2 for photosynthesis.
Photo: Owen Burnell

with a vibrant group of great scientists. There was an array of talks from classical marine ecology (which is great to see!) to novel modelling approaches and plenty of discussion of human impacts in marine systems. In the rare moments of quiet since my return I’ve been thinking about the main message that I took away from the meeting, and it’s this: anthropogenic climate change may be new to the planet, but we were studying the effects of human activities on ecosystems for several decades before we even realised that climate change was happening. So why is it that we seem to have abandoned ecology in our race to understand climate change?

While I was writing my talk for the conference I realised that, in general, research into the effects of climate change in marine ecosystems has been hampered by not looking at the literature on other human impacts. For example, there is a rich and abundant literature on how excess nutrient loads degrade ecosystems and change their structure and function. Yet, it is only recently that we have realised that CO₂ is a “nutrient” or resource in marine systems. This seems logical; after all, plants use inorganic carbon for photosynthesis.  However, the story isn’t that simple, with different algae and seagrasses using different forms of carbon for photosynthesis. Even more confusing is that it looks like the “weedy” species will benefit by switching to the most abundant source of carbon and start to dominate ecosystems (see some of my papers and Harley et al. for the ecosystem effects and Raven & Hurd for the physiological aspects)! But I digress….

The point is that for some reason we don’t seem to draw on this older literature for the general principles of what we may expect to see as CO₂ concentrations increase in the oceans. We’re starting to catch up, but the lost time is frustrating – let’s not make the mistakes of past generations but rather learn by them.

Digital library links for: Connell & Russell 2010

Ocean acidification: will there be ecosystem effects?

Algal turfs overgrowing corals under acidified conditions at CO2 seeps, which we use at “natural experiments”. Note that the seagrass in the middle of the photo also grow well under these conditions.

It has been a while since my last post, for which I apologise, but I have just emerged from a particularly busy period. I was lucky enough to be invited to an ocean acidification round table at the Peter Wall Institute in Vancouver late last year and we have been madly working on multiple papers since then (and I also had a short break over the Christmas week because of sheer exhaustion!). Why are we working so madly to get the papers written? Because the topic of the meeting is both topical and imminently important: how do we predict the ecosystem-level impacts of ocean acidification, and what can we do about it?

Why is this such an important question? The simple answer is that ocean acidification is a direct consequence of increasing CO2 in the atmosphere. It’s simple, indisputable chemistry. As CO2 dissolves into seawater it forms carbonic acid, finally reducing the amount of carbonate in the water (a good diagram of this reaction can be found here). The early research into this field (only 10 years ago now!) focussed entirely on calcifying species, such as gastropods and corals, because they use this carbonate to form their hard structures (calcium carbonate). What we’ve realised more recently, and a big part of my research program, is tha t the extra Carbon in the system (in the form of CO2) is also a resource for some algae and plants, potentially causing a change in the dominant species in ecosystems (see my photo of a “future” coral reef and kelp forest in this post).

Algal turfs dominating under acidified conditions at cold-water (temperate) CO2 seeps, which we use at “natural experiments”. You can just see the fronds of a solitary kelp plant in the right of the photo, otherwise they are rare at the site (when they should be 8 – 10 plants per metre!).

I’m happy to say that we made real progress in trying to understand what the likely ecosystem effects are globally, and more importantly the things that we need to know into the future. I won’t pre-empt our publications, but the synopsis is that ecosystems will change, and for the worse. This has been highlighted before, including for Australia, but for the first time I think we’re starting to get at understanding the ecological mechanisms (which is essential if we are to help the systems resist this change!).

Lead by Prof. Chris Harley, and including an amazing group of contributors, I’d say it was the most successful round-table that I’ve been involved with and we’ll have some good papers coming out soon (I’ll be sure to post about them!). If you’re interested in the topic and want more information, I strongly suggest that you watch this video of the public event we held as part of the week’s activities.

 

Digitial library links for:Falkenberg et al. 2013
Russell et al. 2013

The secret to scientific success….

This post tackles one of the big issues in science – how to be successful. It turns out that the basic principle is simple, publish. But, there’s some detail that needs to be considered. Most of all, it seems that the earlier you start publishing the better for your career (yes Ph.D. candidates, that means you!). To explain the detail much better than I ever could, I have reproduced a post from a colleague of mine, Prof. Corey Bradshaw (see his blog, Conservation Bytes). He can be controversial, but it gets the point across and it’s fun. His post and blog are definitely worth a read if you want to be successful in science…….

Early to press is best for success

by CJA Bradshaw

This paper is bound to piss off a few people. So be it. This is what we found, regardless of what you want to believe.

Led by the extremely prolific Bill Laurance, we have just published a paper (online early) that looks at the correlates of publication success for biologists.

