A keystone species is a species that has a disproportionately large effect on its natural environment relative to its abundance. The concept was introduced in 1969 by the zoologist Robert T. Paine. Keystone species can be any organism, from animals and plants to bacteria and fungi, and they are the glue that holds a habitat together. They are critical to the survival of other species in the ecosystem, and without them, the ecosystem may not survive. Keystone species have low functional redundancy, which means that if the species were to disappear from the ecosystem, no other species would be able to fill its ecological niche. The ecosystem would be forced to radically change, allowing new and possibly invasive species to populate the habitat. Some examples of keystone species include predators, herbivores, mutualists, and plants. A classic example of a keystone species is a predator that prevents a particular herbivorous species from eliminating dominant plant species. If prey numbers are low, keystone predators can be even less abundant and still be effective. Yet without the predators, the herbivorous prey would explode in numbers, wipe out the dominant plants, and dramatically alter the character of the ecosystem. Keystone species are important providers of shelter and habitats for other species, and they can also be apex predators or unassuming plants.