The following section describes some of the basic ecological concepts that underlie the ACE Basin ecosystem. Descriptions include an introduction to ecology and ecosystems, habitats, succession and biodiversity, populations and communities, energy flow through ecosystems, and ecosystem services. Many of these concepts are applicable to the habitats, communities, and ecosystems that are described in the Biological Resources Section and other sections of this product.
What is Ecology?
What is an
Although it has not always been clearly recognized, we are completely dependent on the ecosystems in which we live. The myriad processes that integrate energy and nutrients flowing through the ACE Basin ecosystem provide its human inhabitants a variety of services. Besides providing food and shelter, the ecosystem provides waste treatment (by way of carbon dioxide consumption; oxygen production; and breakdown of sewage), a water filtration system (by the soil), recreational opportunities, and a basis for economic development. Remove any one of these services and the character and function of the other components can be compromised. See the discussion of ecosystem services below.
Natural geomorphic, as well as episodic, events have shaped the overall diversity of habitats and the services they provide. Over the last 10,000 years as early human societies created shell middens and set fires intentionally to capture game, and especially over the last 200 years with the advent of extensive forestry and agricultural practices, ecosystems have been significantly altered by anthropogenic factors. These factors will likely continue to pressure ecosystems in the future. (See related section: History.)
As habitats are modified, ecological processes in these habitats also change and some of these changes may be significant. For example, estuarine marshes are effective traps for many pollutants. When pollutants are introduced at low levels, they may be bound to sediments or degraded through natural processes, and they are less likely to be transferred up the food chain. If the ability of sediments to trap such pollutants is exceeded, however as has happened in some places in the Charleston Harbor Estuary (DHEC 1998), then pollutants may move up the food chain. Some of the changes may not be immediately obvious, such as a reduction in populations of benthic infauna (small worms, crustaceans, and bivalves). Other impacts, such as the loss of oyster beds resulting in the reduced availability of oysters for recreational or commercial harvest, are much more obvious. Part of the solution to these problems lies in appropriate management decisions made at the federal, state, and local governmental levels, as well as by individual property owners and residents of the ACE Basin. (See related section: Management.)
A habitat is defined in relation to a plant or animal species and is the location where the species lives. It is a combination of the physical and biological components of the location and ranges from very large and stable (the earth or the open ocean) to very small and ephemeral (a pond in the dunes). A habitat may be a stand of loblolly pine trees with heart-rot where red-cockaded woodpeckers are found, the estuarine pluff mud where mud snails and polychaete worms thrive, or the town of Edisto Beach where humans reside. Usually, the more diverse the habitat types within a region, the greater will be the variety of species being supported. Coastal areas, such as the ACE Basin, located between the open ocean and upland areas, have a high diversity of habitats and microhabitats, supporting diverse and abundant communities of plants and animals. However, one of the greatest threats to habitat diversity in the ACE Basin is the conversion of existing habitats to structurally and biologically simpler habitats such as agricultural fields, pine plantations, and urban or residential areas. In addition to the direct loss of existing habitat, the resulting fragmentation of the remaining forested and wetland areas results in decreased species diversity (Odum 1997; Meffe and Carroll 1994).
In contrast, due to anthropogenic influences much of the change has become much more long-lasting. Forests have been converted to agricultural fields and suburban and urban land, and seldom have the opportunity to revert back to a natural state. Depending on the land use, anthropogenically altered habitats tend to have a simpler structure with lower habitat diversity and increased edge habitats than do natural systems. As a result, biotic communities shift to favor those species that can utilize open, edge, agricultural, or suburban habitats. Species that require interior forest habitats have a harder time finding appropriate food and other required resources. Thus, there has been an overall reduction in populations dependent on these habitats (Meffe and Carroll 1994).
One result of fragmented habitats arising from development in the ACE Basin is the change of the character of ecotonal habitats. Ecotones where the vegetative communities previously shifted slowly from wetland to upland forest have been changed to sharper boundaries between wetland areas and what are now agricultural fields or suburban developments. Many game species utilize these edge habitats and a common practice of wildlife managers is to create edge-habitat. However, many non-game species require less abrupt transition areas and have declined as a result of the loss of these gradual transition zones (Meffe and Carroll 1994).
