Coastal ecosystems exist as a narrow ecotone between marine and terrestrial realms and form the planet’s longest distance of ecological interface linking land and sea. Coastal zones are defined here as the area within nearshore marine waters where light penetrates throughout (~50m depth) and the adjacent terrestrial areas dominated by ocean influences of tides and marine aerosols (Agardy et al., 2005). Accounting for less than 5% of Earth’s land area, coastal ecosystems are highly productive and concentrate disproportionately high values for biodiversity and ecosystem services (Agardy et al., 2005). Coastal ecosystems are highly dynamic and globally threatened by changing climate, sea level rise, and development. Nearly 40% of the world’s population lives within 100 kilometers of a coastline (Small and Nicholls, 2003; Center for International Earth Science Information Network (CIESIN) of Columbia University, 2006; Center for International Earth Science Information Network (CIESIN) of Columbia University, 2012). Consequently, many coastal habitats have been converted for development, commerce, and recreation (Halpern et al., 2008; Barbier et al., 2011; Wright et al., 2018). Today, just 15.5% of coastal areas worldwide can be considered ecologically intact and having low anthropogenic pressure (Williams et al., 2022). Many of the remaining natural coastal areas are adjacent to human-altered landscapes, highly disturbed, and threatened by development (Neumann et al., 2015; Heady et al., 2018).

The narrow width and highly dynamic properties of coastlines can render them vulnerable at the local and landscape scale to anthropogenic impacts (Alvarez-Romero et al., 2011). Coastal ecosystems are physically and ecologically dependent on adjacent terrestrial and marine ecosystems to provide key inputs (such as trophic subsidies, nutrients, sand supply, freshwater supply) as well as to support wildlife connectivity across the land-sea interface. Consequently, connectivity, condition, and management of adjacent terrestrial and marine areas will affect the maintenance of the character of coastal habitats. Coastal ecosystems are inherently vulnerable to habitat loss, defaunation, and human disturbance due to their constrained location at the land-sea interface and dependence on inputs from both adjacent terrestrial and marine ecosystems (Williams et al., 2022). Land use, resource extraction, and infrastructure – even well inland in coastal watersheds – can disrupt freshwater inputs and flows, alter sand supply, and change erosional and depositional processes leading to an unraveling of coastal ecosystem processes and functions (Defeo et al., 2009; Alvarez-Romero et al., 2011; Merrifield et al., 2011). Coastal development, armoring, diking, dredging, beach grooming, and sand mining along the coast also affect physical processes and habitat quality for a variety of species (Schlacher et al., 2007; Dugan et al., 2008; Hubbard et al., 2014; Torres et al., 2017). Invasive species can alter species composition and structure of habitats, impact trophic dynamics, and alter nutrient and sediment loads to coastal areas (Byrnes et al., 2007; Williams and Grosholz, 2008). Wildlife use of coastal areas, such as resting and feeding areas for marine mammals and migratory birds and foraging grounds for top predators, can be significantly impacted by recreation and human disturbance (Lafferty et al., 2013; Larson et al., 2019). Overharvest of resources and trampling of intertidal and coastal habitats can reduce biodiversity, alter trophic structure, and affect wildlife use of the coast (Addessi, 1994; Crowe et al., 2000; Roy et al., 2003). Oil spills, toxic releases, and poor water quality can also significantly impact the condition of coastal areas (Crowe et al., 2000; Hughes et al., 2015; Bejarano and Michel, 2016).

Direct and indirect effects of climate change (e.g., sea level rise, coastal erosion, more frequent and intense storms, and changing ocean conditions) exacerbate these threats (Heady et al., 2018; Luijendijk et al., 2018; Lorie et al., 2020; Barnard et al., 2021) especially as coastal habitats are squeezed between rising sea levels and topographic or built environment constraints on their upward migration (Vitousek et al., 2017; Heady et al., 2018; He and Silliman, 2019; Leo et al., 2019; Barnard et al., 2021). Climate change is also altering storm frequency and intensity which can drive coastal erosion and coastal change (Zedler, 2010; Lehmann et al., 2018; Barnard et al., 2021). Changing ocean conditions, including rising temperatures, altered circulation patterns, increased acidity, and shifts in species distributions are already altering coastal marine ecosystems and will continue to do so well into the future (Hewitt et al., 2016; He and Silliman, 2019). In this light, shoring up coastal resiliency by protecting the most intact and wild areas becomes an even more urgent priority, as does prioritizing areas of the adjacent terrestrial realm for protection that will be needed to serve as coastal habitat strongholds in the future (Heady et al., 2018).

