The Big Picture

hen viewed from an airplane, the landscape is a patchwork. There is a natural patchiness to the landscape resulting from differences in soil characteristics, topography, microclimate, fire, and biotic factors. Superimposed on this is the modern landscape that has been shaped by humans. For much of the United States, a mosaic of fields, forests, lakes, roads, towns, and cities dominates the landscape. To an ecologist, much of the patchwork represents communities in different stages of succession. For example, from a successional perspective, a cornfield is a pioneer community. This "grass" stage that develops soon after disturbance (plowing) and as long as the land is kept in agricultural production by annual plowing and planting, the pioneer community will persist. If the land were no longer cultivated the site would eventually revert to forest. Similarly, the rice paddies of Southeast Asia, the sugar cane fields of Cuba, slash and burn plots in the Amazon Basin, pastures of Ireland, and pine plantations of the southeastern United States are all communities in early stages of succession. Throughout large sectors of the world, humans have manipulated succession to provide necessary food, fiber, and natural resources. In these areas, only a fraction of the landscape has not been altered. In the eastern United States, less than one percent of the old-growth, virgin forests still exists. While much of the eastern landscape is forested today, these forests are still recovering from being logged as much as three or four times over. Actually there is more eastern land in forest cover today than at the beginning of the century. A century ago Vermont was 30 percent forest and 70 percent cleared (mostly pasture); today these numbers are reversed. Many other ecosystem types have disappeared from the landscape. The tall-grass prairie consists of a few isolated remnants, the grasslands of central California have been converted to cropland or urbanized, coastal development has obliterated the barrier island communities of the East and Gulf coasts, and wetlands of all types have been reduced by at least fifty percent. There have been attempts at ecosystem restoration. For moral, aesthetic, economic, legal, and ecological reasons private and public land managers are replanting and restocking native plants and animals in their former positions in the landscape. If restoration is to be successful, it is imperative to have a sound understanding of ecological succession.

Frequently Asked Questions

• Ecological succession is the natural process of establishing or reestablishing an ecosystem.
• Succession occurs as a series of plants, animals, and microbes colonize a site over time.
• Succession may occur on sites that were previously unoccupied or on sites at which the exiting biotic community was removed or altered by disturbance.
• Primary succession occurs on sites that have not been previously occupied. Such sites exist on newly formed soils, such as exposed sandbars and volcanic ash. The colonization of a sunken ship by coral reef organisms is an example of primary succession.
• Secondary succession occurs when a biotic community has been disturbed, then becomes reestablished. Remnants of the previous biotic community still exist at the site and contribute to recolonization. Secondary succession is much more common than primary succession; much of the landscape is in a stage of secondary succession. One common example of secondary succession is the reestablishment of forest vegetation after an area has been logged. When a forest is logged, only the merchantable timber is removed, undesirable tree species, shrubs, saplings, and seeds remain and constitute the colonizer pool.

• Ecological disturbance is an event that disrupts ecosystem, community, or population structure.
• Disturbance may be initiated by natural events or human activities.
• Common agents of natural disturbance include fires, hurricanes, tornadoes, landslides, floods, droughts, and biogenic disturbance (caused by animals such as prairie dogs, elephants, and outbreaks of insect pests).
• Human-initiated or anthropogenic disturbance dominates the landscape in much of the world. Anthropogenic disturbance includes such activities as the conversion of natural communities to farm field and pasture, logging and deforestation, surface mining, urbanization of natural landscapes, and warfare.
• Disturbance creates ecological opportunities for individuals to become established that would otherwise be excluded from the community.
• Species diversity can be strongly influenced by disturbance. Highest species diversity is often found in ecosystems where disturbance is neither too rare nor too frequent; similarly neither too benign nor too severe. This is known as the intermediate disturbance hypothesis for species diversity.

• Biotic communities change over time. The complete sequence of change is called a sere.
• Seres are made up of recognizable units called seral stages, although the transition from one seral stage to the next is gradual and intergraded.
• Species that appear in early successional stages are called pioneer species. Pioneer species tend to be fast growing, short-lived, and capable of rapid and wide dispersal. The grasses and herbaceous plant species that colonize a farm field soon after its abandonment would be considered pioneer species.
• Late successional species tend to be persistent, longer-lived species.

