The effect of fire on the population viability of an endangered prairie plant

We examined the effects of fire on population growth rate and extinction probability of a rare prairie plant. Lomatium bradshawii (Apiaceae) is an endangered species of western Oregon and Washington prairies that were frequently burned by Native Americans prior to the late 1800s. Using data from mapped plants in two populations subjected to different fall burning frequencies over a six-year period, we constructed stochastic transition matrix models to evaluate the effects of fire on the viability of the populations. Stochasticity was incorporated into the models through two methods performed by widely used computer programs: randomly shuffling whole annual matrices (using POPPROJ2), or selecting each matrix element from a distribution with observed mean and variance (with RAMAS/stage). Log-linear analysis was used to test for site and treatment effects on transition rates. Unburned plots had stochastic population growth rates of 0.905–0.927, depending on the site and stochastic method. Burning twice in six years increased to 0.946–1.091, and three burns yielded values of 0.996–1.173. The risk of extinction (100 yr, extinction defined as less than 10 individuals) was very high (97–99%) in the absence of fire for both methods and sites, but with two burns it declined to 57% and 73% for matrix selection and element selection, respectively, at one site, and equal to or less than 1% (both methods) at the other. Extinction probability was very low ( equal to or less than 1%) for both methods and sites after three burns. Although different methods of incorporating stochasticity gave slightly different quantitative results, they were qualitatively similar. Both site and treatment had significant effects on transition rates in the log-linear analyses. Elasticity analysis suggested that burning decreased the sensitivity of population growth rate to mid-sized plants and increased the importance of seedlings. Fire was an effective tool for maintaining viable populations of this species, although the strength of its effects differed between locations.