Natural system modifications are defined as threats from actions that convert or degrade habitat in service of “managing” natural systems, often to improve human welfare (Salafsky et al. 2008).  

Fire Suppression 

One example of a natural system modification is the suppression of fire. Natural, lightning-caused fire is a dominant force in the western and midwestern portions of the U.S. that naturally perpetuates several community types, most notably prairies and savannahs. However, in the eastern U.S., lightning-strike fires have become rare events. Historically, fire disturbance in this region was primarily the result of Native Americans employing fire as a land management tool. Selective burning employed by Native Americans likely helped create and maintain fire-dependent habitats, especially fire-regenerated species such as those found in coastal pine communities. Fire suppression in Delaware’s natural systems has major impacts on SGCN through alteration of habitat structure and species composition.  

Dams and Water Use 

Another natural system modification identified as a threat to many aquatic and wetland habitats is water management. This broad category includes dam construction, surface water diversion, withdrawals from surface water and groundwater sources, and other operations that alter water flow patterns from their natural range of variation either deliberately or as a result of other activities.  

The American Industrial Revolution produced may mill dams and a long history of water management in Delaware. Of particular concern to fish and wildlife resources is the construction of dams on major rivers that have prevented the migration of anadromous fish to inland breeding locations, while also impeding the inland flow of tidal waters resulting in the nearly total loss of the tidal marsh communities.

Delaware has 83 dams included in the U.S. Army Corps of Engineer’s (USACE’s) National Inventory of Dams (NID). Seventy-eight of Delaware’s NID dams are less than 25 feet high, and all but two are of earthen construction.

Many small dams too small to be state-regulated or included in the NID are present throughout the state. Small dams are especially prevalent along Piedmont streams, where they block passage of diadromous fish, mussels, and other aquatic organisms and contribute to degradation of in-stream habitat by storing sediment. In 2014, the first dam removal in Delaware was conducted. A 40-foot portion of the historic Byrnes Mill Dam on White Clay Creek was removed in late 2014, the first of a number of obsolete, low dams on the White Clay Creek slated for partial or total removal. This removal allowed access to 3.5 miles of upstream reaches by diadromous fish (including Striped Bass, American Shad, Hickory Shad, and River Herring) for the first time since the 18^th century. Numerous dams are present upstream in the Delaware reach of White Clay Creek, Brandywine, and Red Clay creeks. Delaware’s many dams should be evaluated to determine feasibility of removal for those that no longer serve their intended purpose and for which removal would benefit SGCN. 

Surface and Groundwater Withdrawals 

Entrainment and impingement in cooling water withdrawals on the Delaware River are known major sources of mortality for many aquatic species, including several SGCN such as Striped Bass (Morone saxatilis) and Weakfish (Cynoscion regalis), as well as important baitfish such as Bay Anchovy (Anchoa mitchilli) (Kahn 2008). 

Groundwater withdrawals have the potential to deplete aquifers important to wetland system recharge. Simulations of water level reductions in Coastal Plain seasonal pond wetlands suggest that sustained reduction of water levels will result in vegetation shifts, notably encroachment of surrounding forest into these systems (Laidig 2012). Additionally, field observations have confirmed that impacts to groundwater hydrology associated with these systems have resulted in closed canopies and the loss of rare plant species (McAvoy pers. comm.). Other habitats potentially highly impacted by groundwater depletion include karst formations and seepage habitats. 

In addition, excessive groundwater withdrawals in coastal aquifers can exacerbate saltwater intrusion (Sophocleous 2002). The Delaware Geological Survey (DGS) currently monitors groundwater levels in a network of more than 123 wells in Delaware.  

Beach Nourishment 

Beach nourishment is a frequently used technique in Delaware for beach erosion control and widening. Beach nourishment can have dramatic effects on beach invertebrates and their predators that last for at least 4 years post-disturbance (Peterson et al. 2014). Brown (2009) found that beach nourishment affects establishment of Sabellaria tubeworm reefs. Beach nourishment in Delaware typically occurs outside of a time of year restriction for shorebird migration and horseshoe crab spawning, reducing the immediate effects on those species. However, effects are not limited to the beach itself but also include the borrow site and nearby areas of the water column.

On the other hand, nourishing beaches helps provide greater resilience of these habitats to sea level rise and coastal storms and may ensure the continued existence of beach habitat in areas where development precludes inland migration of the beach. In such areas, all beach habitat may be lost to erosion and sea level rise in the absence of beach nourishment.