I have to preface the main message with a little philosophical discussion of that loaded word – ‘success’. What do we mean by scientific ‘success’? There are several bucket loads of studies that have attempted to get at this question, and several more that have lamented the current system that emphasises publication, publication, publication. Some have even argued that the obsession of ever-more-frequent publication has harmed scientific advancement because of our preoccupation with superficial metrics at the expense of in-depth scientific enquiry.

Well, one can argue these points of view, and empirically support the position that publication frequency is a poor metric. I tend to agree. At the same time, I am not aware of a single scientist known for her or his important scientific contributions that doesn’t have a prolific publication output. No, publishing shitloads of papers won’t win you the Nobel Prize, but if you don’t publish, you won’t win either.

So, publication frequency is certainly correlated with success, even if it’s not the perfect indicator. But my post today isn’t really about that issue. If you accept that writing papers is part of a scientist’s job, then read on. If you don’t, well …

So today I report the result of our study published online in BioScience, Predicting publication success for biologists. We asked the question: what makes someone publish more than someone else?

There are a few possibilities here, with some well-known mechanisms, and others that are only suspected. Using the CVs of 1400 biologists in various disciplines (excluding medical) from four different continents, we measured the number of publications they had written by the time they had completed their PhD and ten years later. We also collected information on the scientists’ gender, whether English was their first language, and the international ranking of the university where they obtained their PhD.

Combining the data into a series of linear models, we asked the following questions:

1. Given that our sample included people that stayed in science for at least ten years (i.e., we didn’t include people that gave up their scientific careers in the interim), do males publish more than females?
2. If you went to a highly ranked university for your PhD (e.g., Cambridge, Oxford, Harvard, etc.), were you  likely to publish more than someone who had received theirs from a lower-ranked institution?
3. Most scientific results are published in English these days, so if English is your first language, do you have an advantage and therefore publish more than someone for whom English is a second (or third, fourth, etc.) language?
4. If you start publishing early in your career, does that set the pace for the rest of it?

The results? Drumroll, please.

Most will be happy to read that the most important determinant of your ‘long’-term (10-year) publication success is how many papers you’ve written by the time you’ve completed your PhD. This effect increases markedly if we take the number of papers you’ve published three years after PhD completion as a predictor. To make the point again that publication output is a reasonable metric of ‘success’, we also found that it was highly correlated with the ten-year h-index of the scientists for which we had data.

But there were other effects, albeit of lesser importance. Yes, even after removing the well-known ‘attrition’ effect of female scientists (i.e., leaving their careers earlier than males), men tended to publish a little more than women. There are many potential reasons for this, including still largely male-dominated academic and publishing systems, misogyny and the extra constraints of child rearing. We still have a long way to go here.

English as a first language also gave scientists a publication advantage as hypothesised, although the effect was weak.

Possibly one of the most interesting results was that PhD-university ranking had absolutely no discernible effect on publication output, regardless of which ranking metric one uses.

There a few take-home messages in all of this. First, if you are a PhD student and/or early-career researcher, make sure you put the effort into getting those first papers out. Second, if you’re considering people to hire for a new position and you’re taking a gamble on their potential to publish, you should perhaps place a strong importance on their publication output to date (all other considerations being equal).

However, employers should NOT choose men over women, nor should they blindly hire people with English as a first language. Case in point is that most of my lab’s best and brightest are early-career women from non-English-speaking countries. The gender and language effects were weak at best, and nearly disappeared once we considered the data three-years after PhD completion.

Finally, if an employer is considering choosing one of two recently completed PhD students for a postdoctoral position, and the one from the higher-ranking university has fewer publications than the other from the lower-ranked institution, my advice would be to choose the latter (all other things considered being equal, of course). Maybe students (and their parents) should also put less emphasis on university ranking and more on the people with whom they will be working when considering where to do their postgraduate studies.

CJA Bradshaw

Mediation of global change by local biotic and abiotic interactions

This post is basically a short synopsis of the work done by one of my (now ex-) Ph.D. students, Dr Laura Falkenberg. Laura’s work has turned much of what we thought we knew about the effect of increased CO₂ and nutrients on its head; we found synergies where we didn’t expect them (reviewed in a book chapter) and system resilience and resistance to change beyond what we hoped (via strong competitive interaction and trophic links; published in Oecologia, PLoS One and Marine Ecology Progress Series). Laura has certainly helped us look at things in new ways and given us hope that in marine systems where synergies between stressors exist that management of local conditions could potentially buy us some time in mitigating climate change (e.g. reducing nutrient flows into the marine environment, in Journal of Applied Ecology).

Ph.D. thesis: Mediation of global change by local biotic and abiotic interactions
by Dr Laura Falkenberg.

