ECOVIEWS
SNAKES ARE SIGNS OF A HEALTHY ENVIRONMENT
Could snakes serve as touchstones to healthy habitats? Some people's immediate response might be, "Who wants snakes?"
Despite less than charitable attitudes toward some top-of-the-food-chain predators such as snakes, their absence means that all is not right with the natural world. When many tropical or warm temperate zone ecosystems are operating normally, snakes are present. When they are not around, the biodiversity is anemic.
How can snakes be indicators of a healthy environment? In natural ecosystems, the presence of top-level carnivores, the meat eaters, indicates that the herbivores, the plant eaters, are doing well. The presence of herbivores, in turn, denotes that the plant life is functioning as it should.
No snakes eat plants. All are carnivores. Because snakes vary widely in their dietary preferences, the presence of a wide assortment of snake species in most regions of the southern United States indicates that the food web is sufficiently diverse.
The term "balance of nature" is sometimes used in discussions involving the dependence of some species on others. But the term is overused, misused, and usually is ecologically meaningless, so I will not refer to anything having to do with the balance of nature. Food webs and food chains, however, are tangible, identifiable entities more appropriate for considering the relationships between top predators and their food sources.
A food web refers to the complexity of feeding patterns among organisms, the pathways that energy takes as it passes through the ecosystem--sun to plant to herbivore (prey) to carnivore (predator). Thus, animals eat plants and each other in a bewildering array of patterns.
A food chain is the path of energy flow through a specific part of an ecosystem. Thus, a sweet clover plant captures the sun's energy, which is stored in the plant in the form of calories. Meanwhile, a harvest mouse eats the seeds, and the calories are passed on. The food chain is lengthened when a rat snake eats the mouse. The rat snake could fall victim to a kingsnake or hawk, but in simplest terms, the rat snake sits at the top of a food chain--plant to herbivore to carnivore.
But predators would be unable to persist without prey, which would not be around without the plants. So, the very fact that a predator is present means that lower links in the food chain are operating properly.
Some animals, including some snakes, are dietary specialists, eating primarily one or a few select prey species. For example, hognose snakes do not normally eat mice but instead are restricted to a near-exclusive diet of toads. Adult mud snakes eat primarily large salamanders, and tiny crowned snakes eat centipedes. The prey of snakes may even be predators themselves, making for a longer food chain. Thus, toads require insects, giant salamanders eat crawfish, and centipedes use their pinchers to capture small soil invertebrates for food.
In some regions, where suitable habitats are available, all the snakes mentioned above might be found within a few feet of each other. Their collective presence in the ecosystem indicates that a healthy lower-level food base exists in the form of plants and prey. So, if all the snakes characteristic of a region are around and doing fine, we can have greater confidence that a properly functioning habitat is available to support them. Of course, the environment may suffer from other problems, but having a robust food web in operation is essential.
But if the snakes are missing, we should begin to question what else might be absent. Are hognose snakes missing because no toads are to be found? Are the toads absent because of an insufficient supply of insects? Has something happened at the lower levels, resulting in a problem that is expressed at the top of the food chain? When something goes wrong at the bottom of a food chain, an environmental cascade can result that would not necessarily be apparent by simply looking at the landscape.
So, the discovery of a rich biodiversity of native snakes in a region should be viewed as a positive sign. You may not be fond of snakes, but when they are not found where they are supposed to be you may dislike the deeper-lying environmental problems even more.
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