Succession is a process that encompasses the dynamic biological and physical changes that lead to the development of complex, interlinked, biological communities. For example, habitats that are disturbed by fire or storm go through a series of community changes as the habitat recovers from the disturbance. Succession has been viewed as a step-wise process, whereby early communities typically are dominated by a few opportunistic species. The transfer of energy from primary producers (plants) to herbivores to predators (each is considered a trophic level, see discussion of energy flow through ecosystems below) tends to be less efficient during these early stages. The number of trophic levels also tends to be lower. Early colonizers may change the environment, facilitating later communities of other species to develop and eventually become dominant species. The community of organisms at the final stage of succession is called the climax community. These communities are generally characterized by a wider range of species, a higher number of trophic levels, and more efficient transfer of energy between trophic levels. In some cases, a sequential process that changes the community of plants and animals occurs in which there is initial colonization by fast growing, opportunistic species followed by species that may grow more slowly but which compete better for limited resources (light, moisture, nutrients). The final climax stage reached is dependent on the pool of species available to colonize the area, and the type, frequency, and intensity of initial disturbance (Christensen 1988). Community succession, therefore, is dependent on both predictable and stochastic events.
A successional series begins when some type of disturbance sets back the clock. Landslides, hurricanes, avalanches, catastrophic forest fires, volcanic eruptions, and meteorite impacts may set the clock back to zero by completely destroying the existing communities. These types of events are relatively rare and there are usually some species (e.g. fire-resistant seeds) adapted to surviving these severe types of disturbances. Less severe disturbances from localized storms, hurricanes, or burned forests are more common and generally result in only partial damage to the community.
In coastal forests of South Carolina, including the ACE Basin, this progression commonly starts with communities dominated by grasses and other fast growing opportunistic weedy species that colonize the area within the first few weeks or months after a disturbance. Over the next year or so, the community is slowly taken over by woody shrubs and young pine trees that grow more slowly than the opportunistic species. Over the next 20-30 years, as the pines begin to grow large enough to shade the ground beneath, shade-tolerant species may become established. Shade-intolerant plants that colonized the open habitat, including young pine trees, can no longer establish themselves under a closed canopy. Shade-tolerant species such as hardwood species (e.g. beech and oak) become established, and the community develops into a Southern mixed hardwood community. Between 50 and 100 years after the disturbance, pine trees begin to slowly die, and the community gradually shifts towards a hardwood dominated community, the common climax community of coastal South Carolina upland areas. (See related section: Plants.)
Although fires and storms still may cause significant disturbance in ACE Basin communities, humans impacts through agriculture and forestry practices and urban development have far greater impact on succession in the ACE Basin. One of these anthropogenic influences is the regulation of fire which may result in changes in communities that are dependent on seasonal fires. Management plans usually prescribe burns during the winter without regard to the natural requirements of the species, because this is a period when areas are wetter and fires are less likely to get out of control. This, in part, explains the reduced dominance of longleaf pine forests that require spring and summer fires in the early part of the growing season. Another impact is the clearing of fields (the initial disturbance) and their maintenance as agricultural land (repeated disturbances) preventing the successional sequence that would return them to a forested landscape if left undisturbed. While some areas have been allowed to develop back into forests, more frequently land has been shifted from forest to agriculture and then on to suburban or urban uses. There are extreme cases, fortunately outside the ACE Basin, where habitats have been so severely impacted through salinization, erosion, or pollution that the areas would take thousands of years to recover and may never revert to the habitat that was there originally (Odum 1997). There is a field of study called restoration ecology that deals with restoration and reclamation of severely degraded habitats.
One of the more important characteristics of a habitat is its biodiversity. Individual species in a habitat fill different roles or niches including primary production, herbivory, predation, and decomposition. In most communities these roles are filled by many species with varied characteristics.
