Spring 2015 Ecesis Volume 25, Issue 1
Populations of the coastal cactus wren (Campylorhynchus brunneicapillus) are in decline throughout much of southern California, including San Diego County. Over the last decade or so, large intense fires have damaged coastal cactus wren habitat in San Diego County including lands on the San Diego National Wildlife Refuge. Populations of coastal cactus wrens have also declined in Preserve areas not yet affected by wildfires. This recent trend of cactus wren population decline has been observed in other regions of southern California as well. Genetic researchers have found low population densities and lack of gene flow between some populations of wrens in southern San Diego County and this could lead to genetic bottlenecks. Regional recovery efforts for coastal populations of cactus wrens are intended to stabilize and eventually increase population sizes.
This five-year, grant-funded project was initiated in the fall of 2009 and ended in the fall of 2014. The funding for the project was provided by SANDAG (San Diego Association of Governments) to the City of Chula Vista as part of the TransNet Environmental Mitigation Program (EMP) grant cycle. Successful habitat restoration is a challenge under any circumstances, but restoring cholla-dominated habitat for cactus wrens can be even more challenging than most.
In the Central City Preserve of Chula Vista, which encompasses about 1,300 acres, coast cholla (Cylindropuntia prolifera) patches have declined in the last 10–15 years due to competition with weeds and large shrubs, such as lemonadeberry (Rhus integrifolia). In addition, the below-average rainfall during most of the last decade has caused many patches of coast cholla to die due to the drought conditions. This cholla die-off has likely caused a decrease in suitable habitat for coastal cactus wren. In addition to the drought conditions, it is possible that nest predation by scrub jays, other corvids, and roadrunners have contributed to the observed declines as well.
Project Goals and Habitat Restoration Methods
The project goals and methods used to restore coastal cactus wren habitat were:
- Increase coast cholla patch sizes and density within portions of the Central City Preserve to benefit populations of coastal cactus wrens.
- Restore and enhance patches of coast cholla in a distribution pattern that facilitates dispersal of cactus wrens between areas of suitable habitat.
- Proactive reduction of native and non-native fuels in the immediate vicinity of nesting sized coast cholla patches to decrease the risk of catastrophic fires that could eliminate wren habitat.
- Restore habitat for coastal cactus wrens and other covered species, including coastal California gnatcatcher (Polioptila californica californica) and Belding’s orange-throated whiptail (Aspidoscelis hyperythra beldingi), in areas dominated by weeds.
- Restore and enhance coastal cactus wren habitat through the selective thinning and removal of lemonadeberry, other native shrubs, and exotic annuals that are directly competing with coast cholla to the detriment of cactus wren populations.
Site Preparation Shrub Removal and Dethatching
Field crews began the vegetation-thinning program in the fall of 2009 under the supervision of the project biologists. Chainsaws, loppers, and machetes were used to cut branches of shrubs selected for removal. Shrubs were removed to reduce direct competition with coast cholla for light and water, and also to reduce the risk of catastrophic fire. The openings created by the thinning program were also intended to increase potential open ground foraging areas for the coastal cactus wren.
The shrubs selected for thinning were primarily lemonadeberry and jojoba (Simmondsia chinensis). California sagebrush (Artemisia californica) were occasionally removed as well when they were deemed to present a potential fire risk to the cholla patches, as determined by the project biologist. Vegetation (i.e. cut branches) generated by the thinning program were carried downslope in large burlap bundles from the open space to an access road in Rice Canyon. The cut vegetation was then placed into green waste dumpsters for removal from the site. Cut shrubs were immediately treated with a triclopyr-based herbicide to prevent the plants from re-sprouting.
The vegetation thinning and removal program continued through the fall and shrubs were removed around approximately 45 nesting-sized cholla patches in Rice Canyon and adjacent canyons. The total acreage of vegetation removed was approximately 5.75 acres. Dethatching of weed-dominated areas was also performed during October 2009. Approximately 2.5 acres of weeds at 20 different sites were cut using weed whips. The cut material was raked into piles and removed from the site. Cholla cuttings were planted in all of the dethatch areas, around existing cholla patches to increase cholla density, and within existing openings in coastal sage scrub (approximately 0.81 acre) for a total of approximately 9.04 acres of treated area.
Cactus planting began in early December 2009. Cholla cuttings were taken from existing cholla patches and were distributed into weeded areas or adjacent to existing cholla stands to enlarge the patch size. Cholla segments were either placed horizontally in contact with the soil surface or a small hole was excavated and the base of the cutting was placed in the soil. In areas where weeds were cut and no nesting sized cholla were present, larger cholla stems were planted. These larger stems, approximately two to three feet tall, were planted to encourage wrens to nest in those areas.
