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Olivia: Some animals seem to be everywhere. Like, when you meet one pigeon, there's probably hundreds more around the corner. It hardly seems fair when there are only a few thousand tigers in the whole world. But how can so many of certain animals survive?

The food chain, usually called a trophic structure by ecologists, describes where different living things get their energy, and if they provide energy for other organisms by being food. Plants and some microbes, for example, use energy from the sun to produce their own food. Then, they’re eaten by small consumers, which are then eaten by larger predators. Each of these groups is a part of a different trophic level, and only a fraction of the energy gets passed up to the bigger consumers.

In other words, top predators like orcas probably aren’t going to overrun the planet. And the most abundant animals are usually near the bottom of the trophic ladder, eating really common foods, living in lots of places, and sometimes even taking advantage of us humans. Of course, there are way too many of these guys to actually go out and count them, so scientists have to make educated guesses. But here are 9 animals that we’re pretty sure have huge populations.

Even though city pigeons seem to be everywhere, their numbers pale in comparison to other birds. Like the red-billed quelea, which might be the most abundant wild bird in the world. Despite looking like cute little finches with bright red bills and carefully-woven nests, they strike fear into the hearts of farmers. There’s an estimated 1.5 billion red-billed queleas across about two-thirds of the African continent.

They travel around in flocks of millions that have been known to crush trees when they roost for the night. One reason they’ve been able to become so abundant is their diet: they eat all kinds of seeds. And there are lots of seeds out there. Especially since human farmers are in the business of growing grains.

Flocks of queleas can descend on grain fields like a plague, each bird eating 12 grams of seeds per day. Needless to say, they’re considered to be a huge agricultural pest. When there’s lots of food around, queleas can breed multiple times in the same season, which keeps their populations growing. And because they’re so abundant, red-billed queleas make up a huge part of their trophic level’s biomass, and have a lot of natural predators and scavengers that eat them for energy.

Even though we don’t go down into the deep sea very often, there’s a whole lot of life down there. In fact, the most abundant vertebrate on Earth is a fish you probably haven’t run into: the bristlemouth. Fish belonging to the genus Cyclothone, and others of the bristlemouth family, are estimated to exist in trillions, maybe even quadrillions.

Bristlemouths in this genus are tiny, bioluminescent deep-sea fish that have spiky bristle-like teeth. And they get eaten by bigger, scarier deep-sea fish. We don’t know much about them because they live pretty deep down, up to about three miles. But we do know that the ocean is big, and may account for 99% of the Earth’s habitable space, or biosphere. So if schools of bristlemouths can live pretty much anywhere in the oceans, up to a certain depth... well, that’s a lot of fish.

It’s hard to be completely sure what the most numerous amphibian is, because most species are scattered across lots of habitats. But it’s most likely a frog. One species that’s doing particularly well is the African clawed frog. They’re tough little guys, and they can eat anything they can find, from live insects to fish to chunks of organic waste. Plus, they can live in just about any pool of water, and find ways to thrive in all sorts of places, despite different climates.

Even though these frogs are native to the African continent, humans have introduced them across the world. The reason? Pregnancy tests. At some point around the 1940s, scientists discovered that if you inject a female African clawed frog with the urine of a pregnant person, the frog would lay eggs.

Super weird, but pretty nifty. So humans started breeding these guys in a lot of countries, in labs and eventually as pets. Around the 1960s, less... alive... pregnancy tests were invented, which were smaller technologies that screened for certain hormones. So lots of these frogs were turned loose. Some scientists use African clawed frogs as laboratory models in research even today, but there are plenty of wild ones chilling in ponds all over the world.

The most abundant mammal on the planet might be us, at 7 billion and some. But humans don’t fit the usual rules and survival strategies. There are some mammals that do fit into more traditional trophic structure, though, and that have taken advantage of us. Like the house mouse, which is the reason we have cats.

Some estimates say there are as many house mice as humans in the world. They breed quickly and eat any food we keep around, especially grains, and even things like glue or soap. With humans settling all over the world, bringing food with us and planting crops, it was easy for these mice to come along for the ride.