Saltmarsh Management 

In Delaware, saltmarsh management often focuses on mosquito control or on marsh restoration for enhancing wildlife habitat and ecosystem function. Open Marsh Water Management (OMWM) is a highly effective (James-Pirri et al. 2009) larval source-reduction mosquito control practice that involves selective installation of small, shallow ponds and interconnecting ditches that allow tidal flow and movement of mosquito-eating fish between potential mosquito breeding pools in high marsh areas (Meredith et al. 1985, Lesser 2007). The history and extent of OMWM in Delaware is provided in Meredith and Lesser (2007). The primary rationale for the adoption of OMWM practices is to reduce the need for application of chemical larvicides and adulticides in these systems. Integrated Marsh Management (IMM) is a more holistic approach to managing saltmarsh mosquito populations that considers multiple potential benefits and impacts of OMWM and saltmarsh restoration practices together in relation to saltmarsh restoration, wildlife habitat, and controlling invasive plant species, instead of only focusing on mosquito control or marsh restoration (Rochlin et al. 2012b, Wolfe et al. 2022).  

Several studies have indicated positive effects of OMWM practices on the use of marshes by waterbirds and fish, especially when implemented in degraded sites where the creation of ditches restores tidal hydrology (Rochlin et al. 2012a, Rochlin et al. 2012b). Some alteration of vegetation on OMWM managed sites may include reduction of S. alterniflora short form and S. patens (Mitchell et al. 2006, James-Pirri et al. 2012) and it has been suggested that this may impact obligate saltmarsh birds (Mitchell et al. 2006). In a study conducted in Delaware, relative abundance of Seaside Sparrows (Ammodramus maritimus) was 2.5 times greater on “limited” OMWM sites than on “extensive” OMWM sites, but this was the only species that differed significantly in relative abundance between the two treatment levels (Pepper et al. 2010). Seaside Sparrow territory density and nesting density was also 2 times greater on “limited OMWM” plots than “extensive OMWM” plots (Pepper et al. 2010). This study used a semi-quantitative scoring system to account for the relative amount of OMWM and age of OMWM system across multiple sites. Future studies employing long-term monitoring and Before-After Control-Impact (BACI) designs would help further assess the effects of OMWM installation on marsh fauna.  

James-Pirri et al. (2012) found shifts from fish-dominated to shrimp-dominated nekton and modest changes in bird abundance (primarily an increase in fall use by SGCN American Black Duck) as measured by point count at Delaware sites. Vincent et al. (2015) found significant differences in trophic structure and fish growth for the SGCN fish Mummichog (Fundulus heteroclitus) between ditch-plug ponds and natural saltmarsh ponds. Rochlin et al. (2012b) found a shift from shrimp-dominated nekton to fish-dominated, particularly salt marsh killifish species, in areas where IMM techniques were used. The effects of OMWM and IMM manipulations on SGCN are complex and vary among sites and treatments. Some studies of OMWM and IMM have documented no significant changes in vegetation community or waterbird use (Raposa et al. 2019), while others have shown positive changes in marsh vegetation such as a reduction in the invasive Phragmites australis and an increase in Spartina patens (Besterman et al. 2022, Rochlin et al. 2012b). Further research and coordination are warranted to ensure that OMWM and IMM practices are utilized in Delaware with maximum benefit to SGCN while minimizing potential negative impacts to SGCN and contributing to mosquito reduction goals. 

Wetland Alteration/Drainage and Stream Channelization 

Wetland drainage, especially of isolated freshwater wetlands, has severe effects on many wetland SGCN. The system of tax ditches maintained throughout Delaware was created for the purpose of facilitating the drainage of wetlands for agriculture. These systems are now largely degraded due to channelization, frequent dredging, and poor water quality. 

In northern Delaware, urban and industrial activities have caused extensive habitat modification. Erosion, urban development, dredging, filling, and bulkheading have eradicated many wetlands and continue to have an impact on those that still exist. Shoreline hardening for bank stabilization is also an issue in the Inland Bays and Nanticoke watersheds. 

Many efforts to restore degraded wetlands and streams have been undertaken in Delaware during the past several decades. An example of success is The Northern Delaware Wetlands Rehabilitation Program that was established by DFW in the 1990s to bring together civic and business leaders, scientists, resource managers, and property owners to develop strategies to restore nearly 10,000 acres of wetlands at 31 distinct sites along the Christina and Delaware rivers in New Castle County (Hossler 1994). Many of these sites have had successful restoration of hydrology, but funding challenges must overcome in order to ultimately meet the goal of restoring all wetlands originally targeted. 

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