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Prey for the oceans: How marine predators influence reef ecology

By Dyani Lewis

Marine biologist, Prof Robert Warner, discusses the relationship between marine predators and their prey. He also explains why marine environments may be more robust than terrestrial ecosystems in the face of human impacts.

DYANI LEWIS
Hi I'm Dyani Lewis, thanks for joining us. Predators often get a bad rap. The fearsome teeth of the great white shark terrified people long before the 1975 movie Jaws immortalised our panic. But our view of the natural environments around us has changed drastically over recent years, especially as we've seen the impacts of our own activities on almost every ecosystem that we touch. Even our view of predators is softening, as we recognise their importance in the ecosystems they are a part of. But what exactly happens to an ecosystem when its predators are removed? Does fishing out the marine predators irreversibly change our ocean communities? How effective are marine management practices at protecting the natural balance between predator and prey?
I'm joined on Up Close today by a marine biologist who has been looking for answers to these very questions. Robert Warner is Research Professor of Marine Biology in the Department of Ecology, Evolution and Marine Biology at the University of California, Santa Barbara. Bob is visiting the University of Melbourne on an Australian Academy of Science Selby Scientific Foundation Fellowship. Welcome to Up Close, Bob.

ROBERT WARNER
Thanks.

DYANI LEWIS
When we look at marine environments, are predators particularly vulnerable to human influences?

ROBERT WARNER
They're not only vulnerable to human influences, it's worse than that because generally when we fish, we fish for predators and we eat them. So they're particularly vulnerable mainly because we target them. We want to kill the predators, because the predators are the things that we eat. Everywhere, every fishery initially targets things at the top of the food chain and those are the big predators. We may eventually fish our way down to smaller fish that may not be predators, may be eating seaweeds and algae. But initially we always work on the predators, or at least on the big things. Sometimes we will kill big parrotfish for example, simply because they are big and easy to eat. But most of the time, we're working on the predators, on the carnivores and for every fishery that I know of, humans have been very effective, reducing the numbers down to a fraction of what they used to be.

DYANI LEWIS
When that happens - when an ecosystem suddenly has far fewer predators than it did previously, what happens to the populations of fish that were their prey?

ROBERT WARNER
Yeah the standard view on that is that well of course with fewer predators, then the things that the predators eat must survive better and there will be more of them. Because there are more of them, then they in turn will begin to decimate the things that they eat. We'd see something that is called a trophic cascade, fewer top predators, more of the things that the top predators eat and then fewer of the things below them that they eat. But the interesting thing is that there is very little evidence for trophic cascades in the marine system, in coastal waters. We see it quite often in terrestrial environments, but under water, there are a few examples of it. But, much of the time there are other changes, changes that we weren't ready for, weren't aware of that occur, and that's actually one of the things that I've been talking about on this Selby Tour.

DYANI LEWIS
When you study a marine ecosystem, how do you actually measure what's going on? Is it basically a case of getting in there and counting what you see?

ROBERT WARNER
Yes. It's nice to be an underwater biologist because imagine if you were trying to figure out what was going on in the trees with the birds. There are things in the way and half the time you don't see the birds, but under water you can fly. The animals, the fishes mostly, generally ignore you and so it's a much easier environment to work in. Although you're encumbered by diving gear, the rest of it is really quite easy. Yes you can count, you can observe, you can see what the animals are doing, you can note changes in behaviour for example, and some of those changes in behaviour are spectacular in relation to whether there are predators there or not.

DYANI LEWIS
How easy is it to see evidence of this changing fish behaviour in reefs?

ROBERT WARNER
Sometimes it's actually quite easy and a colleague of mine, Liz Madin, who is at Macquarie University, has noted that you can even see these effects from space. Go on to Google Earth or any of the satellite based observing platforms and you can focus in on a coral reef and there are patch reefs, say the size of the room we are sitting in which might be five by five metres. Those can be seen and then around those patch reefs is a halo of white sand, and that represents where the fishes are feeding, and the reef is where they are sheltering. So the size of the halo actually is an indication of how far the fishes are willing to wander away from shelter. In areas where there are plenty of predators present, the halos are very small. In areas where the predators have been removed, usually through fishing, the halos are quite large. So you can see these effects easily both in terms of just simply watching the fish, they don't venture very far away, or seeing the effects on what the fishes eat.

DYANI LEWIS
It sounds like it's not just about the abundance of prey species then?

