pollination – Conservation Sense and Nonsense

The Light Eaters was written by Zoë Schlanger, a science journalist who covered climate change before writing Light Eaters. (1) She explains her pivot to botanical science as a retreat from the oppressive gloom of climate change. It proved a wise choice, as she found much to cheer us in the remarkable capabilities of plants to adapt to challenges, defend themselves against their predators and competitors, and collaborate with their plant and animal neighbors.

Ms. Schlanger believes that botanical research has lagged behind other biological inquiry partly because of a detour unwisely taken by journalists in the 1960s and 70s that projected human traits onto plants, such as intelligence and consciousness. Humanizing animals and plants is considered a dangerous source of bias by scientists. When scientists described plant behavior in human terms, they were often ridiculed by their colleagues and their research projects weren’t funded. Researchers of the capabilities of plants have been trained to avoid anthropomorphic terms to describe plant behavior. Although Ms. Schlanger tried to observe that rule, I will give myself more leeway because most of my readers are not scientists.

Plants don’t have the mobility that enables them to fly or run away from threats. We might think of them as handicapped compared to the mobility of animals. But what they lack in mobility, they more than make up for with their ability to make they own food from sunlight by photosynthesizing. And with the energy that sunlight provides, plants can create the food—such as pollen, nectar, and fruit—that entices insects and other animals to help them reproduce. So how do plants protect themselves without fleeing from their predators? That’s what Light Eaters is about.

I don’t know the source of this photo. It was sent to me in an email by someone who found it on Facebook.

How do plants perceive threats and react to them?

Plants can sense that they are being attacked by an insect in a variety of ways. They can sense the vibration of the chewing, which is closely related to how animals hear. The attack can also trigger an electrical impulse which can travel throughout the entire plant.

Plants emit chemicals in response to the attack on their leaves and roots. The chemicals can repel the insect by making the plant unpalatable. In a sense, the plant is producing its own pesticide, which has the potential to replace synthetic pesticides.

The chemicals are also wafted into the air to serve as warning signals to their plant neighbors, who can then produce their own chemicals in preparation for attack. Some plants can distinguish between an attack that threatens individuals and those that threaten the entire community. They can tailor their warning messages accordingly, to send messages only to their relatives or to the entire plant community. When plants are sprayed with herbicides, these chemical messages are masked by herbicides. (2) Likewise, pollution can also muddle the chemical messages of plants and reduce their ability to perceive and respond to threats. (3)

Plants sometimes demonstrate a preference for their relatives in other functions as well. They can make room for the roots of close by relatives and move branches to avoid shading their relatives. They can also vary these accommodations depending on available resources, making room when there is plenty of water, nutrients, and light, but not when there’s not enough.

Such warning signals can also be sent via the underground root network, which connects plants in a community to one another through the network of mycorrhizal fungi that attach themselves to plant roots. That network is also used by the community of plants to share resources, such as moisture and carbohydrates produced by photosynthesis. The fungal network enables both communication and sharing of resources. Herbicides that are carried to the roots of trees damage the fungal network, depriving trees of the nutrients they need to survive. (4) The widespread use of these herbicides by native plant “restorations” is one of many reasons why these projects rarely result in new landscapes of native plants.

Can plants hear?

One of the first discoveries of the ability of plants to find what they need is the ability of tree roots to grow in the direction of water sources. Mycorrhizal fungi attached to the roots of plants are clearly involved in guiding that connection. Over 450 million years ago, the evolution of fungi enabled plants to move from water to land by delivering moisture from soil to roots of plants, greatly increasing abundance and diversity of plants. About 80% of plants today receive much of their nutrients and moisture through mycorrhizal fungi. (5)

Now there is evidence that plants may also be able to hear the sound of water to direct the growth of roots. The researcher who made that discovery encased the roots of a plant in plastic pipe so that the roots could not sense the availability of moisture. The plastic pipe formed a “Y” to give the roots the option of growing in one direction or the other. The researcher played a recording of running water at the end of one pipe. The roots grew in the direction of the recording of running water. This is still a controversial discovery, because other researchers have found it difficult to replicate.