Biologically diverse communities, often occurring in stable environments where complex assemblages of species have time to develop, can provide a wide variety of services to the ecosystem as well as to humans interested in the services. As an example, the potential medicinal value of the plants in the biologically rich tropical regions is well recognized, and the loss of these communities through deforestation may reduce this source of alternative medicines. As another example, the loss of habitat for the Red-cockaded woodpecker is directly related to current forest management practices of harvesting pine forests before they have time to age to the point that woodpeckers can utilize them.
The diversity of a community or habitat is, in part, dependent on the stability of the environment surrounding the habitat. If environmental conditions remain relatively constant, the available resources tend to be divided up between many different species. The transfer of energy between trophic levels tends to be more efficient in communities that have a number of trophic levels. These conditions are characteristic of climax communities at the late stages of succession. Examples of climax communities are the maritime forest on barrier islands and hardwood forests in inland areas that do not burn regularly.
In environments where disturbances are relatively frequent, the diversity of plant and animal species may be less. Frequent disturbance may keep the development of the community in the early stages of succession, which tends to have fewer species and a less efficient transfer of energy between trophic levels. An example of this type of community is the pine flatwood community where forest fires occur every 2-5 years, setting back the successional clock.
The thousands of species and billions of individual plants and animals that occupy the ACE Basin can be organized into groups by species (populations), trophic levels (position in food chain), or habitats (communities). The simplest level of organization of individuals is the population, a group of individuals of one species living in a specific area. Every population has attributes associated with it such as number of individuals (or density), range, habitat requirements, reproductive rates, and dispersal characteristics.
Growth Rate and r and
k selected species
In general, r-selected species tend to be opportunists, rapidly expanding their populations in temporarily unlimited habitats. These species can rapidly establish themselves in newly formed or recently disturbed habitats. After a catastrophic forest fire for example, it is r-selected species that first colonize the site. Similarly, the weeds that attempt to invade a corn field (disturbed every year by site preparation) are usually r-selected species. The conditions these species exploit are often characterized by an abundant resources with a limited number of consumers. While able to rapidly invade an area, r-selected species generally do not compete as well as k-selected species when resources become limited. In the absence of frequent disturbances, r-selected species are out-competed for limiting resources by k-selected species that utilize limited resources more effectively. K- selected species tend to dominate stable habitats such as climax communities, where competition for space, light, and nutrients may be important.
Carrying capacity: How many is too many?
Any particular location supports populations of different species. Therefore, a community is defined as all the different populations that live together in an area. (Odum 1997). The area comprising the ACE Basin encompasses many different communities. Communities can have unique boundaries, have varied composition of species, and be variable in time and space. A community may be defined as all the plants and animals (including humans) living on a particular barrier island. Alternatively, a community may also be described in a more specific sense, such as all the parasites living in or on a single fish such as a spotted seatrout.
Different communities frequently have similar characteristics. They are usually composed of both common and rare species. Some species, such as deer, are generalists that live in a wide variety of habitats or feed on a wide range of foods; others species are specialists such as the Red-cockaded woodpecker, which requires specific microhabitats or food sources. Some of these specialists are threatened by reductions of their required habitat. The Red-cockaded woodpecker, an endangered species once found in the ACE Basin, requires pine stands with a minimum age of 50-70 years. The loss of this habitat through timber harvesting has resulted in the extirpation of Red-cockaded woodpeckers from the ACE Basin. In contrast, the marsh hen is also a specialist, inhabiting only the Spartina marshes. But, due to the expanse of this habitat in the ACE basin, this species is very abundant there. National and state level restrictions on impacting Spartina wetlands protect this habitat and therefore protect the marsh hen.
External forces such as geomorphology (primarily affecting plant communities) and episodic events such as storms or fire may also shape the structure of communities. Low-lying areas are generally wet and support only those plant species that can tolerate wet conditions. Sandy soils with limited organic materials do not hold moisture well, and those habitats are dominated by communities of plants and animals that can withstand dry conditions. Intense storms such as hurricanes may decimate an Atlantic maritime forest dominated by hardwoods allowing faster growing, shade intolerant, pine trees to establish themselves and temporarily dominate the plant community.
