Duplicate this page before you start, and write your answers into your own copy. Everything you need is on this page. Work through the sections in order, on your own, and do not read ahead: two of the sections only work if you have not seen the next one.

Section 3 is the long one, and it is meant to be. Do not polish it. Whatever state your twelve rows are in, make sure you reach section 4 before the end of the lesson: that is where today's point arrives, and it will not make sense without them.

🐺 1. The story you already know

Almost everyone has met this story. Biology teachers all over the world use Yellowstone National Park as the standard example of a trophic cascade: take the wolves away and the forest falls apart, put the wolves back and the forest returns.

The version most people carry comes from a short film posted in February 2014 by a small non-profit called Sustainable Man, built out of narration taken from a TED talk by the British writer George Monbiot. Watch it before you read any further.

https://www.youtube.com/watch?v=ysa5OBhXz-Q

✍️ Write here, on your own.

1. In one sentence, what did the wolves do to Yellowstone?

2. Where did you first meet this story: a lesson, a video, a documentary, a parent, somewhere else?

3. Give yourself a mark out of ten for how confident you are that the story is true, and say why you did not give it one mark higher.

📉 2. What happened next

Grey wolves were hunted out of Yellowstone in the 1920s under government programmes. Without wolves, elk numbers grew, and elk eat aspen and cottonwood saplings. Young trees stopped reaching maturity, the old ones died of old age, and the skyline changed. In 1995 wolves were brought down from Jasper National Park in Canada and released into the park.

A photo taken in 1933 shows Yellowstone aspen groves stripped of bark and leaves by elk. (National Park Service, 1935) Source

None of that is in dispute. What came next is.

In July 2025 a team led by Luke Painter, an ecologist at Oregon State University, reported that the returning wolves had set off one of the strongest trophic cascades measured anywhere. Their headline figure was a 152-fold increase in the density of aspen saplings between 1998 and 2021. A second team, publishing that same year, reported a rise of about 1,500% in willow crown volume and also called it one of the world's strongest cascades. The story everybody already believed now had numbers attached to it.

Increased heights of young aspen since wolves, National Park Service Source