• Ecologists recognize at least four patterns or models of succession.
• Facilitation: Early seral stages modify microclimatic and soil conditions that facilitate colonization by later seral stage species. Early successional species so modify the site that the conditions that allowed them to colonize initially are not longer present thus their propagules (progeny) become excluded from the site.
• Interference: Early seral stage species colonize a site and inhibit colonization by later stage species. This is contradictory to the facilitation model. Some early species exude toxins into the soil to prevent other species from occupying the site; this is called allelopathy. Even though these early stage species may persist for an extended time, their shorter longevity will allow late stage species to eventually colonize.
• Tolerance: This model is intermediate between facilitation and interference. Early successional species neither facilitate nor inhibit late successional species, but tolerate the presence of the other. Eventually, the longer-lived late successional species dominate as the early stage species die out.
• Chronic patchiness: In harsh environments or often disturbed environments, early successional species are often favored and late successional species are not able to persist. A familiar example would be the maintenance of grassy lawns (early seral stage species) by frequent mowings (disturbance). Abandoned lawns are soon colonized by shrub and tree species (late seral stage species).

• In addition to changes in species composition that occur during succession, other attributes of ecosystems change, including biogeochemical cycling.
• In the early seral stages of terrestrial succession, plant and animal biomass is low and productivity (growth rate) is high. Inorganic nutrients (e.g., N, P, K, Ca) are incorporated into biomass rather than being leached from the ecosystem. As the community matures, organic matter and nutrients are returned to the soil and again become available for plant uptake; this is called internal nutrient cycling. Dead organic matter and live roots retard soil erosion and thus reduce the associated nutrient loss.
• Biomass and soil are nutrient reservoirs or nutrient sinks.
• Maximum nutrient storage is achieved during the middle stages of succession while both productivity and soil nutrient storage are high. As the ecosystem ages further and productivity declines, there is a net loss of nutrients because productivity does not keep pace with nutrient losses due to soil erosion.
• The nutrient retention of the soil is dependent upon the amount of live and dead organic matter present, and soil texture (the size of soil particles). Nutrients are easily leached from large grained soil particles (sand) but are more readily retained in fine grained soil (clay or silt). Loam is an even mixture of clay, silt, sand, and well decomposed organic matter that maximizes soil nutrient retention and accessibility, and permeability thus is quite favorable for plant growth and root development.

• Disturbance plays an important role in biogeochemical cycling.
• Fire releases nutrients stored in biomass. As ash, these nutrients may be removed from the ecosystem by wind and water; but nutrient-containing ash that remains on-site becomes available for new plant growth. A pulse of available nutrients causes a pulse in plant growth soon after fire.
• Disturbance events such as floods and storm waves, may relocate massive amounts of plant debris and other organic materials (and nutrients) downstream or along high tide or flood lines (also called wrack lines).
• Storms that topple trees create canopy gaps. Below canopy gaps, increased light on the forest floor stimulates plant growth and nutrient uptake. The "tip-up" mounds created by the roots of fallen trees transfer nutrients from soil depth to surface.
• Anthropogenic disturbances that disrupt plant cover or soil structure cause nutrients to be lost form ecosystems. To produce crops, fields are cultivated which results in increased erosion and nutrient loss; additionally, the harvest of plants or plant parts removes nutrients from the system.

• The theoretical end-point of a succession sere is the climax community. At this stage, the community is expected to be self-replicating and persist indefinitely. Individual plants and animals will die but are replaced by species within the same community, thus community composition remains at equilibrium.
• In addition, the classic theories of succession and climax community proposed that at climax, the community would have achieved maximum organic content, maximum chemical (nutrient) storage, and maximum biological diversity. Certainly, many old-growth communities exhibit these characteristics.
• But, ecologists now realized that the classic concept of climax community is not valid and that many of the old-growth communities formerly considered climax communities were not the end-points of successional seres.
• Maximum organic content, nutrient storage, and biological diversity are found in intermediate stages of succession, not at the end-point of a sere.
• The time scale of successional change is difficult to comprehend in human life spans. Successional transitions early in seres may be readily apparent within a few years or decades, however, communities that appear late in the sere may persist for many human generations and changes are not as evident.
• Climates are not stable over long periods of time. Consequently, communities respond as climate changes.

