Conceptual Models

For more information read Chapter 2 of the GULN Final Monitoring Plan,
which can be downloaded on the reports page.

Figure 1

Diagram of the Gulf Coast Network conceptual modeling symbology. The four box types represent levels within the hierarchical ecosystem models. Text within the boxes identifies examples or specific cases of that level. Line-arrows represent (largely directional) superior-level forces or effect on subordinate levels and events. Vital signs for monitoring are mostly drawn from the set of ecosystem responses and indicators, but may be identified at any level within an ecosystem.

Figure 2

The riparian zone of a river, stream, or other body of water is the land adjacent to that body of water that is, at least periodically, influenced by flooding. Riparian zones are important features at BITH, JELA, NATR, SAAN, and VICK. Bottomland hardwood forests are a major riparian ecosystem type in the United States (Mitsch and Gosselink 1993). These systems play a crucial role in the watershed by providing areas to store floodwater, thereby reducing the risk and severity of flooding to downstream communities. Further, these wetlands improve water quality by filtering and flushing nutrients, processing organic wastes, and reducing sediment before it reaches open water. The Barataria Preserve of JELA provides one of the southernmost examples of bottomland hardwood forests in North America (Swanson 1991). Harris (1989) listed characteristics of these ecosystems that are beneficial to wildlife: hard mast production and a phenology (periodic biological phenomena, such as flowering) that is asynchronous with surrounding upland communities, frequent cavity trees, high abundance of invertebrate wildlife, and a linear distribution through the landscape that aids local and regional movement of animals. The seasonal flooding of these habitats makes them less suitable for agriculture; thus, in agricultural landscapes, they are often the only forest refuges available for many mammals, birds, and other species. These characteristics increase in importance in the more arid western subregion. A collection of published models of riparian and bottomland hardwood forests are included in Appendix J and in the GULN Phase II Report (2005).

Figure 3

There are two distinct coastal and marine ecosystems represented in network parks: the riverine-dominated (deltaic) coastal wetlands system (JELA) and the barrier-island systems (GUIS and PAIS). Although these systems are distinct from each other, they share a common set of agents of change, stressors, and ecosystem responses because of their coastal locations. Therefore, they are combined into one conceptual model.

JELA is located in the deltaic plain of the Mississippi River. As such, it is historically a river-dominated system, yet its location in the upper estuary of the Mississippi delta makes it distinct from other riparian areas in the network. The variety of wetland habitats and the stressors that influence those wetlands gives it characteristics more similar to the marine-dominated, barrier-island systems. Deltaic plant communities and associations are determined by the cyclical nature of delta lobe development and degradation (Gagliano and Van Beek 1975); overlapping environments develop and decline as the lobe ages. As delta lobes grow and decay, habitat and biodiversity peak in the early stages of degradation. Vegetation is closely tied to the unique landforms of the delta and proximity of those areas to either continued riverine or marine influence. Coastal wetlands are influenced by the natural processes involved in delta degradation such as subsidence, shoreline erosion, changes in salinity, and changes in availability of sediment. In addition, stressors include altered hydrology, sea-level rise, extreme weather events, recreation, oil and gas exploration and extraction, and contaminants. Coastal wetlands provide storm-surge abatement, water purification, recreational opportunities, and critical habitat for a variety of nesting, wintering, and migrating wildlife species.

Barrier islands are geologically young features; the vast majority are less than 7,000 years in age, and most are probably less than 3,000 years old. Barrier-island formation is dependent upon the complex interaction between waves, sea-level change, and the availability of sediment (http://www3.csc.noaa.gov/beachnourishment/html/geo/barrier.htm). They are a vital part of the coastal and estuarine habitats found in GULN parks. Barrier islands are composed of three general zones: beach, dune, and back dune (Figure 2.7). Each zone provides critical habitat for several state- and federally listed species. These systems serve as key stopover areas for migratory birds (Weber 1983, Moore et al. 1990, Blacklock et al. 1998, Fuller et al. 1998, Cooper et al. 2005c), nesting sites for sea turtles (Nicholas and Jacks 1996, Shaver 2000), year-round habitat for the Perdido Key beach mouse (Oli et al. 2001), and shorebird nesting (Simersky 1972, Mitchell and Custer 1986) and wintering habitat (Nichols 1989, Garza 1997, Gorman and Haig 2002).

