Assateague Island Research Articles

Niche Overlap and Competition in an Insular Small Mammal Fauna: A Test of the Niche Overlap Hypothesis

The niche hypothesis states that maximal tolerable should vary inversely with the of competition. Rigorous testing of this hypothesis has been complicated by problems of measurement and by the difficulty of formulating operational definitions of maximal tolerable overlap and intensity of competition. We test this hypothesis with direct estimates of niche and competition for the species of a diverse insular small mammal fauna. We recorded multiple captures of six species during 9296 trapnights at 1162 trap stations on Assateague Island (Maryland, USA) during the summer of 1978. To describe the structural niche (microhabitat) of each species, we measured nine habitat variables at each trap station where an animal was captured. Using data on population densities and microhabitats, we estimate pairwise niche and competition with discriminant analysis and multiple linear regression analysis, respectively. We report a positive correlation between niche and competition, rather than the negative relationship predicted by the hypothesis. The negative correlation between niche and species diversity reported for previous tests of the hypothesis may reflect a decrease in the average strength of interaction between species as diversity increases, rather than a competition-mediated decrease in niche overlap.

Recreational impacts on the distribution of ghost crabs Ocypode quadrata fab.

Studies were conducted at Assateague Island, Maryland-Virginia, to determine the relative number of ghost crabs Ocypode quadrata Fab. on beaches subject to different recreational uses. The mean density of crabs per 0·1 ha plots was found to be 10 on an undisturbed beach, 19 on a pedestrian-impacted beach, 1 on a light off-road vehicle (ORV)-and pedestrian-impacted beach, and 0·3 in a heavy ORV-use beach. ORVs could be adversely affecting the crabs directly by crushing or burying them or indirectly by interfering with their reproductive cycle or altering their environment. Vehicular disturbance probably results in fewer crabs or no reproduction at all, with new inhabitants migrating from undisturbed areas. Pedestrians appear to have no harmful effects on ghost crabs; instead the crabs may be capitalising on the food scraps scattered across the beach by bathers.

Shoreline Periodicities and Linear Offshore Shoals

Linear shoals off the Delmarva coast are believed to have evolved on the shoreface of Assateague Island in response to inshore processes. Analyses of shoreline rate-of-change data and the spatial distribution of the linear shoals indicate several spatial periodicities. The shoreline periodicities correspond to hypothetical standing edge waves. The linear shoals, however, do not have wavelengths consistent with either the shoreline periodicities or the edge waves. The strongest periodicity in the shoal field has an orientation parallel to relict beach ridges on Assateague Island dated ca 2,000 BP. Thus, the linear offshore shoals may have formed at a time when Assateague Island was shorter and edge waves of a different wavelength were present. Where sedimentary shorelines are strongly concave, such as the 123 km Arc between Cape Hatteras and Cape Lookout, inshore currents associated with standing edge waves seem to inhibit the development of linear shoals. Where the shoreline is convex, such as along the Delmarva coast, currents associated with standing edge waves are less prevalent and thus less likely to inhibit the development of linear shoals.

Vegetation changes along the United States east coast following the great storm of march 1962

Regional-scale vegetation changes on Assateague Island following overwash from the Ash Wednesday Storm of 1962 were investigated using historical aerial photography. Grasses and dunes were more abundant in 1974 in areas that were overwashed, whereas areas that were not overwashed have been more extensively colonized by shrubs. The increase in shrubs on Assateague Island is attributed to the absence of a subsequent storm overwash.

An Inexpensive Suspended Sediment Sampler

This inexpensive and simply operated device can be employed whenever nearly simultaneous samples in a vertical water column are required. This design also allows for the detection of relatively high frequency concentrations of suspended sediment. The sampler has been successfully used in overwash studies at Assateague Island, Maryland, and is presently being employed at Fire Island, New York, for swash research. It is envisioned that this device would have application to other scientific fields for collections of water or sediment samples under similar conditions and data constraints.

Barrier Island Configuration

The 11 Virginia barrier islands are undergoing rapid changes in shore-line configuration. If this trend continues for another 100 years, two capelike features will develop. The process responsible for this island-chain pattern may be a standing edge wave trapped between Assateague Island and Cape Charles.

Barrier Island Configuration

The 11 Virginia barrier islands are undergoing rapid changes in shoreline configuration. If this trend continues for another 100 years, two capelike features will develop. The process responsible for this island-chain pattern may be a standing edge wave trapped between Assateague Island and Cape Charles.

