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As the hours of daylight increase and the ground thaws, as if by magic the northern forests change again. As new leaves appear, trees start producing sugars, and that's good for the fungus in the soil. Entwined with the trees' roots, they can siphon off some of these sugars. But not everything appears so harmonious.

In the land of the lynx something extraordinary is happening to the forest. These trees may have endured the winter, but now it's spring, they are under attack. Some are even dying. But what's happening here now is vital for how this ecosystem functions over the year.

This defoliation is entirely natural. And I might be able to find one of the culprits down here, although they are quite tricky to spot. They live in these fresh green shoots. Now, wrapped delicately in these leaves is a species that is single-handedly influencing the ecology of this entire forest.

Inside this nest is the caterpillar of the spruce moth. The budworm. and it hasn't only wrapped itself up in these leaves to hide from predators, because it's eating them as well. And it doesn't just eat the leaves, it also eats the buds, the flowers, and the cones on the tree here.

Up in the canopy, a tiny budworm caterpillar has just emerged from hibernation. And it's racing to fatten itself up. When it's finished on one branch, it releases a strand of silk, and abseils down to the next.

It's a risky business being a juicy, fat caterpillar – you're in danger of being spied by all of those birds, just back from migration. But the caterpillar has a plan – it uses its silk to weave the needles together and hide in a dense web. Now, the springtime assault by these caterpillars is bad news for the trees. But for other inhabitants of these forests, these caterpillars are heroes.

It's thanks to the behaviour of this species that one of North America's most elusive and charismatic predators – the Canadian lynx – is enjoying a bit of a renaissance in forests like this one. But the caterpillar lives all the way up there, in the canopy. Whilst the cat with the pointed ears is prowling around down here. So, how can a humble insect like this have any impact on a formidable thing like that?

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I bet the lynx never even sees the caterpillar throughout the course of its life. Why does the lynx need the caterpillar? The clue is how they affect the lynx's prey on the forest floor. For decades, scientists have studied budworm caterpillars, and a remarkable pattern has emerged. They've discovered that the population of caterpillars fluctuate dramatically. And at the peak of a cycle there can be tens of thousands of budworms in a single tree.

And this has devastating consequences. Whilst these dramatic natural events might be a catastrophe for the established trees, for anything trying to grow on the forest floor they are an absolute bonus. In here, where it's dark, there is very little, very poor diversity – just some mosses and a few ferns.

But as soon as there is a break in the canopy and the sunlight can flood in, well, look at the difference. Lots of wildflowers, there is a young maple coming through here, a Mountain Ash and most importantly of all, regenerating spruce and fir.

Now, the hares, essentially need these regenerating conifers and shelter. And of course what's good for the hares is also good for the lynx. It's such an elegant connection.

Without the spring emergence of the hungry caterpillars to chew holes in a dense canopy, there wouldn't be enough light flooding the forest floor. And, with less light down here, there will be less growing for our snowshoe hare to forage and to hide in, and then there would be nothing for L1-11 and all those hundreds of other lynx to eat. And that's why the lynx needs a caterpillar.

And now it's spring there's no better time to see what the future holds for the lynx population.

Wildlife biologist Jen Vashan is doing a count. She has detected a signal from a radio-collared female. There she is. But there may be something else here. Safe inside her den, a lynx club. He's just a few weeks old. Jen must work fast before mum returns. Thanks to the timing of the budworm opening the canopy this spring, there is going to be enough prey for these lynx to hunt next winter.