Coastal conservation efforts lag behind those for terrestrial and marine ecosystems (Jones et al., 2018). Indeed, most conservation planning processes still focus on either the terrestrial or marine realm in isolation and do not explicitly address the importance of conserving coastal biodiversity, ecological connectivity, and processes that span the land and sea interface (Beger et al., 2010; Alvarez-Romero et al., 2011; Harris et al., 2014). Coastal areas with intact physical and ecological processes, functions, and diversity provide myriad values to society (Barbier et al., 2011). They support numerous habitat types (such as rocky shores, sandy beaches, dunes, wetlands, and estuaries) and associated species; as well as nursery, foraging, and resting grounds for many species of economic and cultural importance (Beck et al., 2001; Neuman et al., 2008). Coastal areas are the physical and ecological connection between marine and terrestrial ecosystems and play a key role in nutrient, sediment, and water flows; they also buffer human communities from storm surge and sea level rise (Barbier et al., 2011; Arkema et al., 2013; Neumann et al., 2015). Protection of intact ecosystems can be an efficient means of conserving biodiversity, protecting ecosystems services, and slowing extinction rates as demonstrated in terrestrial and marine systems (Di Marco et al., 2019). To inform such efforts along coasts, however, planners need criteria for identifying intact coastal areas. Here we describe attributes of ecologically intact and functional coastal areas, which we further define below as areas of “coastal wildness”.

We demonstrate how these attributes and considerations can be applied in conservation planning and management, using California (USA) as a case study. The state of California (USA) is an ideal study area for examining protection of coastal wildness with its long, ecologically diverse coastline, and history of coastal protection and strong coastal policy (Lester, 2013). About 68% of California’s population of 38.4 million people lives within 48 km of the coast (NOAA Office for Coastal Management https://coast.noaa.gov/states/california.html). Conflicts between coastal access and coastal development led to the passage of the California Coastal Act in 1976, which provides for coastal conservation, coastal access for the public, and relatively strict regulations on coastal development. California has extensive terrestrial protected areas (e.g., national and state parks) along the coast, as well as a network of marine protected areas (MPAs) in nearshore waters (Gleason et al., 2013). Due to data limitations, our study focused on California’s mainland coast, including San Francisco Bay, but not the coast on offshore islands. We note that California’s islands are generally well-protected; their relatively robust populations of breeding pinnipeds and seabirds suggest the conservation values that can be retained by protection of coastal wildness.

We illustrate how lack of protection of coastal wildness represents a significant conservation gap, even in geographies with strong coastal policy and a system of coastal marine and terrestrial protected areas. Our analysis suggests that not only is greater investment in coastal conservation warranted, new designations aimed at prioritizing and preserving wild coastal areas may also be necessary.

We defined coastal wildness as areas that are largely physically and ecologically intact, with limited human disturbance, and containing (or having the potential to contain) the full complement of biodiversity expected for the associated habitat types in their unimpaired states (see also Kormos et al., 2016; Watson et al., 2016). We applied this definition to the mainland coast of California, USA. Using Geographic Information System (GIS) software, we defined a study area that spanned the mainland California coastline to 8 km inland of projected 1.5 m of sea level rise. Readily available spatial data were compiled and aggregated to a grid of 4 km² hexagons to best match the resolution of the data with the resolution needed for analyses. Data were also aggregated by regions, within these geographic bounds: North (Marin County and north to the Oregon border), Central (San Francisco south to Santa Maria), South (Santa Maria south to the border of Mexico), based on established terrestrial and marine ecoregional boundaries (e.g., Bailey, 2004; Spalding et al., 2007).

We characterized four attributes of wildness to identify areas with high ecological intactness, high physical intactness, high species and habitat diversity, and low human disturbance. ( Table 1 ; see Supplemental Information for full methods and Table S1 for data sources).

Each of the four attributes of wildness were mapped statewide, with quintile rank scores for each hexagon, showing broadscale patterns of ecological and physical intactness, diversity, and human disturbance ( Figures S1 – S4 ).

To characterize coastal wildness, we developed a “Coastal Wildness Index” based on the four attributes. Using an additive model, we combined indexed quintile scores with Ecological Intactness and Species/habitat Diversity contributing positively to wildness, and Human Disturbance and lack of Physical Intactness indices contributing negatively to a coastal wildness index (see Stein et al., 2009; Heady et al., 2015). We ranked the resulting coastal wildness index using quintiles with 1 representing low coastal wildness values and 5 representing high coastal wildness values; we considered areas with rank 4 or 5 to be ‘wild coast’.

