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.