Present address: Department of Biology, Pacific University, Forest Grove, OR 97116, USA.
Distance to stable stage distribution in plant populations and implications for near-term population projections
Article first published online: 17 MAY 2011
© 2011 The Authors. Journal of Ecology © 2011 British Ecological Society
Journal of Ecology
Volume 99, Issue 5, pages 1171–1178, September 2011
How to Cite
Williams, J. L., Ellis, M. M., Bricker, M. C., Brodie, J. F. and Parsons, E. W. (2011), Distance to stable stage distribution in plant populations and implications for near-term population projections. Journal of Ecology, 99: 1171–1178. doi: 10.1111/j.1365-2745.2011.01845.x
- Issue published online: 17 AUG 2011
- Article first published online: 17 MAY 2011
- Received 16 September 2010; accepted 19 April 2011 Handling Editor: Hans de Kroon
- matrix model;
- plant demography;
- plant population and community dynamics;
- population growth rate;
- population projection;
- projection distance;
- stable stage distribution;
- transient dynamics
1. Matrix population models capture how variation in vital rates among life stages translates to population dynamics. Analyses of these models generally assume that populations have reached a stable stage distribution (SSD), where the proportion of individuals in each stage remains constant. However, when life stages respond differentially to environmental cues and perturbations, a population may be moved away from equilibrium. Given the multitude of stochastic processes acting in natural systems, populations may never be exactly at SSD. It is thus critical to understand how far away populations are from SSD and how distance from SSD influences near-term model projections.
2. We analysed published matrix models from 46 plant species spanning a range of life histories that reported both a current stage distribution and projection matrix. We examined the distance between observed and theoretical SSD and the associated consequences for near-term transient population dynamics for each species.
3. In the majority of studies, populations were near their expected SSD, with 80% falling within one unit of a projection distance (α0) of zero. This distribution was skewed towards positive values of α0, indicating that the majority of populations had individuals concentrated into stages with high reproductive values.
4. Half of the populations in our survey had projection distances such that transient projections of population size and growth rate were within 10% of asymptotic projections at 5 years. However, in populations where projection distance was > 2, deviations from SSD caused important (more than twofold) differences.
5. We also found that larger deviations from SSD were positively correlated with generation time and matrix size.
6. Synthesis. When some life stages within a plant population are more strongly affected by disturbances or stresses than others, the results of our literature survey suggest that equilibrium projections will tend to underestimate projections that account for the current stage distribution. Measuring the current stage distribution can help determine whether asymptotic measures of matrix model analyses are reliable, and is a crucial step to take when precise population metrics are necessary for guiding conservation and management.