Snakes are important elements in their native environments, regulating the populations of their prey. They're exclusively carnivores, so they're predators, but sometimes they can be prey for other predators, including other snakes. They can present a threat when they're an invasive species in an ecosystem where there's little or nothing to regulate their population. Both the introduction and removal of snakes can have unforeseen effects on an ecosystem.
Snakes as Population Controllers
As carnivores, snakes keep the numbers of their prey down. Rodents provide the best example, as they reproduce exponentially in the absence of predators, as long as there's food. While this is true in a natural environment, it's particularly true in an artificial environment like any kind of food storage area. The University of Nebraska estimates that mice cause $20 million in damage annually in Nebraska alone. Snakes can slowly prowl to hunt, able to enter burrows and tight spaces where other predators like cats or hawks can't go, and traps can't be set.
Snakes Aren't Always on Top
Since snakes aren't always the top predator, they can become prey for higher predators. In that role, they pass the bounty of their prey's population boom up the food chain. When a large prey population attracts and sustains a large snake population, those snakes become plentiful prey for birds like hawks and herons, or mammals like skunks and raccoons. Some snakes specialize in preying on other snakes, like the kingsnake, which can prey on rattlesnakes because they are immune to rattlesnake venom.
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Introducing Snakes Disrupts the Food Chain
Since the 1990s, large constrictor snakes of several species have established themselves in southern Florida. They threaten the populations of mammal, reptilian and avian prey who do not recognize the snakes as predators. An older example of invasive snakes is the brown tree snake, which was introduced to Guam sometime in the 1950s. It hunts in trees among birds who don't expect it. Eradication efforts have gone so far as to air-drop mice dosed with acetaminophen as poison bait.
Snakes Cause Cascading Effects Both Good and Bad
Snakes can play a role in the complicated web of inter-species relationships. For example, timber rattlesnakes in the eastern U.S. prey upon mice who are hosts to black-legged ticks. Those ticks are a vector for Lyme disease, a bacterial infection. When the snakes reduce the number of mice, the prevalence of Lyme disease in the environment is reduced. In the case of the brown tree snake, its predation on native pollinators and seed distributors like birds and lizards has reduced the ability of native plants to regenerate, which has reduced plant cover on Guam..