Jennifer Dunne, professor at the Santa Fe Institute, is working on the first-ever effort to quantify where humans fit into the food webs in our ecosystem. Dunne, co-director for Pacific Ecoinformatics and Computational Ecology (PEaCE) Lab, says our over-fishing of blue fin tuna is a good example of how humans are changing the natural cycles of our food web. It’s obviously bad for the tuna, but what most of us don’t realize is how it might destabilize the entire ecosystem.
How does a food web differ from a food chain?
Food webs reflect the complexity of all the relationships among organisms. The simple way to think about it is a bunch of food chains that are stuck together. You can take a typical food chain and interconnect it with another food chain, and another, and you end up getting a more complex network.
Why is it important to look at it this way?
Everyone learns about food chains in elementary school, and it usually doesn't go any further than that. We all understand the idea of networks, and this is just an example of an ecological network. Looking at this helps us understand what happens when a species goes extinct or if there’s climate change or a change in the habitat. The food web gives us a more complex view.
Can you walk me through an example?
Let’s talk about wolves in Yellowstone. Before humans decided to hunt wolves to extinction, they preyed on a bunch of different species. But then they went extinct, and the things they preyed on--like deer and elk--their populations exploded. And that impacted all the vegetation they fed on. So you end up affecting the plant community in a variety of ways in terms of the plants that elk or deer would feed on. Those plant populations would go down, and the population of the competing plants would go up. So now, wolves are in the system and are feeding on elk and other things, and that affects the plants and animals like coyotes and bears who also feed on elk and deer.
So basically, one simple chain of links doesn’t really get you the full complexity when you reintroduce wolves in Yellowstone. Grizzlies now compete with wolves for at least some of what they eat—elk. And in some cases, the grizzlies are out-competed by wolves in foraging for elk. Again, you continue to get these ripple effects up and down the food web.
If you’re reintroducing a wolf, or reintroducing a species for pest control , you can quickly end up in a situation you didn’t expect because you had too simple a picture in your mind. We see that a lot—you think you’re going to introduce a predator of a bug we don’t like, but at the same time it’ll take out beneficial insects like pollinators.
So It’s important that conservation managers take a look at the bigger picture.
A lot of them have focused on tiny pieces of the system. In fishery systems, we have competition for the fish we like to eat, like seals or sea lions. So some fishery scientists in earlier years have called for killing or removing seals or sea lions, thinking that if we get rid of them there will be more of the fish we want to harvest. Network models show that about half the time the abundances would go up, but the other half, the abundances would go down. That’s because all these fish are not just linked to humans or sea lions. There are all these many different indirect paths and interactions throughout the network.
This is why a network approach in ecology provides a quantitative framework where we can begin to analyze and try to understand more systematically all the effects of us monkeying with the system, and we can avoid the bad unintended consequences.
From the time we were hunter-gatherers, what are some general trends in how humans are affecting the food web?
I’m involved in this study with the Sanak Aleut people in Alaska. It’s the first time that scientists have taken this ecological network approach and plugged humans into the complex ecosystem as predators. We’re talking about humans in the Aleutian Islands over the last 6,000 years , who lived closely tied to the ecosystem. One of the things we wanted to see is are humans just like other predators, or do they play special roles. It turns out humans do play special roles--they are more general eaters; they are eating more types of food than any other predator in the system. So humans are super-generalists and also super-omnivores.
They’re feeding on everything from algae all the way up to higher predators like sea otters, and everything in between—clams, fish, marine mammals. So when you do a quantitative analysis, they’re more strongly omnivorous than any other species is. They play special roles in how they fit into this network. Yet we know they were living on these islands continuously without crashing it. They don’t seem to have driven other species out of existence. So the humans lived as part of this ecosystem for thousands of years.
We use this model in order to understand the conditions by which a species could stabilize the system—either be part of the system without causing other species to go extinct, or destabilize the system. If a species that feeds on many different things enters the system, as long as it’s foraging for just a few of the species it eats, that’s very stabilizing.
What we know from previous modeling is that generalists fit into a system fine as long as they do this switching behavior. If they focus on just one species, its abundances will go down, and it will be harder for the predator to find and capture the species. So it will naturally switch to another prey because they can’t get their preferred thing anymore. So this is what generalist predators do—it’s good for the predators and releases pressure on the prey periodically. You get this cycling, with a predator feeding on something, switching to something else, and then the first prey item starts to recover. That’s very well documented. They’re constantly switching between habitats, between freshwater and terrestrial . This is what non-human generalist predators do.
So in our commercial food industry, are we messing up this natural cycle?
Here’s my take on commercial fisheries. Modern humans are homo-economicus, or humans in an economic system. Think about what happens in a blue fin tuna fishery: They’re becoming increasingly rare, because we are hunting them to extension.
We’re not talking about hunter-gatherers; we’re talking about people motivated by the global sushi industry. So even though they are harder and harder to catch, their value is increasing. With hunter-gatherers, as abundance goes down, the value decreases. In this case, the more rare they become, the more valuable they become. So instead of the system causing the predators to fish something else, it’s increasing the rate of the fishing because they’re trying to get those last few blue fin tuna. It’s obvious it’s bad for the tuna. But what people don’t really understand is that it could be destabilizing for the system--not just for the blue fin tuna but for those organisms that are indirectly connected to the tuna.
So instead of decreasing the rate on low abundance, you are increasing it and making it potentially unstable. We’re just modeling it now for the potential impact of these economically driven activities that introduce an unnatural dynamic to the system. If we fish blue fin tuna to extension, it could have a ripple effect.
A lot of fisheries around he world have failed because fishery managers have thought much too narrowly. There’s a new emphasis to do ecosystem management and to think about the whole system. People are pushing it very much in the context of fishery management. There’s many stakeholders—local fisherman, commercial fisherman, sport fishermen, scientists, managers, policy makers. Getting them all at the table and getting them to agree on a good way forward is difficult. I’m interested in how we can take things from basic science research and how to translate them to management practices.
Tell me about some of your other research areas.
One of the things I’m most excited about is working with paleo-biologists to try to understand food webs from ancient ecosystems—hundreds of millions of years ago, like a dinosaur food web. There’s a lot of evidence in the fossil records from these ecosystems to show what they ate, who ate them. The reason that’s important is that understanding ancient ecosystems gives us a context to understand current ecosystems. We can start to understand whether ancient systems were organized similarly to modern ecosystems. It gives us a baseline, so if we see a change in the fundamental organization of an ecosystem, that’s radical. It gives us a way to understand the severity of changes and to understand big macro-evolutionary patterns. Plus, working with the paleo-biologists is really fun.