Fragmentation is caused by natural disturbances or land-use changes that divide previously contiguous landscapes into separate fragments (habitat patches). As a result of fragmentation, fluctuations in and movements of solar radiation, wind, water and nutrients across a landscape are significantly altered (Convention on Biological Diversity 2004). Direct threats to wildlife resulting from fragmentation are reduction in the size of individual habitat patches, isolation of habitat patches, and creation of barriers to species movements (Noss and Cooperrider 1994, Pringle 1997, Armantrout 1998, Morrison et al. 1998).
Fragmentation in Terrestrial Systems
Reductions in habitat patch size can be detrimental to species requiring large areas of similar habitat, such as Scarlet Tanagers, Wood Thrushes, Eastern and Western Meadowlarks, and American marten (Robbins et al. 1989, Rosenburg et al. 2003). Fragmentation can also influence the size of a landscape feature patch necessary to support species. For example, studies of Scarlet Tanagers found that in landscapes with 70% forest cover, the minimum habitat patch size for breeding was 66 acres. In landscapes with 40% forest cover minimum habitat patch size for breeding increased to 605 acres (Rosenberg et al. 1999). Smaller isolated habitat patches have increased edge to interior ratios and have been found to increase predation rates and brood parasitism, and reduce pairing success for some birds (Gates and Gysel 1978, Faaborg, et al. 1993).
For less mobile species, fragmentation may result from a spatial disturbance as minimal as placing a road through a home range area, separating an animal from the resources it needs. Telemetry studies on massasauga rattlesnakes in southern Michigan indicate a high reluctance to cross an asphalt-paved access road recently constructed through an area frequented by the snakes (Kingsbury et al. 2004). Isolation of populations of less mobile species will lead to reduced breeding potential, disruption of dispersal patterns, and diminished genetic variability (Whitcomb et al. 1981, Kingsbury and Gibson 2002).
Fragmentation also poses significant threats to terrestrial ecosystems and landscape features by disrupting necessary ecological processes. The construction of a drain through a lakeplain prairie will lower the water table, disrupting the natural hydrologic regime and altering periodic disturbance created by seasonal flooding (Albert and Kost 1998). The lack of disturbance will allow invasion by woody plants (Hayes 1964, Roberts et al. 1977).
Michigan's Environment and Relative Risk report (DNR 1992), prepared by the Michigan Environmental Science Board, identified lack of land-use planning as one of the greatest threats to the State's environment. Changes in land-use that are poorly planned, or not planned at all, drastically increase fragmentation of landscapes and disrupt ecosystem integrity.
Fragmentation in Aquatic Systems
In aquatic systems, the most obvious effects of fragmentation are seen in highly migratory species (Edwards 1978, Holden 1979). However, even non-migratory fish species need to move over large areas to use different kinds of landscape features for various reasons (e.g., reproduction, over-wintering, nursery; Schlosser 1993, Schlosser 1995a, Fausch et al. 2002). Similarly, other aquatic organisms also require connectivity across larger scales and experience population declines when isolated (Watters 1996). Human-induced fragmentation of aquatic systems is common throughout Michigan and includes dams, culverts that create velocity, jump, or exhaustion barriers for aquatic organisms, thermal pollution discharge, channelization and hardening of rivers and streams, and stream enclosures.
Dams fragment and alter most of a river's ecological processes by altering flow regimes, sediment transport and distribution, nutrient cycling and availability, energy inflows, and biota (Ligon et al. 1995). Dams transform long river reaches into impoundments with shorter riverine reaches and cause streambed degradation immediately below the dam (Kohler and Hubert 1993; see Dams for more information.)
Like dams, improperly constructed stream crossings may alter water depths and velocities and limit or prohibit passage of aquatic organisms. For example, water flow constricted through an undersized structure will often impound upstream water and increase downstream flow velocity and turbulence. These changes can erode a streambed below the fixed elevation of the outlet, creating a physical barrier. Other causes of partial or total barriers to upstream fish migration at stream crossings may include sediment accumulation in the crossing structure, insufficient water depth, debris collection and ice accumulation.
Numerous industries discharge cooling water into rivers and lakes throughout the state. Discharge of heated water may create thermal barriers for aquatic organisms throughout the entire year or during specific seasons. Aquatic organisms may avoid these areas because temperatures may not be in their preferred temperature range; some species may suffer mortality from thermal shock. These differences in thermal temperature effectively act as barriers to dispersal. Similarly, degradation of water quality through contaminants and eutrophication can create stream reaches that are impassable to many aquatic organisms (Pringle 1997).