Interaction Between Overwash and Eolian Processes on Migrating Barrier Islands: ABSTRACT

Landward barrier-island migration is accomplished by three processes: (1) inlet dynamics, (2) overwash, and (3) eolian transport. Although these processes are well understood conceptually, few studies have been designed to define their relative roles and thus determine the actual mechanics of barrier-island migration in recent times. From field surveys of sites on Nauset Spit, Cape Cod, Massachusetts, and Assateague Island, Maryland, an interaction between the two subaerial sediment transport processes can be recognized. Overwash surges during storm conditions deliver fairly large quantities of sand each year (often exceeding 10 cu m of overwash deposition per meter of dune breach). A large part of this material is then redistributed by the wind, eolian transport being largely governed by the winter northwest (offshore) winds. The net result at Assateague Island is the transport of most of the sand back to the beach face. A small part (less than 10%) of the overwash sand is deposited on the backside of the primary barrier dunes. Although this amount of accretion may seem quantitatively insignificant, this sand may serve as the major source of material to the dunes for their landward translocation concurrent with the migration of the island. This same general model can be applied to the Cape Cod barrier beaches except that drift-line deposits can initiate dune development on the washover fans. These studies can be applied to barrier-island management as well as improving End_Page 486------------------------------ our basic understanding of barrier-island dynamics. End_of_Article - Last_Page 487------------

Migration of Assateague Island, Maryland, by inlet and overwash processes

The northern part of Assateague Island, Maryland, has a history of rapid shoreline erosion, with washovers much in evidence. A comparison of aerial photography shows that the greatest island widths and highest rates of landward migration are associated with inlet dynamics. The over-wash process, at maximum transport conditions in this sand-starved area, is effective only in maintaining the island as a low, narrow barrier.

Freshwater reserves of mid-atlantic coast barrier Islands

A multiple regression model was constructed for the purpose of predicting barrier island hydrology from easily measureable island characteristics. The model was developed using data obtained from 17 sites on the Outer Banks of North Carolina. The accuracy of the model for predicting key hydrologic variables was evaluated by statistical and graphic procedures. In general, agreement between observed and predicted values of the hydrologic variables was very good, suggesting that the quantity of potable water at various island sites can be estimated without resorting to extensive field investigations. The model was then applied to Assateague Island, a barrier island located off the coasts of Maryland and Virginia. Results indicate that the original model developed for the Outer Banks may be applied to other barrier islands but that errors involved may necessitate corrections in detailed studies. Correction for bias in predictions for Assateague was shown to be possible with limited field data from surface resistivity surveys.

Overwash sedimentation associated with a large-scale northeaster

The December 1, 1974 northeaster was a significant event in terms of sediment transport with 20 m 3 of sand per meter of dune breach being carried onto the backdune area of Assateague Island, Maryland as overwash. Previous investigators have reported larger transport rates for landfall hurricanes, but this is the largest amount recorded for a winter northeaster. The Ash Wednesday Storm, March 6–8, 1962, was a much larger event, but no quantitative data exists for overwash deposition. The resulting sedimentary deposit can be interpreted by defining sedimentation units. Inverse sediment grading was predominant, but normal grading occurred when the material was monomineralic and the unit totally depositional in nature. The timing of the storm surge was recorded in some sections as a distinctive heavy mineral enrichment in the sediment.

Impact of Hurricane Belle on Assateague Island washover

The occurrence of Hurricane Belle on August 9, 1976 supplied additional data on the role of overwash on Assateague Island. Approximately 19m 3 of sand per metre of washover centerline (oriented normal to shoreline) was deposited at the survey site as a result of the storm. The major source of sand appears to be the beach and near-shore regions as opposed to the foredunes adjacent to the washover throat, because there is a lack of measured dune erosion. Unlike most winter storms, there was no concurrent offshore wind to deflate this deposit as the storm subsided. However, strong offshore winds in January 1977 eroded some 16 m 3 /m from this centerline and redeposited this sand on the beach. This study suggests that overwash of the magnitude experienced during storms of less than major proportions may not be processes for significant long-term sand accumulation to this barrier island.

BARRIER ISLAND DYNAMICS: OVERWASH PROCESSES AND SOLIAN TRANSPORT

The northern 5 miles of shoreline at Assateague Island, Maryland are presently being eroded. During storms, swash surges are able to overtop the most landward (storm) berm as overwash with deposition occurring on the barren flats. Where primary barrier dunes still exist, sediment-charged surges are funneled through breaches in the dune field for deposition of the entrained material on the washover fan. Sediment budget computations show that there has been a small net loss of material at each washover area, in spite of 7 discrete overwash events during a 26 month time interval. The predominant northwest winds effectively eroded the overwash material, transporting the majority of the sand back to the beach. This analysis indicates that there exists a balance between overwash and eolian processes with wind transport being slightly dominant.

