Science: Habitat Connectivity

herd of elk running across landscape dotted with small shrubs
Herd of elk

Connectivity refers to the degree that organisms or natural processes can move unimpeded across habitats – both terrestrial and aquatic. Natural and semi-natural components of the landscape must be large enough and connected enough to meet the needs of all species that use them. Without connectivity, processes like nutrient flow, gene flow, seasonal migration, pollination, and predator-prey relationships, cannot occur. Increasing connectivity is a critical strategy for addressing habitat loss and fragmentation, a top threat to biodiversity. A functional network of connected habitats is essential to the continued existence of California's diverse species and natural communities, in the face of both human land use and climate change. Climate change may impact both the quality and distribution of habitat, and the ranges of some species ranges are already shifting. Thus, connectivity also is important to allow for wildlife to adapt, adjust, and move in response to climate change.

CDFW Connectivity Work

CDFW’s mission is to manage California’s diverse fish, wildlife, and plant resources, and the habitats upon which they depend, for their ecological values and for their use and enjoyment by the public. In managing wildlife resources and habitat, many efforts underway at CDFW are tied to connectivity. CDFW works closely with federal, Tribal, state, and local agencies on three primary strategies to ensure habitat connectivity for wildlife:

  • Protect connectivity where habitat is still intact, through permanent conservation and adaptive management
  • Avoid further fragmentation of habitat (e.g., cluster urban development, roads, and other infrastructure projects where they are least likely to disrupt connectivity)
  • Minimize or mediate the effects of existing barriers to connectivity (e.g., create wildlife crossings or fish passage structures)

In order to implement these strategies, scientists must understand how habitats are connected, the corridors that animals use to move through the landscape, and the locations of barriers that impede movement.

Terrestrial Habitat Connectivity

map of california with different shades of color depicting levels of connectivity - click to enlarge in new windowTerrestrial habitat connectivity is measured by how well habitats in the landscape are connected to allow for the flow of ecological processes and animal movement. Habitats are generally defined by the vegetation type, vegetation structure (e.g., trees, shrubs), geologic features, and water features present at a location. Together, these features determine which species can live and survive in the area. Terrestrial habitat connectivity allows organisms to move through the landscape and between habitats. This includes daily movements to find food and resting places, yearly movement (e.g., seasonal migration), and occasional movement (e.g., dispersal to new territories or to find mates or movement to flee a natural disaster). Habitat connectivity also allows organisms to shift their distributions slowly over time due to climate change. Fragmentation due to human development and infrastructure creates barriers in the landscape that impede habitat connectivity.

Over the past several decades, scientists have been working to map habitat connectivity across California and understand which swaths of habitat, or “habitat linkages,” are most important to maintain habitat connectivity in California. In 2000, the Missing Linkages project (PDF) was the first to identify and map important habitat linkages statewide. This was followed by the California Essential Habitat Connectivity  (CEHC) project in 2010, commissioned by CDFW and the California Department of Transportation (Caltrans), which identified remaining blocks of contiguous natural habitat in California (i.e., natural landscape blocks) and modeled linkages between neighboring large natural landscape blocks. The CEHC map represents a coarse-scale, “20,000-foot” view of remaining intact habitats in California and linkages between those areas. The project report (PDF) recommended that the next step in improving habitat connectivity information in California would be to develop species-specific linkages at a finer-scale for use in regional conservation planning. As of July 2019, regional, species-specific linkage models had been developed for approximately 60% of the state. More information on the development of regional linkage models can be found in the CDFW Guidance Document for Fine-Scale Connectivity Analysis (PDF).

In 2018, the California Biodiversity Initiative Roadmap (PDF) directed CDFW to develop an updated statewide habitat connectivity map. The first step in this effort was to compile the regional linkage models that had been developed to-date in California and bring them together with the CEHC linkages and other state-of-the-art connectivity models published in recent years. The resulting Statewide Terrestrial Connectivity Map (PDF), available through the CDFW Areas of Conservation Emphasis (ACE) project, shows a comprehensive statewide overview of essential corridors and linkages (PDF) that have been mapped in California. The map can be used to distinguish the connectivity attributes that have been identified in each area across multiple studies. CDFW is currently working to fill remaining data gaps, including completing the picture of fine-scale linkage studies across the state and modeling species corridors with GPS collar data. The map will be updated over time as new connectivity data for the state become available.