ROBERT WARNER
That's right, the classic view would be the effect of increases or decreases in predators would be increases or decreases in the things that they eat. But, the other effects, the behavioural effects, when predators are around, the fishes are scared. They act differently and they call it the ecology of fear, can have much greater effects, pervasive effects throughout the community that are more important often than the effect of just removing some of the prey by having the predators eat them.

DYANI LEWIS
So, what are some of those effects?

ROBERT WARNERWhen there are lots of predators around, the fishes - generally we've been watching fishes - don't stray as far from shelter as they would under circumstances where we have observed them in the absence of predators. They stick closer to home, they feed less often, because they are vigilant, they're on the lookout for predators and when you're on the lookout for predators you can't be eating algae on the bottom, you have to look around. You have to be close enough to shelter to get away from the predator, if they do appear. Now that sounds natural, it sounds like what you would expect any animal in the presence of something that was to eat them to do, but the interesting thing is the knock on effects of that. Less feeding leads to less reproduction, less feeding leads to lower growth and so the entire transfer of energy from whatever they were eating, whatever those prey fishes are eating, up through the food chain is slowed down. It looks like essentially a loss of fitness in the sense that these animals are not reproducing, they're not growing as fast as they were before. On the other hand there is a reward, they're not being eaten. It's a different situation, the ecosystem is operating in a different way, some processes are slowed down, but it's what the system used to be like everywhere.

DYANI LEWIS
It does sounds like a bad thing though when you've got predators, you have this reduction in productivity.

ROBERT WARNER
Right. It's not exactly a reduction in productivity because the plants down below the prey - the fishes - are still producing as much or even more now because they're being eaten less, than before. But the transfer up through the food chain is slowed down a bit. It's always a question of whether a very fast transfer of energy up through a food chain is a good thing or a bad thing. It's an ethical question that's hard to gauge. It's a situation that existed before, it's a more natural situation - the existence of many trophic levels, many predators, eating prey or even scaring prey so that prey are not eating as fast. That's how it used to be everywhere. I'll leave it up to you to judge whether or not that's a good thing or a bad thing. I actually enjoy being in ecosystems in coastal communities swimming around and seeing the organisms like they used to be. The whole community like it once was. That's important to know, not just from an interest point of view like taking a walk in the woods, it's interesting to know that simply because these are the conditions under which almost all of these organisms evolved.

DYANI LEWIS
In those situations, what actually keeps the predators in check?

ROBERT WARNER
Maybe nothing. That's the interesting thing about it. If you go to areas where no removal of predators has occurred, some of the pristine unique areas that are - there is a few of them still out there. What you see and what researchers at Scripps Institution of Oceanography have documented is something called an inverted trophic pyramid. That is, generally we think, oh there will be lots of plants around, fewer of the animals that eat the plants and a few top predators drifting around picking off those prey items. But it's exactly the opposite. You see an enormous number of large predators. You see a good number of the prey that they eat and you see the plants as they always have been. The difference is that the plants turn over very rapidly. They're producing all the time. That energy goes up into the next trophic level into the prey. They eat at whatever rate they want to and they live a moderately long time. Then the predators up top can live an enormously long time. So this energy trickles upward and is stored essentially at the top levels. When man first appeared in any ecosystem, underwater or above on land, what they probably first saw was an enormous number of big animals. Unfortunately we are very good at taking those out of the environment and we're left with the ecosystems that we see now both on land and in the water.

DYANI LEWIS
So even in terrestrial environments there would have been an inverted pyramid, do you think?

ROBERT WARNER
Right, whether that inverted pyramid consisted of lots of large carnivores, like exists in the water, or lots of large herbivores, like you've seen models and pictures of these wonderful large herbivores that roamed on Australia, North America, everywhere. They were big, and there was a lot of them. Because they could store up this energy over a long period of time and then we mined that energy down.

DYANI LEWIS
This is Up Close. I'm Dyani Lewis and in this episode we're talking about the role of predators in marine ecosystems with Marine Biologist, Professor Robert Warner. Bob, people are increasingly recognising the importance of protecting marine ecosystems by establishing things like marine reserves. Can predators easily recover when a reserve is established?

ROBERT WARNER
The answer is yes and no. Certainly if you stop killing them, there will be more of them around and they can grow to larger sizes. There is no question that fishing results in populations of predators that are younger and smaller. The complete recovery, that is, more young growing up and increasing the population size of the predators, can take a longer time, simply because a lot of these predators, like sharks, are slow reproducers. They grow slowly and they reproduce in large packages. A female shark will lay large eggs or give birth to large young. That means that the rate of increase, the potential for an increase in the population, is less than that of say a large fish, which is producing thousands or even millions of young that can grow up fairly rapidly.
DYANI LEWIS
Many large predators are migratory, does this make them also harder to protect?

