It seems like they produce and trade with each other, even across species. Are trees and the fungi being selfish or altruistic? Adam Smith and Thomas Malthus make an appearance.
See The Social Life of Forests: Trees appear to communicate and cooperate through subterranean networks of fungi. What are they sharing with one another? by Ferris Jabr of The NY Times. It highlights research by Suzanne Simard, a professor of forest ecology at the University of British Columbia. Excerpts:
"Underground, trees and fungi form partnerships known as mycorrhizas: Threadlike fungi envelop and fuse with tree roots, helping them extract water and nutrients like phosphorus and nitrogen in exchange for some of the carbon-rich sugars the trees make through photosynthesis. Research had demonstrated that mycorrhizas also connected plants to one another and that these associations might be ecologically important, but most scientists had studied them in greenhouses and laboratories, not in the wild."
"By analyzing the DNA in root tips and tracing the movement of molecules through underground conduits, Simard has discovered that fungal threads link nearly every tree in a forest — even trees of different species. Carbon, water, nutrients, alarm signals and hormones can pass from tree to tree through these subterranean circuits. Resources tend to flow from the oldest and biggest trees to the youngest and smallest. Chemical alarm signals generated by one tree prepare nearby trees for danger. Seedlings severed from the forest’s underground lifelines are much more likely to die than their networked counterparts. And if a tree is on the brink of death, it sometimes bequeaths a substantial share of its carbon to its neighbors."
"Since Darwin, biologists have emphasized the perspective of the individual. They have stressed the perpetual contest among discrete species, the struggle of each organism to survive and reproduce within a given population and, underlying it all, the single-minded ambitions of selfish genes. Now and then, however, some scientists have advocated, sometimes controversially, for a greater focus on cooperation over self-interest and on the emergent properties of living systems rather than their units."
"An old-growth forest is neither an assemblage of stoic organisms tolerating one another’s presence nor a merciless battle royale: It’s a vast, ancient and intricate society. There is conflict in a forest, but there is also negotiation, reciprocity and perhaps even selflessness. The trees, understory plants, fungi and microbes in a forest are so thoroughly connected, communicative and codependent that some scientists have described them as superorganisms. Recent research suggests that mycorrhizal networks also perfuse prairies, grasslands, chaparral and Arctic tundra — essentially everywhere there is life on land. Together, these symbiotic partners knit Earth’s soils into nearly contiguous living networks of unfathomable scale and complexity."
"Mycorrhizal networks were abundant in North America’s forests. Most trees were generalists, forming symbioses with dozens to hundreds of fungal species. In one study of six Douglas fir stands measuring about 10,000 square feet each, almost all the trees were connected underground by no more than three degrees of separation; one especially large and old tree was linked to 47 other trees and projected to be connected to at least 250 more; and seedlings that had full access to the fungal network were 26 percent more likely to survive than those that did not.
Depending on the species involved, mycorrhizas supplied trees and other plants with up to 40 percent of the nitrogen they received from the environment and as much as 50 percent of the water they needed to survive. Below ground, trees traded between 10 and 40 percent of the carbon stored in their roots. When Douglas fir seedlings were stripped of their leaves and thus likely to die, they transferred stress signals and a substantial sum of carbon to nearby ponderosa pine, which subsequently accelerated their production of defensive enzymes. Simard also found that denuding a harvested forest of all trees, ferns, herbs and shrubs — a common forestry practice — did not always improve the survival and growth of newly planted trees. In some cases, it was harmful."
"Many were perplexed as to why trees of different species would help one another at their own expense — an extraordinary level of altruism that seemed to contradict the core tenets of Darwinian evolution."
"It’s now well accepted that resources travel among trees and other plants connected by mycorrhizal networks. Most ecologists also agree that the amount of carbon exchanged among trees is sufficient to benefit seedlings, as well as older trees that are injured, entirely shaded or severely stressed, but researchers still debate whether shuttled carbon makes a meaningful difference to healthy adult trees. On a more fundamental level, it remains unclear exactly why resources are exchanged among trees in the first place, especially when those trees are not closely related.
In their autobiographies, Charles Darwin and Alfred Russel Wallace each credited Thomas Malthus as a key inspiration for their independent formulations of evolution by natural selection. Malthus’s 1798 essay on population helped the naturalists understand that all living creatures were locked into a ceaseless contest for limited natural resources. Darwin was also influenced by Adam Smith, who believed that societal order and efficiency could emerge from competition among inherently selfish individuals in a free market. Similarly, the planet’s dazzling diversity of species and their intricate relationships, Darwin would show, emerged from inevitable processes of competition and selection, rather than divine craftsmanship. “Darwin’s theory of evolution by natural selection is obviously 19th-century capitalism writ large,” wrote the evolutionary biologist Richard Lewontin.
