The Coyote Brush Invasion of Southern California Grasslands and the Legacy of Mechanical Disturbance

Abstract

This study investigates the recent invasion of exotic grasslands by coyote brush in La Jolla Valley, California. We test the “event dependent” hypothesis that mechanical disturbances during the past century were a key cause. To examine the relationship between past mechanical disturbances and vegetation dynamics we first conducted a review of the historical literature on practices of shrub removal and documented disturbance history using historical imagery. We next analyzed vegetation‐cover change over time using remotely sensed imagery and a vegetation map to document the history of native shrub advancement into exotic grassland by species association. Finally, we determined the topographic characteristics associated with different phases of shrub advancement. We found that mechanical disturbances historically varied by topography with upper and steeper slopes being least intensively disturbed. We found that shrub advancement rates, following release from grazing, varied by slope, elevation, and time period, and that Baccharis pilularis (coyote brush) was the main species to invade the more intensively disturbed sites at lower elevations. Our results indicate that mechanical disturbances played an important role in modifying the original vegetation cover with long‐lasting consequences, including a facilitating role for the subsequent Baccharis pilularis invasion. We concluded that the practice of grazing often included exotic mechanical disturbances that had long‐lasting impacts on native plants.

Key words:

• Baccharis pilularis
• California sage scrub
• invasive species
• native invasives
• grazing

The authors would like to thank the many Geography and Environmental Science and Policy students from California State University Long Beach as well as those from our local high schools involved in Geoscience Diversity Enhancement Project (GDEP) for their efforts to gather soil and vegetation samples in the field without which this study would not have been possible. We also thank Michael Shensky for his help on the figures. Finally, we thank Suzanne Goode at Point Mugu State Park and Marti Witter at the Santa Monica Mountains National Recreation Area for their helpful assistance.

The authors would like to thank the many Geography and Environmental Science and Policy students from California State University Long Beach as well as those from our local high schools involved in Geoscience Diversity Enhancement Project (GDEP) for their efforts to gather soil and vegetation samples in the field without which this study would not have been possible. We also thank Michael Shensky for his help on the figures. Finally, we thank Suzanne Goode at Point Mugu State Park and Marti Witter at the Santa Monica Mountains National Recreation Area for their helpful assistance.

Notes

The authors would like to thank the many Geography and Environmental Science and Policy students from California State University Long Beach as well as those from our local high schools involved in Geoscience Diversity Enhancement Project (GDEP) for their efforts to gather soil and vegetation samples in the field without which this study would not have been possible. We also thank Michael Shensky for his help on the figures. Finally, we thank Suzanne Goode at Point Mugu State Park and Marti Witter at the Santa Monica Mountains National Recreation Area for their helpful assistance.

1. California sage scrub is also commonly referred to as coastal sage scrub. Both have the initialism CSS. California sage scrub is preferred as it is useful for distinguishing between sub‐types such as interior and coastal sage scrub plant communities (Riordan and Rundel 2009).

2. According to Pearce (2016) when the journal Science carried a series of articles quoting ecologists who questioned the pervasive fear of invasive species, a full suite of “heavy‐hitting” conservation leaders wrote in reply. They warned that such talk risks trivializing the action that is needed to address one of the most severe threats to global biodiversity.

3. This statement is based on a discussion which broke out among park service employees, conservationists and the authors of this paper following an oral presentation of the paper at a conference sponsored by the Santa Monica Mountains National Recreation Area in 2015.

Additional information

Notes on contributors

Paul Laris

Dr. Paul Laris is a professor and chair of geography at California State University, Long Beach, Long Beach CA 90803; [[email protected]].

Sean Brennan

Ms. Kyra Engelberg is a mitigation specialist with WRA Environmental Consultants; [[email protected]].

Kyra Engelberg

Mr. Sean Brennan currently works with the U.S. Forest Service; [[email protected]].