Nativists who favor “native pollinators” often believe that these insects – bees, butterflies, moths and similar insects – rely on native plants. The plants and insects co-evolved, goes the argument; and so to protect one, you must plant the other.
We came upon a recent article in the journal Nature that notes such specific interdependent relationships are rare. There’s a good reason: it would make both plant and insect vulnerable to changes in the weather or other environmental factors.
Instead, plants are pollinated by a variety of insects (and even birds); and these same pollinators use a number of different species of plants.
Including non-native ones.
WHAT’S BEST FOR BEES
In the article, “The Pollinator Crisis: What’s Best for Bees” author Sharon Levy starts by describing numerous bees, native and non-native, buzzing around a patch of red flowers.
“All these insects are drawn to a clump of red vetch (Vicia villosa), an invasive weed. Just down the road is a patch of native lupins, laden with purple blossoms. But the lupins bloom in silence: no bees attend them.”
The article (which was reprinted in the Scientific American) is based on the work of a number of researchers, including Alexandra Harmon-Threatt (who just finished her doctorate at UC Berkeley) and Rachel Winfree of Rutgers. (In case you want more information, the article cites published research on plants and pollinators from a number of scientists.)
A BUMBLE BEE’S POINT OF VIEW
Harmon-Threatt looked at it from a bumble-bee’s viewpoint. She studied three species of bumble-bees, and analyzed the pollen they’d collected. Instead of favoring particular plants, she found that they looked for quality and quantity of pollen.
“What matters to most bee species is the abundance and quality of pollen — and if an introduced plant, such as the red vetch, offers more protein-rich food than the natives around it, the bees will collect its pollen.”
A pollination expert quoted in the article agrees. “Until the past five or ten years, people thought that exclusive pollination relationships were more common,” Rachel Winfree of Rutgers says. Instead, they found that bees collect pollen from plants in proportion to how common the flowering plants are in the landscape. “I don’t see why bees would know or care whether a plant was native or exotic,” Winfree says, according to the article.
WEEDS, BLOOMING WEEDS!
So what do pollinators want? In a word, weeds.
More specifically, thriving, abundant and diverse collections of flowering plants, ideally blooming over a long period. Weeds work just fine. Birds and insects really don’t care.
We’ve seen this for ourselves in San Francisco. The early-blooming oxalis and ivy and the winter-flowering eucalyptus provide food for pollinators of all sorts and the wildlife that depends on them. As we described in our article Interwoven and Integrated: Native and Non-native Species in Life’s web, the so-called “invasive species” that the Sutro Stewards and the Significant Natural Resource Areas Program are trying to destroy are in fact rich and valuable habitat in our ecosystem.
The fact that animals and insects are as likely to eat and/or use non-native plants as native plants cannot be said too often. Native plants advocates cling tenaciously to the fiction that insects (and other animals) are dependent upon native plants. Science is finally catching up to that fiction, but the information hasn’t yet penetrated.
We recently attended a lecture by a local bee expert. In the first half of his lecture he said repeatedly that native bees prefer native plants and don’t visit most non-native plants. In the second half of the lecture he talked about creating hedgerows in agricultural fields to accommodate native bees so that they could provide pollination services to the crops.
Astute audience members pointed out the contradiction. That is, virtually all agricultural crops are not native. So, if native bees won’t use non-native plants, how can we expect them to pollinate non-native agricultural crops?
The bee expert seemed flummoxed by this question. His answer was vague and unconvincing. A second questioner persisted with the same result.
Native plant advocates seem to have so much invested in their ideology that reality is invisible to them.
There are many kinds of pollinators. There are those that can adapt to a wide range of flowers (provided that the shape, cues, time of bloom, etc. are the right ones) and there are the specialists. The latter include, not just the yucca moth, the senita moth, the fig wasp and a handful of others that are mutualists with only one species of plant; but many other pollinators that can only extract pollen from a limited number of species. The plants can be all members of one family or a few related families. There are many examples of specialists. The names say it all: squash bees, azalea bees, spring beauty bees, blueberry bees, to name just a few. Needles to say all these specialists would do very poorly when offered only non-native plants. By the same token, some flowers are generalists, others are dependent on just a handful of pollinators or only one.