• Old Field Succession. An often used and classic example of succession is old field to forest succession (See A Closer Look 9.2).
• Old field succession is especially apparent in the eastern United States. During the first half of this century, much eastern farmland was abandoned due initially to changing economic conditions of the Great Depression and later a demographic shift from an agrarian to industrial and urban society.
• Land that was no longer cultivated underwent a successional transition. A typical pattern in the middle Atlantic region is colonization by grasses and herbaceous species first, followed by fast growing trees such as loblolly pines, black locust, tulip poplar, or sweet gum. Later as these relatively short-lived species began to die out, they were replaced by long-lived hardwood species, such as oaks and hickories. The transition from pines (and other early successional trees) to hardwoods is still very apparent in these forests today.
• In New England, white pine, pin cherry, and white birch are the pioneer trees later to be replaced by sugar maple and American beech.
• During succession, not only are there changes in plant species composition (autotrophic succession) but animals as well (heterotrophic succession); changes in bird species are good examples. Small seed-eaters and insect eaters, such as quail, meadow larks, buntings, and sparrow inhabit the grass and herbaceous vegetation stage. Woodpeckers, thrushes, owls, and wild turkeys prefer woodland stages.
• Aquatic Succession
• Lakes and ponds are temporary features of the landscape, although large and deep lakes such as Lake Baikal in Russia, have persisted for millions of years. Lakes and ponds are sediment and nutrient sinks, thus they accumulate organic and inorganic materials on the lake bottom, becoming shallower and eventually filled. Terrestrial plants may eventually colonize sites that were once open water.
• Oligotrophic is a term used to describe the nutrient status of geologically young, nutrient-poor lakes and ponds. As sediments and nutrients accumulate and aquatic plant growth increases, the waters become eutrophic. This process is called eutrophication.
• Human activities that cause soil erosion or nutrient input may accelerate the processes of eutrophication and aquatic succession.
• Sand Dune Succession
• Wind and water cause the shifting sands of dunes to be geologically unstable.
• Pioneer plant species of sand dune ecosystems must anchor the sand with extensive root systems. Once stabilized, other plant species may colonize and begin to modify site conditions, facilitating the establishment of late successional species.
• In the absence of subsequent disturbance forest communities dominate the site.
• Sand dunes are usually located along lake or oceanic shorelines thus are exposed to erosive forces of winds, tides, and waves. Dunes nearest the shoreline are the most frequently disturbed and receive the most severe storm impacts. These fore dunes are maintained in a state of early succession. Further away from the shoreline, more stable communities develop.
• Shorelines change, however, fore dunes can be breached by storm waves, inlets can migrate, and the more stable, interior dunes can be disturbed.

