Native plant advocates use many arguments to justify the destruction of non-native species and we have debunked many of those arguments here on Million Trees. Now we will examine the claim that non-native species must be destroyed because their mere existence reduces biodiversity by out-competing native plants and animals. Because eucalyptus trees are one of the primary targets for eradication, we will focus on the specific claim that the eucalyptus forest is a “biological desert.” We are frequently told that “nothing grows” under the eucalypts and that they are not providing food or habitat to insects, birds, and other animals.
- Species of plants in the understory
- Species of invertebrates (insects) in samples of equal size and depth of the leaf litter
- Species of amphibians
- Species of birds
- Species of rodents
He reported his findings in Global Ecology and Biogeography*:
“Species richness was nearly identical for understory plants, leaf-litter invertebrates, amphibians and birds; only rodents had significantly fewer species in eucalypt sites. Species diversity patterns…were qualitatively identical to those for species richness, except for leaf-litter invertebrates, which were significantly more diverse in eucalypt sites during the spring.”
Professor Sax also surveyed the literature comparing biodiversity in native vs non-native forest in his article. He reports similar findings for comparisons between non-native forests and local native forests all over the world:
- In Spain, species of invertebrates found in the leaf-litter of eucalyptus plantations were found to be similar to those found in native forests, while species richness of understory plants was found to be greater in the native forests.
- In Ethiopia the richness of understory species was found to be as great in eucalyptus plantations as in the native forest.
- In the Mexican state of Michoacán, species richness and abundance of birds were found to be similar in eucalyptus and native forests.
- In Australia species richness of mammals and of soil microarthropods were found to be similar in native forests and in non-native forests of pine.
The only caveat to these general findings is that fewer species were found in new plantations of non-natives less than 5 years old. This helps to illustrate a general principle that is often ignored by native plant advocates. That is, that nature and its inhabitants are capable of changing and adapting to changed conditions. In the case of non-native forests in the San Francisco Bay Area, they have existed here for over 100 years. The plants and animals in our forests have “learned” to live in them long ago.
We recommend that you visit the SaveSutro website for a description of the richness of the non-native forest that thrives on Mount Sutro in San Francisco. It is the perfect illustration of these scientific principles. We can discuss scientific principles in the abstract, but there is no substitute for a walk in the forest to confirm with our eyes what science tells us.
*Dov Sax, “Equal diversity in disparate species assemblages: a comparison of native and exotic woodlands in California,” Global Ecology and Biogeography, 11, 49-52, 2002.
Conservation Sense and Nonsense 
Very well-written and convincing article!
from the conclusion of the Sax article:
“These results may not at first seem to be intuitively reasonable. The negative impacts of exotic species on many native systems have been catastrophic, e.g. the effects of the brown tree snake in Guam (Fritts & Rodda, 1998). However, in this study, the substitution of eucalypt trees for native ones has not had catastrophic effects on species diversity. The reasons for this are most likely complex, but the most important aspect of the explanation seems to be the existence of a well-established understorey of plants.”