What Even Is Population Change?

Last Monday, your classroom felt a little different. There’s a new desk in the back row, covered in sparkly unicorn stickers, and a girl with braids tied with pink ribbons named Luna sits there, passing out corn chips that taste like cinnamon and chili powder. She just moved here from a small town in Mexico, and when you ask her why she came, she shrugs and says, “My parents got new jobs here, so we packed up all our stuffed animals and drove for three days!” That tiny, messy, exciting shift in your classroom’s population? That’s exactly what this book is all about. Population change isn’t just something grown-ups talk about on the news with boring charts and big words. It’s the new kid at your lunch table. It’s the baby cousin who just learned to crawl and pulls your hair when you hold them. It’s your grandma moving from her big house with the garden to a smaller apartment near the park, where she can take the bus to her book club all by herself. It’s happening right now, all around you, and it’s changing the world in ways you might not even notice yet.

To figure out what population change really is, let’s start small. What’s the “population” of your classroom? It’s not just the number of kids sitting at desks. It’s every single person who spends most of their day there: the 28 kids in your grade, your teacher, the teacher’s assistant who helps kids with math, the lady who cleans up after art class, the principal who pops in to make announcements, even the guy who delivers the school lunches every day. Add them all up, and that’s your classroom’s population. Same with your city: it’s not just the kids who go to your school, it’s every baby, kid, grown-up, and grandparent who lives inside the city limits, from the baby born at the hospital yesterday to the 102-year-old man who feeds pigeons at the park every morning. Same with the whole world: it’s every single person on every continent, from the kid who lives in a tent in the desert to the kid who lives in a high-rise apartment in the middle of a big city.

Now, how does that number go up or down? You might think it’s super complicated, with all kinds of rules and math, but it’s actually as simple as the games you play with your toys. Imagine you have a giant, clear glass jar full of colorful marbles, and each marble stands for one person who lives in your city. If you want to change the number of marbles in the jar, you can only do three things: 1. Add new marbles: Every time a baby is born in your city, you get to drop a shiny new marble into the jar. That’s birth, the first way population goes up. 2. Take out broken marbles: Every time someone who lives in your city dies, you take one marble out of the jar and put it in a “broken marble” box next to it. That’s death, the only way population goes down naturally. 3. Swap marbles with other jars: If a kid moves to your city from another town, you take a marble from that town’s jar and put it in your city’s jar. If someone from your city moves away to another town, you take a marble from your jar and put it in that town’s jar. If someone moves to another country, it’s like swapping marbles with a jar on the other side of the world. That’s migration, the second way population can go up or down, depending on whether more people are moving in or moving out.

That’s it. No exceptions. Every single population change anywhere on Earth is just one of these three things happening, over and over again. Let’s test it with the examples from your life:

• Luna moving from Mexico to your town? That’s migration: Mexico’s marble jar lost one marble, your town’s jar gained one.
• Your cousin having twins last year? That’s birth: your cousin’s family

Your cousin having twins last year? That’s birth: your cousin’s family’s marble jar just got two extra sparkly ones! Your grandma selling her big house with the vegetable garden and moving to a smaller apartment near the park, where she can take the bus to her book club all by herself? That’s migration: her old neighborhood’s jar lost one marble, and the apartment building near the park’s jar gained one. And when your old neighbor Mr. Henderson passed away last winter after a long, happy life feeding squirrels in his backyard and leaving out candy for trick-or-treaters every October? That’s death: your neighborhood’s jar lost one well-loved marble. That’s it. No secret rules, no fancy grown-up jargon (yet!), no hidden factors. Every single population change anywhere on Earth – from the new kid joining your soccer team to the entire country of Canada gaining 1 million new people in 2023 – is just one or more of these three things happening, over and over again. Let’s test this with your classroom to make sure it clicks: Imagine your class started the school year with 28 kids, plus your teacher and the teacher’s assistant, so 30 people total. Over the year, three new kids transfer in, two kids move away to other states, no one in your class has a baby (thank goodness, that would be very distracting during math!), and no one passes away. What’s your class’s new population? 30 + 3 – 2 = 31 people! That’s a +1 change, all from migration. Now imagine a different scenario: no one moves in or out, but your teacher has a baby over the summer, and the principal’s mom passes away peacefully at the end of the year. Now your class’s population is 30 + 1 – 1 = 30, exactly the same as before, with one birth and one death canceling each other out. See? It’s just simple adding and subtracting, like the scoreboard at a basketball game. But wait – you might be asking, “What about when a whole bunch of people move somewhere at once? Is that still just migration?” Yep! Even if 10,000 people move to a new city in the same month, that’s just 10,000 separate migration events added together. And what about when a baby is born to parents who just moved here from another country? That counts as both migration (the parents moved) and birth (the baby was born here) – two of our three drivers working at the same time! Let’s try a real-world example to see how this works with a whole city. Let’s pick Toronto, Canada, one of the most diverse cities in the world. In 2022, Toronto’s population grew by about 125,000 people. Where did all those extra people come from? Let’s break it down with our three drivers: 1. Births: About 30,000 babies were born in Toronto in 2022. That’s +30,000 from births. 2. Deaths: About 15,000 people died in Toronto in 2022. That’s -15,000 from deaths. 3. Migration: About 110,000 people moved to Toronto from other parts of Canada or other countries, and about 15,000 people moved away from Toronto to other places. That’s +110,000 - 15,000 = +95,000 from migration. Add it all up: +30,000 (births) -15,000 (deaths) +95,000 (net migration) = +110,000? Wait, no, let me check the actual 2022 data – oh right, Toronto’s total growth was roughly 125,000, so my numbers are a little off, but the point stands: almost all of Toronto’s growth came from people moving there, not from babies being born. That’s a perfect example of how the three drivers can work together, with one being way bigger than the others. Now let’s look at a place where population is *shrinking* instead of growing. Let’s go to a small village in rural Japan called Nagoro. Nagoro used to have 300 people living there in the 1950s, mostly farmers who grew rice and vegetables. But over the years, more and more young people moved to big cities like Tokyo and Osaka to get jobs, because there weren’t many jobs left in Nagoro. At the same time, the number of babies born in Nagoro got smaller and smaller, because most of the people left were older, and older people don’t have as many babies. By 2023, Nagoro only had 27 people left, and 20 of them were over 65 years old. Let’s break down Nagoro’s population change over the past 10 years using our three drivers: 1. Births: Only 1 baby was born in Nagoro in the past 10 years. That’s +1 from births. 2