The enduring fiction of the native plant movement is that the existence of non-native plants threatens the existence of native plants by “crowding out” native plants. If that were true, we should expect to see some evidence of such a causal relationship after 250 years of steadily increasing numbers of non-native plant species. But we don’t.
Marcel Rejmanek (UC Davis) is the author of the most recent report on plant extinctions in California, published in 2017. At that time there were 13 plant species and 17 sub-species native to California known to be globally extinct and another 30 species and sub-species extirpated in California but still found in other states. Over half the globally extinct taxa were reported as extinct over 100 years ago. Although grassland in California had been converted to Mediterranean annual grasses by grazing domesticated animals decades before then, most of the plants now designated as “invasive” in California were not widespread over 100 years ago.
Most of the globally extinct plant species had very small ranges and small populations. The smaller the population, the greater the chances of extinction. Most of the globally extinct plants were originally present in lowlands where most of the human population and habitat destruction are concentrated. Although there are many rare plants at higher altitudes, few are extinct. Plants limited to special habitats, like wetlands, seem to be more vulnerable to extinction. The primary drivers of plant extinction in California are agriculture, urbanization and development in general.
Non-native plants are the innocent bystanders to disturbance
“Invasive species” are mentioned only once in the inventory of extinct plants published by California Native Plant Society and only in combination with several other factors. However, the identity of this “invasive species” is not clear. Rejmanek suggests that the “invasive species” rating refers to animal “invasions” by predators and grazers. He says, “Indeed, one needs quite a bit of imagination to predict that any native plant species may be driven to extinction by invasive plants per se.” (1)
Although climate change is not cited as the cause of any of the known plant extinctions in California, Rejmanek predicts that climate change is likely to be a factor in the future, not only because of the impact of drought and higher temperatures, but also because non-native plants may be better adapted to changed conditions.
There are over 1,000 naturalized non-native plant species in California. Their presence is associated with human disturbance. Naturalized non-native plants are a symptom of disturbance, not the cause. The impact of non-native plants on native plants cannot be separated from other factors that created the conditions for success of non-native plants.
Specialized insects are exaggerated
Another popular fiction among native plant advocates who love to hate non-native plants is that specialized insects—especially pollinators—require specific native plant species. Again, the record of plant extinctions in California does not support that myth: “…there is no indication that the loss of pollinators was an important factor in plant species extinctions in California. [For example, one of the native plant species extirpated in California] has many documented non‐specialized pollinators. There does not seem to be any particular dispersal mode associated with presumably extinct plants in California.” (1)
Putting plant extinctions into context
Setting sub-species aside, there are 5,280 identified native plant species in California and 28 known extinctions of native plant species, including 15 plant species known to still exist in other states. Only .53% of California native plants are known to be extinct in California, about one-half of one-percent. Does that seem like a lot? Rejmanek compared the extinction rate in California with other Mediterranean climates. The extinction rate of native plants in California is similar to those in the European Mediterranean Basin, South Africa, and Australia, but a little greater than the rate in Chile, where there are fewer endemic plants that exist only in Chile. Endemism is associated with small native ranges and small populations that are more vulnerable to extinction.
Why are there many endemic plants in California?
About 40% of native plant species in the California Floristic Province are endemic, found only in California and in most cases only in small areas within California, including our off-shore islands. Their small populations in isolated geographic areas, sometimes within unique ecosystems, such as alkaline sinks, make them particularly vulnerable to extinction.
The evolutionary history of endemic plant species explains why there are so many in California. Endemic plants are close relatives to plants that exist elsewhere and are sometimes plentiful where they came from. For example manzanita is a genus of chaparral shrub that is plentiful in California, but there are also many rare endemic manzanita species that occur only in small areas and small populations. There are several endangered manzanita species in the Bay Area (pallid, Raven’s, Franciscan).
The geography of California explains why the evolution of a plant species diverged from its plentiful ancestors to become an endemic species in a small geographic area. Plants move around in a wide variety of ways, most natural, without the aid of humans. Their seeds are dispersed by animals and birds that eat them or inadvertently carry them to another location. Sometimes their seeds are carried on the wind or brought to islands by storms and currents.
When a plant arrives in a new location that is isolated from its original home and therefore cannot mate with its relatives, it begins its own, independent evolutionary history. Each successive generation is reacting to its new environment, rewarding its fitness with its new home with a successful new generation. Each generation rolls the genetic dice, its genome drifting away from its ancestors in a random way. Occasionally a mutation will occur that alters the evolutionary trajectory. Eventually, the plant in its new home is sufficiently genetically distinct that taxonomists are ready to call it a separate species. Naming a new species is a judgment call, often questioned by some taxonomists, called “lumpers” as opposed to the “splitters” who are ready to name it a new species.
The factors that result in endemic species are many, but broadly speaking they are mobility and, ironically, isolation. California is one of the most geographically diverse states in the country, with corridors for mobility, but many barriers that create isolation. Gordon Leppig describes California’s geographic diversity in Beauty and the Beast: California Wildflowers and Climate Change, published by California Native Plant Society: “The state’s natural wonders include five deserts, the highest and lowest points in the continental United States, the third-longest state coastline (about one thousand miles), the most national parks (nine), the most federally designated wilderness areas (more than 140), the highest percentage of wilderness in the contiguous United States (14%), the most diverse conifer assemblage outside the Himalayas, the most federally listed species….” The multitude of different ecosystems with unique microclimates produces one of the most diverse floras in the world.
Human activities penetrate the barriers that created genetic isolation in the past. Our roads become corridors for the biological exchange that threatens small, isolated pockets of rare plants. Trade and travel has ended the isolation of off-shore islands. Our roads and dams also create new barriers for mobility. In other words, we are altering pre-settlement corridors and creating new ones. We should expect consequences for our ecosystems for the changes we have made.
Given the number of rare and endemic plants in California and the changes in the environment required to accommodate nearly 40 million human Californians, it seems that extinction of less than one-half of one percent of native plants is a surprisingly small loss.
(1) Marcel Rejmanek, “Vascular plant extinctions in California: A critical assessment,” Diversity and Distributions, Journal of Conservation Biogeography, 2017