Are humans a keystone species?

“Humans are just a fossil-fueled outbreak that will go away”, stated one of my professors yesterday.

In the world investigating the human dimensions of natural resource management there’s two schools of thought as to how humans fit in: a) as just another particularly troublesome species in the ecosystem or b) a special kind of species with the ability to predict and change the future.  This latter formulation hinges on humans as a species with “motivation” and “intent”, according to C.S. Holling.

Otherwise, however, humans should be able to fit into the ecological definitions and models we all learned in intro biology.  At an extreme case, it means we’re the next trilobite or dinosaur, living in our 15 minutes of fame now but soon to disappear.  In the process, we will pave the way for a new species to become dominant.

Check out the layering caused by Pisaster, thanks wallawalla.edu

Robert Paine first coined the term keystone species after working with his sea stars in the Pacific Northwest.  He observed that even though they had low abundance, they controlled the structure of the rocky intertidal community.  Therefore they had disproportionate effects in the system given their abundance and were dubbed “keystone”.  The concept has been taken outside his study system and the definition tinkered with to best fit different areas, as reviewed in Conservation Biology.

Davic, the author of the review, proposes the following to be a more useful tailored definition: “a keystone species is a strongly interacting species whose top-down effect on species diversity and competition is large relative to its biomass dominance within a functional group.”

Even a few people can have a big effect, thanks aaagiclee.com

This definition opens up the possibility to identify more keystone species within a system, as divided by functional group.  Either way, humans meet the first criteria of causing drastic changes in community diversity and structure.  Arguably, they always have, even when population (or biomass) levels were low.  Just think of the fate of the buffalo in the North American plains, the control of the fire-dominated forests in boreal areas, or the promotion of useful agricultural or medicinal species over others.

However, as the human population nears 7 billion, many of the environmental and ecosystem impacts we cause are more related to sheer numbers than particularly intense resource use.  Resource use still dominates the impacts in developed countries such as the US, where a baby born today will be expected to have 125 times the impact as a baby born the same time in Indonesia.  Whatever the implications of this are for conservation, people are still arguably a textbook keystone species.

In areas of the world where space is a premium, humans not only affect the system but dominate it just by their physical presence.  For example, there are 22000 people per square kilometer in Mumbai.  It’s hard to envision that kind of human presence without some creative stacking.

So as more places head in the direction of Mumbai, should we still be thinking of humans as a keystone species?  Or are we something else, still with disproportionate per capita impact but also multiplied by an incredible number of capitas?  Ecosystem biology and the world have not faced this conundrum before.  And it might have many implications for how to manage our impacts and protect the ecosystem functions we care about and depend on.

~Bluegrass Blue Crab

  1. I thought keystone species was usually defined as having a relatively small biomass as compared to the rest of the species in an ecosystem. The Davic definition you provide is what I thought was a dominant species.

    It’s still possible humans are a keystone species, though. You’d just have to define whether or not you’re using a local or global scale, because depending on which, the classification of humans could be very different. If we’re talking about a rural setting, say in the prairies of South Dakota, humans probably are more of a keystone species, while if we’re talking about New York City, humans would be what I understand to be a dominant species. Globally, who knows?

  2. We will always seek the most logical reasons for our evolution. Keystones just doesn’t fit in my thinking either I believe what my religion dictates or Charles Darwin. Keystones can be used to tell time not the human anatomy.

  3. keystone species are more of non-living things that humans. it is not something combined to be whole again..if we deem human sa keytones, here should be categories or kinds or whatever that categorizes humans.

  4. To places having massive population such as Mumbai, i agree (and quote) that ” It’s hard to envision that kind of human presence without some creative stacking”

  5. Amy,
    I see you reference my Conservation Ecology paper on keystone species (KS). My first goal with this review was to return the KS concept to the original 1966 intent of Paine as reported in his Seastar experiments. A second goal was to highlight that his Seastar were biomass dominant within a well defined predatory “functional group” (even if not apex). Thus the Seastar of Paine represent KS because their predatory actions had effect on diversity in lower trophic levels (in his case, the effect was to increase prey diversity, but, in other situations the effect could lead to lower prey diversity at multiple scales). I called attention to the observation of Paine that Seastar were biomass dominant within a top predatory functional group, and used this to advance a simple way to predict the possible presence of KS within any ecosystem, a topic of importance to managers of natural areas.

    You raise the very important topic of how do humans fit into the KS concept as I defined it. I did not discuss human KS status, but, humans clearly are biomass dominant within a unique vertebrate predator functional group of many ecosystems, and they have potential to alter species diversity of lower trophic levels. Thus, I would agree with your claim that humans have potential to function as keystone species in ecosystems where density would allow (I mean, a single human living along the shore of a lake is not any type of keystone).

    It is important to mention that, whereas the Seastar of Paine was a KS whose presence was to “increase” prey species diversity via competitive interactions, humans are the antithesis, they would function as a KS to “decrease” prey species diversity. Of course, not all human populations interact with environment as KS, their actions may have no measured effect on prey species diversity. Thus perhaps it is best to say that humans are potential KS, and where they are, their effects are predicted to be negative on prey species diversity.

    The experimental test of KS effect is to remove all humans from an ecosystem and monitor effect on prey species diversity. The outcome will depend on human density prior to removal and the degree of pre-human impact on species diversity. Where pre-human density/impact is high, a KS effect should be observed when humans are removed (i.e., species diversity should increase), were pre-human density/impact is low, little change in species diversity is predicted after humans removed. This experiment is ongoing where lands once modified by humans (i.e., corn field, clear cut forest) return to natural ecological succession.

    In addition to being potential keystone species, humans are also potential strong ecosystem engineers and habitat modifiers. Such ecosystem effects differ from KS effects because they are not directly related to trophic dynamics of the ecology (but may have indirect effects). Your picture of the high numbers of humans in Mumbai documents how the distinction between KS effect (as dominant predator within a functional group) and habitat modifier effect (as dominant form of life in the ecosystem as a whole) becomes fuzzy, yet predictive. No matter how humans are labeled in Mumbai, keystone or otherwise, the predicted outcome is strong negative effect on species diversity. Humans are unique in that they are the only species that can know they are KS. Thus humans can act to ensure that they minimize negative KS effects on species diversity and maximize positive KS effects. Actions such as preservation and restoration of natural areas can serve as KS models for humans. The Seastar can serve as a useful educational model to teach how each human can become an keystone of their community, use metaphor to raise environmental awareness.

    Finally, you are correct that my re-definition of the KS concept highlights the possibility that ecosystems can have many different KS acting jointly within many different functional groups. This aspect of nature is not well studied.

    • Thank you for such a thoughtful response! I especially like the point about environmental awareness, as that’s what I was trying to achieve with this post: education both in terms of ecological principles and stewardship ethics.
      I had the privilege of meeting Robert Paine last spring and a good part of our interaction was about conservation lessons from his work. It wasn’t exactly what I was expecting to hear, but it definitely made me think.