The replication of breakthrough scientific discoveries is one of the ways that science moves forward. It is a not a reliable method of confirming or rejecting a new discovery because there are always many variables operating simultaneously that are difficult to control, particularly in field studies, and researchers have rarely identified all the variables involved in the phenomenon they are observing.

The academic career of David Rhoades is an example of the dangers of being too far ahead of your academic colleagues and a reminder of the conservatism inherent in academic science. Rhoades was a chemist at University of Washington and the author of a study that made the first report of warning signals that plants under attack send to their neighbors via volatile chemicals in the atmosphere.

The forest on Rhoades’ campus was being killed by tent caterpillars. He studied the spread of the caterpillars until the insect infestation was stopped by the chemicals that the unaffected trees infused into their leaves. The chemicals killed the caterpillars and the spread of the insect in the forest was stopped. Backed by a mountain of carefully accumulated data, Rhoades concluded: “This suggests that the results may be due to airborne pheromonal substances!”

Rhoades was met with resistance to this new information from his colleagues. Then he had trouble replicating his original study. When his grant applications were rejected, he gave up. He left academia and taught chemistry in a local community college to make a living. Years later, other researchers figured out why he was unable to replicate his original study. The airborne chemicals that trees produce are seasonal. Rhoades’ original study was done in the spring and Rhoades was trying to replicate the study in the fall. The scientists who eventually confirmed Rhoades’ finding did so in the laboratory where conditions are easier to control.

Plants collaborate with animals to protect themselves and reproduce

The Light Eaters reports many remarkable observations of interactions of plants and animals. Here is a sampling of these stories:

If bumblebees emerge from hibernation before plants begin to bloom, the hungry bee bites the plant’s leaves to trigger the bloom that delivers the nectar the bees need.
Plants must use their limited resources to make pollen and nectar. Some plants can ration the delivery of the pollen and nectar that attracts their pollinators by timing the delivery with the anticipated arrival of the pollinator. The plant estimates the time of arrival of the insects based on its memory of past visits.
Bats find the plants they pollinate by using echolocation sonar to locate them in the dark. Some plants that are pollinated by bats have evolved saucer-like petals that act like a satellite dish to receive the sonar ping to help bats find them.
Some corn, cotton, tomato and tobacco plants can emit chemical distress signals to summon tiny parasitic wasps to kill caterpillars such as tobacco budworm and corn ear worm.
Many orchids are pollinated by wasps. Some orchids attract wasps by mimicking the chemical pheromones of the female wasp. The orchid is pollinated by the attempt of the male wasp to mate with what he supposes is a female wasp.
Some plants form partnerships with ants by secreting a sugary substance that feeds the ants, who eat the insect predators of the plant.

Can plants see?

The observation that plants are capable of mimicking animals and other plants is not new. In the early 1900s, a Russian agronomist observed that weeds in food crops have sometimes mimicked the food crop and thereby evaded the hand-weeding that was the method used by farmers to eliminate competition for their crop. Rye, oats, and lentils were initially considered weeds of wheat and rice. Over time, they evolved the seed heads that qualified them as food crops.

More recently, weeds that are killed by herbicides within crops that have been genetically modified to be resistant to the herbicide have engaged in mimicry at the biochemical level to also become resistant to the herbicide. Those who engage in chemical warfare against plants do not seem to understand that it’s a war they can’t win because evolution will enable plants to develop resistance to their poison.

Like many of the remarkable capabilities of plants, scientists can observe the phenomenon, but they are rarely able to explain the mechanism that makes it possible, beyond the evolutionary force of natural selection, which achieves a better adapted plant or animal through a series of mutations and genetic and epigenetic drift. Each change in the species is a trial balloon. If the change works, it’s a keeper. If it doesn’t, it’s in the dustbin with some 99% of the estimated 5 billion species that have lived on Earth. The dominant evolutionary force is random, irrepressible, complex change. The notion that humans are capable of stopping evolution is absurd.

In 2014, a Peruvian ecologist discovered a vine in the Chilean rain forest that is capable of quickly taking on the shape of almost any plant that it grows beside. Nicknamed the chameleon plant, many tests proved that the vine can mimic many different species of plants. Presumably this mimicry enables the vine to become invisible in the sense that it blends in with whatever plants it grows amongst. It’s a disguise, if you will, that protects the plant from its predators.

