Why do leaves change color in the fall?

Fall has arrived in the northern hemisphere, and leaves will soon be changing colors. We’ve all heard why they do this: the green color is created by chlorophyll, a substance key to photosynthesis, and when the chlorophyll is lost at the end of the summer, leaves reveal their true colors.

Now, some scientists think this story is wrong. They believe that, instead, trees actively work to create their brilliant fall colors.

Why would trees do this? One challenge plants face in life is being attacked by insects. To combat this, plants produce a variety of chemicals that deter insects from eating their leaves or burrowing in their bark.

Just as chlorophyll creates a green color in leaves, some of these insect-deterring chemicals create bright yellows, oranges, and reds. The more chemical a tree stores up, the more vibrant the colors.

In the same way that a male bird puts on showy colors in spring to prove that he is a healthy mate, trees display dramatic autumn hues to tell insects, “I’ve invested in defenses against you; don’t bother trying to attack me.”

Colorful fall leaves may therefore be not only a defensive strategy, but a method of communication – providing further evidence that plants are intelligent.

What are wildlife corridors?

As a general rule, bigger habitat patches are better.

Since a small yard cannot be a big patch, this may sound like disappointing news. However, suburban yards can play a crucial role in helping surrounding patches be bigger.

How does this work? Imagine two patches – say, two stands of forest – that are relatively near each other. Some species, like birds, might be able to easily move back and forth, in order to take advantage of both patches. But this isn’t an option for other species – like small mammals, slow-moving reptiles, or birds reluctant to leave the forest interior. These less-mobile animals have to make do with the resources of just one patch.

Now, imagine there is a strip of woods connecting the two forests. The less-mobile animals can move along this strip, enabling them to use both patches. In effect, the addition of this strip makes the patch twice as big!

This kind of strip is called a wildlife corridor. For forest animals, it might be a narrow belt of forest, as described above. For fish, it could be a river between two lakes.

While some wildlife corridors are naturally occurring, some are deliberately built. A common example of this is tunnels under roads, to help small animals cross without being run over. To help larger animals move, California is considering building a bridge over the 101 Freeway – the so-called wildlife overpass.

Halfway between the natural and the artificial, property owners can landscape their yards in ways that help connect neighboring habitat patches. This could mean offering food and water for migrating birds, or cover for small animals to travel under.

By thinking outside our property lines, we can choose to make our yards vital parts of the habitat networks that surround us.

What is a patch?

A patch is an area of habitat.

Patches can come in any size. For example, balsam fir trees enjoy a huge patch of habitat across Canada’s boreal forest. But, a small Wisconsin yard with the right site conditions could also be a habitat patch for balsam fir.

If you own a yard – or even a balcony that could host a few flowerpots – then you have a patch. It is your choice what to do with it.

Some people choose to have a lawn, maybe with some non-native ornamental flowers or shrubs, and to prevent any other plant or animal from living on their property. Other people choose to make space for many species in their patch.

Even in a small yard, it is possible to have multiple patches. For example, one corner of the yard might include trees, along with plants that like shade. Squirrels may nest here, and forest birds might drop by. Another corner of the yard could host sun-loving plants, and the insects that frequent them. A third corner could feature a pond or rain garden, providing a habitat patch for wetland plants and birds that like to bathe.

Patches differ in quality as well as size. While a grouping of woodland plants in a corner of a suburban yard is not as good as a forest, it is better than a single aggressively-pruned tree standing in a lawn. Even relatively small, low-quality patches can provide critical resources for struggling species.

By taking small steps to improve the quality of our very own patches, we can enjoy seeing species not normally observed in the suburbs.

What is habitat?

Habitat is an area where a plant or animal can live.

Whether or not a particular area is habitat depends on which species’ perspective you’re looking at it from. For example, the ocean is habitat for a whale, but is not habitat for a squirrel.

To count as habitat, an area must provide for a plant or animal’s needs. For example, habitat for a bird must include water sources, appropriate food, safe shelter, and a place to nest. Habitat for dandelions must include plenty of sun.

Lawns count as habitat, since by definition grass lives in them. However, few other species can meet their needs in or around a lawn. Overall, lawns are not very good habitat.

Natural yards provide better habitat by inviting in many plant species. These plants, in turn, provide much of what an insect or bird needs to call a place habitat.

Habitat loss is a major factor in species extinctions. Hundreds of North American bird species, as well as popular insects like bumblebees and monarch butterflies, are in decline. If we exchange poor habitat in our yards for better habitat, we can increase these species’ chances of survival.