Biodiversity and the battle for Planet Earth: The graphic novel

[Editor’s note: It’s been a big month for the science of biodiversity and an exciting time to be a part of it. Last week, Nature came out with its issue commemorating the 20th anniversary of the Rio meeting that first put biodiversity on the world’s radar screen and spawned the Convention on Biological Diversity. The Nature issue included our consensus paper outlining what we feel confident science knows, after two decades, about how changing biodiversity affects ecosystems and the services they provide, and a second paper showing that impacts of biodiversity loss on ecosystems are just as important as other major global changes like rising temperatures and nutrient pollution (see full citations below).

In today’s issue of Science, they’ve let three of us off our leashes to speculate on where this field may, or should, be headed. In lieu of recapping the paper’s contents, I thought it might be amusing to tell the story (well, a story) in pictures—not a proper graphic novel admittedly but perhaps a (very) short story.

Being a visual guy I find that a picture is indeed worth a thousand words and sticks better than a block of text or, god forbid, an equation. Most people do. My colleague Paul Heidemann, a prof of biology at the College of William and Mary, has posted some fascinating reflections on how we learn, along with a set of practical tools for optimizing learning through drawing and studying quick sketches here.

So, onward – and with apologies in advance for the hyperbole, an illustrated play in three acts . . .]

Act 1. Rise of the Hoard

Act 1. Rise of the Hoard

Mists rise from the brightening waters, and the towering forest canopy. Across the edenic landscape creatures fantastic, large and small, teem.

But there is trouble afoot. One species, and you know which one, has eaten the forbidden fruit. All goes well for a while, as humans gradually, then more quickly, bring a rapidly growing swath of the planet surface under submission. Great pyramids are raised, we travel to the moon, gigantic ships trawl the ocean depths, Facebook is invented. Humanity waxes, most everything else wanes. The multibranched tree of life (shown here in its mystical, circular manifestation) starts looking a bit ragged.

 

Act 2. The dawn of consciousness

Act 2. The dawn of consciousness

The morning after. In the bright light, the picture changes. We learn that the multifarious living world around us is not merely an ornament for our amusement but, almost literally, an inseparable part of our collective body. The daily activities of the innumerable creatures—magically capturing sunlight, eating, breathing, building homes, reproducing, dying—meld together in impossibly complex ways to turn the cosmic wheel that we now call, prosaically, biogeochemistry, moving the microscopic building blocks of nature in a perpetual cycle. We, like the others, are connected together. When nature coughs, we cough. And it’s starting to hack.

 

Act 3. Connection

We learn, hopefully not too late, that the only way to save the world is to understand it, make peace with our fellow travelers rotating with us through space, and tread lightly. What happens next? The answer will have to wait, alas, for the sequel. But it should be noted that this is not Hollywood so it’s not at all clear whether we’ll live happily ever after.

Act 3. Connection

 

Dramatis Personae

So, toward the goal of understanding our fellow travelers, let’s deconstruct the last figure, which I initially sketched (and Shahid masterfully finished) with the aim of concisely summarizing the interconnected dimensions of biodiversity:

The big picture: We see four panels, representing two linked ecosystems (the columns), observed both before (lower row) and after (upper row) humans have had at them. On the left is a lake, on the right a nearby forest. Within each panel is a stylized depiction of a food web, made up of the various animals and plants and arrows showing who eats whom, with trophic levels (plants, herbivores and detritivores, predators) separated by horizontal dotted lines.

Species diversity: The different shapes represent different species. For example, in the lower left panel we see two species of fishes, both predators, two species of water bugs at the intermediate level, and a basal level with several species of rooted plants, various types of planktonic algae (in the circle), and leaves of three species of trees that have washed in from land (arrow from forest panel). The leaves sustain a detritus-based food chain. Because different species eat different things, grow at different rates, and so on, the number of species in the system can have important impacts on how the system works. This has been the main focus, and a robust finding, of the research synthesized in the papers cited below.

Genetic diversity: But diversity has many other components. Individuals of a species are not identical (just look around you!). They differ genetically, among other ways, and these differences can have large consequences for what they do in ecosystems. In the figure different colors of the same shape (e.g., water bugs) represent different genetic variants within a species. There is growing evidence that, because many traits important to ecosystem processes vary within populations, genetic diversity within species can be comparably important to species diversity in influencing ecosystem processes.

Interaction diversity: This is my personal favorite. Ultimately, the ways that organisms interact with one another are the wheels that drive ecosystem processes and biogeochemical cycling. Plants make food and release oxygen, animals eat the food and consume oxygen, in turn releasing CO2 and inorganic nutrients that plants consume, and so on. We’re beginning to learn that an ecosystem with a diverse net of interconnected pathways connecting its species often functions more efficiently and more stably than one with only a few links.

Landscape diversity: Finally, there is the diversity of habitat types within a landscape, in this case aquatic and forest patches. Research on biodiversity and ecosystem functioning has hardly touched this component, but it is obvious that it’s important. Connections between habitat types in a landscape are well known to influence productivity, nutrient and carbon cycling, food web processes, and so on.

The battle: Alas, the rise of humanity is stressing these connections. As time passes from the bottom panels to the top ones we can see that native species have disappeared, such as fishes in the lake and the birds and bobcat in the forest, resulting in loss of entire trophic levels. Invading species (catfish, rat) have stepped into the breach, but without restoring interaction diversity — for example, the original five links (red arrows) in the lake web are reduced to one. Diversity has also been lost as the remaining species become more genetically homogeneous, compromising their ability to respond to further environmental change. And habitats become isolated as extinction of flying insects cuts the supply of aquatic production to the nearby forest, and declining forest birds no longer excrete nutrients back to the lake (arrows between lake and forest).

As a disclaimer I should probably add, just to be clear, that the original paper in Science did not stray into this flowery stuff–we endeavored to be scrupulously scientific. But there’s more than one way to skin a cat, as they say. And just for fun, here is the original pencil sketch I dashed off that evolved into the last figure:

Credits

Cardinale, B. J., J. E. Duffy, A. Gonzalez, D. U. Hooper, C. Perrings, P. Venail, A. Narwani, G. M. Mace, D. Tilman, D. A. Wardle, A. P. Kinzig, G. C. Daily, M. Loreau, J. B. Grace, A. Larigauderie, D. S. Srivastava, and S. Naeem. 2012. Biodiversity loss and its impact on humanity. Nature 486:59–67.

Hooper, D. U., E. C. Adair, B. J. Cardinale, J. E. K. Byrnes, B. A. Hungate, K. L. Matulich, A. Gonzalez, J. E. Duffy, L. Gamfeldt, and M. I. O’Connor. 2012. A global synthesis reveals biodiversity loss as a major driver of ecosystem change. Nature 486:105–108.

Naeem, S., J. E. Duffy, and E. Zavaleta. 2012. The Functions of Biological Diversity in an Age of Extinction. Science  336:1401–1406.

 

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