Habitat linkages can be impeded by barriers, including human development and infrastructure. In particular, long linear infrastructure, such as roads, railways, canals and fences, can act as barriers to animal movement through otherwise intact natural habitats. These barriers can cause genetic isolation by preventing gene flow between neighboring populations, which has long-term impacts on populations and can lead to local extinctions. In some locations, such as roadkill hotspots, animals may be frequently killed as they attempt to cross a barrier. In March 2020, CDFW released the Wildlife Barrier Priorities report (PDF), which identifies the top wildlife barriers in each CDFW region of the state. This report represents CDFW’s first effort to collate an initial suite of priority wildlife movement barriers across the state. Identifying priority wildlife barrier locations will help focus limited financial resources in areas with the highest need for improvements for wildlife movement. The report will be updated annually.

Wildlife Corridors

black bear emerging from rectangular undercrossing
A black bear (Ursus americanus) emerges from Sagehen Grade Underpass after a successful crossing of Highway 89 (photo courtesy Highway 89 Stewardship Team)

California is home to multiple large mammals from ungulates including mule deer, black-tailed deer, elk, pronghorn, and bighorn sheep, to carnivores including black bears, mountain lions, and wolves. These large mammals inherently need large landscapes to persist and are particularly vulnerable to habitat loss and fragmentation. Connectivity between habitat patches is therefore crucial for conserving these charismatic species and facilitating their movement across the landscape. Due to regional climate variation, large mammals display diverse movement behavior across California. These include seasonal migration between separate ranges, daily movement within a home range, and dispersal which can lead to shifts in species distribution and connect isolated populations. Human disturbance and habitat fragmentation due to human infrastructure can inhibit these types of animal movement. However, strategies, such as corridor conservation and road crossing structures, can help maintain connectivity.

In ungulates, migration is a learned behavior. This means it may be difficult to re-establish population-level migration routes once they are lost, resulting in dramatic population declines. Migration is essential for ungulates to access seasonally available resources and to sustain larger populations. Ungulates in California exhibit both resident and migrant strategies. It is also possible for a single population to contain both strategies with migrants and residents overlapping during part of the year (generally during the winter). Residents remain on a single range year-round while migratory individuals use distinct summer and winter ranges connected by a migration route which may or may not include stopover sites.

CDFW is currently working to map where important migration routes occur on the landscape. By collaring animals with satellite GPS collars, biologists can track where animals are moving during migration. GPS collars are programed to record locations at a fixed interval (e.g., every 2 hours) which means there is some uncertainty about where an animal was between satellite fixes. Models, including the Brownian bridge movement model, incorporate information from collar data such as step lengths between consecutive locations and turning angles to identify which route the animal likely took between points. After using this method to identify individual migration routes and stopover sites, managers can explore which routes are most important at the population level by estimating what proportion of the population uses a particular route. Mapping migration corridors also allows managers to locate potential movement barriers and prioritize routes for conservation.

Human encroachment and development within wildlife habitat are also concerns for non-migratory individuals and species. In addition to conserving migration corridors, it is also important to maintain connectivity within an animal’s home range and between populations to facilitate dispersal. California’s human population is approximately 40 million and humans and wildlife are increasingly coexisting in semi-urban environments. Mountain lion P-22 is perhaps the most famous example of this. After crossing multiple major freeways, he established his home range alongside the Hollywood sign in Griffith Park.

As with migration corridor mapping, GPS collar data can be used to identify potential movement barriers which may prevent an individual from safely moving between habitat patches within a home range or may impede dispersal into isolated populations. CDFW has previously collaborated with other state and federal agencies on the Highway 89 Project to implement wildlife road crossing structures. Efforts like this help to maintain connectivity across various scales, from the individual home range scale, to the species distribution scale. While this is important for all species, it is especially important for large mammals that require extensive habitat.