In addition to external physical forces, a number of biological processes within a community (such as species interactions) help to establish and maintain its structure. Examples include territorial behavior, predator-prey interactions, inhibitory chemicals used by plants to slow the growth of other plant species, overstory shading that reduces the growth of shade intolerant species, and parasites and diseases. All these processes interact to control the abundance and diversity of the community of plants and animals in an area. Consider the example of Edisto Beach: humans have greatly altered geomorphic features and wetlands of the island. The structure of the community has changed considerably through the modifications that humans have made to make the island habitable for them. This may favor raccoons, egrets, and house cats at the expense of salamanders and bald eagles.
The energy supplied by the sun is the only source of truly renewable energy on the earth. Other sources of energy, such as fossil fuels, are not renewable. Nuclear energy, although potentially unlimited, has disadvantages related to its waste products. Hydroelectric energy is renewable, but is dependent on solar energy and weather patterns to replace the water that flows through the dam. Most of the sunlight falling on the earth is absorbed as thermal energy (Energy Dissipation of Solar Radiation as Percentage of Annual Input into the Biosphere ) and this drives physical processes such as the hydrological cycle through evaporation and rainfall, and weather patterns through the heating of the air and land. Only about 1-5% of energy from the sun is stored by photosynthesis while the rest is radiated as thermal energy or heat (Odum 1997).
Energy flow through the biosphere (the thin biological layer at the earths surface) begins with the conversion of the radiant energy of sunlight to chemical energy by organisms called autotrophs. These include all the species, such as plants, algae and some bacteria, that can convert solar energy to high-energy molecules through the process of photosynthesis. These species convert raw materials such as elemental nitrogen, carbon dioxide, water, and potassium into organic molecules, and in the process store energy in those molecules. The organic materials synthesized in this way are used by autotrophs to grow and survive. This conversion of raw materials and solar energy to organic materials is called primary production. Gross primary production is the total amount of raw materials converted by plants to organic molecules. A percentage of the gross primary production is used to maintain the plants themselves. Any extra production is called net primary production and it is used for growth and reproduction or it may be lost as dissolved organic materials.
Energy stored by primary producers is used by primary consumers which include herbivores and detritivores. Some herbivores include deer, rabbits, seed-eating birds, caterpillars, and carp which feed on living plant material. Detritivores are less conspicuous but no less important because they eat dead plant material and start the decomposition processes that reduce the organic material back to simpler, lower energy forms. From an energy storage and transfer point of view, these levels of primary producer, primary consumer, and secondary consumer are called trophic levels. Energy stored in high-energy organic molecules is transferred between these levels by herbivores grazing on plants and carnivores preying on herbivores. If it is hierarchical, from plant to herbivore to carnivore, this flow of materials is called a food chain. However, movement of materials from primary producers to consumers to decomposers typically follows many different paths resulting in a complex network called the food web as depicted in this figure from Odum (1997).
Energy Subsidies in Managed
Humans have substantially changed the flow of energy in some systems by subsidizing the energy input (Odum 1997). In the course of history, two major changes in energy flow have allowed the human population to expand significantly. The first was during the agricultural revolution, which occurred approximately 8,000 years ago. The primary mechanisms of subsidizing energy flow through agriculture became the addition of fertilizers (energy rich organic material ) and removal of competitors (energy removed by unwanted sources such as weeds and pests). The second change occurred with the industrial revolution, when advances in medicines increased human life expectancies and the use of fossil fuels increased the productivity of agriculture through mechanization.
The global economy is currently using more energy than is renewable over the long term and we are therefore heavily dependent on non-renewable fossil fuels and nuclear energy. Our technologically advanced societies are using more energy than can be replaced by renewable energy sources. Thus, energy use is currently subsidized by fossil fuels, a limited resource; and nuclear energy an unlimited resource that has a significant impact on the environment through the release and disposal of radioactive byproducts. Renewable energy sources such as hydroelectric power and solar power, although cleaner, have limitations as well as undesirable impacts on the environment.
