Eucalyptus plantations in California have been criticised, because they compete with native plants and typically do not support native animals. Eucalyptus has historically been planted to replace California’s coast live oak population, and the new Eucalyptus is not as hospitable to native flora and fauna as the oaks. In appropriately foggy conditions on the California Coast, Eucalyptus can spread at a rapid rate. The absence of natural inhibitors such as the Koala or pathogens native to Australia have aided in the spread of California Eucalyptus trees. This is not as big of an issue further inland, but on the coast invasive eucalypti can disrupt native ecosystems. Eucalyptus may have adverse effects on local streams due to their chemical composition, and their dominance threatens species that rely on native trees. Nevertheless, some native species have been known to adapt to the Eucalyptus trees. Notable examples are herons, great horned owl, and the monarch butterfly using Eucalyptus groves as habitat. Despite these successes, the Eucalyptus generally has a net negative impact on the overall balance of the native ecosystem…
Due to similar favourable climatic conditions, Eucalyptus plantations have often replaced oak woodlands, for example in California, Spain and Portugal. The resulting monocultures have raised concerns about loss of biological diversity, through loss of acorns that mammals and birds feed on, absence of hollows that in oak trees provide shelter and nesting sites for birds and small mammals and for bee colonies, as well as lack of downed trees in managed plantations. A study of the relationship between birds and eucalyptus in the San Francisco Bay Area found that bird diversity was similar in native forest versus eucalyptus forest, but the species were different.[109] One way in which the avifauna (local assortment of bird species) changes is that cavity-nesting birds including woodpeckers, owls, chickadees, wood ducks, etc. are depauperate in eucalyptus groves because the decay-resistant wood of these trees prevents cavity formation by decay or excavation. Also, those bird species that glean insects from foliage, such as warblers and vireos, experience population declines when eucalyptus groves replace oak forest.
Birds that thrive in eucalyptus groves in California tend to prefer tall vertical habitat. These avian species include herons and egrets, which also nest in redwoods.[110][111] The Point Reyes Bird Observatory observes that sometimes short-billed birds like the ruby-crowned kinglet are found dead beneath eucalyptus trees with their nostrils clogged with pitch.[41]
Monarch butterflies use eucalyptus in California for over-wintering, but in some locations have a preference for Monterey pines.
A friend notes:
There are a couple of other problems about Eucalyptus other than that they generally “sterilize” the area where they dominate of native plants and animals
The first is that they are not trees that can survive frost. When Oakland California had its devastating fires that wiped out half the homes in the hills, the previous year or (maybe a couple of years earlier) a frost had killed the trees. Even when alive and green Eucalyptus trees are full of oil and resin and burn easily. When they are dead they are even more dangerous tinder.
The second is that they are known as “widow makers” in Australia because often large branches fall from even seemingly healthy trees. If a person is unfortunate enough to be under the tree when this happens, they will be seriously injured or killed. Even if no one is there, the fallen branches will damage whatever they land on.