On Ecological Diversity

What do ecologists mean when they say ‘diversity’?

Nick Minor
Aug 11, 2015 · 5 min read

This sort of writing is what I would call a ‘heuristic essay’. Its purpose is to crystallize thoughts — thoughts that are part of my learning process — through externalization. In other words, this essay is the product of my use of writing to clarify. Its aim is not to convince, not to inspire, but rather to shed light on principles which may remain otherwise veiled by ambiguity to most readers. I hope that in doing this sort of writing for myself, I can add to the richness of your own learning experience. More than anything else, the message of these essays is to focus on learning instead of knowing. Why? Because what we still have to learn has far more profound implications in our lives that what we know. With that, I bid you read on, all ye curious…

So often ecologists find themselves pondering diversity. Much of the strength in an ecosystem is based in its propensity to sustain species, to lift up a rich variety of organic forms through evolutionary time and push them forward into new radiations, all while retaining the diversity of yore. It is a common presumption that ecosystems closest to their carry capacity — by which I mean a theoretical saturation point of species, not individuals, in which all possible ecological niches are filled — are the most stable through time.

Health, then, is diversity.

Field ecologists are often the most fulfilled after returning from voyages into luxuriously rich ecosystems. Manu National Park in Peru. Yasuni National Park in Ecuador. Sinharaja Forest Reserve in Sri Lanka. These are places to go where the biodiversity is overwhelming, where the vigorous radiative power behind life is fully realized. In these places, our minds can be fully engaged, can grasp utterly novel insights, and can sink into a sort of biologist’s immersion, where boredom is far from possible in the face of so much life. As living things interested in living things, these are the places where life feels its fullest, both within ourselves and without.

But here’s the problem with the common usage of diversity: it’s vague. It’s lofty. It implies at quantification — a cornerstone of science — without any inherent measures to add. A simple count of species in a void means nothing, providing no insight that could fuel the fire of scientific progress, nor the churning motor of conservation. No, what we need with such quantification is context, and then we will achieve clarity.

Enter Robert H. Whittaker, a prolific American ecologist known for ideas like gradient theory and the five-kingdom system. One of his many contributions to ecology was a quantified method to consider diversity. In a 1960 monograph, Whittaker introduced the concepts of alpha-, beta-, and gamma-diversity. Each form of diversity was distinct, denoting a real quantity in an ecosystem. Even more powerful, though, was the power they have when mathematically linked.

The grandest of the three, and coincidentally the simplest, is gamma-diversity. Gamma-diversity represent the total species diversity in a given landscape. In an ideal world, this quantity would be every single species found in one location, like, say, Manu National Park. All bacteria and all fungi; all trees and all birds; all reptiles and all amphibians; this quantity works best when there is no ignorance to force any omissions. Unfortunately, though, biology is still in what could be called its infancy in cataloguing this planet’s species, so what we know will have to do.

Alpha-diversity is the next simplest quantity, representing the mean species diversity at a more local scale. For alpha diversity, we have to do a little more math than simply counting. Regardless, context is still paramount in considering diversity. In this case, the transition from gamma- to alpha-diversity is like zooming in on Google Maps from your neighborhood to your yard. There may be a seasonal, weekly, or even hourly fluctuation of species, but it’s important for this to be an average quantity.

Finally, we have the contentious mess that is beta-diversity. What does beta-diversity quantify? Not the number of species in a specified area, but rather the differentiation of diversity between areas. This could be thought of as the extent to which species switch between areas in the landscape, helping you to understand the dynamics of diversity. It could also be thought of as the number of distinct subunits within a landscape, as species exchange from one area to another depends intimately on the number of areas.

We must remember, when working with quantities in ecology, that few things are as fluid as ecosystems. When doing eco-math, change is the only constant, and beta-diversity accounts for this wisely.

But how can we call this rate diversity? Alpha- and Gamma-diversity make sense to us because they are static in time, or in other words, continuous. The mean species diversity, as alpha-diversity calculates, should be unchanging through time. The number of species in a landscape should remain the same through time. These quantities work in our brains because we can expect them to remain true.

But this sort of thinking commits a common human pitfall. In thinking abstractly, it’s often useful to think of concepts in a void. This sort of reductionism makes it easier on our brain. But it also allows fallacious thinking in the context of ecology. In order to understand fluid ecosystems, we need to quantify that fluidity exactly. In other words, we need to quantify the diversity that causes the change. This is exactly where beta-diversity comes in, providing a number to understand the dynamics of diversity within the specified landscape.

With this dynamic quantity, we now have the power to create amazingly simple equations to understand diversity. In the context of ecology, where calculus and statistics come up all too often, simple, three-variable algebra like β = γ/α is refreshing. Even more refreshing is that in using these variables, we have the ultimate sort of clarity in considering diversity. More than just quantified species counts, we now have quantified relationships between species.

Now these relationships, as useful as they can be, are not set in stone. There are still areas where our equations don’t quite describe reality accurately. Perhaps even more significant is the limits on the data behind the diversity quantities to begin with; as mentioned above, it’s difficult to know all the species in an area, no matter how “obvious” they may be.

But quantifying and relating sorts of diversity in these ways provides a powerful springboard in understanding how diversity works through time. Diversity is no longer a lofty consideration, vaguely describing the richness we see in ecosystems. Instead, we have a basis for monitoring the changes in ecosystems over time through the species they are composed of. Here, the door is opened to citizen science, to rapid inventories, to a wide range of evolutionary questions.

In these equations, we come closer to understanding what makes an ecosystem robust, and therefore, we come closer to understanding exactly what conservation should aim at.

For all this, we have Robert H. Whittaker, and all the giants whose shoulders he stood upon, to thank.

Thoughts? Questions? Critiques? Speak up! A wise man once said we should try to be wrong as fast as possible. Sharing your perspective will only help my learning process and that of anyone who reads this.


Stories about biodiversity & why earth has so much of it

Nick Minor

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Aspiring explorer-naturalist with a passion for birds, big questions about biodiversity, storytelling, & adventure.


Stories about biodiversity & why earth has so much of it