Evaluating the power of monitoring plot designs for detecting long-term trends in the numbers of common guillemots
Article first published online: 12 MAY 2006
Journal of Applied Ecology
Volume 43, Issue 3, pages 537–546, June 2006
How to Cite
SIMS, M., WANLESS, S., HARRIS, M. P., MITCHELL, P. I. and ELSTON, D. A. (2006), Evaluating the power of monitoring plot designs for detecting long-term trends in the numbers of common guillemots. Journal of Applied Ecology, 43: 537–546. doi: 10.1111/j.1365-2664.2006.01163.x
- Issue published online: 12 MAY 2006
- Article first published online: 12 MAY 2006
- Received 22 July 2005; final copy received 30 January 2006 Editor: Rob Freckleton
- linear mixed model;
- monitoring programme;
- population change;
- power analysis;
- variance component
- 1In recent years concerns have been raised regarding the status of the common guillemot Uria aalge in the UK. Numbers have declined in several regions, highlighting the need for continued monitoring of this internationally important population. However, the extent to which the current monitoring scheme is capable of detecting declines and options for improving efficiency has received little attention.
- 2We investigated the power of different monitoring design options for detecting long-term trends in abundance at a colony of guillemots. The ability to detect trends in abundance was reduced by the large temporal and spatial variability in colony attendance. Taking a linear mixed model approach, we obtained details on the sources and sizes of the variance components using count data collected from monitoring plots on the Isle of May, Scotland, and assessed how best to allocate sampling effort in the light of the count variability structure.
- 3Our results indicated that trend detection will be improved by counting birds in more plots rather than by increasing the number of counts at existing plots.
- 4The revisit pattern of counts at the monitoring plots during the seasonal counting period had little effect on trend detection power. However, given the practical issues associated with counting guillemots, alternative revisit patterns to the current approach are preferred.
- 5For a fixed number of visits per plot, power is strongly influenced by the choice of revisit design if the day-to-day variation in colony attendance is increased.
- 6Synthesis and applications. Aspects of the UK seabird monitoring scheme can be improved. Changes to the allocation of sampling effort and the plot-revisit pattern will improve both the statistical power to detect long-term trends and the efficiency of conducting the survey. We stress the importance of considering the structure and magnitude of the count variation in a power analysis because judicious design decisions depend on the relative magnitude of these variance components.