Anywhere we live, house mice can too. And they love to set up shop in the nooks and crannies of buildings, meaning we provide extensive habitats for them too. We may consider them pests, but they were pretty shrewd to get in on the ground floor of the whole “humans taking over the planet” thing.

The most abundant mollusc, which is the phylum that includes the squid and octopus, plus lots of shelled creatures, has to be a snail or slug. And one possible candidate is the brown garden snail. These snails are native to Europe, and are food for lots of small predators, but humans have introduced them to all continents besides Antarctica.

Sometimes it was by accident, the little guys like to stow away on all kinds of plants, which are their main source of food. But other times it was on purpose, because people like eating them too. Escargot, anyone?

Garden snails have done pretty well in spite of hungry humans, though. They’re considered an agricultural pest, because they’re good at settling down anywhere there’s moisture and plants they can eat. Which is a lot of places.

Now, let’s get into the arthropods, which is a phylum of invertebrates that puts every other animal group to shame in terms of sheer numbers. Of the insects on Earth, ants outnumber all the rest, with estimates of trillions of them worldwide. And of the thousands of species of ant, one of the most abundant is the Argentine ant.

The Argentine ant is native to South America, but they’ve hitchhiked on human cargo shipments to every continent except Antarctica. Ants are social animals, and Argentine ants are especially good at settling down in huge fast-growing colonies, thanks to many breeding females instead of just one baby-making queen. Some colonies even stretch for hundreds or thousands of kilometers down coastlines in the Mediterranean, western Japan, and in the U.S. state of California.

These ants can take advantage of all kinds of food sources, and have been known to farm other insects, like aphids, that produce sugary honeydew. Because of sheer force of numbers, these ants are tough. And we’ve accidentally brought them all around the world.

These next animals, called springtails, used to be considered insects until recently. So they get their own category. They’re six-legged creatures, named for a special springy organ that lets them jump away when they feel threatened. Springtails feed on all kinds of fungus pieces or rotting dead stuff using internal mouthparts, instead of external chewing bits like insects. Basically, they’re tiny, and researchers think that they live in soil all over the world – up to tens or hundreds of thousands of them per square meter.

And that includes snowy places, where you might notice the snow flea as a little black speck. These snow fleas can survive temperatures lower than most arthropods, by producing a natural antifreeze-like protein to keep their tissues from getting damaged by the cold. So, since springtails are pretty much everywhere, they’re important parts of ecosystems and a stable food source for anything that eats tiny invertebrates.

As for crustaceans, krill are the poster child for abundance in the ocean's trophic structures. Of particular note, the Antarctic krill pretty much support the entire food chain of the southern oceans, with an estimated population of around 500 trillion. Dense swarms of small Antarctic krill can stretch for kilometers and even make the water look orange.

They feed on phytoplankton, and are nutritious meals for basically everything in the cold ocean waters, like whales, penguins, seals, and fish. And the biomass of all these krill is estimated to be huge, hundreds of millions of metric tons, which means that they could outweigh all the humans on the Earth. And that’s kind of mind-boggling, since one human weighs a lot more than one krill.

But one crustacean outnumbers every single animal on the planet, and maybe even all multicellular organisms. I give you the copepod, a humble zooplankton, which are tiny animals that drift around in watery environments.

Some of the world’s most important fisheries depend on copepods of the genus Calanus. Like krill, they live in huge groups, eat phytoplankton, and provide nutrients to the rest of the food chain. But copepods are better survivors than they seem: while many ocean creatures are filter-feeders, eating whatever’s floating in the water nearby, some copepods can detect phytoplankton at a distance and actively attack them. And even though lots of things eat them, they can make speedy escapes from predators.

Finally, copepods are good at meeting up and making babies, instead of just drifting in currents and reproducing asexually like many other small organisms. Sexual reproduction on a regular basis gives them an evolutionary edge, keeping them ready to change and adapt.

If we can learn anything from copepods, and all of these abundant animals, it’s that being near the bottom of the food chain isn’t so bad. That way, they have plenty of food, can take advantage of bigger creatures like humans, and spread out across the world. So we tend to think that huge predators like tigers are really good at what they do, but these little guys seem to have it figured out too, and are really important parts of ecosystems.

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