The Up And Downsides of Invasive Species

Blue gum eucalyptus in Marin County | Photo: James Gaither, some rights reserved

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. )

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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.)

Environmentalists are trying to save this invasive species | Photo: BugMan50, some rights reserved

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 nat

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