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

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

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

May 11, 2010 • 5:29 pm