Demography and population viability of Fassett’s locoweed (Oxytropis campestris var. chartacea), a rare endemic plant growing on lake shores in Wisconsin

Background/Question/Methods

Fassett’s Locoweed (Oxytropis campestris var. chartacea) is a threatened plant species endemic to eight Wisconsin lake shores with frequently fluctuating water levels.  In the long term, increasing threats, such as water drawdown by high-capacity irrigation wells, could reduce the amount of habitat available to the locoweed by allowing competitors to establish on the upper shores of these lakes.  During the summers of 2009-2011, I conducted experiments and collected data from two of the larger natural populations (Plainfield Lake and Pickerel Lake) to parameterize vital rates (survival, growth, and reproduction) of seeds, seedlings, and non-seedlings of different sizes.  I used these data to construct projection matrices for each population in each year, and estimated both long-term and short-term population growth rates for each population in each year.  I also tested for effects of plant density on vital rates.  To assess the long term effects of water drawdown from high-capacity irrigation wells on population viability, I used estimates of drawdown levels and lake maps to determine the change in lakeshore habitat area as water drawdown occurs. 

Results/Conclusions

In these populations, seed production and/or seed survival in the soil may be most important for determining population persistence, yet these vital rates are among the most difficult to estimate.  Projection matrices indicate that populations may be growing in the short term, but may decline in the long term if conditions remain constant.  However, population persistence of O. campestris var. chartacea may be affected by changing conditions on the lakes.  In the short term, rising water levels have already decreased the numbers of non-seedling plants at Plainfield Lake, which will slow seed production.  These declines may be buffered by potentially large numbers of seeds remaining in the seed bank.  In the long term, water drawdown could reduce the locoweed habitat by more than twenty percent.  In summary, these two large populations of this globally rare plant species appear to be growing in the short term.  In addition, changing conditions driven by water levels, and changing threats to existing populations create highly uncertain estimates of population viability in this species.