| Biodiversity at the VCR
Long-term Ecological Research Site |
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Authors: John H. Porter and Linda K. Blum
Abstract
| Biodiversity at the VCR
Long-term Ecological Research Site |
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Superimposed upon the landscape determinants of biodiversity are disturbances, which, again, occurs at a variety of scales. During the Pleistocene, a lower sea level meant that the Delmarva Peninsula was substantially wider and that the landforms that gave rise to present-day islands were inland ridges. Biogeographic theory would therefore predict that the peninsula would be undergoing a general decline in diversity through a process of relaxation. Over the last 400 years, human-mediated biotic exchanges have increased dramatically, leading to an influx of exotic species. McCaffrey and Dueser (1990) found that 15 of the 217 plant species found during a 1974 survey of the islands were naturalized. Some introduced species, such as the house mouse (Mus musculus), have achieved dominance in some components of the landscape. On the decadal temporal scale, large storms cause extensive flooding and reshaping of the barrier islands.
The diversity of landforms in the barrier island system lends itself to formulating and testing hypotheses about determinants of biodiversity. Although experimental manipulations of biodiversity are not always possible, the physical diversity of islands lends itself to statistical and modeling approaches. Island areas vary from over 7,000 ha (Assateague) to less than 30 ha (Crescent, Little Cobb, Skidmore), and lengths vary from 58 km (Assateague) to less than 1 km (Crescent, Little Cobb, Raccoon, Skidmore). Maximum elevations on islands are all low, varying from 10 m to 1 m. The islands are similarly diverse in their flora and fauna, although both are depauperate in comparison to the adjacent mainland (Dueser and Brown 1980, McCaffrey and Dueser 1990). Plant diversity varies from single species on Godwin Island to 138 on Smith Island, with a median of 42 varieties (McCaffrey and Dueser 1990).
Research in biodiversity on the Virginia Coast Reserve LTER takes multiple forms. The first of these is collection of new data on the composition of insular biotas. Prior to establishment of the VCR/LTER, The Nature Conservancy (TNC) conducted an ecosystem study which included surveys of the plant and animal communities of the Virginia Barrier Island Complex. This information base has been supplemented by LTER permanent plots and faunal surveys. These include mammalian, vegetation, fish and microbial sampling efforts.
The second type of activity is repeated surveys designed to detect changes and trends in biodiversity. The Nature Conservancy and affiliated volunteers have made annual surveys of nesting shorebirds since 1974 (Williams et al. 1990). Repeated small mammal live-trapping surveys have been conducted on an irregular basis since 1974 (Dueser et al. 1976, 1979, Dueser and Brown 1980). Semi-annual trapping of small mammals on Hog Island has been conducted since 1989. Networks of permanent plots for assessing vegetation changes have been established by Donald Young, Frank Day, John Porter and Hank Shugart on barrier islands. Bob Christian and Mark Brinson are also using permanent plots in a mainland marsh to determine the role of inundation/long-term chronic, sea-level rise and acute short-term storm disturbance on diversity in marshes. The VCR/LTER is also cooperating with The Nature Conservancy efforts spearheaded by Barry Truitt to monitor the progress of invasive plant species, particularly Phragmites communis which is spreading in the upland marsh boundary throughout the Virginia Coast Reserve. Affiliated PI John Albertson is investigating topographic and hydrologic controls on Phragmites distribution.
A third type of activity is research on the specific processes responsible for changes in biodiversity. Intensive studies of small mammal genetics conducted by Raymond Dueser and Nancy Moncrief are using electrophoresis and mitochondrial and microsatellite DNA techniques to infer patterns of island colonization based on genetic similarities between insular and mainland populations (Loxterman et al. in press). Direct observations of mammalian movements between islands were made by Forys (1990, Forys and Dueser 1993). Randy Carlson and others are conducting high-accuracy topographic surveys using the Global Positioning System coupled to a laser total survey station. The resulting measurements are being used to assess the susceptibility of vegetation and mammal sampling plots to flooding. Studies of shrub gaps are examining the effects of small-scale disturbance on local biodiversity (Young et al. 1995). Mike Erwin of the USGS Biological Resources Division, in collaboration with the VCR/LTER and TNC is examining bird nest productivity for species showing long-term declines.