• Ecosystems are unimaginably complex; to assume that an ecosystem can be completely restored by human efforts and technology is unrealistic and presumptuous. Nonetheless, attempts at ecosystem restoration are justifiable. In some circumstances, land managers may be legally required to restore ecosystems or managers wish to restore ecosystems for aesthetic, ecological, economic or moral reasons.
• Ecosystem restoration requires a sound understanding of ecology in order to determine what has been altered, why and how the system has been altered, and what steps must be taken to achieve the targeted restoration condition. The landscape is cluttered with species that have been introduced in an attempt to improve environmental quality but without understanding the ecological risks. Purple loosestrife, kudzu, mongoose, and European starlings, are but a few species that have been introduced with good intentions but undesirable results.
• It is important to set realistic goals for restoration. For example, the reestablishment of a fire-dependent grassland ecosystem in a residential land use area is unrealistic.
• Ecosystem restoration must take into account social and economic conditions of the surrounding community. For example, the restoration of a single species, the gray wolf, in the Northern Rocky Mountains has precipitated considerable debate within the community.
• Most terrestrial ecosystem restoration entails planting appropriate species of vegetation in a manner that accelerates the successional process but that is ecologically sound, or creating the appropriate habitat conditions that will permit natural colonization of the site. The restoration of animal populations is primarily dependent upon natural colonization, but animals may be introduced if the appropriate habitat conditions have been established.
• The process of ecosystem restoration requires a thorough knowledge of site conditions (natural and land use history, soils, hydrology, species interaction) and the desired target condition. From this information a restoration plan must be developed.
• The restoration plan must be implemented in the proper sequence and during the appropriate season. For example, soil preparation would have to be accomplished before planting begins, and planting must be started and completed within the appropriate seasonal time frame (usually winter) to maximize seed or sapling survival. Remedial plantings are often necessary to replace plants that have died.
• A strategy for weed and pest control is usually necessary. This may require the use of herbicides, pest or predator deterrents, cultivation, mechanical removal, biological controls, or a combination.
• If fire is to be used as a management tool, a burning regime must be implemented.
• In order to gauge restoration success, a monitoring program must be implemented.
• Long term protection of the site may be achieved through local, state, and federal incentive programs that provide tax breaks, cost share opportunities, and direct payments.
• Ecosystem restoration may be undertaken by individual landowners, environmental consulting contractors, environmental groups, and government agencies.

• Biomes are major ecological divisions of the terrestrial environment (See Chapter 7). Biomes (Clements and Shelford) are but one of several classification schemes that have been developed for the Earth's biota; others include biogeographic realms (Wallace), life zones (Holdridge), and ecoregions (Crowley and Bailey).
• Biomes classification is based primarily on similarities in plant growth form. Growth form is in response to climatic factors, principally temperature (along a latitudinal gradient) and moisture (Figure B2). Although species differ within biomes, growth forms are consistent. For example, temperate grasslands occur in North America and central Asia; plant species are different between these regions but the dominant growth forms (grasses, sedges, and herbaceous plant species) are common to both.
• Biomes are discussed in this chapter because they represent the most stable seral stage for a given region and climate. In the absence of major disturbance by humans or natural factors, the biomes listed below would persist in the landscape.
• Most ecologists recognize at least nine major biomes; your text lists ten terrestrial biomes as well as major aquatic and marine systems (See the tables that follow).  

Ecology In Your Backyard

• This exercise can be used to track primary succession. Fill greenhouse trays with sterile potting soil. Potting soil should be heated in an autoclave beforehand to kill any existing seeds. Place replicate trays in a natural area that is protected from human or animal disturbance. Make a data sheet to record biweekly counts throughout the growing season of seedlings that germinate in the trays. You will have to identify species of seedling and count the number of individuals for each species. Seedling identification can be very difficult; if you cannot identify the species, make a sketch of each unidentified type of seeding and give it a name (i.e., Species A, Species B, Species C, etc.).
• Average your biweekly data, and calculate the Shannon Diversity Index and evenness values (See In Your Back Yard in Chapter 7). Record these values along with species richness. At the end of the growing season, generate bar graphs for each of these parameters.
• If you would like you participate in forest or prairie restoration projects or in an urban forestry project, there are several conservation groups you may contact. The National Arbor Day Foundation and The American Forest, Global Releaf programs are but two organizations specializing in ecosystem restoration. You may wish to contact your city parks or horticulture department of county extension office for additional information about on-going projects, volunteerism, and planting techniques.
• Please respond to these questions or send your thoughtful examples and comments to:

Hardcopy Links In The Library

• National Research Council. 1992. Restoration of Aquatic Ecosystems. National Academy Press, Washington, D.C. 552 pp.
• Restoration Ecology. 1993+. Blackwell Science. A peer-reviewed scientific and technical journal published quarterly and available through the Society for Restoration Ecology.
• Restoration and Management Notes. 1981+. Published twice yearly by the University of Wisconsin Press for the Society of Restoration Ecology; nontechnical.
• Thayer, G.W. (ed.). 1992. Restoring the Nation's Marine Environment. Maryland Sea Grant College, College Park, Maryland. 716 pp.

Ecolinks On The Web

• http://www.cep.unt.edu/serecores.html - This site is maintained by the University of Wisconsin-Madison Arboretum. It contains perspectives on and definitions of ecological restoration.