As a consequence, in the majority of cases, both pollinators and plants do better when exposed to members of their own biotic community, that is to say to native organisms.
Pollinator, thanks for commenting here, and for the link. You’re right that specialists need their own specific plants (usually for reproduction rather than food, though). But that doesn’t necessarily translate to a “biotic community” because nature is generally not static. Biotic communities keep changing over time – from climate, changes in populations of plants and animals, and even just through random events such as a fire or landslip.
This means that if you want to encourage particular specialists, you need to encourage specifically the plants they need, not a broad group of others that share the same historical background. I think “native” is a biologically meaningless classification. Mission Blue butterflies for instance, need lupine. It won’t do to say, well, lupine is a native plant and so is hairy gumweed. We’ll just plant gumweeds, and the Mission Blues will be fine because it’s all native. What plants do is more important than their historic antecedents.
And sometimes, even for native insect specialists, the plant they need isn’t a native: Anise swallowtail butterflies need fennel as a nursery plant.
I read your article, and it’s insightful. I think, though, that the real difference you may want to consider is not native vs non-native, it’s cultivars vs weeds. Dandelions get visitors (as does oxalis) because no one has developed their beauty at the expense of their nectar and pollen.
After all, what are “garden flowers” bred for? Looks. Ease of cultivation. Pest resistance. Most people don’t actually want a lot of bees in their gardens, especially in the flowers, though they don’t mind them in the trees. They like butterflies but hate caterpillars…
” bees collect pollen from plants in proportion to how common the flowering plants are in the landscape. “I don’t see why bees would know or care whether a plant was native or exotic,” Winfree says, according to the article.”
First of all, Rachael (notice the spelling) was quoted out of context. This is the whole paragraph: “In highly disturbed habitats, bees make greater use of alien plants — not because the bees prefer them, but simply because introduced plants are more common where people have transformed the landscape. That makes sense to Winfree. “I don’t see why bees would know or care whether a plant was native or exotic,” she says.” The key idea is that pollinators can switch to non-natives, not because of preference but because of abundance. As I said before, some generalist pollinators can make the switch relatively easily, others, with difficulty, others not at all. Still, most of them have preferences. Rachael knows this quite well. She and her team have been working on lists of the best plants to restore pollinator populations in farmland; no introduced plants passed the test so far.
Balance is important, especially dynamic balance. You wouldn’t be able to ride a bicycle without it. Fortunately for us, the balance of nature is far more robust than riding a bike, with numerous fail-safe devices and built-in redundancies. It can survive disturbances, as you mentioned, including the continued and growing human-caused disturbances. Nature is in a constant search of a new balance. But there are numerous examples of severely damaged ecosystems as a consequence of human interventions. Take the loss of most elms, American chestnuts, hemlocks and more recently viburnums. Probably we will not cause the complete destruction of nature, but we are headed toward an impoverished world because of our ignorance and our actions.
Your example of lupines and hairy gumweed has no value. Biodiversity is very important, as any ecologist knows, and nobody would think of planting only one species.
The anise swallowtail has co-evolved with plants of the Apiaceae family. It needs members of that family, this is why it can live on fennel, if the native species are absent from the landscape. It doesn’t exactly need or prefer fennel.
Those gardeners who don’t like caterpillars are missing something very important. Without caterpillars, there are no butterflies. Also, without caterpillars, many birds cannot feed their young. As for native bees, there is nothing to fear from most of them. They are very gentle, hardly ever sting. Many of them are so small that they can’t penetrate human skin; their venom, if they happen to sting, is very mild.
It is too bad that many gardeners lose sight of the larger picture. Visual beauty isn’t everything. An ecosystem, with all its intricacy, is like a beautiful symphony. And, like a symphony, it wouldn’t sound good if you kept introducing fragments of other equally beautiful musical compositions into it. I hope that you and your readers try to take a look at the multiple interactions in nature and how they develop. The new awareness may change some of your ideas on gardening, as it is happening to many people I know.