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OVERVIEW OF THE ECOREGIONAL PORTFOLIO*
As in other ecoregions, we adopted a 'coarse filter/fine filter' approach to selecting conservation targets. In the Chesapeake Bay Lowlands ecoregional portfolio, the coarse filter consisted of three types of large-scale targets: matrix forest blocks, aquatic ecosystems, and estuarine, coastal & marine targets assembled in 'significant conservation areas'. The fine filter consisted of natural (terrestrial) communities, and plant and animal species. A brief description of each of these target groups is presented below.
Matrix Forest Blocks: Areas of at least 10,000 acres and 60% natural forest vegetation cover that are relatively unimpacted by interior roads, agriculture or development. Blocks are presumed to be dominated by matrix-forming natural communities, with the spatial and temporal structure and composition of the forest determined primarily by regional disturbance processes. Matrix forest blocks are intended to act as coarse filters for common native species, and often contain embedded occurrences of smaller (large, linear and small patch) natural communities and rare species populations. Six broad types of matrix forest blocks were identified in CBY, based on surficial geology and landform/topography.
Aquatic Ecosystems: Generally, networks of freshwater stream segments within local watersheds that support diverse and viable communities of native aquatic species (vertebrates, invertebrates, plants), as a result of good water quality and good habitat quality in the adjacent upland or watershed. Occurrences may be of variable length, and may include mainstem stream segments alone, or mainstem segments plus adjacent tributaries. Different aquatic ecosystems are presumed to occur in the different freshwater system types - defined by geology, hydrology, chemistry and biology - found in the ecoregion. Freshwater systems in CBY were classified into 11 types, two tidal and nine non-tidal.
Estuarine, Coastal and Marine Targets: Species and habitat types common in, and significant for ecosystem functioning of, tidal waters and adjacent coastal uplands in the Chesapeake Bay and Atlantic coastal bays. Ten species and four habitat types were identified as conservation targets; habitat types included submerged types (e.g., SAV, oyster reefs), emergent types (e.g., tidal wetlands) and terrestrial types (e.g., dunes and beaches). Diverse aggregations of multiple, abundant targets, in water of good or high quality, were defined as 'Significant Conservation Areas' in CBY.
Natural (Terrestrial) Communities: Natural (terrestrial) community targets were set at the association level of the National Vegetation Classification and included all types known to occur in the ecoregion. Natural communities were categorized into four size groups: matrix-forming, large patch, linear patch and small patch, based on their typical size and the scale of the processes affecting their structure and composition. In CBY, 113 natural communities belonging to 18 vegetation groups were identified as ecoregional targets, with at least 38 others as potential targets.
Plant and Animal Species: Species targets were designated as Primary or Secondary targets, generally according to their global rarity. Primary targets included most globally rare (G1-G3G4) plants and animals native to the ecoregion, all federally listed species, and other special cases (e.g., significant disjuncts, rapidly declining species) where appropriate. Secondary species targets were generally G4 or G5 species that were in decline or otherwise vulnerable in the ecoregion. Only viable populations of Primary targets were explicitly included in the portfolio; the capture success of portfolio sites for occurrences of Secondary targets was assessed subsequently. In CBY, 58 species of plants and animals were identified as Primary targets, while 46 were identified as Secondary targets.
Portfolio Occurrences The complete Chesapeake Bay Lowlands ecoregional portfolio includes 20 matrix forest blocks, 51 aquatic ecosystem occurrences, 18 Significant Conservation Areas (SCA's) for estuarine, coastal and marine targets, and 274 sites for target plant and animal species and natural communities (Map 1, Table o1). Portfolio occurrences for each of the conservation target groups are described in more detail below.
The majority of the matrix forest blocks occur in Maryland, with three in Virginia and four in Delaware; three of the latter overlap into Maryland (Map 2). They range from 10,500 to 225,000 acres in size (966,000 ac total), averaging about 48,000 acres. They group into 6 general types based on an analysis of ecological land unit composition (below; Map 2). Thirteen of the blocks were selected as 10 Year Action sites (Map 2).
The aquatic ecosystem occurrences range from small, linear segments to large, local networks or lengthy stream corridors (Map 3). They occur throughout the three states, in each of the four Ecological Drainage Units, and in every freshwater system type. Thirty three of the occurrences were categorized as Tier 1, the highest quality, and 17 were categorized as Tier 2.
Significant Conservation Areas (SCA's) ranged in size from 1300 to 262,000 acres (1,276,986 ac total), and occur throughout the salinity gradient in the ecoregion, from freshwater (i.e., Susquehanna) to saline (e.g., Cape Henlopen, Lower Bay, Lower Eastern Shore; Map 4). Eleven SCA's fall all or in part in Virginia (including Nanjemoy and Blackwater/Bay Islands), while nine occur in Maryland and one occurs in Delaware.
Two hundred and thirty three viable occurrences of natural communities (Map 5), and 303 viable occurrences of plant and animal species (Map 6) were included in the portfolio, at 274 sites (i.e., places assigned a name by a state Natural Heritage Program). Almost 20% of these occurred within a matrix forest block (Map 1, Table o1). Including 36 of these within blocks, 99 sites for natural community and species targets were chosen as 10 Year Action sites.
Note that portfolio occurrences of several different conservation targets cluster together at a number of places in the ecoregion. Most notable are areas where matrix forest blocks, aquatic ecosystems, natural community and species occurrences occur together upstream of a Significant Conservation Area. These include Blackbird-Millington and Redden-Ellendale in Delaware, Nanticoke (DE and MD) and Nassawango in Maryland, and A.P. Hill and Dragon Run in Virginia. Occurrences of at least three conservation targets cluster together at numerous other places in the ecoregion (Map 1).