The Up And Downsides of Invasive Species
In our recent look at what makes a native species native, we contrasted species that are native to a place with those that have been brought in from somewhere else. People familiar with the concept of introduced species often assume that every species that’s introduced causes problems. But that’s not true, and even the worst troublemakers can offer occasional benefit to their new habitat’s native denizens.
It’s the most important thing to keep in mind when we’re talking about introduced species: they’re not all the same. For every species like kudzu, star thistle, or feral pig that wreaks havoc in its new home, there are perhaps dozens of introduced species that don’t cause much problem.
Even the most ardent native plant activists, for instance, will admit there are some introduced plants that behave themselves. It’s hard, for instance, to find a California native plant fan who’ll say bad things about daffodils, and if bearded irises have detractors, it’s usually because they provide shelter for snails.
It’s not just introduced plants that can seem benign additions to an ecosystem. Opossums have made themselves comfortable in California cities without obviously harming native wildlife, and while the several species of introduced parrots in California cities may outcompete native birds for tree fruits, that’s usually only the case where the fruit trees are themselves introduced, so the effect on the native ecosystem is hard to suss out. (Here’s some handy terminology: When an introduced species has established a self-sustaining population that doesn’t necessarily seem to be causing much trouble, biologists often refer to the species as having “naturalized.”)
There are even introduced species that are considered so valuable by your typical environmentally oriented person that we engage in prolonged social media campaigns to protect them — even though they may actually be much more problematic for local native species than opossums or daffodils.
Take for example the beloved honeybee, which in its few centuries of habitation on the North American continent has definitely offered beleaguered native pollinators some stiff competition. (There’s little evidence that honeybees have directly damaged native bee species, to be fair.) And yet despite their introduced status, it’s hard to pass a week online without reading entreaties to ban pesticides to save them. Which is fine: those same pesticides are as likely to harm wild native pollinators, and we do rely on those introduced bees for a lot of pollinating work.
There’s also the earthworm, beloved of bait-dunk anglers and home composters, most of which you’ll see in North America evolved in Europe and were brought here after the 1500s. There are earthworms native to California, and they still thrive in habitats where the land hasn’t been disturbed or artificially irrigated, but it’s the common exotic earthworms of coastal gardens that are generally heralded by enviro-types as indicative of robust soil health and fertility.
In other words, there seems to be a number of exceptions in the public mind to the notion that introduced species aren’t a good thing. Still, many ecologically aware folks have more destructive species come to mind when asked about introduced species: feral pigs, for instance, or pampas grass, tamarisk and tumbleweeds, gorse and blue gum eucalyptus.
All new species introduced into an ecosystem have an impact. Ecosystems tend to be zero-sum games. Take the aforementioned daffodils: even if that three-foot wide clump of bulbs next to where an old farmhouse once stood took a century to get that big, it has occupied a space that could have been occupied by a native species, consumed a bit of soil moisture that could have gone to native insects and earthworms, and so forth. It may be a small impact, and it may be outweighed by small benefits the organism offers to other members of the ecosystem. (A daffodil clump in Pacoima might offer shelter to native slender salamanders, for instance.)
Even if a new species is taking advantage of space and resources, that no other organism seems to be using, it can still have a decided impact on an ecosystem. In the California desert, the grass red brome grows in places that aren’t usually occupied by other plants: the wide open spaces of mineral soil between creosotes and other shrubs. In wet years, the brome competes for space and water with native annuals, but in dry winters and springs the introduced grass grows where no other plant wants to, using moisture in the top couple inches of gravel that would likely evaporate anyway.
That might sound like a perfectly innocuous way of surviving in the desert. But red brome’s success at colonizing the spots no one else usually wants makes it a serious threat to native plants and animals in the desert. Those formerly bare stretches of mineral soil between desert shrubs meant that fires in the desert rarely spread before the advent of red brome. Now, a lightning strike that might have burned ten square feet of desert back in the 1930s will ignite the carpet of brome, which will spread the fire from shrub to shrub, perhaps for miles.
Introduced species that turn out to be invasive can disrupt ecosystems just by crowding, as in the case of red brome, or of yellow star thistle, a fiercely armed plant that has covered more than 15 percent of California’s landscape in thick, nearly impenetrable mats. They can compete with their native neighbors for food and other resources, even chasing them away from prime habitat, as Eurasian collared doves sometimes do with native mourning doves, and red-eared sliders do with Western pond turtles.
Or, as in the case of bullfrogs, brown trout, feral pigs, and domestic cats, introduced species can change local ecosystems by chowing down on the native inhabitants.
However they alter the ecosystem, introduced species that become invasive are a serious ecological problem. One recent study published in the journal Nature found that of 8,868 species the International Union for the Conservation of Nature has listed as Near Threatened, Vulnerable, Endangered or Critically Endangered, invasive species threatened 2,298 of them: a quarter of the total.
Generally, only about a tenth of introductions end with the species surviving in its new home. Of those that survive, only about a tenth become problems. It’s estimated that six new invasive species are introduced to California every year, which means about 594 species are introduced to the state with little damage resulting.
