The conventional wisdom is that human knowledge, especially scientific knowledge, moves inexorably forward. We are sometimes amused by the misconceptions of the past and marvel at the primitive knowledge of our ancestors. However, we rarely stop to think that our descendents will probably do the same when they consider our current state of knowledge.
Historical botanical beliefs
The Doctrine of Signatures seemed a logical botanical belief at a time when plants were one of man’s few medicinal tools and religion was a powerful influence in human society. The Doctrine of Signatures, which was actively promoted by the church in 17th century Europe, was based on a belief that God had “signed” plants with certain suggestive shapes and colors to inform humans of their medicinal properties. For example, a heart-shaped leaf was considered God’s message to us that a particular plant would be beneficial to the human heart and this message was strengthened by a flesh-colored flower. Every plant was believed to be useful in some way if man could only discern its use. Else why would they have been created, since the Garden of Eden was created for the benefit of man? The church encouraged man’s study of plants as a way to worship God’s creation. (1)
Many botanical myths originated from ancient Roman and Greek horticultural treatises and persisted for hundreds of years. For example a belief in the influence of the moon on plants is first found in the writings of Pliny in first-century Rome and also found in writings as late as 1693: “[w]hen you sow to have double Flowers, do it in the Full of the Moon.” (2)
The origins of many horticultural myths are unfathomable but probably began with a particular event because we often confuse coincidence with causal relationships. (2)
- Planting bay trees and beeches near homes will prevent lightning strikes
- An apple tree that fruits and flowers at the same time is a bad omen
- The parents of a child who picks red campion will die
- A pregnant woman who steps over cyclamen will miscarry
Modern botanical beliefs
Now we will turn to the theoretical underpinnings of the native plant movement to see how they are holding up to the scrutiny of current science and ask the rhetorical question, Is it time to relegate invasion biology to the dust heap of discredited science?
The field of invasion biology upon which the modern native plant movement is based, originates with the publication in 1958 of The Ecology of Invasions for Animals and Plants by Charles Elton. Elton postulated that every plant and animal occupies a different ecological “niche” and plays a specific role within that niche:
“…every species will have a slightly different role, or niche, and often, he believed, every niche will be filled. Some animals eat grass, others leaves; some plants grow on wet soil and some grow on dry; some birds nest in dead trees, others in live ones. When new species are introduced, the theory goes, they can get a foothold and start reproducing only by finding a vacant niche or by throwing some other species out of its niche…” (3)
Elton’s corollary to the exclusivity of the niche is that the introduced species will have a competitive advantage because its predators are absent in its new home. The predicted result of Elton’s theory was that introduced species will exterminate previous occupants, mass extinctions will occur, and the result will be a simplified ecology composed of few surviving species.
The problem with Elton’s theory is that it doesn’t correspond with reality. More and more scientists are finding that the frequency of introductions far exceeds the frequency of extinctions.
- In 2002 Dov Sax reported that introduced species greatly outnumbered extinctions on oceanic islands: “In the case of plants, islands are now twice as diverse as they were before humans started moving things around.” (3)
- In 2012, Erle Ellis, et. al., reported that “…while native losses are likely significant across at least half of Earth’s ice free land, model predictions indicate that plant species richness has increased overall in most regional landscapes, mostly because species invasions tend to exceed native losses.” (4)
- In San Francisco, the second-most densely populated city in the US, ninety-seven percent of the 714 plant species known to exist in San Francisco in 1850 are still found there, despite the fact that most plants and trees in the city are introduced. (5)
The dire predictions of invasion biology have not come to pass nearly 60 years after their inception. Many scientists are clearly ready to abandon invasion biology because it does not conform to reality. Can we finally breathe a collective sigh of relief and move on to a less gloomy view of ecology? Some day our descendents will look upon this episode in human history and laugh, as we laugh at the 17th century Europeans who examined plants, looking for the clues from God that revealed their purpose.
(1) Richard Mabey, Weeds: In Defense of Nature’s Most Unloved Plants, Profile Books Ltd, London, 2010, page 87-91
(2) Andrea Wulff, The Brother Gardeners, Alfred Knopf, 2008, page 11-12
(3) Emma Marris, The Rambunctious Garden, Bloomsbury, USA, 2011, page 102-104
(4) Erle C. Ellis, et. al., “All Is Not Loss: Plant Biodiversity in the Anthropocene,” http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0030535
(5) Duncan et al, “Plant traits and extinction in urban areas: a meta-analysis of 11 cities,” Global Ecology and Biogeography, July 2011