Someone has joked that, for animals, the concept of niche can be defined by what an organism eats, and what eats the organism. In truth, the whole business of who eats whom in the animal world is fun to think about.

First of all, in our backyards natural predators such as bobcats, foxes, weasels, skunks, certain snakes, and even turtles (who snatch ducklings from below the water) are usually absent. However, too often there's one predator who decimates backyard wildlife -- the house cat. If house cats were kept in houses, numbers of squirrels, birds, and many other wild backyard animals in our towns and cities would blossom dramatically.

Besides such outright predation, most wild animals, including those in our neighborhoods, are being eaten alive all the time -- by parasites. One spectacular case of that is shown at the left. This is a Tomato Hornworm caterpillar parasitized by Braconid wasps. I just spotted this caterpillar on the Bell Pepper plants in my organic garden, so this is one of the wonderful things you get to see when you garden without lots of chemicals. Anyway, the Braconid wasp laid eggs on the caterpillar, the larvae hatched from the eggs and burrowed inside the living body of the caterpillar as it ate my Bell Pepper plants. Gradually the caterpillar began feeling bad and the larvae eventually burrowed to the caterpillar's outside and spun the white cocoons you see. When I scanned this picture the hornworm appeared to be barely alive. It wasn't eating at all and would hardly react to my touch. You can bet that after scanning the picture I returned the hornworm with its cocoons to a pepper plant in the garden so that later adult Braconid wasps will emerge from the cocoons and fly away to mate and lay their eggs on more hornworms. And the poor hornworm in the picture will eventually die. This is a beautiful example of natural biological control of a garden pest.

Other forms of parisitism are more subtle. For instance, even healthy, ordinary songbirds are typically loaded with parasites. Feather lice and mites can completely destroy a bird's feathers. Certain kinds of fly eat the outer layers of a bird's skin. Mites and tongue worms bore into nasal cavities, lungs, and connecting tubes to feed on secretions. Fleas, lice, fly larvae, mosquitoes, midges, certain bugs, leeches, and ticks suck the bird's blood from outside. The list goes on and on. A.L. Rand describes the common park pigeon as a "living house" that can be home to 71 or more different kinds of plants and animals. The situation is similar with other kinds of animal.

We shouldn't let these thoughts about parasites cause us to lose interest in animals. Similar lists can be compiled of parasites on and inside humans, even those of us who look squeaky clean and smell of rose-scented soap. Inside, we are all hosts to large metropolitan areas of protozoans, worms, flukes, bacteria, and creatures we may have never heard of, such as the minuscule items who every now and then cause our noses to itch, and our eye-winkers to twitch...

Parasites by themselves seldom kill wild animals, but they do often weaken them so that when stressful situations arise, such as cold weather, they may die when otherwise they would survive. Parasites can slow animals down, both physically and mentally, so that they don't react fast enough if, for instance, a house-cat comes stalking...