A keystone factor is a small factor that has disproportionately large effects on the whole system. Analysis of keystone factors aims to identify such few critical factors, so that the whole system could be changed by merely altering the keystone factors.
In architecture, a keystone is a final stone that is placed to lock and hold all other stones together. The keystone itself is under the least pressure, but it is structuring the whole arch—without it, the arch collapses.
In conservation biology, a keystone species constitutes a small portion of a particular ecosystem’s biomass, and yet it has disproportionately large effects on that ecosystem. One example of keystone species is the jaguar, which makes up the small percentage of the biomass but it structures the whole ecosystem: without it, various herbivorous animals would grow in large numbers, destroying dominant plants and altering the whole ecosystem.
The keystone species are not limited to large ecosystems—in the human gut, for example, a few small microbial communities (Bacteroides fragilis and Bacteroided stercosis) have disproportionate influence on the structure of the gut microbiome.
Keystone Factors in Social Systems
The idea of the keystone species is also relevant in social systems. In politics, for example, a few interest groups or decision-makers have a very strong influence on the overall structure of the system. In the market, likewise, there are keystone consumers who have disproportionate influence on a structure of a particular market; for instance, the demand created by wealthy Chinese consumers for shark fin soup has structured the market of shark fins and has even caused the collapse of several shark species.
Strategic Analysis of Keystone Factors
Keystone factors have an enormous potential for strategic analysis. Some strategists have long ago realized that they could achieve an enormous overall effect if they could only identify keystone factors and influence them; in a sense, this is the extreme version of the 80/20 analysis.
The best example comes from a pharmaceutical industry (though not necessarily the best example from an ethical standpoint). Pharmaceutical companies long ago realized that a relatively few medical professionals, such as editors of prestigious medical journals, have a structuring effect on the medical knowledge in the field. Therefore, to influence the whole system of medical knowledge, pharmaceutical companies only needed to affect those few key decision-makers.
 Charles K. Fisher and Pankaj Mehta, Identifying Keystone Species in the Human Gut Microbiome from Metagenomic Timeseries Using Sparse Linear Regression, PLOS One, 9(7): e102451 (2014); doi:10.1371/journal.pone.0102451, <www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0102451>
 Jennifer Jacquet, Keystone Consumers, in John Brockman (ed.), This Will Make You Smarter: New Scientific Concepts to Improve Your Thinking (Harper Perennial, 2012) pp. 174-176, available at <http://edge.org/response-detail/10336>