ROBERT WARNER
Yes. It makes them harder to protect but it's not impossible. Sometimes the job that we have to do is to protect part of the life cycle, part of the places where the predators occur. If the predators are migrating over large distances, one might protect the migration pathways. But most importantly, you'd protect the places where reproduction's occurring and a lot of these migrations are for reproduction. If you can identify where the predators are gathering to reproduce, just as fishermen over the ages have identified the same areas, if you can find those areas and protect them, you can get a much stronger result than just protecting a piece of the ocean where the predators are occasionally found.

DYANI LEWIS
Does the limiting the size of fish that you are able to catch, so that no small fish are captured, help to ensure that a population isn't fished to extinction?

ROBERT WARNER
Yes. It's hard to fish - regular fishes, not sharks - to extinction simply because a few individuals as I mentioned, can produce a lot of young. But the number of individuals, the number of young that a fish can produce, goes up exponentially with its size. So it's a good idea to preserve some of the larger fishes in a fishery because then you can get lots of recruitment. That is, arrival of young back into the population. Fishing limits in the past often were set as, you can't take anything smaller than a certain size. That's fine except that often that size limit was set just at the size of maturity. If the fishery was very efficient, the only thing that would be left, are fishes that can't reproduce, yet. A more efficient method and more and more we're seeing this, is something called slot limits, that is, yes don't take the smallest fish, but also don't take the largest fish. That leaves some of those large fish that have very, very high fecundity or producing lots of young, still around to help to replenish the population.

DYANI LEWIS
Humans are very good at driving species to extinction though. So in the cases where a predator does go extinct, perhaps only in one reef, is it ethical to, I guess capture it from another area in the world and bring it back in, can you do those sort of reintroductions?

ROBERT WARNER
You could, you probably don't need too. Because one of the prime features of marine species is the fact that not only are they having these small, small young, many, many small young, but those small young spend some of the time up in the water column drifting in the pelagic realm. They're essentially part of the plankton. That means that they have the potential to travel great distances before they settle out, after sometimes weeks, sometimes months, up in the water. They can settle out then on to proper habitat, on to a reef that they can grow up and be an adult on.The fact that these animals can travel long distances as very small young, means that even if there is a local extinction that occurs, if you stop whatever process led to that local extinction and wait, usually not too long, you can get recruitment. You can get individuals arriving back in there. You don't have to transport adults from somewhere else into that environment. You just wait and the young will appear, if you're lucky and most of the time this is true. There is a lot of connectivity, that is connections between populations, in marine species. But that works only for those species that have very small pelagic young that can drift in the water. Things like sharks, things like marine mammals, turtles, animals that have large young, don't have that dispersal phase and so in that sort of a situation, you may have to try other measures.

DYANI LEWIS
But it does seem like there's an inbuilt resilience that isn't necessarily the same as what terrestrial environments face?

ROBERT WARNER
That's very correct. The fact is that most marine organisms, probably about 90 per cent have a life cycle, which involves the production of lots of very tiny young that can disperse and so therefore, a few individuals have the potential at least to repopulate large areas. On land, you simply can't do that, a very small tiny young would dry up and die. So terrestrial plants, terrestrial animals all produce fairly large young and therefore have a slower reproductive rate, a greater risk of local extinction, and a slower rate of recovery in an area.

DYANI LEWIS
I'm Dyani Lewis and my guest today is marine ecologist, Professor Robert Warner. We're talking about marine conservation and the role of predators, here on Up Close.Bob, if we were to restore our marine ecosystems to reverse some of the human activity, or the impacts of human activity, what are we aiming for? Are we aiming to go back to pristine environments like the ones that you've said are very few and far between now?

ROBERT WARNER
That's a good question and it's actually a question that I can't answer specifically. What I can tell you is what the state might be under various forms of protection. Let's say we stop fishing, we can predict to a certain extent what the environment will look like after that. We can't go back to the original conditions, because as you've already pointed out, there have been some extinctions. We simply can't replace those organisms, but there have been actually relatively few extinctions in the marine environment and so we can get closer to what it used to be like. Now the question to me is always, is that what we want? If we went back to a completely pristine condition everywhere and wanted it to stay that way, wanted a sustainably pristine condition, we couldn't remove any of the protein from that environment and use it for human consumption. Right now about a third of the world's protein consumption comes from coastal waters. So the object may not be to produce a sustainably pristine environment, the answer may be to restore it to a very productive - sustainability productive environment, at least in some areas, which would improve fisheries rather than prevent them. In other areas perhaps, they have to be large enough to contain all the organisms that you want. But in other areas, complete protection so that we can see what the wilderness used to be like. But it's up to you, it's an ethical decision and a political decision rather than a single scientific answer.