Energy use at the local level of the ACE Basin is no exception. Millions of dollars in fuel, fertilizer, and pesticides are used to increase the production of agricultural products well beyond natural levels. When fossil fuels become expensive and scarce, how will the current production levels be maintained on ACE Basin farms? One of the problems faced by society is how to maintain current production levels as non-renewable resources such as fossil fuels become scarce. This issue is not restricted just to the agricultural community but will impact many of the ways that human society uses fossil fuels to subsidize ecosystem services.
Ecosystem ServicesA Global Perspective
Ecosystem services are defined by Daily (1997) as the conditions and processes through which natural ecosystems, and the species that make them up, sustain and fulfill human life. Humans benefit from the materials and processes provided by these services. Some of these materials such as seafood and lumber are clearly apparent while others are less conspicuous including waste assimilation, carbon cycling, storm buffering, and the hydrological cycle.
From a global perspective, ecosystems within the ACE Basin contribute to the maintenance of the atmosphere by removing carbon dioxide and producing oxygen, by transforming nitrogenous wastes into less toxic forms, and by trapping pollutants. Global ecosystem services (Ecosystem services and functions ) are large-scale processes that remain stable over long periods of time, thereby allowing biological systems to develop. Loss of these ecosystem services would result in significant changes in the stability of the global ecosystem, affecting all life on earth. However, the scale and complexity of the global ecosystem make it exceedingly difficult to detect changes; to attribute causes for the change; and to stop, or reverse, the processes leading to the change.
ACE Basin Perspective
As an example, estuarine ecosystems absorb certain anthropogenic wastes (such as nitrogen released from municipal waste-treatment plants and fertilizers applied to agricultural fields) as well as trap and detoxify pollutants. However, if these services are overloaded or the capacity of the estuary to process these nutrients is reduced through wetland loss, the free service provided by the estuary will have to be replaced by expensive technological substitute services such as more efficient and expensive waste treatment plants.
Another example involves the dunes and maritime forests of the ACE Basin which provide protection from storms. Dune lines serve to buffer coastal areas from high seas and water levels during episodic events such as strong storms and hurricanes. Human activities such as building on dunes, changing the movement of sand with groins or jetties, or removing large areas of maritime forest may compromise this protective buffer allowing increased erosion or greater property damage (Thieler and Bush 1991).
Wetlands and estuaries trap many pollutants such as oil (PAHs), pesticides (DDT, Chlordane), and heavy metals (lead, copper) in their sediments. Some of these pollutants, such as PAHs, may then be further degraded by natural microbial activity to less toxic byproducts or to simple compounds such as carbon dioxide and water (Peterson and Lubchenco 1997). Pollutants that are not readily degraded (PAHs, chlorinated compounds, and metals) are often bound up with the fine particulates that make up the sediments and therefore are less likely to be absorbed by wildlife. Sediments may temporarily trap the pollutants, but biogenic and oxidative processes periodically facilitate the release of sediment-associated pollutants. Therefore, pollutants may cause long-term effects on biological resources of the ACE Basin. Although not yet identified as a problem in the ACE Basin, many highly urban areas on the east coast of the United States have exceeded the capability of these services, and are experiencing the undesirable effects of these contaminants being released into the environment and affecting wildlife as well as human populations. These effects are seen in changes in plant and animal populations, increases in fish mortality, reduced growth and deformities, and harmful algal blooms (SCDHEC 1998). (See related section: Phytoplankton: Algal Blooms.)
Some ecosystem services can be difficult to assign a monetary value. Ecologists and economists have made some recent progress in assigning dollar values to ecosystem (Value of biome types ) operations or functions to elucidate the economic value of pertinent non-market goods (Colgon 1990). While these estimates are considered rough ones and are thought to underestimate any actual dollar value equivalent of aggregate services of the biome (Costanza et al. 1997), they make the value of these services comparable with more easily recognized services such as the value of commercial fisheries (Total value of commercial fishing in South Carolina. ) or the ability of aquatic systems to recycle municipal waste.
G. Riekerk, SCDNR Marine Resources Research Institute
E. Wenner, SCDNR Marine Resources Research Institute
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