Modeling efforts are also a prominent component of VCR/LTER biodiversity research. We have experimented with a number of ways of developing and testing "assembly rules" for island faunas. Monte-Carlo simulation models are being used to test whether observed patterns of island occupation are consistent with different forcing functions or are random. Individual-based dispersal models have been used to clarify sampling issues related to dispersal.
Data Resources
The ability to answer questions regarding biodiversity is dependent upon the quantity and quality of data available. There is a substantial quantity of data available for the Virginia Coast LTER (Table 1). A major resource is a volume summarizing the species lists for the Chesapeake Bay region by Wass et. al. (1972). Although the amount of information on the distribution of species varies widely between taxonomic groups and is always general, the species lists in this volume set the context for understanding the diversity in the region.
Other resources are more specific to the VCR/LTER research sites, particularly the barrier islands. In 1974 The Nature Conservancy commissioned a study of 14 newly-purchased barrier islands. The report which resulted provides critical island-by-island information on species distributions (Dueser et al. 1976). This study inspired symposium on the Biota of the Virginia Barrier Islands in 1989 sponsored by the VCR/LTER which led to a special publication of the Virginia Journal of Science (Dueser 1990).
There are also a large number of individual papers which include biodiversity information for specific barrier islands. VCR/LTER researchers are an important source of new biodiversity information, from permanent plots and intensively studied areas. Biodiversity information is being specifically solicited from VCR/LTER researchers as part of LTER data and information management.
In order to better use this information, the VCR/LTER has created interactive WWW-based biodiversity databases. A comprehensive biodiversity database along with specialized vegetation, fish and bird databases are online at http://www.vcrlter.virginia.edu/data.html. The comprehensive database of biodiversity information is being compiled in conjunction with a series of biodiversity classes taught by Bruce Hayden, David Smith and John Porter. Class activities have included development of a data model which allows input of species, location and observation information and population of that database with over 6,000 individual observations of over 2,000 species. The database is searchable based on location, and taxonomic information (Family, Genus, and Common Name). Information on class activities and the database is available at http://www.vcrlter.virginia.edu/biod96.
Biodiversity Summary
The biodiversity for different taxonomic groups on the Virginia Coast and associated estruarine waters varies by taxon and habitat (Table 2). There are some taxonomic groups where there is little extant information. Data on insects and microbes are particularly lacking.
The islands are particularly important nesting habitats for colonial waterbirds. A survey conducted in 1993 revealed that the islands and lagoons of the Virginia Coast supported nearly 70,000 pairs of colonial waterbirds including 23 different species (Watts 1994). The site supported over 50% of the known Virginia population for 18 of these species and over 80% for 11 species. In the state of Virginia, 7 of these species have been proposed for a status of "special concern" and 4 have been proposed for a status of "threatened". In addition to the colonial waterbirds the site supports an annual population of over 100 pairs of the federally endangered Piping Plover and 40 pairs of the state endangered Wilson's Plover (Williams et al. 1990, Beck et al. 1990, Patterson et al. 1990).
Biodiversity and Productivity
The relationship between species richness and productivity can be approached conceptually in at least two different ways at the VCR LTER: as an estuarine ecosystem or from an island biogeography viewpoint. Using the first approach, the "communities" at the VCR can be categorized as either subtidal (water), intertidal (marshes), or subaerial (upland) based on the relationship between the three dominant "free surfaces" that structure the VCR LTER: fresh-water table, land and sea level. The concept of free surfaces provides the theoretical framework guiding work at the VCR. The relationship between diversity and productivity at this scale for these three "community" types is negative and significant when primary producer species richness is considered, as well as, when all reported species are examined.
Using an island biogeography approach, there is a direct relationship between island area, maximum elevation, and vegetation complexity, measured as the observed number of vascular plants (Dueser and Brown 1980). Larger islands tend to have a greater variety of plants, particularly woody plants, than smaller islands. Similarly elevated islands tend to have a greater variety of plants, particularly woody plants, than low-lying islands. McCaffrey and Dueser (1990) suggest that these patterns reflect the greater substrate diversity, the more reliable supply of fresh groundwater, and the reduced frequency of overwash and flooding associated with larger area and higher elevation.