The chameleon vine is able to mimic plants that are native to their locations as well as plants that are foreign to the region. In other words, mimicry is not the result of a long evolutionary co-existence. This finding is another blow to the nativist myth that plant and insect associations are the result of co-evolution that makes insects dependent on native plants. The associations between plants and insects evolved long before the plants and insects moved into new regions. Plants and insects retain that association as they change in response to their new environment and as the result of mutations and genetic drift.

Until recently, there was a debate among scientists about how the chameleon plant morphs itself into an entirely different shape. One school of thought speculates that plants have an organ that performs much like our eyes. Another school of thought is that horizontal gene transfer (6) from the bacteria inhabiting the plant being copied to the plant doing the copying achieves this transformation.

A study (7) published in 2022 seems to support the hypothesis that some plants have some type of organ that functions like our eyes. The study found that the chameleon vine was capable of mimicking an artificial leaf. The plastic leaf contains no chemicals or bacteria.

In conclusion

The Light Eaters reports many other capabilities of plants that aren’t covered in this article. If it’s a topic of interest to you, the book is well worth reading. It’s well researched and well written. It is also thoughtful because it asks us to ponder the philosophical question of whether or not this new(ish) knowledge of plants adds up to intelligence, consciousness, and agency. Ms. Schlanger dodges that question by reminding us that there is not consensus agreement about what any of those descriptions actually mean.

Now we must add a few caveats that we hope will put this important topic into perspective:

Not every plant species has all of the capabilities described in The Light Eaters.
Those that do have such capabilities may not consistently use them because every plant is responding to a specific environment in a specific place. Plants are inseparable from their environment. A plant that has plenty of water and plenty of light behaves differently than plants with less resources. Sweeping generalizations about plants are usually ridiculous. For example, it makes no sense to claim that native berries are more nutritious than non-native berries. (8)
Plants have the potential to develop such capabilities, depending on their specific circumstances.
Without a brain or a nervous system, plants seem to organize a response to stimuli by functioning as a decentralized network.

The Light Eaters says as much about science as it does about plants. There are fads in science, just as there are fads in every human endeavor. Presently, much scientific investigation of botanical phenomenon is focused on genetics, which has misled the public to underestimate the plasticity of plants and animals. In fact, the genome of a species is a flexible repertoire, with many genes unexpressed until triggered by a change in the environment in which the plant lives. For many characteristics of species, the environment is a more powerful influence than genes.

Science is better at observing than it is at explaining. Explaining requires speculation and academic science studiously avoids speculation. The reader of scientific studies is often left in a quandary. Conclusions are often a contradictory list of maybes with a plea for funding for further investigations. That’s one of many reasons why science journalism is important to the general public’s understanding of scientific issues. Ms. Schlanger goes out on a limb for us by speaking in comprehensible terms that many scientists refuse to use. Thank you, Ms. Schlanger, for helping the public understand the plant world.

Shortly before publishing this article and after I had drafted my article, I received the following review of The Light Eaters from Arthur M. Shapiro, Professor Emeritus of Ecology and Evolution, UC Davis. He has given permission to add his review to my article.
– Conservation Sense and Nonsense

“Elizabeth Kolbert has a collective review of Schlanger and two other, similar books–“The Nation of Plants” by Gregory Conti and “Planta Sapiens” by Calso and Lawrence–in the new NY Review of Books (Oct.3). Her review is only lightly snarky because it’s clear she doesn’t know quite what to make of the “plant neurobiology” fad.

“When I read Schlanger (I haven’t read the others) I dug back into my library to find my copy of “The Secret Life of Plants” by Peter Tompkins and Christopher Bird (1973). I doubt that Kolbert realizes that the current fad is a rerun of the 70s! Unlike Schlanger and perhaps the others reviewed by Kolbert, Tompkins and Bird is packed with overt woo-woo and makes little attempt to be “science-based.” The frank woo-woo is very 70s. But the underlying motivation for both waves is the same: philosophical panpsychism, the notion that consciousness is ubiquitous in Nature.

