Cactus Wren Recovery
The coastal cactus wren Campylorhynchus brunneicapillus is named for the nests it makes in mature prickly pear cactus Opuntia or cholla Cylindropuntia stands within coastal sage scrub. This cactus scrub habitat has become rare due to extensive urban development. The remaining habitat fragments are often degraded due to the spread of invasive species and frequent wildfires.
Only a handful of coastal cactus wren populations remain in San Diego County, with arguably the most robust population occurring behind the Safari Park, on the Park’s 800-acre biodiversity reserve. This population is part of a network of populations, which biologists call a “meta-population,” occurring throughout the San Pasqual Valley.
When the 2007 Witch Creek Fire spread across the San Pasqual Valley, the birds were gravely impacted. After the fire, the Applied Plant Ecology Division of the Institute for Conservation Research invested in cactus and coastal sage vegetation restoration to aid the recovery of cactus wrens in the Park’s reserve. Within six years of the fire, the robust cactus wren population is recovering on the reserve. Unfortunately, other areas within the San Pasqual Valley have not fared so well. The Applied Plant Ecology Division was awarded a grant from the City of San Diego to duplicate their successful restoration strategies within the greater San Pasqual Valley. The question is, where should restoration efforts be focused?
For a meta-population to survive long-term, there needs to be a delicate balance between population connectivity and refuge from disturbance. On the one hand, connectivity allows individuals to migrate within subpopulations, increasing the genetic pool and allowing re-colonization after a disturbance. On the other hand, it is not a good idea for all populations to be so connected that a disturbance affects all of the populations simultaneously. In other words, don’t put all of your eggs in one basket.
In the San Pasqual region, fire is a frequent disturbance that illustrates these properties. Within the meta-population, we want populations that are isolated enough that not all populations burn in a fire, but are connected enough that cactus wrens can seek refuge in unburned areas in the event of a fire. Thus, when choosing a restoration site it is important to consider the total population of birds, the distribution of those birds over populations, the spatial isolation of the populations, the risk of fire, and the population growth rates of cactus wrens (e.g. annual survival and fecundity).
These issues are suitably complex for a mathematical simulation model.
The goal of the population model is to find potential restoration sites that increase connectivity between current populations while minimizing the extinction risk due to catastrophic fire, environmental fluctuations, and random demographic events (for example, a year in which a lot more males are born than females may lead to reduced breeding in the following year). The model combines information on cactus wren life history, suitable cactus habitat, and existing cactus wren territories with spatially explicit fire predictions.
Comparing historical fire data to environmental variables such as climate, land use, and vegetative cover makes fire predictions. Once such a model is in place, a variety of potential restoration scenarios can be considered. Specifically, should cactus wren habitat restoration focus on building corridors, enlarging existing habitat patches, or selecting locations primarily based on minimizing fire risk?
After exploring a variety of restoration strategies and the model’s sensitivity to particular inputs (such as initial population size, rates of dispersal between patches, growth rates, etc.), the best restoration strategy in the immediate future was to enlarge an existing habitat patch by restoring cactus in the Lake Hodges area. In the long-term, a corridor connecting the Safari Park’s biodiversity reserve to the Lake Hodges population is important, but not before the Lake Hodges area is capable of sustaining more cactus wrens.
This winter, the Applied Plant Ecology Division and San Diego Zoo Global volunteers will be digging holes and wrestling three-foot tall prickly pear cactus into the ground around Lake Hodges. If you would like to help with the habitat restoration efforts please contact the San Diego Zoo volunteer department.
Erin Conlisk, Ph.D., Applied Plant Ecology