Each season after winter rains had germinated weed seeds, glyphosate was used to control non-native annuals in dethatch and shrub thinning areas and in locations that were immediately adjacent to these sites. Non-native species that were controlled included primarily annual grasses such as wild oat (Avena spp), black mustard (Brassica nigra), crown daisy (Chrysanthemum coronarium), and filaree (Erodium spp.). Herbicide was applied by licensed applicators
Cactus and other Native Plant Growth
Near normal rainfall in 2009–2010 was conducive for the cactus cuttings planted in the fall and winter to root and begin growth. Existing cholla patches in shrub treatment areas also exhibited new growth as well. In some dethatch areas, cholla plants that had appeared to be dead prior to the implementation of weed control efforts began to show new growth. With continuing weed control efforts, these plants showed amazing recovery. Over time, these areas have exhibited dense cholla growth that will benefit the coastal cactus wren by providing additional nesting areas.
Shrub clearing areas supported populations of annual natives such as cryptantha (Cryptantha sp.), Indian tobacco (Nicotiana sp.) and Nuttall’s snapdragon (Antirrhinum nuttallianum). These species acted much like fire-following annuals that germinate and grow for the first few years after a fire moves through. Perennial subshrubs such as wishbone plant (Mirabilis laevis var. crassifolia) grew well in recently opened areas. Sensitive plant species that benefited from the shrub thinning includes coast barrel cactus (Ferocactus viridescens) and snake cholla (Cyldropuntia californica) both listed as rare by the California Native Plant Society; both are also Multiple Species Conservation Plan covered species.
Weed Control and Cactus Growth Results Due to the continued maintenance efforts over the five-year project, weed cover at the shrub thinning and dethatching sites remained low in spring 2014. The average absolute non-native cover at the shrub thinning sites was 0.0 percent in 2014. The relative cover of non-natives at the shrub thinning sites also remained very low at 0.4 percent in 2014. At the dethatching sites, average absolute non-native cover was 0.0 percent in 2014. The relative cover of non-natives at the dethatching sites also remained very low at 0.2 percent in 2014. These numbers indicate that weed control efforts have been successful and are maintaining low levels of non-native cover.
During Year 5, even though rainfall was well below normal during the 2013–14 season, cactus cuttings and existing coast cholla patches continued to expand — in part, because weed cover and competition were reduced and more water became available for native plant growth. The positive effects of this additional water are reflected in the large number of new coast cholla and prickly pear stems that have appeared each year on the cuttings originally planted in 2009. This new growth is particularly evident at the weed dethatching areas. Coast cholla, prickly pear, and cuttings planted in 2009 flowered each year and have been observed being visited by pollinators such as native bees and honey bees. Pollinated cactus flowers are producing numerous fruits that provide potential food for coastal cactus wrens and other wildlife. Over time, the dethatching areas have filled in with dense coast cholla and prickly pear that will benefit the coastal cactus wren by providing additional nesting areas.
Quantitative data show that the cover of coast cholla at the shrub thinning sites increased about 8 percent since 2010, while the average cover of coast cholla at the dethatching sites increased about 14 percent. The most noticeable change in the coast cholla at the dethatching areas was the increase in height of the plants which is important for successful cactus wren nesting . The percentage of coast cholla that were one to three feet tall increased from just 4 percent in 2010 to nearly 57 percent in 2014.The percentage of coast cholla over three feet tall increased from 5 percent in 2010 to over 21 percent in 2014. Also, the average cover of coast cholla relative to the total plant cover at the dethatching sites increased from 50 percent in 2010 to over 92.9 percent in 2014. Figure 2 depicts the changes in health, size, and cover of a coast cholla patches that were formerly dominated by non-native mustards. The dethatch and subsequent weed control program have dramatically improved the growing conditions for the coast cholla.
Cactus Wren Use for Future Restoration Efforts Based on survey results from 2003, several cactus wrens pairs were historically present in the project area, but by 2009, at the start of this restoration and enhancement project, only one pair of wrens remained. In 2011 an additional pair of cactus wrens moved into and started using the one of the shrub thinning areas in the southwest portion of the restoration area. Those birds occupied the area for about one year, but moved on in 2012. This illustrates the challenge of performing restoration for cactus wrens as the birds have multiple threats to overcome.
Additional cactus wren restoration projects are underway in other areas of San Diego County including in the Otay Valley area in southern San Diego County. In addition, a restoration planning effort is underway for these wren populations, headed up by The Nature Conservancy in collaboration with the San Diego Management and Monitoring Program and with funding by SANDAG. The goals of this planning effort are to protect and enhance suitable cactus scrub and coastal sage scrub habitat within the Otay River and adjacent areas and to increase connectivity between these south San Diego County populations. We hope these focused restoration efforts will stabilize and increase the wren populations so that this important member of our native fauna is preserved in perpetuity. — Mark Dodero, RECON Environmental