“There is nothing in the actual data discussed by Schlanger that obliges one to embrace panpsychism. The main reason to do so is that one WANTS to. That is, for some (many?) people it is very reassuring to believe that at least the biosphere, if not the entire universe, is sentient. (This has resonances with the Gaia Hypothesis.) This notion is an integral part of a number of cultural cosmologies, of which the most familiar to most Americans is probably Native American, broadly speaking. In the 70s many hippies embraced the Native American notions of “tree people,” “stone people,” etc. Some still do.

“Remember that I have taught community ecology for some 50 years, with an emphasis on coevolution. Things like inducible anti-herbivore defenses (chemical or morphological) and communicable defensive messages (plant pheromones, if you will) come as no surprise. Rather, they are predictable consequences of natural selection: if something can evolve, it probably will. There is no logical necessity to invoke intelligence or consciousness to account for them. If you want to, go right ahead. But don’t call it science!

“I have never had a chance to pull up a mandrake plant. In the Middle Ages it was widely believed that if you did it would shriek, and the sound if heard would drive one mad. Thus one must cover one’s ears when doing so. Now, that is framed as a testable hypothesis!

“Are you familiar with the walking fern? If not, Google it. I am very fond of it, but never for a moment would I claim it has the property of wanderlust.“

Arthur M. Shapiro, Professor Emeritus of Ecology and Evolution, UC Davis

Zoë Schlanger, The Light Eaters: How the Unseen World of Plant Intelligence Offers a New Understanding of Life on Earth, Harper Collins, 2024. The Light Eaters is the source of information in this article unless otherwise noted.
Behrend*, J.E., & A.L. Rypstra (2018) Contact with a glyphosate-based herbicide has long-term effects on activity and foraging of an agrobiont wolf spider. Chemosphere 194:714-721 doi: 10.1016/j.chemosphere.2017.12.038
“Polluted Flowers Smell Less Sweet to Pollinators,” New York Times, February 16, 2024
K. Hage-Ahmed, “Arbuscular mycorrhizal fungi and their responses to pesticides,” Pest Management Science, September 25, 2018
Thomas Halliday, Otherlands, A Journey Through Earth’s Extinct Worlds, Random House, 2023
Conservation Sense and Nonsense, “All Life on Earth is Related”
Jacob White and Felipe Yamashita, “Boquila trifoliolata Mimics leaves of an artificial plastic host plant,” Plant Signaling Behavior, 2022
Conservation Sense and Nonsense, “Baseless Generalizations in Doug Tallamy’s Nature’s Best Hope”

The native plant movement is based on the fundamental assumption that native species of plants and animals are inherently superior to non-native species. The basis of this assumption seems to vary. Sometimes the explanation offered is as simple as “the non-native doesn’t belong here.” It’s not clear what that statement means. Putting it in the best light, it implies that there is some optimal ecology that is best represented by exclusively native species. A less generous interpretation would be that non-native plant and animal species are the non-human equivalent of illegal immigrants.

We will examine this claim of the superiority of native species in the context of bees to make the point that nature is complex and cannot be oversimplified by such a sweeping generalization.

Professor Gordon Frankie, our local expert on the bees of the Bay Area, says that native bees are superior pollinators to the European honeybee. If that were true, we would consider that a legitimate basis for the judgment that, in this case, the native bee is superior to the non-native bee. However, the evidence available to us suggests that a comparison of the native to the non-native bee is more complicated.

In considering this question, we will focus on agriculture rather than residential gardens, because agriculture is economically more important and for the same reason more is known about the role of bees in agriculture.

Why would native bees be superior pollinators than non-native bees?

We know of two specific examples of native bees that are more effective pollinators of agricultural crops. Both cases illustrate the pros and cons of native bees as agricultural pollinators.

There are some crops—tomatoes, cranberries, blueberries, eggplants, and kiwi fruits—that are effectively pollinated by native bumblebees (Bombus) because of their unique method of pollination. This method is called “buzz-pollination” or “sonication” and it is described as “an intense vibration, like a tuning fork being struck, pollen gathered from other flowers literally exploded off Bombus.”(1)

Unfortunately, though its pollination technique is superior, other characteristics of the native bumblebee have limited its usefulness in agriculture. The crops with which it is most effective produce only pollen. Therefore, the bumblebee must be provided with an alternate source of nectar to fulfill its dietary needs.(2)

The bumblebee, like most native bees, is solitary. It does not live in hives like the social European honeybee. Therefore, it cannot be transported where and when it is needed, as the honeybee can. An attempt at a high-tech solution to this limitation ended in disaster: “In the 1990s, a bumblebee species Bombus occidentalis, was made extinct when experimenting breeders mixed species in Europe and shipped queens back to America. The queens carried with them an exotic disease that Bombus occidentalis has no immunity for.” Growers of tomatoes are now “forced to resort to less efficient pollinators.” (Schacker 2008).

