Pockets of California grasslands, such as serpentine outcrops and perennial-grass refugia, remain dominated by native species. I use a population-modeling approach to understand how interacting global change drivers affect the persistence of native species in these areas.
Serpentine grasslands are typically dominated by endemic annual forbs but are occasionally invaded by non-native annual grasses. In collaboration with Professor Richard Hobbs I am using long-term demographic data to model these dynamics. We are discovering that native and non-native species become abundant for different reasons: native forbs are highly fit but also strongly self-limited, whereas non-native grasses have low potential growth rates but also minimal self-limitation. As a consequence, slight, rainfall-driven increases in the growth rate of non-native grasses can cause large spikes in their abundance. Fortunately for management, this indicates that long periods of drought can be leveraged to eradicate non-native populations.
In contrast, in a separate collaboration with the East Bay Regional Park District, Dr. Loralee Larios and I are discovering that climate variability has the opposite implication for perennial grass conservation. Sustained drought reduces adult survival, which can facilitate conversion from perennial to annual-dominated grasslands. Here, interacting global change factors (extreme drought and widespread non-native species) may create a threshold dynamic that requires intensive intervention to prevent or reverse.