DYANI LEWIS
Are the pristine environments the only way that we can get an idea of what marine ecosystems were like in the past?

ROBERT WARNER
Well we can guess and we can do research, we can do something called historical ecology, and that is to use every piece of evidence that we have to see what the oceans were like before humans were there. In some of those exercises we've discovered some fascinating things, one of which was it seems impossible that there could be this many large predators around and then we found out that it is actually true when we went to some of these still pristine environments. But you can try and recreate what was there simply by using every piece of evidence that you have from middens, which are old refuse heaps, from people that originally lived in coastal environments. You can use fishermen's logs, you can use Columbus' log, we used that as well. Reports from travellers to areas that had not been affected much by people and the reports of huge concentrations of - unbelievably huge concentrations of animals. So yes we can assemble that kind of information and guess what the environment was like. But perhaps one of the most satisfying things is then to go to a pristine environment and see whether we were right. But there are very few places like that.

DYANI LEWIS
Fishing is far from the only human activity that impacts marine ecosystems. What about effects that happen from the bottom up? Rather than the top down, things like pollution or increased sediment from dredging and those kinds of impacts?

ROBERT WARNER
Right those are sometimes easier to control. Again, it's always an economic decision of if you stop this sort of human activity, we'll get this in the ocean. Is it worth it to you to stop it? That's something that a manager or a politician or the people will have to decide. But those can be controlled and you can get often a very rapid response, unless the environment has been horribly degraded and then it might take a long time or it might be impossible to bring it back. The other changes, more global changes are the ones that are harder to control and harder to predict the impact. So there are a lot of scientists that are attempting to predict what the biological responses will be to environmental change. It's a hard thing to do because these organisms have never been exposed to this sort of environment before. It's not going to be a sudden change, we're going to slowly increase the acidity of the oceans. We're going to slowly increase the temperature of the oceans and the real question that many of us are dealing with right now is, can organisms keep up? Adaptation is a real thing, either both within an organism's lifetime, getting used to something and behaving appropriately, just like we were talking about risks and prey, and over time through evolutionary adaptation. The real question is, given the rate of change and given the ability of animals to adapt, will there be enough time or not?

DYANI LEWIS
Is there any way to know whether predators are going to be particularly vulnerable to these kind of changes? Or is it going to be all species affected across the board?

ROBERT WARNER
Right, all species will be affected, there's no question about that. Predators are in a little more precarious position because they're at the end of the food chain. So they're depending on a chain of organisms from plants that are producing things to things that eat the plants et cetera. Anything that happens within that chain is then going to affect them. So they're a little bit like at the tail end of the train and if the train starts whipping around you might have a greater effect at the end.The other problem with many of the predators, especially things like turtles, like sharks, like big marine mammals, is that they do reproduce slowly. With a longer generation time, you don't adapt as quickly. So they may be vulnerable, simply from a life history point of view. More vulnerable than say a planktonic organism that has a lifetime of a day or two.

DYANI LEWIS
Outside of reefs, what's happening further afield in the deeper ocean?

ROBERT WARNER
Yes you know there isn't as much work being done in the deeper ocean, simply because the protein that we extract, the fisheries, are involved in more shallow water. There are some deep water fisheries that are catching animals on tops of seamounts and in fairly deep water, but most of the fisheries are concentrated in shallow water, simply because that's where the sunlight penetrates, that's where the plants grow and that's the base of the food chain.The deep water in terms of what is going to happen over the course of the next hundred years is of some concern, but interestingly enough, deep water may be a model for what the shallower ocean will be in the future. Simply because deep water is more acidic already, it's not warmer that's for sure, but it is more acidic. We see a good number of adaptations of organisms in the deep sea to the acidic conditions. This actually gives us some hope that the same sort of adaptations may begin to occur in shallow water organisms.

DYANI LEWIS
Bob Warner, thank you very much for being our guest today on Up Close.

ROBERT WARNER
You're welcome, thank you.

DYANI LEWIS
Robert Warner is Research Professor of Marine Biology in the Department of Ecology, Evolution and Marine Biology at the University of California, Santa Barbara.