Another example of a native bee that is a superior pollinator of an agricultural crop is the alkali bee which is the most efficient pollinator of alfalfa, a crop that is essential to the dairy and beef industries. “Alfalfa flowers…keep their sexual parts hidden, under tension like a spring. Bees must trip the spring to get at the pollen, and in so doing, they are hit on the head—something honey bees are not particularly fond of. The alkali bees…don’t mind getting hit on the noggin and will happily pollinate a field of alfalfa.” (Schacker 2008).

Native bee at Albany Bulb2 — Native bee approaching nest in ground, Albany Bulb

Unfortunately, the alkali bee, like 85% of native bees in the US, nests in the ground, in particular the alkaline soils of the western US for which it is named. As cropland in the west expanded, the alkali bee was virtually wiped out by plowing up the ground in which it nested. A leafcutter bee was imported from Canada as a substitute, but a fungus is now infecting its larvae. (Schacker 2008)

These disadvantages of native bees can be compensated for by providing nesting and nectar sources adjacent to croplands. These hedgerows must be large enough to provide sufficient nesting opportunities and nectar sources.

However, such hedgerows do not solve all the potential problems of using native bees as pollinators. Because the bees are resident year around and cannot be transported, they cannot be removed when the crops are sprayed with pesticides. And the pesticides are very damaging to the bees. Therefore, a commitment to providing hedgerows for a resident bee population is also a commitment to organic agriculture, i.e., without using pesticides.

This is not to say that the honeybees aren’t being adversely affected by the use of pesticides in the crops they are pollinating. The impact of pesticides on honeybees would be exacerbated if they were resident when pesticides were applied to the crop. As it is, the honeybee is being affected by the residues of the pesticides on the crops they pollinate. This is considered one of the primary reasons for Colony Collapse Disorder of commercial honeybees which has been destroying about one-third of commercial honeybee hives in the past few years.

Unlike most native bees, the European honeybee does not hibernate. It is therefore available year around to be transported where and when it is needed. Most native bees hibernate, but not necessarily at the same time. Different species of native bees hibernate at different times and are therefore available for pollination at different times.

Most native bees are more selective in their pollination than the European honeybee which is an extreme generalist: “honeybees have the greatest pollen dietary range…of any known pollinator.” Although there are “only a handful of well-documented cases in North America of truly monolectic bees [a bee that visits only one kind of flower]” (Buchmann 1996), the flower preferences of native bees are narrower than that of the European honeybee. While some native bees may prefer native plants, honeybees are willing to pollinate both native and non-native plants. This is important because virtually all of our agricultural crops are non-native.

Native bees are not inherently superior to non-native bees

honeybee hives — Honeybee hives, USDA photo

In summary, the European honeybee has several important advantages over native bees as pollinators of agricultural crops:

Because the honeybee is a social bee that lives in hives, it can:
- Be transported where and when it is needed
- Be removed from the agricultural crop when it is sprayed with pesticides
- Does not need to be provided with nesting space and an alternate food supply
The honeybee is available for pollination services year around because it does not hibernate.
The honeybee pollinates a wider range of flowers than most native bees.

While native bees may be more efficient pollinators of residential gardens, there are a number of disadvantages to using native bees for agricultural pollination. Although many of these obstacles can be overcome with greater use of resources, we cannot agree with the assumption of native plant advocates that native bees are inherently superior to the non-native European honeybee. As with all sweeping generalizations, the truth is usually more complicated because nature is complex and man’s understanding of it is limited.

(1) Schacker, Michael, A Spring without Bees, Lyons Press, Guilford, Conn, 2008.

(2) Buchmann, S, and Nabhan, G, The Forgotten Pollinators, Island Press, 1996

Tag: pollination

The Light Eaters: Plants will find a way to survive…if we let them