ENHS Nature Trails May 2004 Article 2

President's Column: "The One in Twenty Rule"
by David Wagner

There are several cases from the past where well-meaning botanists, in the pursuit of increasing knowledge about plants, have destroyed the existence of a species at certain sites by documenting their discoveries with collections. The case most familiar to me concerns one of the world's rarest ferns: Botrychium pumicola (the pumice grape-fern). A student searching for new sites found two plants on Tumalo Mountain in 1954. He proudly collected them, digging them up to make complete herbarium specimens. In the late 1970's I searched the top of Tumalo Mountains with friends. We all had experienced eyes, but no plants were found. My belief is that the two plants removed in 1954 eliminated the possibility of establishment of a population of this fern. We hope that this won't happen today, that botanists finding only one or two plants would document their discovery with photographs and notes. Good photographs and careful field notes are increasingly acceptable for recording plant discoveries.

Nevertheless, from time to time a field worker is likely to encounter a small population of an unknown plant and feel it is necessary to collect a bit of it for positive identification and documentation. The Native Plant Society of Oregon's Guidelines and Ethical Codes for botanists urges that a collector use good judgement and rules of thumb when deciding whether or not to collect. What is a good rule of thumb? When this question came up at a rare plant conference many years ago I had no answer but at some time in the late 1980's I began using a rule of thumb which I now call the "1in 20 Rule."

Simply put, the "1in 20 Rule" means that a botanist should never collect more than one out of twenty plants. It means NOT collecting ONE plant UNTIL you have found at least TWENTY. This runs counter to the traditional collector's mentality. As a teenage fern collector, the sight of a rare fern sent my hand out to pluck it from the rocks as a prize. Having the plant safely in my vasculum, I started looking around for more. Now, if I run across an unusual plant I suppress my traditional impulse and first think, "Can I find twenty?" Only if twenty are found will I consider collecting one plant. And forty should be present before two are taken, etc. Leave at least nineteen for every one taken. This applies to parts of plants also: remove no more than five percent (one twentieth) of a shrub, one frond from a clump of twenty, 5% of a patch of moss. I use the "1in 20 Rule" whether I am doing rare plant work or gathering common species for educational use. I now make a point to call myself a field botanist rather than a plant collector.

The "1in 20 Rule" does not obviate the need for good judgement. Only when a botanist has the knowledge to assess if collecting is both ecologically justified and legally permitted should a specimen be taken. Any pertinent factor relating to the survival of a population needs to be superimposed on the "1in 20 Rule" The main value of a rule of thumb is to provide a clear point of reference from which to begin assessing a situation. It helps a botanist determine how much time should be spent inventorying before sampling is appropriate. I suggest the "1in 20 Rule" as a minimal criterion to be met before any taking of a plant be considered.

There is at least a modicum of scientific logic behind this rule. When we do statistical analyses of populations, we generally consider the difference between two populations to be not statistically significant if results of what we are counting or measuring are within 95% of each other. A population sample of nineteen is not significantly different from a sample of twenty. Note that I recognize that one plant out of a total of twenty is indeed significant and statistics has nothing to do with it.

Although the statistical argument for justifying the "1in 20 Rule" is weak when dealing with small populations, it is the most reasonable one I can imagine. One population geneticist I consulted, advised me that contemporary statistical theory would support the "1in 20 Rule." Another pointed out, however, that repeated collecting would tend to reduce every population to nineteen individuals. This caution serves to emphasize that the "1 in 20 Rule" is a rule of thumb, not a license to ravage. An interesting line of argument in support of the proposal has developed since I first published the idea in the newsletter of the Botanical Society of America. Shortly after the note appeared, I received a letter from James Grimes of the New York Botanical Garden querying whether or not I hadn't gotten the idea from a similar article he and several other botanists had promoted in the newsletter of the Idaho Native Plant Society some years prior to my publication. I honestly believe I did not see that note. Then, in 1994, four botanists from Australia and New Zealand published an article in the international journal, Taxon, which made essentially the same recommendation.

My paper was published, with minor revisions, in a number of newsletters over the next several years. Eventually Adolf Ceska placed it in an issue of BEN, an electronic newsletter, where it was noticed by Dan Branton of Canada. Dan sent me an article he had written for a local newsletter that promulgated exactly the same notion: use 1in 20 as a collecting rule of thumb. He assured me that he had arrived at the notion quite spontaneously. Thus, four botanists or groups of botanists, deliberating independently, arrived at the same standard. I submit that this unconscious concurrence from four separate sources speaks strongly for the sensibility of the "1in 20 Rule."

David H. Wagner
Northwest Botanical Institute, April 2001

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