Invasion biology is the scientific discipline that spawned the native plant movement. Charles Elton published a book in 1958 that is considered the origin of the modern version of invasion biology, although there are precursors centuries earlier. These are the basic tenets of modern invasion biology:
- Plants and animals that are “native” to a specific location are considered members of an ideal ecosystem that have co-evolved over thousands of years so that members of the community are dependent upon one another.
- Plants and animals introduced to an ecosystem by humans are assumed to disrupt the equilibrium balance of the community and threaten its existence because introduced plants and animals do not have predators that would control their spread. All introduced plants and animals are therefore considered potentially invasive.
- Animals are believed to be dependent upon the plants with which they evolved—and only these plants–and these mutually exclusive relationships are disturbed by the introduction of new plants and animals.
- Adaptation and evolution of introduced plants and animals is believed to be too slow for introduced plants and animals to successfully enter the food web.
- Native members of the ecosystem are presumed to be inherently superior to introduced plants and animals. Invasion biology does not acknowledge that introduced plants and animals are often functional members of the ecological community.
- Native ecosystems are said to be in “balance” and introduced species are presumed to cause “imbalance.” Introduced species must be eradicated to restore balance to the ecosystem, presumed to be the ideal for a particular location.
Hundreds of empirical studies have been conducted since the 1960s to test these assumptions. Little scientific evidence has been found to support them. Current knowledge of ecology explains why the assumptions of invasion biology are mistaken.
What is native?
The native plant movement defines native as the plant species that lived in a specific location prior to the arrival of Europeans. In the San Francisco Bay Area, “native” is defined by native plant advocates as the plants and animals that lived here prior to 1769 when Europeans first laid eyes on San Francisco Bay. When Europeans arrived, the San Francisco Bay Area was already occupied by indigenous people who had arrived approximately 10,000 years earlier.
The arbitrary selection of the pre-European settlement period to define the ideal landscape was based on the mistaken assumption that the indigenous human population had not radically altered the land. Anthropological and paleontological research informs us that the landscape was essentially gardened by the indigenous population to provide food and cultural implements.
The landscape found by Europeans at the end of the 18th century was not “natural.” It was altered by humans to serve humans who lived as hunters and gatherers. Since modern society no longer hunts and gathers for its food and shelter, the landscape that served that lifestyle cannot be maintained without mimicking the land management practices of native people such as frequent burning of the landscape and grazing by animals. Indigenous people in California did not have domesticated animals (except dogs), but the grassland was grazed by wild deer, elk, and antelope.
Plants and animals have migrated around the world without the assistance of humans since life began. The seeds of plants are carried in the stomachs of migrating birds and on the winds of storms. Animals, including humans, move to wherever they can find what they need to survive. Migration is natural and often necessary for survival. Making a distinction between species moved by humans and those moved by natural forces is pointless and usually impossible to distinguish.
Climate change renders the concept of “native plants” meaningless because when the climate changes, the vegetation changes. The plants that live in tropical climates will not survive in arctic cold and vice versa. Introduced plants are often better adapted to current climate conditions than their native predecessors because the climate has changed and it will continue to change.
Mistaken assumptions about evolution
Animals rarely depend upon a single plant species for survival. Such mutually exclusive relationships rarely exist in nature because they are evolutionary dead-ends. Animals can, and often do, adapt quickly to changes in the environment. Transitions from native to introduced plants are routinely made by animals, including humans. Indigenous hunter/gatherers quickly incorporated plants introduced by European settlers into their diets. Plants in the same family and genus are often chemically similar, making the transition more likely.
Native plant advocates assume that evolution only occurs slowly, over thousands of years, but evolution can be faster than they assume. Rapid environmental change accelerates the speed of evolution because extreme weather events caused by climate change increase the speed of natural selection, the primary tool of evolution. When cataclysmic events such as hurricanes, droughts, floods, extreme temperatures kill many members of a species population, these are selection events in which the fittest members survive to breed and the next generation inherits the genetic traits that helped their parents survive. The classic example of this principle is the finches in the Galapagos Islands who died if they didn’t have big enough beaks to eat the seeds of the only plant that survived extreme drought. The next generation of finches had bigger beaks.
Evolution occurs when genetic changes enable future generations to inherit the genetic change. Adaptation occurs when animals respond to environmental challenges by changing behaviors that aren’t necessarily inherited by the next generation. Adaptation to changed environmental conditions is even more rapid than evolution and equally effective to ensure survival. Genetic changes are not required for an insect to make the transition from a native host plant to a chemically similar introduced plant. Extreme temperatures require that plants and animals move to more temperate climates. “Native” ranges must change to survive changes in the environment. A plant or animal that cannot survive extreme heat will migrate (if it can) into regions where temperatures are not as warm. They should not be prevented from doing so.
Plant and animal species with large populations and short lives, such as insects, evolve more quickly. This more rapid pace of evolution enables a more rapid transition from native host plants to closely related introduced plants.
Nativism and the native plant movement
The native plant movement is based on the belief that native plants are superior to introduced plants, that native plants are somehow “better” than immigrant plants. That assumption of superiority is the definition of nativism. It is as specious an assumption in the natural world as it is in human society and it is equally dangerous.
There are pros and cons to everything living in the natural world and there is no right answer to the question of which species is “best.” When evaluating introduced plants, nativists consider only the negative aspects. They refuse to acknowledge that there are also advantages and a death verdict should take both into consideration. For example, native plant advocates want all eucalyptus trees in California cut down because they were planted here after European settlement. This negative judgment of eucalyptus does not take into consideration that 75% of monarch butterflies who spend the winter in California use eucalyptus trees for their safe haven. Also, eucalyptus blooms in California from November to May, providing nectar to butterflies, hummingbirds, and bees at a time of year when native plants are not blooming. Eucalyptus trees are also nesting homes of owls and other raptors. Cutting down eucalyptus trees simply because they are not native in California ignores the many benefits they provide to wildlife.
Confusing cause and effect
The native plant movement mistakenly assumes that the mere existence of introduced plants threatens the existence of native plants. They believe that native plants will magically emerge if introduced plants are eradicated. They have spent 25 years eradicating non-native plants and do not seem to have noticed that native plants have not returned. They make this mistake because they do not acknowledge the changes in the environment that make non-native species better adapted to current environmental conditions.
Many of the changes in the environment that are inhospitable to native species are caused by structural changes made to accommodate human activities, not by introduced species. For example, all the major rivers in California have been dammed to prevent floods and store water for use during the dry season. These dams have fundamentally altered the ecology of our rivers. There are no longer cleansing spring floods that clear rivers of accumulated mud and vegetation. Channeled rivers are deeper and warmer. Salmon can no longer get to their spawning grounds past the dams. The altered structural conditions are more hospitable to bass than to trout. Aquatic plants from tropical regions become invasive in warmer water. None of these conditions are reversed by spraying aquatic plants with herbicide or killing introduced bass.
Wherever “invasions” are observed, no thought is given to why. Instead, a convenient plant or animal scapegoat is found and poisoned. That death sentence doesn’t reverse the underlying reason for the invasion. Therefore, the invasion persists. Society is unwilling to make the sacrifices, even inconveniences, needed to address the underlying cause of the “invasion.” We have done little to address the causes of climate change. We are unwilling to destroy the dams and the system of supplying water to serve agriculture needs. Invasions are the symptom, not the cause of the changes in nature.