Briefing Paper, March 2003
Overview
The Regionally Significant Ecological Areas (RSEA) Assessment is a product of the Minnesota Department of Natural Resources, Central Region. The assessment covers the seven-county Twin Cities metropolitan area.
The assessment is a science-based, landscape scale, coarse-filter assessment that relies on satellite derived land cover and other region-wide databases. Its purpose is to identify areas with a high likelihood of having intact native plant communities and/or high quality native animal habitats for further site level analysis.
The assessment includes the development of GIS models to evaluate terrestrial and wetland areas based on:
- ecological principles
- land cover characteristics (size, shape, connectivity, species diversity, and compatibility of adjacent land uses)
Central Region DNR staff from Regional Operations, the Ecological Resources Division, and the Management Information Systems Division worked on the assessment as part of the region's biennial work plan, with advice from other DNR staff and partners from other agencies.
This document overviews the Terrestrial and Wetland Assessment. The Metropolitan Council provided GIS support staff and satellite land cover data for the RSEA (Terrestrial and Wetlands) Assessment. In addition the Metropolitan Council partnered with DNR Fisheries staff in the development of an aquatic assessment, which will be available in the future.
Definition of Terrestrial and Wetland RSEA
The assessment identifies places (referred to as Regionally Significant Ecological Areas) where intact native plant communities and/or native animal habitats occur.
RSEAs may include high quality native plant communities mapped by the Minnesota County Biological Survey (MCBS), which are defined by their most prominent features (vegetation, hydrology, landform, soil, and natural disturbance cycles) and form recognizable units (oak forest, prairie) across the landscape. These communities may or may not provide significant habitat for animal species.
RSEAs not meeting MCBS criteria are identified and evaluated based on land cover characteristics (size, shape, adjacent land use, connectivity, and native species diversity) and the habitat requirements of selected animals sensitive to habitat fragmentation.
RSEAs do not include areas dominated by intensively managed vegetation such as croplands, pastures, orchards, sod farms, and mowed grasses. Some RSEAs do include areas of managed vegetation, such as infrequently mowed grasslands.
Collectively, RSEAs:
- Provide habitat for game and non-game, including threatened, endangered, and special concern animals
- Maintain biological diversity
- Maintain connectivity
- Contribute to groundwater recharge and improved water quality
- Represent high to outstanding examples of native plant communities, or contain populations of state-listed rare plants, rare animals and/or animal aggregations mapped by the Minnesota County Biological Survey
Ecological Principles Informing the Model:
- Size of a patch is important (bigger is better)
- Shape of a patch is important (rounder is better)
- Connectivity is important
- Adjacent land uses are important (the more compatible the surrounding land uses, the better the chances for persistence)
- Higher native species diversity contributes to higher productivity and resiliency
Model Process:
- A hybrid land cover map was created by the DNR and the Metropolitan Council
- Ecological models were developed by the DNR
- Models were applied and integrated and all native plant communities mapped by the Minnesota County Biological Survey were incorporated
- Sites were evaluated against the land cover characteristics and ranked as 3, 2, or 1 (3=highest rank)
- Documented rare species occurrences were incorporated
- Riverine corridors for wildlife were mapped
Summary of Ecological Models:
Four sets of models were developed to map significant habitat. Literature reviews and expert opinion were used to select native animals that could serve as indicators of significant habitats. Minnesota County Biological Survey animal survey staff acted as primary consultants for this process. Habitats identified by the models harbor many other plants and animals in addition to the indicator species.
Forest Model:
The forest model included two components: 1) forest interior (core) and 2) riparian forest. The habitat requirements of 5 bird species, the red-eyed vireo, wood thrush, scarlet tanager, ovenbird and eastern wood pewee, were used to map forest core. Interior forest (core) patches were identified and ranked based on:
- Forest patch size (minimum patch size was 24 ha)
- Edge effect (edges, by definition not forest interior, were 120 m wide)
- Percent of total patch that was core
- Distance to a source patch (i.e., forest patch 100 ha or greater in size with more than 40% core)
- Additional forest areas at least 150 m wide and connected to a forest core patch were included for their habitat, buffer, and connectivity benefits
The habitat requirements of 3 birds, the cerulean warbler, Louisiana waterthrush, and the red-shouldered hawk, were used to map riparian forest areas (forests adjacent to streams). Riparian forest areas had to be at least 90 m wide and a minimum of 4 ha in size.
Grasslands Model:
This model included two components: 1) "tall grasslands," which are relatively large areas of unmowed grasses, both native and non-native; and 2) "dry tall grasslands," which are tall grasslands that occur on sandy soils and harbor a number of rare animal and plant species. The habitat requirements of the bull snake, pocket mouse, and Blanding's turtle were used to map dry tall grassland habitat.
Grasslands were identified and ranked based on:
- Size (minimum size for tall grasslands was 16 ha with a minimum width of 90 m; minimum size for dry tall grassland was 6 ha with a minimum width of 90 m)
- Maintained grasslands (i.e., infrequently mowed hayfields and pastures) at least 90 m wide and connected to tall grasslands 16 ha or greater in size were included for their habitat, buffering, and connectivity benefits
- Compatibility of adjacent cover types; adjacent wetlands and open water increased patch score because they may provide valuable habitat for many species (amphibians, waterfowl, and small mammals)
- Incompatible adjacent cover types, such as forest and impervious surface
Wetlands Model and Wildlife Lakes:
While many wetlands are regulated under state and/or federal laws, this model evaluates wetlands on three characteristics: 1) connectivity to uplands and other wetlands, 2) diversity of upland cover types associated with the wetland, and 3) size diversity.
Wetlands were identified and ranked based on:
- Isolated wetlands (minimum size 10 ha)
- Wetland complexes (3 or more wetlands connected by natural vegetation within 150 m of a wetland; minimum wetland size 60 ha, including the natural vegetation within the 150 m zone)
- Wetlands associated with tall dry grasslands (minimum wetland size of 8 ha)
- Wetland forest complexes (0.5 to 4 ha wetlands adjacent to forest and connected by forest to a wetland 10 ha or larger and not more than 200 m away)
- Wildlife lakes, identified by the DNR's Wildlife Division Shallow Lakes Program as significant for wildlife, were also included (minimum size 20 ha)
Riverine Corridor Model:
The riverine wildlife corridor model identifies the shortest path between two or more RSEAs using rivers and streams as natural, linear, landscape features for movement. Terrestrial and aquatic routes were identified.
Terrestrial routes are the shortest path between two or more RSEAs that follows the banks of rivers, streams, lakes and wetlands. Terrestrial routes have a minimum width of 150 m. When adjacent to a river, lake, or wetland, the width was measured 150 m from the shoreline of the water body. When the terrestrial route was along a stream, the width was measured 75 m from the center of the stream for a total width (including the stream) of 150 m. The model assumption was that streams could be crossed by terrestrial species.
Land cover within terrestrial routes included natural (shrubland, forest, grassland) and semi-natural (maintained tall grasslands, short grasses, and agricultural land) vegetation. The lack of a 150 m natural or semi-natural vegetation zone around open water features (river, lake, or wetland) or a high concentration (>10%) of impervious surface within the route was considered a barrier to movement and the route was eliminated.
Aquatic routes connect two or more wetland RSEAs using rivers, streams, lakes, and wetlands that could serve as movement corridors for aquatic species. The width of aquatic corridors was variable. Any lake or wetland that intersected a river or stream connecting two or more wetland RSEAs was included as part of an aquatic corridor.
This model does not rank riverine corridors, nor does it evaluate RSEAs based on the presence or absence of a riverine corridor.