From a train a man was observed lying face down in a field, on the train’s return journey the next day he was still in exactly the same position leading some of the passengers to think him dead. He was in fact in the fortunate position (depending on your point of view) of being not dead, but a plant demographer. Years of pain-staking data collection face down in fields have led to the construction of a data-base of thousands of population measurements for over 50 plant species. This has enabled us to determine two surprising facts: 1. The longer you study a population for the more its population growth rate declines and 2. The variability in population growth rate through time was more predictable than we had expected, a good year was quite likely to be immediately followed by a bad year.
As every 1st year science student knows correlation does not imply causation so we cannot recommend putting ourselves out of a job by halting all long term demographic studies in order to prevent rare plant declines. This might instead tell us something about how we go about setting up our studies – perhaps we should be less inclined to add excitement to the daily grind of being face down in a field by choosing only the charismatic fast-growing populations to study and instead include both fast and slow growing populations as starting points. Initially fast growing populations must at some point slow down or take over the world, so perhaps it’s inevitable that we find the large showy charismatic populations to be ultimately disappointing.
As populations vary more in time than in space with good years followed by bad and vice versa, the short-cut of studying multiple populations for short periods of time in lieu of following populations over longer time periods is not necessarily a good one. The peril of appearing to be dead will continue to be faced by plant demographers as they collect data year after year instead of flitting about from field to field.
Buckley,Y.M., Ramula, S.R., Blomberg, S.P., Burns, J.H., Crone, E.E., Ehrlen, J., Knight, T.M., Pichancourt, J.B., Quested, H. & Wardle, G.M. (2010). Causes and consequences of variation in plant population growth rate: a synthesis of matrix population models in a phylogenetic context. Ecology Letters 13(9): 1182–1197.