Aquatic Connectivity and Fish Passage

underwater shot of two fish with green heads and red bodies swimming in a river
Kokanee salmon (CDFW photo by Andrew Hughan)

Anadromous fish, such as salmon, steelhead, coastal cutthroat trout, pacific lamprey, and sturgeon, hatch in freshwater systems like rivers and streams, spending their early life as juveniles in these systems. Much of their once-accessible habitats in California have been partially or fully blocked by road-stream crossings, irrigation diversions, dams, long concrete channels in urban areas, and other in-stream structures. In some cases, previously installed fish passage structures that were built to provide passage past a barrier, such as fish ladders for salmon and steelhead, act as barriers because of poor design or construction, lack of continued maintenance or were not built with passage in mind for all anadromous fish, particularly lamprey.

These obstacles affect adult and juvenile fish by delaying or preventing: 1) upstream migration to spawning and rearing habitat in streams and rivers and 2) downstream migration to the ocean. Not only does this prevent fish from using available and suitable habitat, but it also can result in injury or mortality. Delays in downstream migration to the ocean can prevent fish from entering the ocean during optimal conditions, causing further stress to populations. Meanwhile, delays in upstream migration can hinder fish from reaching their spawning grounds in time and lead to spawning in less desirable habitat or no spawning at all. These obstacles have negatively impacted populations of anadromous fish and are a major factor in their decline, representing a continued threat to their survival.

Senate Bill 857 (SB 857) mandated that the California Department of Transportation (Caltrans) locate, assess, and remediate fish passage barriers on the state highway system for any project using state or federal transportation funds and to submit an annual report to the California State Legislature. Since SB 857 was enacted, Caltrans has developed an active program for assessing, prioritizing, and remediating barriers and has established regional groups of Fish Passage Advisory Committees.

Barriers to fish passage and barrier remediations are tracked in the California Fish Passage Assessment Database. Since there are thousands of barriers to fish passage in the state, CDFW has been systematically prioritizing them by annually publishing a list of top priorities. This list is linked in proposal solicitation notices for several grant programs (refer to Funding Sources section below) include CDFW’s regional fish passage priority list, assisting in the removal of these high priority barriers.


California Wildlife Conservation Board’s Proposition 68 grant program funds wildlife corridor projects that address barriers to terrestrial wildlife movement in California. The Natural Community Conservation Planning (NCCP) Local Assistance Grants Program provides state funds for urgent tasks associated with the implementation of approved NCCPs or NCCPs anticipated to be approved within 12 months of grant application. Additionally, there are several grant programs in California that fund fish passage improvement projects, including the Fisheries Restoration Grant Program and Proposition 1 and 68 grant programs managed by CDFW, California State Coastal Conservancy, and California Wildlife Conservation Board.

Connectivity Data

Connectivity data developed by CDFW and partners are available in the Habitat Connectivity Viewer, which is a part of the CDFW Biogeographic Information and Observation System (BIOS). Maps can be viewed in the online BIOS map viewer, and the GIS datasets can be downloaded.

The 2010 California Essential Habitat Connectivity (CEHC) Project was commissioned by CDFW and the California Department of Transportation (Caltrans) to identify remaining blocks of contiguous natural habitat in California (i.e., natural landscape blocks) and modeled linkages between neighboring large natural landscape blocks. Data from the CEHC Project:

Project Highlights

CDFW is working with partners across California to implement connectivity projects.

