by Abigail Kingston
In an ecosystem, almost every organism depends on every other in some way for survival. However, the concept of symbiosis is taught to us in school like it’s a specific, situational partnerships where two organisms turn from the usual path of competing for resources or trying to eat each other and offer each other some kind of benefit, as with clown fish and sea anemones. Otherwise, ecosystems are made of organisms all looking out for their own interests, and sometimes “taking advantage of” or “utilizing” opportunities created by other organisms.
The problem is, this idea makes it hard to guess just how much organisms in an ecosystem depend on one another. In fact, it’s possible that the way we talk about relationships in ecosystems is seriously hindering our ability to understand them.
I have noticed strange patterns in the language we use to describe interactions in nature, particularly in when and how we assign living things agency. It is discouraged to talk about processes in nature like they happen “on purpose.” But, we have no problem describing plants as active agents when they behave in a competitive, self-interested way. Crabgrass “colonizes” a patch of bare ground, young saplings “choke out” light-loving pasture weeds, tall trees “starve” smaller plants of sunlight. Allelopathic plants “wage war” on other plants. Two species can engage in an “evolutionary arms race.” Researchers on mycorrhizal associations use the metaphor of a “market” where goods are bought and sold to explain how fungal and plant participants select symbiotic partners.
This type of language disappears, however, when an interaction between two living things seems to be cooperative.
Late in fall, the ground is carpeted in a layer of fallen leaves shed by the deciduous trees that dominate forests here in Kentucky. Many people rake up and burn or throw away these leaves, not knowing their importance. This has led to the circulation of articles and images on social media imploring people to leave the leaves.
When researching the importance of fallen leaves in the forest ecosystem, I learned that the leaf layer is necessary for the survival of almost all moths and many other insects, including nine-spotted ladybugs, stick bugs, and many butterflies. These insects need the protective blanket of leaves to survive the cold winter, where they wait for spring in eggs or hibernate as pupae or adults.
Early in spring, the leaves shelter the earliest spring flowers from frost, and in my own garden, I have noticed fallen leaves protecting young plants in the same way late in fall. Spiders, frogs, and salamanders hunt for their food in the leaf layer. The leaf layer suppresses the germination of weeds, while improving germination rates for other seeds, such as acorns. The leaf layer stops erosion, regulates the temperature of the soil, and improves the soil’s ability to absorb rain. Perhaps most importantly, the leaves directly serve as food for a diverse array of invertebrates, microbes and fungi, which break the leaves down into rich, nourishing food for plants. Fallen leaves are the fertilizer that keeps a deciduous forest lush and green. Rivers and streams carry this fertilizer far and wide, creating the fertile soil characteristic of river floodplains.
Why do we throw away this valuable bounty? Could it be that we no longer see the rhythms of the trees as connected to the survival of everything else—including us?
In the articles, papers, and blog posts that tell me the importance of leaves, the tree’s role has disappeared—the trees are no longer mentioned. Leaves just so happen to be on the forest floor to feed and shelter an entire living community.
Isn’t it strange that we can say that a tree starves or poisons another living thing, as though the tree is acting intentionally, but we don’t say that a tree feeds or houses one? As a forest grows, it “outcompetes” or even “chokes out” and “starves” light-loving plants, but merely “allows for” the growth of shade-loving plants.
This passive language affects what insights we can have about nature. For example, it’s hard to miss that some plants prefer shade and will die in full sun, but no one seems to attribute this to the protective role of canopy trees. Instead, the shade of the forest is described as a hardship the plants on the forest floor have managed to overcome. From this point of view, sunlight is a resource, and plants compete to have more of it.
But something doesn’t add up. Most plant species will struggle to survive when planted in sunny bare ground—construction sites and recently tilled gardens are overtaken by a select few tough, stubborn annual weeds. If competition were the main force in the ecosystem, we would expect these environments to be more hospitable than a forest, but instead, they have little diversity and support only plants that can survive extreme hardship.
But this makes sense—the fallen leaves in a forest hold moisture, regulate temperature, protect young plants from frost, and decompose into rich, fertile soil. The trees themselves provide protection from weather and sun to plants and animals alike.