Conversion of a natural stream channel to a trapezoidal shape or hardened channel can fragment the aquatic environment by creating velocity barriers, reduced depths and increased distance, which result in exhaustion barriers for aquatic organisms.
Enclosure of a stream results in a loss of ecological connectivity and creates an ecological barrier. An enclosed stream segment is isolated from natural inputs of sunlight, groundwater, surface water and organic terrestrial material. The absence of sunlight deters movement of some fish into or through enclosures (Pinkham 2000). Enclosures also alter channel processes and change adjacent channel character and shape by affecting flow path and velocity of water, movement of debris, sediment, and flood flows.
Conservation Needs to Address Fragmentation Threats:
Many of the following conservation actions, research and monitoring were patterned after recommendations in the 2003 Michigan Land Use Leadership Council Report (prepared for Governor Jennifer Granholm and the Michigan Legislature) that were designed to minimize negative effects of current and projected land-use patterns on Michigan's environment and economy.
Land & Water Protection
- Develop statewide land-use goals
- Protect identified large tracts and systems
- Provide assistance and incentives to private landowners to conserve larger habitat patches and corridors
- Consolidate ownership of high quality natural assets by eliminating in-holdings
Land, Water & Species Management
- Incorporate identified areas of high biological significance into local, regional and statewide planning and management efforts
- Improve coordination between natural resource conservation organizations and transportation planners at local, regional and statewide scales
- Address thermal pollution issues to eliminate or minimize fragmentation
- Promote closure of non-essential resource management roads and seek other road closure opportunities that do not conflict with appropriate uses
- Incorporate identified linkage areas (between isolated patches of priority landscape features) into local, regional and statewide planning and management efforts
Law & Policy
- Initiate local, regional and statewide planning efforts or regulations that encourage retention of larger landscape blocks and promote establishment of corridors between landscape fragments
- Improve collection, analysis and access of data used for local, regional and statewide land-use and transportation planning
- Enhance ecosystem planning efforts by coordinating management of adjacent public and private lands
- Minimize new dam construction, including lake-level control structures, and promote removal of old dams that have exceeded their useful life
- Incorporate passage facilities at dams to increase movement of aquatic organisms
- Incorporate best management practices in construction, repair and replacement of stream crossings
- Promote policies and requirements for drainage and channel modification practices that ensure stream form, function, continuity and aquatic organism passage are maintained
- Implement requirements of the Biological Diversity Act, Part 355 of the Natural Resources and Environmental Protection Act (Act 451, P.A. 1994), including interdepartmental coordination requirements
- Provide example and model ordinances/regulations for local, regional and statewide planning efforts
Education & Awareness
- Educate the public, land-use planners, transportation planners, planning commissioners and local government officials about the consequences of habitat fragmentation
- Improve accessibility of information to local, regional and statewide land-use and transportation planners
- Initiate and support programs that identify the importance of the natural environment and how it contributes to economic prosperity and the quality of life of all citizens
- Provide training and education in the means and methods of managing land-use change, transportation systems and community development at multiple scales
- Provide information to private landowners on opportunities to support land conservation, including management, easements, land donations and local or State land conservancy purchase
- Develop initiatives that promote urban green space and other activities to encourage repopulation of urban areas and reduce population out migration
- Initiate and support programs that explain the role and value of protecting particular landscapes (e.g., wetlands protection, natural river designation, critical dune protection), as well as the role of other local, regional and statewide environmental land-use programs, in protecting and enhancing natural environments
Research, Surveys & Monitoring
- Develop science-based land-use goals
- Identify large tracts and systems to target for protection
- Identify areas of high biological significance
- Identify lands that serve as important linkages between isolated patches of priority landscape features
- Test the effectiveness of local, regional and statewide planning efforts in retention of larger landscape blocks and corridors
- Test the assumption that remnant natural landscape features are widely dispersed and becoming more fragmented, resulting in a loss of species diversity
- Develop and test techniques for passage of aquatic organisms at dams
- Identify practices that fragment stream systems and develop best management practices and policies to prevent and reverse fragmentation
- Conduct research to determine whether linear landscape features act as corridors for native species, reducing negative effects of fragmentation
- Conduct research to determine whether mosaics of similar features (e.g., prairie, hayland, old field) have the same wildlife value as similarly sized patches of a single feature