Seed Limitation in Native and Exotic Catchfly Silene Sp. Plants in Central CaliforniaEPA Grant Number: U915786
Title: Seed Limitation in Native and Exotic Catchfly Silene Sp. Plants in Central California
Investigators: Satterthwaite, William H.
Institution: University of California - Santa Cruz
EPA Project Officer: Jones, Brandon
Project Period: August 1, 2000 through August 1, 2003
Project Amount: $98,152
RFA: STAR Graduate Fellowships (2000) RFA Text | Recipients Lists
Research Category: Academic Fellowships , Ecological Indicators/Assessment/Restoration , Fellowship - Ecology and Ecosystems
The objective is to assess the importance of seed versus safe-site limitation in native and exotic catchflies Silene sp. in central coastal California.
A combination of experimental and modeling work will be used to assess seed versus safe-site limitation of two species of catchflies in the Santa Lucia mountains of Monterey County, California, where both the native Silene antirrhina and exotic Silene gallica are found. Seeds of each species will be collected, and several populations of each species will be found and monitored. Permanent plots of three types in each population will be established: control plots, seed addition plots, and seed removal plots, maintained over the course of the study. Baseline model: Within each control plot, individual plants will be marked and followed through entire growing seasons (both Silene species are annuals), recording survival, growth, and fecundity of all individuals. Bags of seeds of each species will be buried to assess survival and germination dynamics of each species? seed bank, and varying densities of seeds will be sown to generate density-dependent estimates of single-year germination rates. These data will be used to generate periodic matrix models of both species? population dynamics. These models will generate estimates of current population growth rates, aiding in prioritization of Silene gallica invasion as a management concern, and sensitivity analyses of these models will aid in developing specific management plans. Manipulations: In seed addition plots, a quantity of seeds equivalent to 10 times the estimated total seed production of plants naturally occurring in a plot will be added. In safe-site limited populations, no change in the density of plants the following year is predicted, whereas in seed-limited populations, increased densities will be observed. In seed removal plots, all flowers from half of the plants in each plot will be removed before they set seed. This is predicted to lead to reduced densities in subsequent years for seed-limited populations, but not for safe-site limited populations. Interpretation of these results may be complicated by the seed bank, which might be able to compensate for short-term reductions in seed output but become depleted over the long term. This possibility will be accounted for by maintaining the seed removal treatment for several years and by using demographic models to estimate whether and how fast the seed bank would be depleted in such a scenario. For the native Silene antirrhina, new populations will be established in which it can be assumed there is no seed bank, and similar experiments will be performed.
Knowing the relative importance of seed versus safe-site limitation is crucial for successful management of invasive exotic plants. Management efforts focusing on reductions of fecundity (e.g., through biocontrol agents or manual clipping of plants) are most likely to be successful in the case of seed-limited plants. By contrast, such approaches would be ineffective for safe-site limited plants, for which a better strategy might be management of the native community to reduce openings suitable for the establishment of the invader. Comparing seed versus safe-site limitation in native and exotic congeners may help make generalizations about whether exotic species are particularly likely to be seed or safe-site limited.