If you were told that beaver, bees, an oak tree and a purple sea star were keystone species, you might wonder, “What does that mean?” Simply stated, this means that each of these species fills a critical niche in its ecosystem which no other species can. This makes them vital to the function of the part of nature they inhabit – their habitat. It can be a plant, animal, insect, bacteria or fungi.
Let’s back up a bit and see how the idea of one species being vital and far more important than others (in its own niche) has developed. Until the mid 20th century, the study of nature was a rather general, called Natural Science. Ecosystems were just beginning to be studied in detail and conventional thinking held that a system with a variety of plants, animals and insects would be in balance. Not a lot of thought had been given to the variety of living things, the numbers of each species or the ways in which they interacted with each other. Nature took care of everything.
However, Environmental Science was beginning to come into its own. In 1962 Rachel Carson published her ground-breaking book “Silent Spring” which documented the detrimental effects of pesticides on the environment. People began to ask questions and began to notice that ‘nature’ which had always been taken for granted, was changing.
A group of researchers began to think about ecological niches, and balance of species asking the question, “Why does the world stay green?”. Why don’t insects eat every leaf, every blade of grass? We know they are voracious, so what keeps them in check? They posited the Green World Hypothesis which states that the balance of predatory carnivores and herbivores prevents the total destruction of plant life. This seems perfectly logical now, but this top down view was a new way of thinking.
Previously, it was thought that the balance of life at any level was dependent on the level below it, not above.
Robert T Paine set about to prove that top carnivores affected species diversity. In the 1960’s he was teaching Zoology at the University of Washington in Seattle. He began removing ocher sea stars from an intertidal area on the Pacific coast. He found that the sea stars ate 15 different species of sea life, including anemones, sea urchins, limpets, snails, mussels and barnacles. Paine began visiting the test area weekly, tossing star fish out to sea and keeping meticulous records of the life in the tidal area.
With in 18 months the variety of species was close to halved. Acorn barnacles had crowded out almost everything, and within months, California mussels overran the barnacles, resulting in a mussel monoculture. Species diversity was gone.
Paine used the term ’keystone species’ to describe a species of life which has a disproportionally strong influence in its niche. He also used the phrase ‘trophic cascade’ or ecological collapse to describe what happen when a keystone is removed. The purple sea star is a keystone species in the intertidal zone of Mukkaw Bay.
Since that ground-breaking work, Keystone Species have been found at all levels of existence. Examples:
- Keystone predators: wolves, grizzly bears, lions, sea stars
- Keystone Prey maintain healthy populations in spite of being preyed upon: snowshoe hares, wildebeests, lemmings, krill
- Ecosystem Engineers change the habitat to make it better for all: beavers, elephants, African termites
- Mutualists are beneficial to other species while benefitting themselves: bees, hummingbirds, wasps
- Keystone plants: aspen, willow, wild strawberries, white oak, Saguaro cactus, Big Bluestem grass
*A Keystone Species may not be key in a different habitat
* Keystone Species are always native to their environment
Keystone species are often removed from their niche by accident or by human intervention with disastrous consequences. The removal of wolves in Yellowstone National Park in Wyoming, USA is a famous example. Without that apex predator, the number of elk skyrocketed. They ate so much of the willow and aspen that the erosion in riparian areas increased. Beaver had nothing to eat and they moved on or died. The health of the entire ecosystem deteriorated, and species diversity plummeted.
Read about the reintroduction of wolves to Yellowstone and how a balanced ecosystem works here:
When Keystone hardwoods such as oak and hickory are removed from a forest (usually by clear cutting) the food sources for bears, birds, deer, and rodents are gone. There are no nesting sites, no leaves to feed insects and the understory no longer hosts rotting logs and leaf litter for worms, snails, salamanders, ants and all the other vital decomposers. The area becomes a wasteland with no biodiversity – no life.
Why do keystone species matter in our gardens? Earth needs our assistance to return to even a small proportion of its health and biodiversity. By using native plants in our gardens, we create a food web which will increase the number of pollinators, small animals and songbirds. More trees, shrubs and plants cool the local climate through transpiration and by actively growing during the hottest season of the year. Native plants can deter weeds and invasives. They are appropriate for the region’s soil, temperatures, rainfall and are naturally resistant to many pests and diseases. They are also more drought tolerant than introduced species due to their deep root systems. These extensive roots stabilize soil, sequester carbon, and hold organic matter.
A White Oak is a Keystone species in southern Ontario. You may not have space for one, but you can plant Blazing Star, Black-eyed Susan, Bee Balm, Goldenrod, Native Asters and perhaps a Downy Serviceberry to encourage pollinators and birds. If you need to replace a tree or have space for a new one, consider a Keystone tree, which will provide food and homes for dozens of species. Possibilities: Pin Cherry, Red Maple, Sugar Maple, native Willow. Non-natives such as Norway Maple or Ginkgo support very few, if any insects.
If you’d like to know more, Doug Tallamy is a wonderful resource.
His books include Nature’s Best Hope; Bringing Nature Home; and The Living Landscape,. Or check out the following YouTube video:
submitted by Leslye Glover