Letter 5: Top Predators

One of the most thrilling moments of our trip to Africa was seeing this beautiful Cheetah.

(Click on any image to enlarge it)

Before we arrived, she had killed a little Thomson's Gazelle, such as these.

We could still see the herd in the distance. They knew it was safe to continue grazing nearby, since the Cheetah had already made her kill.

Thomson's Gazelles
Thomson's Gazelles
Ngorongoro Crater

As I'm sure you know, all of the top predators in the world today, such as that Cheetah, are in trouble because of loss of habitat. Cities, such as Nairobi, spread to take over more of the land. Here you can see Nairobi and its airport just beyond where these Zebra are grazing.

Of course, it's not just Nairobi, but human habitations and farms all over the world, that are the problem. And it's not just that we take up space the animals need. Another part of the problem is fragmentation of their habitat; that is, we cause their habitat to be broken into smaller and smaller pieces.

Top predators, such as the Cheetah, need a large territory in which to hunt, or else they will kill all their prey animals and starve. But they must also be able to find other Cheetahs with whom to mate, which they can't do if there are roads or cities or people with guns between their territories. And even if they can find a mate, their children must also be able to find mates who are not closely related to them, since mating between close relatives results in weak children. Thus, the large predators die out unless they have territory enough to hunt as well as access to the territory of suitable mates.

These top predators are so magnificent that seeing them or just knowing that they exist makes our lives richer. (It would be a sadder world if there were no Cheetahs or Polar Bears or Cougars.)

Zebras and Nairobi
Zebras and Nairobi
Nairobi National Park

But, even more important is the fact that ecosystems tend to collapse, to unravel, when the top predator is removed. You might think that doing away with a big predator would help the other animals, but in fact it usually causes many of them to die out too. This has been seen all over the world, whenever a large carnivorous mammal or bird has been hunted to extinction or has died out because of habitat loss. (Ecologists call this a "trophic cascade".)

Some years ago, Lee and I saw an example of this very near you. There is a park outside Miami that used to be famous among us birdwatchers for its heron rookery. A rookery is a place where large wading birds, such as this egret, come together to build their nests atop a group of trees. Building their nests near one another keeps them safer from predators, because of having many eyes to watch for danger.

Lee and I were in Florida to see our relatives, so we decided to make a detour to visit the park. When we got to the gate and had paid the entrance fee, we asked for directions to the rookery. The jolly man at the gate said, "Oh, we have no birds! We removed all the alligators and the birds went away."

We were puzzled by this at first, but then we understood. The alligators had been removed because they were thought to be a danger to people. Once the alligators were gone, there was nobody to keep the raccoons under control. That meant that the raccoons climbed the trees of the rookery and ate the eggs and the baby birds, so the birds had left. Hundreds of herons, egrets, ibises, and spoonbills no longer had a safe place to live and breed. The ecosystem had become less rich.

Little Egret
Little Egret
Lake Baringo

We humans are still learning what makes ecosystems work. The complexity of it all is amazing.

Aunt Melinda