The monarch butterfly population rose sharply this year, increasing by about 60 percent at their wintering grounds in Mexico, one of the strongest rebounds in recent years.

But zoom out, and the picture changes.

In the 1990s, monarch winter colonies in Mexico typically covered 37 to 50 acres of forest. In recent years, they have often occupied under 12 acres, and sometimes less than 3. Even with this year’s increase, the population remains roughly 70 to 90 percent below those earlier levels.

Scientists estimate that a more stable eastern monarch population would require around 6 hectares or more of winter habitat. Many recent seasons have fallen short of that threshold.

The difference comes down to habitat.

Monarchs depend on milkweed. It is the only plant where they lay their eggs, and the only food their caterpillars can eat. Without it, the life cycle stops.

A single caterpillar can consume an entire milkweed plant or more as it grows. That means a few scattered plants rarely support more than one or two butterflies. What works better is a visible cluster, enough plants in one place to be found, used, and replenished.

Adult monarchs need something different. They rely on nectar from flowering plants such as coneflower, bee balm, goldenrod, and asters to fuel their long migrations.

Supporting monarchs is not about planting a single species. It is about building a small system:

• Milkweed for eggs and caterpillars
• Nectar plants for adults
• Clusters, not isolated stems

This year’s increase shows how quickly monarchs can respond when conditions improve. It also shows how far they still have to go.