Environmental change, shifting distributions, and habitat conservation plans: A case study of the California gnatcatcher

Abstract

Many species have already experienced distributional shifts due to changing environmental conditions, and analyzing past shifts can help us to understand the influence of environmental stressors on a species as well as to analyze the effectiveness of conservation strategies. We aimed to (1) quantify regional habitat associations of the California gnatcatcher (Polioptila californica); (2) describe changes in environmental variables and gnatcatcher distributions through time; (3) identify environmental drivers associated with habitat suitability changes; and (4) relate habitat suitability changes through time to habitat conservation plans. Southern California's Western Riverside County (WRC), an approximately 4,675 km2 conservation planning area. We assessed environmental correlates of distributional shifts of the federally threatened California gnatcatcher (hereafter, gnatcatcher) using partitioned Mahalanobis D 2 niche modeling for three time periods: 1980–1997, 1998–2003, and 2004–2012, corresponding to distinct periods in habitat conservation planning. Highly suitable gnatcatcher habitat was consistently warmer and drier and occurred at a lower elevation than less suitable habitat and consistently had more CSS, less agriculture, and less chaparral. However, its relationship to development changed among periods, mainly due to the rapid change in this variable. Likewise, other aspects of highly suitable habitat changed among time periods, which became cooler and higher in elevation. The gnatcatcher lost 11.7% and 40.6% of highly suitable habitat within WRC between 1980–1997 to 1998–2003, and 1998–2003 to 2004–2012, respectively. Unprotected landscapes lost relatively more suitable habitat (−64.3%) than protected landscapes (30.5%). Over the past four decades, suitable habitat loss within WRC, especially between the second and third time periods, was associated with temperature‐related factors coupled with landscape development across coastal sage scrub habitat; however, development appears to be driving change more rapidly than climate change. Our study demonstrates the importance of providing protected lands for potential suitable habitat in future scenarios.

Subspecies differentiation and range‐wide genetic structure are driven by climate in the California gnatcatcher, a flagship species for coastal sage scrub conservation

“…Given that the northern‐most sampled populations of Ventura, Chino Hills, and Coyote Hills appear to harbor unique genetic variation associated with seasonal extremes in precipitation and temperatures, maintaining and restoring habitat in the most northern and eastern range edges could help preserve this reservoir to facilitate continued range expansion, particularly as warmer temperatures and more frequent and intense drought with variable and extreme precipitation events are predicted throughout southern California (Berg & Hall, 2015 ; Cayan et al, 2010 ; Kam & Sheffield, 2016 ; Swain, 2015 ). At least in some areas of southern California, climate change and development have already led to a loss of gnatcatcher habitat (Hulton VanTassel et al, 2017 ). Efforts to identify and protect future suitable habitat, and protect existing adaptive potential that allows for range expansion, may be important strategies to ensure long‐term persistence of the California gnatcatcher.…”

“…At least in some areas of southern California, climate change and development have already led to a loss of gnatcatcher habitat (Hulton VanTassel et al, 2017). Efforts to identify and protect future suitable habitat, and protect existing adaptive potential that allows for range expansion, may be important strategies to ensure long-term persistence of the California gnatcatcher.…”

Impacts of competition and herbivory on native plants in a community‐engaged, adaptively managed restoration experiment

“…In the coastal sage scrub system of Southern California, a combination of global change stressors can convert open space from a diverse, shrub-dominated habitat to lowdiversity, invaded annual grassland (Kimball, Goulden, Suding, & Parker, 2014;Talluto & Suding, 2008). Loss of diverse coastal sage scrub vegetation means less habitat for threatened bird species such as the California gnatcatcher (VanTassel, Bell, Rotenberry, Johnson, & Allen, 2017). The system can be stuck in a degraded state as established nonnative annual plants out-compete native shrubs at the seedling stage, making re-establishment of native shrubs difficult (Cox & Allen, 2011).…”