As Darwin well knew, however, ruthless competition was not the only way that organisms interacted. Ants and bees died to protect their colonies. Vampire bats regurgitated blood to prevent one another from starving. Vervet monkeys and prairie dogs cried out to warn their peers of predators, even when doing so put them at risk. At one point Darwin worried that such selflessness would be “fatal” to his theory. In subsequent centuries, as evolutionary biology and genetics matured, scientists converged on a resolution to this paradox: Behavior that appeared to be altruistic was often just another manifestation of selfish genes — a phenomenon known as kin selection. Members of tight-knit social groups typically share large portions of their DNA, so when one individual sacrifices for another, it is still indirectly spreading its own genes.
Kin selection cannot account for the apparent interspecies selflessness of trees, however — a practice that verges on socialism. Some scientists have proposed a familiar alternative explanation: Perhaps what appears to be generosity among trees is actually selfish manipulation by fungi. Descriptions of Simard’s work sometimes give the impression that mycorrhizal networks are inert conduits that exist primarily for the mutual benefit of trees, but the thousands of species of fungi that link trees are living creatures with their own drives and needs. If a plant relinquishes carbon to fungi on its roots, why would those fungi passively transmit the carbon to another plant rather than using it for their own purposes? Maybe they don’t. Perhaps the fungi exert some control: What looks like one tree donating food to another may be a result of fungi redistributing accumulated resources to promote themselves and their favorite partners.
“Where some scientists see a big cooperative collective, I see reciprocal exploitation,” said Toby Kiers, a professor of evolutionary biology at Vrije Universiteit Amsterdam. “Both parties may benefit, but they also constantly struggle to maximize their individual payoff.”"
"plants and symbiotic fungi reward and punish each other with what are essentially trade deals and embargoes, and that mycorrhizal networks can increase conflict among plants. In some experiments, fungi have withheld nutrients from stingy plants and strategically diverted phosphorous to resource-poor areas where they can demand high fees from desperate plants."
"Some researchers have proposed that cooperation within or among species can evolve if it helps one population outcompete another — an altruistic forest community outlasting a selfish one, for example. The theory remains unpopular with most biologists, who regard natural selection above the level of the individual to be evolutionarily unstable and exceedingly rare. Recently, however, inspired by research on microbiomes, some scientists have argued that the traditional concept of an individual organism needs rethinking and that multicellular creatures and their symbiotic microbes should be regarded as cohesive units of natural selection. Even if the same exact set of microbial associates is not passed vertically from generation to generation, the functional relationships between an animal or plant species and its entourage of microorganisms persist — much like the mycorrhizal networks in an old-growth forest. Humans are not the only species that inherits the infrastructure of past communities."
[At a mycorrhizal network] "She handed me a thin strip of root the length of a pencil from which sprouted numerous rootlets still woolly with dirt. The rootlets branched into even thinner filaments. As I strained to see the fine details, I realized that the very tips of the smallest fibers looked as though they’d been capped with bits of wax. Those gummy white nodules, Simard explained, were mycorrhizal fungi that had colonized the pine’s roots. They were the hubs from which root and fungus cast their intertwined cables through the soil, opening channels for trade and communication, linking individual trees into federations. This was the very fabric of the forest — the foundation of some of the most populous and complex societies on Earth."
"Five hundred million years ago, as both plants and fungi continued oozing out of the sea and onto land, they encountered wide expanses of barren rock and impoverished soil. Plants could spin sunlight into sugar for energy, but they had trouble extracting mineral nutrients from the earth. Fungi were in the opposite predicament. Had they remained separate, their early attempts at colonization might have faltered or failed. Instead, these two castaways — members of entirely different kingdoms of life — formed an intimate partnership. Together they spread across the continents, transformed rock into rich soil and filled the atmosphere with oxygen.
Eventually, different types of plants and fungi evolved more specialized symbioses. Forests expanded and diversified, both above- and below ground. What one tree produced was no longer confined to itself and its symbiotic partners. Shuttled through buried networks of root and fungus, the water, food and information in a forest began traveling greater distances and in more complex patterns than ever before. Over the eons, through the compounded effects of symbiosis and coevolution, forests developed a kind of circulatory system. Trees and fungi were once small, unacquainted ocean expats, still slick with seawater, searching for new opportunities. Together, they became a collective life form of unprecedented might and magnanimity."
No comments:
Post a Comment