Sediment Sources and Sedimentation Rates, Chincoteague Bay, Maryland and Virginia

ABSTRACT Chincoteague Bay receives an estimated 90,000 m3 of sediment annually, of which roughly half is sand and half mud. Sand is derived principally from the barrier, Assateague Island, with storm overwash being twice as effective as eolian transport in supplying sand to the bay. The mainland of Delmarva Peninsula is the primary source of mud-size sediment for the bay. Annual contribution of mud from shore erosion is approximately eight times that introduced by streams. Sediment transported from the Atlantic Ocean through the two active tidal inlets is important only in the immediate vicinity of the inlets. Present average rate of sedimentation in Chincoteague Bay estimated from annual sediment supply is 0.3 mm/yr. This is far less than the rate of 1.5 mm/yr, obtained from sediment thickness in borings in the bay and believed to represent average conditions over the past 5,000 years. The present anomalously low sedimentation rate and accompanying decline in salt-marsh growth are attributed to a decrease in ocean-derived sediment caused by the closing of former tidal inlets through Assateague Island. If this interpretation is correct, it underscores the wisdom of a recent decision against dune stabilization for prevention of storm overwash on Assateague Island. Overwash is a major means of providing sediment for landward accretion of barrier islands in response to relative sea-level rise. Elimination of storm overwash in the absence of sediment influx from the sea via tidal inlets almost certainly would have resulted in long-term net erosion on the lagoonal shore of Assateague Island.

OVERWASH PROCESSES ON ASSATEAGUE ISLAND

The primary barrier dune on the northern portion of Assateague Island, Maryland is presently being scarped on its seaward face and breached by storm-generated surges. During storms, sediment-ladened water moves across the dune line onto the barrier flats as overwash. The objective of this project is to determine the role overwash plays in barrier island sedimentary dynamics. An overwash model has been suggested. The nonvegetated overwash fan serves as a reservoir for the eventual distribution of the storm-deposited sand. Eolian processes, after the storm, determine the net contribution of overwash sand to each of the different morphological features, i.e., dunes, marsh, beach, etc.

Occurrence of American Eels, Anguilla rostrata, in Waters Overlying the Eastern North American Continental Shelf

Eleven reproductively maturing specimens of the American eel, Anguilla rostrata, were collected during three independent off-shore trawling operations. Three females were taken on December 5, 1967 southeast of the mouth of Chesapeake Bay in 10–13 fathoms, one male and one female on November 5, 1969 southeast of Cape Cod in 35–45 fathoms, and six females east of Assateague Island on December 22, 1971 in 5 fathoms. Morphometrical analysis showed that the specimens were within the range of "silver" phase of Anguilla rostrata. Gonadal observations were made on all specimens.

Autumn Migrations of Peregrine Falcons on Assateague Island, 1970-71

Autumn Migrations of Peregrine Falcons on Assateague Island, 1970-71

Checklist of Mammals of Assateague Island

Twenty-five mammal species are listed from Assateague Island, off the Atlantic Coast of Maryland and Virginia, of which 11 are native land species, 6 are feral or semi-feral forms introduced by man, and 8 are marine. The land species are discussed in relation to their habitats which include coastal dunes, fresh and salt water marshes, swamps and forests.

The Common Blackfish in Virginia Coastal Waters

Early in May, 1956, a single blackfish came ashore on the outer beach at Fishing Point, Assateague Island, Accomack County, Virginia. It is not known whether the whale was alive or dead when it stranded. As shown by colored photographs taken at the time, the whale was in good condition when it was discovered by Mr. Jacob Valentine, Manager of the Chincoteague National Wildlife Refuge. On May 15, with the assistance of Mr. Valentine, the animal was photographed, measured, and the skull secured for the U.S. National Museum. This specimen has been identified as Globicephala melaena Traill, and it proves to be the first valid record of the species from Virginia waters. Although current literature gives the range of G. melaena as extending south in the western Atlantic to the coast of Virginia, the coast of New Jersey has until now been the southernmost valid locality. Dr. Charles O. Handley, Jr., U.S. National Museum, has pointed out to me that all specimens from Virginia, recorded as G. melaena by Handley and Patton ( Wild Mammals of Virginia , p. 204, 1947) …