Throughout my Ph.D., I assessed the conceptual model that while cross-scale abiotic stressors can combine to synergistically favour shifts in marine habitats from kelp forests to mats of turfing algae, management of local conditions can counter this change. My experimental manipulations found broad support for the hypotheses that; 1) cross-scale factors (i.e. local and global) can have interactive effects which increase the probability of expansion of turfs but not kelp and, 2) management of local conditions (e.g. maintaining intact forests, limiting nutrient enrichment) can dampen the effects of global change (e.g. forecasted carbon dioxide). I published the results from my thesis in four papers. In the first, I showed that experimental enrichment of CO₂ and nutrients influence the biomass accumulation of turf and kelp differently, with turf responding positively to enrichment of both resources while kelp responded to enrichment of nutrients but not CO₂. Given that such direct responses could be mediated by interactions with other taxa, in the second paper I considered a key competitive interaction and revealed that the presence of kelp can inhibit the synergistic positive effect of resource enrichment (i.e. CO₂ and nutrients) on their turf competitors. Similarly, in the third paper I highlighted the importance of herbivory by showing that under enriched CO₂ conditions rates of this process were increased to counter the expansion of turfs. Finally, in the fourth paper, I considered a scenario in which these biotic controls were absent and identified that where multiple resources had been enriched and prompted a synergistic response (i.e. the expansion of turf where CO₂ and nutrients are modified), subsequent reduction of the locally-determined factor alone (i.e. nutrients) substantially slowed further expansion of turf algae, but that the legacy of nutrient enrichment was not entirely eradicated. Together, these results represent progress in ecological tests of hypotheses regarding global climate change as they incorporate comprehensive sets of abiotic and biotic community drivers.

You can access all of Laura’s publications from the University of Adelaide’s digital library, or email her for a copy.

Recovery of seagrass from overgrazing depends on species morphology.

Above: A meadow of seagrass (Amphibolis antarctica) that has been heavily grazed by sea-urchins to the point where only dead shoots and detritus remain. Below: A moderately dense meadow of Posidonia sp. with aggregations of sea-urchins (Amblypneustes pallidus). This genus of seagrass appears to have a much greater capacity to recover from grazing than Amphibolis antarctica. Photo credits: Andrew Irving (Above), Owen Burnell (Below)

Following on from my last post on how sea-urchins alter how much they eat in response to nutrients and CO₂, here Owen Burnell describes his latest paper (in as many months!) that shows why Amphibolis antarctica and other morphologically similar species of seagrass may be so susceptible to grazing.

Sea urchins are important marine invertebrates, which in many parts of the world can shape sub-tidal habitats via their grazing. In South Australian seagrass meadows the short-spined sea urchin Amblypneustes pallidus generally occurs in low densities, however, population increases of the species have recently been documented by researchers at The University of Adelaide.

It was observed that the grazing activity of these urchins was impacting seagrass meadows, in particular the species Amphibolis antarctica, when compared with adjacent Posidonia spp.  By manipulating urchin density to measure seagrass loss and then simulating urchin grazing to study seagrass recovery, we found that while urchins grazed equally upon both seagrass species, Posidonia sinuosa recovered much faster from simulated grazing than Amphibolis antarctica. It appears the different morphology of these two seagrass species, in particular the meristem location (or centre of growth) of Amphibolis spp., which is elevated within the canopy and thus exposed to grazers, is likely to be the cause of these asymmetric grazing impacts. In essence, if the urchins eat the meristem that seagrasses grow from they don’t recover as quickly!

While sea-urchins are by no means a rampant force destroying local seagrass meadows, population expansions such as these are important to document, particularly if they have deep seated connections with changing trophic interactions or urchin fecundity. In many marine systems worldwide population expansion of macro-grazers such as urchins can be linked to over-exploitation of their predators (e.g. fish or crustaceans, or before their protection, sea otters!) or changes to temperature that affect their reproduction and metabolism.

For more information, check out the abstract (below), journal website (subscription required), or link to the full manuscript

The persistence of seagrass meadows reflects variation in factors that influence their productivity and consumption. Sea urchins (Amblypneustes pallidus) can over-graze seagrass (Amphibolis antarctica) to create sparse meadows in South Australia, but this effect is not observed in adjacent Posidonia sinuosa meadows despite greater densities of inhabiting urchins. To test the effect of urchin grazing on seagrass biomass, we elevated the density of urchins in meadows of A. antarctica and P. sinuosa and quantified seagrass decline. Urchins removed similar amounts of biomass from both seagrass species, but the loss of leaf meristems was 11-times greater in A. antarctica than P. sinuosa. In a second experiment to assess the recovery of seagrass, we simulated urchin grazing by clipping seagrass to mimic impacts measured in the first experiment, as well as completely removing all above ground biomass in one treatment. Following simulated grazing, P. sinuosa showed a rapid trajectory toward recovery, while A. antarctica meadows continued to decline relative to control treatments. While both A. antarctica and P. sinuosa were susceptible to heavy grazing loss, consumption of the exposed meristems of A. antarctica appears to reduce its capacity to recover, which may increase its vulnerability to long-term habitat phase-shifts and associated cascading ecosystem changes.