Some thoughts on ecology, evolution and economics

The McCanny Pattern

It began with a mislabelled graph.  My comment on that simple mistake led to my name being linked with those of the leading ecologists of my day and the designation of an entirely new pattern of seed dispersal and survival.

dandelionI was a graduate student at the University of Western Ontario in 1984 and I was studying the seed dispersal of an agricultural weed. Pretty tame stuff as far as seed dispersal goes.  My species was an annual grass with no special adaptations to fling itself or glide through the air or attach to the nearest animal. Proso millet is better known as bird seed. Nonetheless, it was infesting the corn fields of Ontario and my professor wanted to know how it was dispersed.

The most interesting work on seed dispersal was being done in the tropics.  In particular, Dan Janzen, a colorful and iconoclastic scientist who would be awarded the first Crafoord Prize for ecology later that year, had proposed the Escape Hypothesis to explain why tropical tree saplings rarely grew near their mother tree.  Basically, Janzen reasoned that seeds were most concentrated near their parent and faced a disproportional risk of being found and eaten by predators in such large numbers.  Stephen Hubbell, who later became well known for his Unified Neutral Theory of Biodiversity and Biogeography, challenged Janzen.  He argued that even with a feeding frenzy all around the mother plant you will “almost always”  have a greater density surviving near the parent than farther away. His reasoning was based, in part, on a graph which showed greater survival at the parent tree even when the probability of survival changed as rapidly as seed density with distance from the tree.

The more I looked at this graph, the less sense it made.  Finally, I realized that in the example given, seed density actually decreased more quickly with distance than the probability of survival was increasing.  He had drawn the graph wrong!  It was not inevitable that survival was best in the shade of the mother tree.  In fact, a whole range of options were possible. One of these possibilities was that the chance of survival could decrease with distance from the parent tree.  The correction was published as a note in the journal Oikos in 1985.  My first publication!

Nineteen years later, Ran Nathan and Renato Casagrandi published a paper on seed dispersal.  They named three distinct patterns: the Janzen-Connell pattern, where the density of surviving offspring increased to a peak at some distance from the parent; the Hubbell pattern, where the peak of survival occurred at the base of the parent tree despite decreased odds of survival there; and the McCanny pattern, which has the same peak of survival at the parent tree but shows better odds of survival there than further away.  The last of these is the only one that does not support the Escape Hypothesis.    The McCanny pattern occurs when seed predators, often mammals, do not specialize on one species and actually get over-full on a dense collection of seeds.  This pattern also arises when there is something special about where the parent is growing, moisture or minerals,say, that leads to seedlings not surviving so well at a distance.  In the 9 years since Nathan and Casagrandi published their paper it has been commented on by at least 100 papers. Not famous, but pretty well known among the folks that think about seed dispersal.

So, for those of you who wonder how far to move away from your parents, the McCanny pattern- named after me- suggests that Mom and Dad are not a threat.  Interestingly, after living many years in a different time zone than my own parents I decided to come closer and settle within visiting distance. I didn’t know that I was taking my own advice!

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