We then compiled spatial data on Conservation Management Status (CMS), Marine Protected Areas (MPAs) and ownership (federal, state, or private) to identify existing terrestrial and marine protected areas that could most likely support effective management and conservation of wildness attributes ( Table S2 ; CMS dataset modified from Heady et al., 2018, data derived from the California Protected Areas Database CPAD, 2016 and other sources; see Heady et al., 2018 for data sources). We considered CMS category A (highly conserved) and B (conserved) to contribute most to the protection of wild coast attributes, with the more highly conserved category A likely providing more protection of wildness attributes. We then overlaid existing marine and terrestrial protected areas with the Coastal Wildness Index spatial layer to calculate and assess gaps in protection of wild coast areas. Additionally, we identified areas of wild coast that are publicly or privately owned (CMS category C and D), but not conserved, to identify potential opportunities for future conservation of wild coast.

Approximately 35% of the California coastal study area can be characterized currently as wild coast ( Figure 1 ; Table S3 ). This varies by terrestrial ecoregion, with about 39% of northern and central California regions having wild coast attributes ( Figure S5 ; Table S3 ), including some well-known areas such as the King Range-Lost Coast, and Big Sur. In southern California, 27% of the coast has wild attributes, including Vandenberg Space Force Base, the Point Conception area, Malibu-Santa Monica Mountains, and Camp Pendleton Marine Corps Base ( Figure 1 ; Table S3 ). Some wild coast areas, especially in northern California, likely have proxy protection by their remoteness and low population density. Other areas, such as military lands in southern California, have some level of de facto protection from recreation and access but are subject to military uses and priorities.

Statewide, approximately 60% of the area characterized as wild coast is currently protected in CMS A or B, amounting to about 21% of the total California coastal area ( Figure 2 ). Only about half of that, 11% of the total coastal area, is managed for values consistent with sustaining wild coast attributes in the most protected category (CMS A, highly conserved; see Figure S6 ; Table S3 ). Areas of wild coast with protection spanning the land-sea interface are limited. Only about a quarter (26.6%) of areas characterized as wild coast are protected terrestrially (in CMS category A or B) and adjacent to an MPA, amounting to about 9% of California’s coastline ( Figure 2 ; Table S3 ). Some parts of the wild coast area characterized as well protected on the marine side are adjacent to agriculture or development on the terrestrial side. Similarly, other wild coast areas with strong terrestrial protection lack protection of coastal and marine biodiversity in an adjacent MPA.

Remaining wild coast is distributed non-equally among regions of California. Of the 35% of the remaining wild coast, 13.5% is in the North Coast, 14% in the Central Coast, and 7.9% in the South Coast ( Figure 1 ; Table S3 ). Levels of protection for remaining wild coast areas also vary by region with ‘highly conserved’ area (CMS A) of 29.3% of wild coast of the North Coast, 43.6% of Central Coast, and 10.6% of South Coast, and ‘conserved’ area (CMS B) of 2.5% of wild coast of the North Coast, 3.0% of Central Coast, and 5.0% of South Coast region ( Figures 1 , 2 ; Table S3 ).

We found that about 40% (1,974 km²) of the area characterized as wild coast is in private ownership ( Table S3 ; Figure S6 ). This amounts to about 14% of the total coastal area highlighting that even in a relatively well-conserved geography like coastal California there remain opportunities for conserving wild coast values through conservation easements or other mechanisms on land, especially where that can be paired with marine protection in nearshore waters.

Ecologically intact and wild areas are essential for effective biodiversity conservation and resiliency in the face of global change, and areas with these attributes are poorly protected globally (Di Marco et al., 2019). Coastal areas with high wildness values have been undervalued by conservation planning and often fall in the cracks between terrestrial and ocean protection, management, and governance. California has some of the strongest coastal policy of any place in the world. Yet despite a robust policy framework and decades of conservation, we found that much of California’s remaining wild coast is vulnerable to threats to biodiversity and ecosystem function. This is largely due to the paucity of areas with management focus of protecting wildness attributes, the importance placed on providing public coastal access, and the lack of integrated marine-terrestrial conservation planning and management oversight.

Conservation of wild coastal areas urgently needs more attention in conservation planning, policy, and practice around the world, especially where there are opportunities for adjacent land-sea protection and management. With nearly half of the global population residing near the coast and increasing threats focused in coastal areas, we recommend a multi-tiered approach to conservation of wild coastal attributes based on four key elements:

Data for the original contributions presented in the article are available on the Knowledge Network for Biodiversity (KNB) repository https://knb.ecoinformatics.org/view/urn%3Auuid%3A2f1c0de5-30ae-42b3-b05e-7214a5874115.

All authors contributed to the conceptualization of the article, contributed to the article writing and approved the submitted version. KE and WH led the analysis of the data. All authors contributed to the article writing and approved the submitted version.