Northeastern California Wildlife Connectivity Symposium

map of northern California with shades of color depicting levels of habitat connectivity

On January 8, 2020, CDFW hosted a Wildlife Connectivity Symposium in northeastern California with support from Pew Charitable Trust. The purpose of the Symposium was to engage a diverse group of stakeholders within the focal area to: 1) gather data and develop a map of wildlife connectivity areas of interest based on scientific data, expert opinion, and stakeholder input, 2) identify partnership opportunities and funding sources to address connectivity needs; 3) Identify potential focal species that could be used in developing a fine-scale regional connectivity assessment; 4) Brainstorm criteria and explore parameters that could be used to develop a transparent and repeatable method to prioritize barriers for remediation that can be replicated in other parts of the state; and, 5) Develop recommendations for replicating the process statewide. Results of the symposium are being used to identify priority barriers for the implementation of wildlife crossings and to inform a fine-scale regional connectivity assessment.

Remediation or removal of fish passage barriers

As of 2020, there have been over 780, or roughly 12%, of fish passage barriers remediated or removed per the California Fish Passage Assessment Database. Several of these were culverts replaced by full channel-spanning bridges, which allow for both aquatic and terrestrial species migration. For example, in October 2014, Caltrans replaced a culvert in Siskiyou County on State Route 96 at Fort Goff Creek within the Lower Klamath sub-basin with a channel-spanning bridge (JPG) that allows for aquatic and wildlife connectivity. Before replacement, the culvert was a barrier to juvenile salmonids and for adult spawning fish due to high water velocities and shallow depths. After construction of the bridge, sixty-four chinook salmon were observed upstream. This location is within an essential connectivity area and potential riparian connection area. The project was funded by USFWS National Fish Passage Program, National Fish and Wildlife Foundation, CDFW Fisheries Restoration Grants Program, Caltrans, American Association of State Highway and Transportation Officials, and the Federal Highway Administration.

Grass Lake Wildlife Crossing, Highway 97, Siskiyou County

Rocky Mountain elk have been crossing Highway 97 near Grass Lake for many years, and there have been numerous accidents involving collisions with elk in the area. Grass Lake is a large ephemeral lake that fills in the winter and spring and attracts wildlife throughout the summer. In late 2019, Caltrans began collaborating with numerous agencies and partners to reduce wildlife mortality in the area by considering the installation of the first prefabricated fiber-reinforced polymer wildlife bridge crossing in North America. The Western Transportation Institute is designing the bridge and evaluating design options, various pros and cons of the available prefabricated bridges, and the best placement of the crossing that will move the most animals safely across the highway. This project was selected by Caltrans as a priority location for a dedicated wildlife crossing and should serve as a model for other dedicated crossings throughout the state.

North Central Region, Highway 89 Stewardship Team

deer emerging from metal corrugated undercrossing
An ear-tagged deer uses the flagship Highway 89 Stewardship Team's underpass at Kyburz Flat (photo courtesy of Highway 89 Stewardship Team)

In 1976, three bridge underpasses and a box culvert were placed under US 395 along the Hallelujah Junction Wildlife Area with wildlife fence and one-way gates. Four years later, a crossing was built on Highway 89 near Markleeville in Alpine County. For unknown reasons, that was the extent of such mitigation until 2002 when the Highway 89 Stewardship Team formed and partnered to construct three fenced underpasses with jumpouts between Truckee and Sierraville. Since then, the partnership and vision have continued within Caltrans District 3, resulting in a variety of wildlife crossings on US 50 in El Dorado County, Interstate 80, and Highway 193 in Placer County; Highway 49 in Nevada County; and plans for more on Highway 20 in Lake and Nevada Counties. In addition, a million-dollar project is underway on Sierra US 395 to continue the existing deer fence at the south end of Hallelujah Junction to the state line and replace all non-functional one-way gates with jumpouts. Repairs will be made to low or damaged fencing. A smaller augmentation of box culverts that existed under I 80 east of Truckee is in the works to add wildlife fencing to direct animals to the structures. The Highway 89 Stewardship Team has expanded to include US 395 and I 80 in partnership with the Nevada Department of Transportation and the Nevada Department of Wildlife, as well as Wildlands Network. All these crossings have been or are being monitored and have documented use by multiple species, including deer, bear, mountain lion, coyote, bobcat, and other birds and small mammals.