Constraints to Connectivity
Overview of Constraints to Connectivity
Climate Change
In the context of a changing climate, barriers to animal movement can include non-physical features such as an altered range of temperature that is outside the species' temperature tolerance (Nunez et al. 2013), as well as secondary effects such as a perennially flowing stream segment becoming intermittent or water level decreasing to the point it presents a barrier. Increased erosion and turbidity resulting from climate change effects could also interfere with aquatic connectivity . Additionally, longer term climate change may cause drastic changes to habitat structure and composition such that natural areas currently amenable to species movement will become barriers, although Opdam and Wascher (2004) found that insects on the leading edge of their range were adapted to a broader range of habitats. Such information is rare but would be highly useful for determining the degree to which different habitats may act as barriers.
Terrestrial Landscapes
Depending on the organism or system in question, different features on the landscape – either natural or man-made – may act as partial impediments or complete barriers to species movement or landscape permeability. What may act as a barrier to one species may not impede the movement of another. There may be a tendency to assume that natural features or habitats are crossable and anthropogenic features (e.g., roads, developed areas) or habitats (e.g., old fields) are not. However, habitats and landscape features that are “natural” (e.g., rivers, mountain ranges, large areas of open habitat) may not be traversable by some species. Similarly, some anthropogenic infrastructure (such as low-volume roads) may be readily traversed, depending on the species. For example, the gray wolf may travel through semi-natural habitat or semi-developed areas to reach suitable habitat, while the American marten will avoid even clearcuts or large openings within a forested matrix.
Some features may not act as complete barriers, but instead cause avoidance behavior. For example, noise associated with gas operations is avoided by some species. Although wind towers and transmission lines do not create true barriers on the landscape, sage grouse are documented to avoid these for significant distances. Such features can create a zone of avoidance that, in conjunction with physical barriers or unsuitable habitat, create pinch points or otherwise constrain the species movement across the landscape.
In addition, a species may not be inhibited from attempting to navigate some features, but may experience higher mortality as a consequence. Artificial lighting attracts and disorients many bird species during migration and causes increased mortality. Similarly, many terrestrial animal species will cross roads and experience increased mortality as a result.
Some features may mitigate the impact of barriers and should also be incorporated in a connectivity assessment if possible. For example, culverts, bridges, pipes, and other structures may mitigate the impacts of roads as barriers for many terrestrial species. However, it is important to determine what types of structures the species of interest will readily use; a given species may be limited by its size and preferences to a particular type of culvert.
Aquatic Ecosystems
Freshwater ecosystems are spatially discrete components of a larger landscape and inherently constrain the movement of aquatic species to the water body. In aquatic systems, connectivity is most often considered for native fish species or other organisms that may travel or disperse up or downstream within a stream network. In such constrained systems, connectivity is also a critical consideration for the spread of aquatic invasive species. While the system is inherently constrained, and the modeling tools and data sets are different, the step of identifying which natural or anthropogenic features eliminate or reduce connectivity for aquatic species is still the same. Dams and improperly built or maintained stream culverts commonly reduce or eliminate connectivity for many fish species. Aquatic system characteristics such as turbidity and other pollutants, water temperature, or flow volume may create unsuitable habitat and limit the movement of species to varying degrees; however, such characteristics may not be sufficiently mapped in order to assess their potential impacts on connectivity.
Structural Connectivity/Landscape Permeability
When considering general habitat connectivity, or landscape permeability, anthropogenic infrastructure (such as roads, railroads) and land uses (such as urban/suburban development and row crop agriculture) that significantly alter or remove natural habitat are treated as “barriers.”