Distribution, Occupancy and Population Genetic Structure of California Gnatcatchers in Southern California

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The coastal California Gnatcatcher (Polioptila californica californica), a federally threatened species, is considered a flagship species for southern California conservation planning. Dr. Barbara Kus collaborates with Dr. Amy Vandergast and local agencies to provide information to managers that can be used to conserve this species and its habitat.

sage scrub

Typical composition of coastal sage scrub habitat in California. (Credit: Barbara Kus, USGS, Western Ecological Research Center. Public domain.)

Many conservation activities in southern California are linked directly to the California Gnatcatcher or the scrub vegetation it inhabits, with the goal of establishing persistent populations within an interconnected reserve system. Efforts have concentrated on tracking direct loss of habitat, establishing regional population trends of gnatcatchers, and identifying areas for conservation. Prior to 2000, concerns over habitat loss associated with urbanization led to the implementation of several Natural Community Conservation Plans which established large preserves of coastal sage scrub. This effort regulated development and reduced habitat loss.  More recently, several catastrophic wildfires burned hundreds of thousands of acres of coastal sage scrub throughout the gnatcatcher’s southern California range, making wildfire and the associated habitat loss and degradation the most significant threat to gnatcatcher persistence.  Gnatcatchers currently occupy remnants of sage scrub habitat in a highly fragmented landscape, raising further concerns for the long-term conservation of gnatcatchers. Small, isolated populations can lose genetic variation through genetic drift, which can reduce the genetic variability needed to adapt to changing environmental conditions.

Our research addresses three major topics related to California Gnatcatcher conservation and management:

Genetic Structure

The goal of this project is to begin to determine how genetic variation is arrayed across remaining populations of gnatcatchers, allowing inference about individual movement and gene flow patterns among those populations. Our work focuses on determining the extent to which gnatcatcher populations function as metapopulations, with aggregations being interconnected across a fragmented landscape and freely capable of re-establishing in patches from which they have been previously extirpated. Combining genetic data with habitat and corridor indices will allow us to determine whether genetic connectivity is facilitated by stepping stone arrays of suitable habitat, or whether birds are unable to move across unsuitable habitat regardless of distance. These genetic data can also provide insight on how far individual birds will disperse for breeding.

Rangewide Occupancy

Determining occupancy of California Gnatcatchers at a regional scale allows us to track the status of the species throughout its entire southern California range and link population trends to environmental and anthropogenic factors. Because California Gnatcatchers will be managed through management of coastal sage scrub vegetation, our research addresses how gnatcatcher occupancy is related to plant structure and composition, as well as other biotic and abiotic variables. This information will provide guidance on where and how coastal sage scrub could be managed and restored to benefit California Gnatcatchers and other inhabitants.

Post Wildfire Recovery

Photo of a burning southern California landscape

An active wildfire in southern California. (Public domain.)

In this study, we monitor the recovery of California Gnatcatchers and their coastal sage scrub habitat following wildfires with the goal of informing management before, during, and after fires. Focusing on areas burned during three large fires in 2003, 2007 and 2014, we conduct surveys to determine occupancy as a function of time since fire, and compare these to occupancy in unburned reference areas to assess the length of time needed for gnatcatcher populations to rebound following fire. We collect vegetation data to determine the relationship between California Gnatcatcher occupancy and vegetation structure, composition and cover, and evaluate patterns of vegetation recovery based upon time since fire, spatial distribution, previous land use, and environmental conditions.