New research is showing that the forest ecosystem is intensely interdependent. Nowhere is this better demonstrated than in the mycorrhizal network. Most plants cannot sustain their needs for nitrogen and phosphorous alone, and instead obtain those nutrients through a symbiotic relationship with fungi that live attached to the plants’ roots. In exchange, the fungi receive the products of the trees’ photosynthesis.
But it gets weirder. Any single tree in a forest is attached to many species of fungus, which in turn may be specialists or may attach to a wide variety of trees. The plant life of a forest is interconnected through a vast network of fungal mycelium, which transmits water, nutrients, and chemical messages between trees. Researchers have documented young saplings receiving nutrients from parent trees, understory plants with little access to light receiving sustenance from canopy trees, and trees in extreme drought receiving water from fungi digesting rocks!
Perhaps most shocking of all, the melding together of fungus and plant is not a bizarre new event in the history of our world. Instead, we have found the fossil remnants of mycorrhizal fungi in 400-million-year-old fossils of the oldest land plants known.
These discoveries have challenged a deeply held assumption about nature, generating intense controversy. Isn’t it fanciful to say that trees engage in altruistic behavior—that trees are caretakers? Plants are widely accepted to colonize land, conduct transactions, and wage war, but when the evidence suggests that they also nurture families and care for one another, op-eds sternly warn that anthropomorphizing plants in this way is misleading.
Once I noticed this hang-up with describing cooperation in nature, I couldn’t stop. An article about pollinators tells me that insects “inadvertently” spread pollen from flower to flower, as if it is important to remind me that any benefit insects give to flowers is nothing more than an accident.
Of course pollinating insects didn’t intentionally choose to evolve a mutualistic symbiotic relationship with flowering plants, but no species chooses to evolve anything! An “evolutionary arms race” is just as accidental.
You may ask, “Does it matter how we talk about relationships in nature? No matter what, describing them using humanizing terms will be inaccurate.” To answer that question, we must return to the leaves.
When walking in my neighborhood a few weeks ago, I saw piled at the curb black trash bags filled with fallen leaves. When the garbage truck passes through the neighborhood, those leaves will be taken to a landfill, where they will be buried forever amongst truckloads of non-biodegradable plastic trash, along with the sleeping eggs and caterpillars of beneficial insects and the helpful millipedes and isopods. Their nutrients will never return to the soil beneath the trees where they fell.
What has happened? People have become unable to see nature through the lens of cooperation and mutual dependence. If we characterize trees as being self-interested and indifferent, and their benefits to other creatures as accidental, then it makes sense to assume that most of a tree’s activities don’t really benefit other living things. What is it to a moth or butterfly if a tree loses its leaves?
We focus only on the interests of individuals, rather than the relationships between them, and it seems almost silly to step outside of this narrow mindset—of course fallen leaves don’t have a purpose; they’re no longer useful to the tree once they have fallen! It would be fanciful to imagine that trees take care of other living things by providing a protective and nourishing layer of fallen leaves to be eventually recycled into rich soil.
But that’s exactly what they do. Instead of deciding that trees-as-caretakers contradicts the evolutionary principle that a successful organism looks out for its own survival, maybe we should consider that observations show cooperation and “caretaking” to be vital to the health and survival of every organism in the forest.
By providing shelter for pollinating insects and food for decomposers, fallen leaves ensure that viable seeds will be distributed onto rich, fertile soil, where they can grow into the next generation of trees. The trees cannot flourish alone; they flourish only through belonging to a community that provides for them in return.
And no, I don’t think this means that trees are selfless and compassionate in the way humans can be. I think it means something else, something with implications that are far more uncomfortable and far-reaching—that millions of years of evolution show us the danger and foolishness of pure competitive self-interest.
I began to call trees caretakers because they are, and I also started to think of them as teachers. If millions of years of evolution and adaptation through harsh ice ages and cataclysmic events produced a natural environment where almost everything engages in mutualistic symbiosis with several other creatures at once, that meant mutual dependence was not contradictory to the laws of evolution. Humans did not invent cooperation and caretaking in spite of our nature—these things arose as mechanisms of survival. Like the trees, we depend on the birds and insects to survive, and like the trees, we can protect them so our children will grow in a world that is flourishing and green—and the trees have already shown us how.