When the total vascular plant species richness for an island was plotted as a function of total island productivity, there was no apparent relationship between diversity and productivity. When upland and marsh communities were considered separately, there was a strong positive relationship between upland productivity and the number of upland plant species, but no relationship was evident for the marsh communities. When the residuals obtained from a species area plot for the islands were considered as a function of productivity no relationship between diversity and production was observed for the upland community. There is positive relationship between total island productivity vs total island vascular plant numbers based on residuals from the species area plots but this relationship is weak (r2 = 0.461) and can be explained by island size and shape.
Taxa | Number of Species | Source |
|---|---|---|
Amphibians (entire penninsula) | 43 | Musick, J.A. 1972. Herptiles of the Maryland and Virginia Coastal Plain. In Wass, M.L. et al. A check list of the biota of the lower Chesapeake Bay. Virginia Institute of Marine Sciences, Gloucester Point, VA. Special Scientific Report No. 65: 213-242. |
Amphibians (islands only) | 8 | Conant, R., J.C. Mitchell and C.A. Pague. 1990. Herpetofauna of the Virginia Barrier Islands. Virginia J. Science 41:364-380. |
Birds | 223 | Wass, M.L. 1972. Birds dependent on open water or wetlands. In Wass, M.L. et al. A check list of the biota of the lower Chesapeake Bay. Virginia Institute of Marine Sciences, Gloucester Point, VA. Special Scientific Report No. 65: 243-286. |
Mammals | 41 (includes aquatic mammals) | Anonymous. 1972. Mammals of water, wetlands and barrier islands. In Wass, M.L. et al. A check list of the biota of the lower Chesapeake Bay. Virginia Institute of Marine Sciences, Gloucester Point, VA. Special Scientific Report No. 65: 287-290. |
Mammals (islands only) | 21 | Dueser, R.D., W.C. Brown, G.S. Hogue, C. McCaffrey, S.A. McCuskey, and G.J. Hennessey. 1979. Mammals on the Virginia barrier islands. Journal of Mammalogy. 60: 425- 429. |
Fishes | 287 | Musick, J.A. 1972. Fishes of the Chesapeake Bay and adjacent coastal plains. In Wass, M.L. et al. A check list of the biota of the lower Chesapeake Bay. Virginia Institute of Marine Sciences, Gloucester Point, VA. Special Scientific Report No. 65: 175-212. |
Vascular Plants | 217 | McCaffrey, C. A. and R.D. Dueser. 1990. Preliminary vascular flora for the Virginia Barrier Islands. Virginia J. Science 41:259-281. |
Phytoplankton (shelf waters) | 103 | Marshall, H.G. and M.S. Cohn. 1987. Phytoplankton distributions along the eastern coast of the U.S.A. Part VI. Shelf waters between Cape Henry and Cape May. J. Plankton Res. 9:139-149. |
Phytoplankton (one pond on Smith Island) | 146 | Marshall, H.G. 1983. Phytoplankton studies within the Virginia Barrier Island. III. Phytoplankton composition of a saline pond on Smith Island. Virginia J. Sci. 34:215-222. |
Reptiles (entire penninsula) | 59 | Musick, J.A. 1972. Herptiles of the Maryland and Virginia Coastal Plain. In Wass, M.L. et al. A check list of the biota of the lower Chesapeake Bay. Virginia Institute of Marine Sciences, Gloucester Point, VA. Special Scientific Report No. 65: 213-242. |
Reptiles (islands only) | 21 | Conant, R., J.C. Mitchell and C.A. Pague. 1990. Herpetofauna of the Virginia Barrier Islands. Virginia J. Science 41:364-380. |
Table 2. The total number of species reported for estuarine and associated barrier island communities of the VCR LTER
TAXA UPLAND MARSH WATER
Amphibians 8 0 0
Reptiles 17 3 3
Birds 223 28 31
Mammals 21 21 14
Vascular Plants 233 47 2
Fish NA 6 85
Invertebrates ? ? ?
Macroalgae NA 21 21
Microalgae ? 192 192
Insects ? ? ?
Microbes ? ? ?
Total No. Recorded
Species 502 318 348