Defending Biodiversity
Oriole feeding from a Banksia tree. Photo by Tim Bray
In last month’s Black Oystercatcher I described the removal of large Himalayan Blackberry thickets on the Noyo Headlands that had been used by many birds for feeding and nesting. Similar actions occur in many places as species classified as invasive are targeted for removal. These actions result from a belief that non-native species do not offer ecosystem values comparable to native species, despite abundant evidence to the contrary.
Many plant species that have naturalized in California now provide valuable habitat for wildlife. Eucalyptus trees are disliked by many people, yet they are preferred nesting habitat for Red-tailed Hawks, Great-horned Owls, and many songbirds. Their flowers provide abundant nectar for hummingbirds as well as many bees and other insects, which in turn provide food for a variety of songbirds. As with Himalayan Blackberry, Eucalyptus invaded so long ago that many generations of wildlife have adapted to its presence here. Yet in the San Francisco Bay area, public agencies are planning to eradicate immense numbers, drastically altering the local ecosystem while transforming the landscape.
Rufous/Allen’s hummingbird feeding from a Grevillea bush. Photo by Tim Bray
Birds will readily utilize the shelter and food resources provided by plants, without regard to their origin or length of residency. The Banksia trees planted 20 years ago in the Rose Memorial Park in Fort Bragg are now a local hotspot for migrant songbirds, especially in winter when the trees (from Australia) produce flowers whose nectar draws Orioles and Hummingbirds, as well as Warblers and other small insectivorous songbirds. Grevillea bushes (also from Australia) that bloom year-round at the Mendocino Coast Botanical Garden are host plants for many resident and migratory hummingbirds, and their dense network of limbs and foliage provides nesting habitat for songbirds.
Much of what we read about invasive species focuses on the harm they might cause to ecosystems. Frequently that harm is not yet evident but is predicted based on extrapolating current growth rates of the invader. It turns out that in most invasions, those growth rates peak and then decline, and the eventual net result is simply an increase in biodiversity. The exceptions are mostly in island or island-type ecosystems, where biodiversity is relatively low, refugia scarce, and introduced predators or pathogens can take out entire populations. Introduced plant species can alter ecosystems, sometimes to the benefit of some species while detrimental to others, but do not cause extinction by competition.
Rufous/Allen’s hummingbird feeding from a Grevillea bush. Photo by Tim Bray
Plants and animals have always moved about, expanding their ranges wherever conditions favored them. Every widely distributed species was “invasive” at some point in its evolutionary history. How long must a species live somewhere to be considered native? Was there ever a time when everything lived only where it belonged?
Our landscape management decisions are often influenced by our preference for some idealized state, and science is often misused to provide justification for those personal preferences. Nature however has no preferences, is constantly changing, and prioritizes only fitness and adaptability. We should judge species by their ecosystem functions and habitat values, rather than on their origins.