• http://nabalu.flas.ufl.edu/ser/SERhome.html - The Society for Ecological restoration home page. See their Library for abstracts on a variety of restoration topics.

• http://www.evergreen.ca/restorationandbiodiversity.html - Evergreen Foundation (Canada). This web site provides an extensive discussion of biodiversity and ecological restoration.

• http://volcano.und.nodak.edu/vwdocs/msh/p_a/p_a.html - Mount St. Helens. This web site contains a wealth of annotated photographs about the geology and biology of the Mount St. Helens, a Washington state volcano that erupted in 1980. Find out more about plant and animal succession at this site.

• http://www.connix.com/~harry/forest.htm - Forest Succession. This web site contains a brief discussion of forest succession and the ecosystem changes that accompany succession.

• http://ibsasc-nbii.asc.nbs.gov/Glba/nps/glacier/newland.htm - Glacier Bay National Park and Preserve. This is a National Park Service web site about plants, animals and succession at Glacier Bay National Park and Preserve, Alaska.

• http://marine.usgs.gov/fact-sheets/baikal/title.html - Lake Baikal. This USGS web site contains facts and data summaries about Lake Baikal. Lake Baikal, like all lakes, is a sediment sink, note that sediments in excess of 7 km thick have been reported.

• http://pegasus.cc.ucf.edu/~arbor/fire.html - Fire Ecology . This University of Central Florida web site provides a brief discussion of fire ecology.

• http://whyfiles.news.wisc.edu/013tornado/ecology_tornado.html - Wind disturbance. This University of Wisconsin-Madison web site describes the effects of wind disturbance in forests.

• http://www.prairienet.org/gpf/homepage.html - Prairie restoration. This web site is maintained by Grand Prairie Friends and contains several links to information and activities about prairie restoration.

• http://www.marlborough.la.ca.us/depts/science/biomes.html - Biomes. This web site contains a listing of major biomes and pertinent links about those biomes.

• Note: If any of these links are not working, please see if alternative links are available at the Ecolink Update Site.

Ecotest Online

1. When a forest is logged, remnants of the community (i.e., seeds and saplings) are left on site and contribute to site recolonization. This is an example of _____ succession.

a. residual

b. cyclic

c. primary

d. secondary

2. The entire successional transition from an abandoned field to a forest is called a _____.

a. pattern

b. sere

c. series

d. procession

3. An event that disrupts ecosystem, community, or population structure is called a (an) ______ event.

a. interference

b. disturbance

c. nonsequential

d. inconsistent

4. Which statement about biogeochemical cycling during succession is false?

a. Maximum nutrient storage in soil occurs during the late or climax community stage.

b. Soil texture is a determining factor in nutrient storage.

c. In early seral stages, nutrients are rapidly incorporated into plant biomass, reducing the amount of nutrients lost from the ecosystem via leaching.

d. As communities mature from early to intermediate seral stages, internal nutrient cycling increases.

5. Lakes that are of poor nutrient status (i.e., low nutrient concentrations) are called _______ lakes.

a. eutrophic

b. oligotrophic

c. microtrophic

d. autotrophic

6. Short-lived species that are capable of rapid and wide dispersal and are usually the first species to colonize an unoccupied site are called _____ species.

a. phalanx

b. competitive

c. facultative

d. pioneer

7. In the _____ model of succession, early species modify the microclimate, making it more favorable for later colonizing species.

a. competition

b. tolerance

c. facilitation

d. mediation

8. If a site is frequently disturbed so that only early successional species can survive, this successional pattern is called:

a. patch dynamics

b. chronic patchiness

c. induced patchiness

d. biogenic patchiness

9. The ______ biome is characterized by cold winters, cool summers, dense stands of conifers (spruce, fir, and larch) and low tree species richness.

a. temperate forest

b. taiga

c. temperate woodland

d. tundra

10. The ______ marine environment is characterized by strong wave action and alternating exposure and inundation by saline water; algae and mollusks are attached on rocky substrates; on sandy substrates burrowing animals dominate.

a. intertidal

b. open ocean

c. benthic

d. upwelling

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