Coastal salt marshes are characterized by high productivity and biodiversity, and their limited range and unique abiotic traits increase their tendency to have specialized and adapted plant species (Teixeira et al. 2014; Grewell 2008). These specialized plants, particularly the halophytes, are highly adapted to environments with high salinity and periodic tidal inundation, and their presence provides critical habitat for other organisms, including crustaceans, arthropods, and estuarine birds (Teixeria et al. 2014). Halophytes also reduce soil salinity and improve oxygen conditions in marsh soil that would otherwise often be under anoxic conditions (Grewell 2008); thus, their presence mitigates abiotic stressors for other plants in the salt marsh and creates a more habitable environment for other less salt-tolerant species. Therefore, the presence of halophytes is critical to coastal salt marsh biodiversity and resilience, and their loss will change the composition of the plant communities, reducing species richness.
Coastal salt marshes consist of low, middle, and high marshes, creating unique habitat niches for plants and wildlife that are likely to be negatively impacted by future sea level rise (SLR). Globally, SLR is projected to increase by 0.25-3m (IPCC 2014), which will cause a decrease in salt marsh habitats and their endemic halophytes (Lee et al. 2014; Simas et al. 2001; Thorne et al. 2018). This decrease in abundance will be particularly pronounced if the salt marsh remains static, with no sedimentation; it is projected that 20–60% of the world’s coastal wetlands will become inundated within the century if sedimentation is unable to keep pace with SLR (Kirwan et al. 2010). Therefore, the persistence of salt marsh ecosystems depends on the ability for halophytes to vertically shift their elevation ranges to keep pace with SLR (Nelson and Zavaleta 2012).
California coastal salt marshes are particularly vulnerable to SLR, due to relatively low sedimentation rates, steep topography, and or the presence of urban environments where landward transgression would be inhibited (Thorne et al. 2018). On the western coast of the US, up to 95% of salt marshes under high SLR scenarios are likely to become mudflats or inundated entirely by 2110 (Thorne et al. 2018). Endangered and endemic species in California salt marshes are therefore particularly vulnerable to SLR without upland habitat to support landward transgression (Fagherazii et al. 2019).
Cordylanthus maritimus (Salt Marsh Bird’s Beak) is a hemiparasite found primarily in the middle marsh and is protected by the federal and state endangered species acts (Zedler et al. 2001). The California Natural Diversity Database (CNDDB) classifies this species as a California Rare Plant Rank of 1B.2, meaning it’s listed as rare, threatened, or endangered in CA and elsewhere. Historically, C. maritimus was widely distributed but its range has become increasingly restricted due to a variety of factors including warmer temperatures, a lack of genetic diversity, extreme tide events, and urbanization (Milano et al. 2020; Noe et al., 2019). Currently, C. maritimus remains in only eight salt marshes in California, with five occurring in southern half of the state (Noe et al. 2019). In recent years, the presence of invasive species has impeded efforts to restore C. maritimus populations. Limonium ramosissimum (Algerian Sea Lavender) was recently introduced from the Mediterranean and became naturalized in disturbed salt marshes throughout California (Barbour et al. 2007). In contrast to the small, realized niche of C. maritimus, L. ramosissimum has a wide distribution in both salt marshes and upland habitats, and causes a decrease in species richness (Archbald & Boyer 2014). Since C. maritimus relies on native plant species as host, the loss of native plant species richness due to invasive species will have an additional negative affect on their recovery. Moreover, it remains unclear how populations of C. maritimus will respond in the future both SLR and interspecific competition with L. ramosissimum.
Here, we address the response of C. maritimus to the combined impacts of SLR and invasive L. ramosissimum. To create habitat niche models for each species, we combined the distributions of C. maritimus and L. ramosissimum with LiDAR data in the Upper Newport Bay Ecological Reserve (UNBER) located in Orange County, California. Using these habitat ranges, we then used various SLR projections to model future habitable ranges for each species. Our habitat models will assist land managers with predicting the future ranges of both species and with prioritizing potential restoration sites for reintroduction of C. maritimus. We also provide an estimate for likely locations of invasive L. ramosissimum, both now and in the future, so that restoration efforts can be maximized with limited resources.