Changes in coastal geomorphology are normal processes; however, human-made structures (e.g., jetties) may alter offshore sediment transport and increase shoreline-erosion rates (Williams 1999). Extreme weather events are also normal coastal processes, but increased fragmentation and/or reduced system integrity prior to any given event will likely adversely affect system resilience and subsequent recovery. In response to events that cause substantial beach erosion, beach nourishment is often conducted. However, beach nourishment is linked to a decrease in species richness and density, shifts in the assemblage structure, and greater instability of these indices (Rakocinski et al. 1993, 1994, 1996). Excess foot traffic and off-road vehicle use disrupts natural dune processes and has adverse consequences on dune integrity by destabilizing the vegetation (Shabica and Shabica 1978; Shabica et al. 1979; McAtee and Drawe 1981; Blum and Jones 1985; Cousens 1988). However, a balance in recreation and preservation is necessary to fulfill the multiple purposes of designation (Psuty 1988). Oil and gas extraction is linked to subsidence (Denslow and Battaglia 2002, Morton et al. 2001), disturbance, and possible contamination of the activity site. Although contaminants have been detected in bird eggs and tissue, levels do not exceed those known to adversely affect survival and reproduction (Michot et al. 1994, Mora 1996). However, contaminants are associated with decreasing macrobenthic trophic diversity (Rakocinski et al. 1997, Brown et al. 2000) and could have cumulative effects in higher organisms (Carls et al. 1995). In the absence of extreme weather events (i.e., prolonged period since large disturbance), fire maintains barrier-island plant structure, specifically in the back-dune/maritime forest zone. Reduced fire frequency and fire suppression results in the loss of the herbaceous component to a woody overstory of shrubs and trees (Sheaffer 1998).

Figure 4

Upland systems in the GULN are diverse, ranging from slash pine stands at GUIS to more xeric brushlands at PAAL and SAAN. Fire is the primary disturbance mechanism in the systems; however, the increasing wildland¬–urban interface makes fire management increasingly difficult and often results in fire suppression. Ecosystems respond differently to fire suppression. Temperate ecosystems where frequent, low-intensity wildfires had occurred in the past are more likely to have been adversely affected by fire suppression (Agee 1993). Because of altered fire regimes (i.e., fire return interval), many fire-adapted upland systems in the GULN have shifted from herbaceous-dominated to woody-dominated (Table 2.2), thus altering structure and species dominance and reducing species diversity and overall site viability. Overgrazing has further facilitated an increase of woody components in fire-adapted GULN systems.

The greatest effect of fire suppression on biological diversity is not on the diversity within a particular habitat (Whittaker 1977), but on the diversity of habitats across a landscape. Landscapes with high diversity resulting from fire perpetuate high species diversity by providing opportunities for the establishment and maintenance of early successional species and communities (Connell 1978, Reice 1994). Fire suppression, on the other hand, increases uniformity in habitats as competition eliminates early successional species, leaving only shade-tolerant understory plants to reproduce (Stuart 1998).

Fire suppression has helped change the ecosystem dynamics of communities adapted to frequent, low-intensity wildfire. Complex landscapes are made simpler, some early and midsuccessional plants and animals are extirpated, shade-tolerant tree populations rapidly expand, and the relative importance of fire as a disturbance agent is reduced, while the importance of insects and pathogens as agents of disturbance is elevated (Covington et al. 1994). During droughts, for example, excessively dense forests become further stressed, enabling pathogens and insects to reach high population levels (Johnson et al. 1994). Trees killed by drought, insects, or pathogens create abundant fuel that exacerbates fire hazard. When fire occurs in such a system, it is often larger and more severe than one expected in areas with a natural fire regime (Stuart 1998). Southern pine beetle (Dendroctonus frontalis) invasions often result in habitat loss and altered forest structure while further increasing fuel loads in fire-suppressed areas, possibly resulting in catastrophic fires.

Previous land use has permanently altered the upland habitats of GULN parks. Water withdrawals, agriculture, and forest clearing are examples of the types of land uses that continue to impact these systems. Disruptions of native habitats make these areas susceptible to invasion by non-native species, further altering the biotic community.

Erosion is a concern at VICK, where surface soils consist of highly erodible loess soil.