Distinction between food chain and food web – and rise of mesopredators.

So what’s the difference between a food chain and a food web? So actually scientists really often use the idea of food chains to simplify the ecosystem we’re working in so we might think of a food chain that’s just a line that goes from a mountain lion eating a deer that’s eating grass but obviously that’s an over simplification and the reality is that there’s also coyotes bobcats foxes other herbivores other carnivores and it’s really looks more like a web than it does like a straight line like a chain you can picture a food web even when I’m saying over and over again Another important – controversial lyric – everyone’s talking about it the rap genius boards are just exploding here with the line now this is sung from the perspective of the mesopredator – these middle tier carnivores that have been released because apex predators were wiped out.

Now is this a real thing that a mesopredator could kind of rise up into that apex niche? pr not? So it can happen there are occasions where mesopredators (when apex predator is removed) will take over that niche more or less but it’s important to point out that no two species are alike so even if the mesopredator expands or changes its function it will never fulfill the same role that an apex predator did but what we see a lot more commonly is that rather than climb up the food chain like the song says often what happens is mesopredators grow in numbers expand and range but retain their trophic position so they still eat mostly the same things and that can have important effects. “Sharks getting killed for the fins? (Stew) Boom time for the rays and skates!” But they eat all the bivalves – humans left with an empty plate Because a lot of ecosystem functions that we care about – supporting biodiversity, filtering water, cycling nutrients, all of these things oftende pend on that food chain being long and so when the trophic system is downgraded, when there’s no more apex predators, than sometimes you lose those ecosystem functions.

So the lyrics should have been That would probably be the more common situation okay well I’ve got questions about that because it’s not entirelyclear to me why the mesopredators cause so many issues, where the apex predatorsdon’t. So, what’s what’s special about these middle tier predators that leadsto problems when they are released? There’s a few characteristics of themthat give them the potential to cause these impacts when apex predators areremoved so we know apex predators are large, really big animals. They typicallyhave huge ranges. Often solitary or territorial and so they occur in lowerdensities than mesopredators generally do.

So when they’re removed, mesopredators can occur in much higher densities and because mesopredators occupy that lower trophic position, their diet is more generalist they can eat a lot more things and so their impacts are more widespread as well and the other side of that is that because they have this more generalist diet they can really take advantage of subsidies that humans are providing like trash or crops or other things that come from the human dominated world and so it just makes them especially able to thrive when that top-down pressure from the apex predators is removed How do we know this stuff?

Like, how how can we tell these stories about, “Oh when apex… predators are removed, mesopredators come up?” It’s a nice story to tell but how do scientists figure this stuff out? ALEX: we do it in a few different ways the first is to use good old-fashioned experimentation where we control a situation in the laboratory setting and try to manipulate the variables and that’s often done with insects or other smaller animals that are easy to do that with in a laboratory. Going from there we want to actually go out into the real world and so people look for simplified ecosystems. Famous example where starfish is eating mussels and there’s only a few players in the ecosystem so you canreally isolate and understand what’s happening which basically means removethe starfish and then see what happens exactly and then the last way is there’sjust an abundance of evidence where over and over and over we see when apexpredators are removed meso predators generally expand grow in numbers and wesee the accompanying effects of that okay so you get all this data coming infrom all these different places and you just get this like simple beautiful story like the the Yellowstone story for example in our version we we chose tohighlight the idea that, “okay when wolves were removed the Coyotes were released and they started eating all these little mammals and that species used to eatthose mammals suffered” but that’s not the most famous Yellowstone story so hit me with the the big Yellowstone story and and any additional nuance thatpeople need to know about yeah so it’s important to explain this becauseeverything in ecology is a little bit more complicated than it seems and eventhen the stories we tell and so there’s a famous example of a food chain wherewolves eat elk, elk eat willows and by reintroducing the Wolves you’re actuallyallowing the Willows to recover and a bunch of ecosystem functions to recoveras well but the story is a little bit more complicated the Wolves did play arole they do have an important role to play in regulating elk numbers but whatwe’ve learned recently is that drought, climate change, hunting by humans outside of Yellowstone National Park, the return of other predators like Grizzlies, thesecan also play important effects and complicate that ecosystem turning itfrom a food chain into a food web So the main point is not that that story is wrong It’s just that as, like, criticalconsumers of science, we need to always remember that there’s probably a littlemore complicated variables to the situation.

for example, elk hunters if we’re using the Yellowstone example might actually have their livelihoods suffer because of some of these changes so it’s really important to bear in mind that there’s a lot more to ecology than just the science and we have to think about both the wildlife and the human aspects of it if we want to do a good job I think that’s a cool question to leave the audience with. Think about the largest wild area that you live near. What is that ecosystem looking like what would change if apex predators were doing better in that particular area? Who would the winners and losers be?