The "conservation of variety," as Carson writes, is a concept articulated by Charles Elton, a British ecologist. What Carson (and Elton) mean by this phrase is directly relevant to her analysis of the massive mortality of Dutch Elm trees in American cities in the mid-twentieth century. Carson shows that attempts to fight the disease focused on destroying the beetles that carried the fungus that killed the trees. These included the use of pesticides that had the unintended consequence of killing or sterilizing many different birds that fed on the beetles and other insects that were exposed to the poisons. The real problem, Carson argues, was that city planners and other people did not contemplate the problems that could emerge when they planted their city streets with a single species of tree. This made it more likely that a single disease could destroy every tree in the city, and the frantic...
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attempts to save them led to the ill-advised use of pesticides. Carson argues that a better approach would have been one rooted in the conservation of variety, which would allow for more biodiversity and render outbreaks of disease less catastrophic. This would not have the unintended consequences that spraying dangerous pesticides to stem an outbreak would have. As she wrote, "What is happening now is in large part a result of the biological unsophistication of past generations." People didn't understand the consequences of planting Dutch Elm trees everywhere:
Even one generation ago, no one knew that to fill vast areas with a single species of tree was to invite disaster. And so whole towns lined their streets and dotted their parks with elms. Today, in consequence, the elms die and so do the birds.
References
The primary focus of Carson's claim is the idea that human beings take an active role in fostering as many different types of natural elements as possible. Carson believed that the deep notion of environmental ethic can only be accomplished if individuals protect and conserve the environment in as many ways as possible. The development of different types of Dutch Elm tress is but one example. Carson takes this same approach in discussing the conservation of the shorelines of the United States, which need to be conserved in a variety of ways and manners, ranging from protecting the species that live and balances the ecosystem in the water to ensuring that steps are taken to not further pollute such a setting. The conservation of variety is the term Carson employs to ensure that the natural diversity and variance found in nature continues under the mindful and protection of human beings.
Dutch Elm Disease was a disease, spread by a beetle, that devastated elm tree populations in the United States. The disease was introduced to the United States in 1920s and has destroyed huge percentages of the elm trees in this country.
When she discusses replanting of elm trees, Rachel Carson advocates for "conservation of variety." By this, she means that not just one variety of elm tree (and that other kinds of trees, and not just elms) should be planted. The reason for this is that diseases that attack one variety of tree are not likely to attack all varieties.
If there are many different kinds of trees planted, it is much less likely that any one disease will kill them all.