Population monitoring and viability analysis for Horkelia congesta ssp. congesta at the Long Tom Area of Critical Environmental Concern
Monitoring for Horkelia on land managed by BLM was initiated in 1993 at the Long Tom Area of Critical Environmental Concern (ACEC). Since that time, monitoring was conducted annually from
Monitoring for Horkelia on land managed by BLM was initiated in 1993 at the Long Tom Area of Critical Environmental Concern (ACEC). Since that time, monitoring was conducted annually from 1993 through 1999 by BLM or ODA and from 2002 through 2004 by Institute for Applied Ecology (IAE) staff. The objective of this report is to summarize population monitoring data for Horkelia congesta ssp. congesta at the Long Tom ACEC and discuss population trends from 1993-2004. In addition, the data available to date make it possible to conduct a population viability analysis and compare population size predictions from computer simulations with those observed in 2004.
Observed population trends of Horkelia congesta at Long Tom ACEC were consistently positive from 1993 through 1996, then declined slightly from 1997 to 2002, when the population reached its lowest recorded size since monitoring began. The population made an upturn in the subsequent two years, increasing to 1997-98 levels in 2004. Plant sizes have also remained relatively stable throughout the monitoring period. All of the largest average plant sizes were recorded in 2002. The reason for this larger average plant size in 2002 is unknown, but may be linked to favorable climatic conditions that year, or mortality of smaller plants, leaving larger plants to dominate the population.
Using computer simulation we have created a population viability analysis of collected data. Our results from this analysis concluded that:
- The structure of the Long Tom ACEC Horkelia congesta population in most years has been fairly stable. Small vegetative plants were usually the most abundant stage while seedlings have been the most infrequent.
- The average transition matrix for Horkelia congesta ssp. congesta yielded a population growth rate, or lambda, equal to 1.035, suggesting that the long-term population dynamics of this species at the Long tom ACEC are stable, increasing at the average rate of 3.5% per year.
- Stochastic projection through the year 2013 predicts that the Horkelia congesta ssp. congesta population will continue to increase at a very gradual rate.
The elasticity analysis shows that small changes in the probability of stage VI plants remaining at stage VI (0.1315) would have the single greatest effect on the population growth rate, with stage II plants remaining at stage II being the second most important transition.