The Ties That Bind: A Closer Look at Interdependencies Between Species

The Ties That Bind: A Closer Look at Interdependencies Between Species

This article relates to The Call of the Honeyguide

In The Call of the Honeyguide, applied ecologist Rob Dunn examines the many ways that living things in an ecosystem are synergistically connected by reciprocal relationships called mutualisms—defined as interactions between two or more species in which each benefits.

As the book shows, some of the connections between species are so intimate that they challenge our assumptions about the nature of living things. "The world as we perceive it is composed of species, living as individuals," Dunn writes. "The world as it exists is composed not only of species, but also of the connections among those species." Look closely at those connections, and the boundaries between different species can start to blur.

Consider the complex interdependencies connecting termites, protists, and bacteria. Microscopic protists called Mixotricha paradoxa live inside the intestines of termites. The termites host and supply food for the protists. The protists help the termites digest the cellulose in their wood-based diets. In turn, four distinct species of bacteria live inside the protists. The protists host and nourish the bacteria. The bacteria help the protists move and digest food. Linked by these nested mutualisms, the termites, protists, and bacteria function together as a unit, forming a composite—a compound being "composed of symbiotic partnerships in which different species work together to achieve ends that neither could achieve on its own."

Like termites, humans host an array of other organisms that we depend on for our proper functioning. Trillions of microorganisms live on and inside the human body, including hundreds or even thousands of different species of beneficial bacteria that are essential for good health. Indeed, the genes of all the microbes in our bodies far outnumber the genes in our own DNA. Technically, all these microorganisms are separate from us, each with its own genome. Together, we form a unified composite.

As Dunn puts it, we are holobionts, a term coined by scientists to describe the ecological unit made up of a host organism and the multiple organisms that coexist with it and contribute to its overall function and survival. "It isn't just our genes and genomes that influence our being and behavior, but instead our hologenomes—the sum total of genomes for both us and the species that live on and in us," he writes.

Quoting evolutionary biologist Lynn Margulis and her son Dorion Sagan, Dunn compares this dynamic to the way myriad minute dabs of paint combine to create a coherent picture in a pointillist painting. "Scrutinizing any organism at the microscopic level is like moving ever closer to a pointillist painting by Georges Seurat," Margulis and Sagan wrote in a 2001 article. "The seemingly solid figures of humans, dogs, and trees, on close inspection, turn out to be made up of innumerable tiny dots and dashes, each with its own attributes of color, density, and form."

As Dunn argues in The Call of the Honeyguide, no less intimate than these links at the microscopic level are the connections that weave together different species on a macro level—the complex networks of interdependence that connect different organisms in an ecosystem, the way trees in a forest, for example, are interlinked with a vast web of fungi in the soil underground.

Like every other living thing, humans are connected to countless other species in our shared environment, from the trees that sequester our carbon dioxide to the grains that nourish our bodies. Zoom in close enough, and you can see that at the microscopic level each of us contains a multiplicity of smaller organisms functioning as a unity. Zoom far enough out, and you can see that on the macroscopic level each of us, in turn, is just one small part of a larger, interconnected whole—the overarching ecosystem that contains us all.

Image of termite by Sanjay Acharya, CC BY-SA 4.0 International.

Filed under Nature and the Environment

This article relates to The Call of the Honeyguide. It first ran in the September 24, 2025 issue of BookBrowse Recommends.