When those species do cause damage, they cause big damage. A pair of clam species introduced from Asia have pushed the ecology of the San Francisco Bay Delta to the point of collapse. The red brome already mentioned isn’t the only plant that aids potentially destructive wildfires: many of the conflagrations the reporters call “brushfires” are actually fueled by introduced grasses like wild oats. And in the desert, red brome isn’t the only plant that spurs fire fears, or even the worst. Top billing there almost certainly goes to the herb Sahara mustard, with fountain grass close behind where it’s gotten a foothold in the Sonoran Desert. Any native plants in the Delta that survive those clams diverting their nutrients have to compete with a literal raft of invasive aquatic plants. Scotch broom and gorse crowd out other shrubs in Northern California’s coastal chaparral.
With well-publicized introduced destructive invaders like these, it’s no wonder that some folks think of introduced species and invasive species in the same breath. But not all invasive species are introduced. The California deserts, for instance, are experiencing a massive problem with ravens, a native bird whose numbers have skyrocketed in recent decades, and which are a serious threat to desert tortoises. At the turn of the 20th Century ravens were present but rare in the Mojave Desert. I just spooked two dozen of them out of my back yard two hours before writing this sentence. And that’s bad for the tortoises: ravens can eat a prodigious number of baby tortoises, and biologists are debating how best to address the problem.
Another example of an invasive native species: the Canada goose, hunted to near extinction in many parts of its range in the late 19th Century. Now, there are so many Canada geese in less-arid parts of the continent, especially cities, that they’re often considered a nuisance animal. Their droppings can pollute waterways, often leading to eutrophication of ponds and lakes. Their size and habit of bursting into flight in large flocks makes them an aviation hazard, especially during migration seasons. And they’re ill-tempered at times, and have been known to attack people, causing occasional significant injuries.
The common factor in the population explosions of ravens and Canada geese? Humans’ redesigning of the landscape in which the once-manageable populations of birds live. Canada geese thrive in places with a lot of short grass, which provides both food for the goose and good visibility of potential approaching predators. And in the last 125 years we’ve replaced a very large percentage of native vegetation on the North American continent with just the kind of short grass the geese really like: lawns. More goose habitat equals more geese.
In the deserts, ravens have benefited by human encroachment into once inhospitable places. In the post-Route 66 era, as more and more people came to the desert either for a visit or to stay, we brought water with us, and food waste, and billboards and transmission lines and broadcast towers for perching, and structures for a source of shade in the Mojave summer.
In other words, we created more habitat for both Canada geese and common ravens, and their numbers grew to problem proportions, allowing them to multiply in places tat once lacked them. In other words, they became invasive — though many biologists will tend to use a different adjective.
It’s basically the same dynamic at work whether the invasive species we’re talking about is native or introduced. A species is introduced to a whole new ecosystem, either because we moved it or because we changed the ecosystem it was already in. If that species finds some aspects of the new ecosystem advantageous to its survival it may prosper, especially if the new ecosystem doesn’t harbor a lot of potential predators. Introduced species are often imported without the predators and diseases that kept them in check back home. (It’s worth noting that the above-mentioned invasive natives, common ravens and Canada geese, lack serious predators to keep their numbers in check as well.)
Whether native or introduced, then, members of a species generally just do what they do: try to survive and reproduce as best they can. When we come along and mix things up, some of those species will find advantage in the turmoil. Other species that don’t find value in the turmoil can dwindle in numbers.
And just to keep things complicated, there are cases where invasive species can actually help those endangered native species here and there. Imported, moderately invasive blue gum eucalyptus trees along the Northern California coast are an important habitat for migrating monarch butterflies, which are in serious trouble these days. Filaree, an annual plant that takes advantage of the same open desert soils that red brome does, turns out to be a nutritious snack for the federally threatened desert tortoise; tortoises gain weight effectively when filaree is available, and tortoises need all the help they can get these days. (They also did just fine on the native herbs and grasses that filaree and other invasive plants are outcompeting, so don’t go spreading filaree seed far and wide in the desert out of kindness to the tortoises.)
Even very destructive invasive organisms can offer some benefit to native wildlife. Tamarisk, a highly drought-adapted tree imported from Eurasia, is so effective at taking groundwater out of deep desert soils that it can kill and displace native trees and shrubs like cottonwood and willow along desert watercourses and oases. About 16 years ago, scientists began experimenting with an imported leaf beetle that can stunt and even kill tamarisk. The problem was that the southwestern willow flycatcher, a federally listed Endangered bird, had used those native woodlands for nesting. Deprived of those trees, the birds started using tamarisk as nesting habitat in the southern part of their range. Some of the birds apparently turned out to like tamarisk just fine, using it even when native trees were available. Faced with an ironic threat to the bird, the federal government ended the beetle introduction program in 2005.
That’s not a slam-dunk win for the tamarisk. The trees still deplete groundwater, helping to dry up desert streams and thus threatening species like the endangered Colorado pikeminnow.
Even the maligned yellow star thistle, bane of hikers, campers, and ranchers, might have its upside. Those introduced bees love its flowers for honey-making, for one thing. And as Native Californian herbalist Sage LaPena put it to me recently, it might be worth looking at the bigger picture. "The thing about star thistle," LaPena said, "is it might well be a way the earth heals itself by keeping people off it."
"I usually say the same thing about poison oak," LaPena continued. "It is a sign that says 'Beware' or 'Keep Out.'" When it comes to native and introduced and invasive species, even the nuances have nuances.