Temporal Dynamics in General

Different kinds of developmental processes are associated with different kinds of ecologic systems; are controlled by either dominantly extrinsic or intrinsic adjustments or turnovers in composition; and may occur within the lifespan of one system or involve replacement of a previous system by a newly organized system. Smaller, more localized systems are associated with faster process rates; larger systems that contain such systems have more sluggish behavior and processes that encompass longer time spans. The important points are: different temporal dynamics characterize different levels in the ecologic hierarchy, and succession is only one of several kinds of changes that eco-logic systems may undergo.

Primary succession—The initiation of a new local ecosystem at an unoccupied site. Earliest colonists have adaptations for broad dispersal, utilization of abundant resources, and rapid growth rates and high fecundities. As the system develops, these organisms are displaced or replaced by other organisms that were present from the beginning or that arrive subsequently, having weaker dispersal ability, resource specializations, and comparatively slow growth rates and low fecundities. Late arrivals are often superior competitors. The buildup of species in a local system produces spatial heterogeneity and ameliorates physical-chemical factors, potentially making way for the addition of other species. Species richness and diversity are low at first, then increase rapidly; if the system remains disturbance-free, super-competitors will eventually dominate the system, producing low diversity again, but for different reasons. In this view of primary succession, the embedded processes result in the initiation of a new local ecosystem largely by means of colonization and internal interactions. Some ecologists refer to the end-product of succession as an equilibrium system known as the climax (better known as developmental maturity). The developmental stages are called seres. Biomass increases, nutrients are conserved more efficiently, and productivity declines in the course of succession (see Table 1). This entire process takes on the order of one to ten years in most aquatic environments, and ten to a thousand years in terrestrial systems, with much variation. A well-documented example is the reestablishment of terrestrial ecosystems now taking place on the slopes of Mt. St. Helens, a volcano that erupted in 1980 in Washington state.

Secondary succession— Recovery from a disturbance that is primarily controlled by internal dynamics and does not involve total collapse of the system. Many of the generalizations about primary succession apply. The internal dynamics consist of a changing network of population components and their interactions involving what are called facilitation (early arrivals modify a site to make survival of later arrivals more likely); tolerance (accumulation of species able to tolerate the developing conditions at a site, primarily reduced availability of resources); or inhibition (adding species with specialized survival strategies in the crowded neighborhood of populations undergoing gradual turnover). Secondary succession probably takes about the same amount of time as primary succession; it may occur repeatedly and often is reinitiated before a system has reached a dynamic equilibrium. Because of this, some ecologists think that the climax is an idealization, a condition rarely achieved in local ecosystems consisting of spatial mosaics at different stages of succession at any one time. In other words, ecosystems are almost always recovering from

Table 1

Immature versus Mature Stages of Succession of Ecosystem Properties

Table 1

System Properties

Immature

Mature

Ecosystem energy flow

Production/system respiration

>1 or <1

«1

Production/biomass

Relatively high

Relatively low

Net system production

High

Low

Food web geometry

Simple

Complex

Nutrient cycling

Mineral cycles

Open

Closed

Exchange rates

Fast

Slow

Importance of detritus

Little recycling

Efficient recycling

System structure

Species diversity

Low

High

Heterogeneity

Disorganized

Organized

Symbiosis

Relatively rare

More common

System stability

Overall stability

Relatively low

Relatively high

"Information" content

Low

High

Entropy

High

Low

Nutrient conservation

Low

High

Properties of organisms

Habitat/resource specialization

Low

High

Body sizes

Often small

Relatively large

Life cycles

Often short

Long

Population growth

Rapid, unbounded

Resource-

constrained

Competitive strategies

Colonization

Competitive

exclusion

Source: Based on Odum, Eugene P. 1969. "The Strategy of Ecosystem Development." Science 164: 262-270.

Source: Based on Odum, Eugene P. 1969. "The Strategy of Ecosystem Development." Science 164: 262-270.

Note: More recent work on succession shows the process to be less predictable than characterized here and to be subject to continual "resets," with few systems ever attaining idealized maturity or successional "climax."

the last disturbance. An example is the patchwork of benthic marine ecosystems in Long Island Sound, each recovering from a different episode of dumping of dredged sediments.

Community response—Temporal changes in a local ecosystem paced by external environmental factors, such as seasonal fluctuation in climate or aperiodic habitat changes, not leading to a complete collapse of the system. This would include many examples of so-called allogenic succession; it occurs in stressful environments, settings characterized by frequent shifts in the environment, or locations that experience strong, unpredictable disturbances. An example would be the annual changes in species composition and community organization in a high-latitude lake controlled by high-amplitude seasonal cycles of temperature, runoff from the adjacent landscape, and chemical concentrations.

Figure 1. Community Replacement Documented in the Marine Fossil Record

Figure 1. Community Replacement Documented in the Marine Fossil Record

Source: Miller, William and J. R. Dunbar. 1988. "Community Replacement of a Pleistocene Crepidula Biostrome." Lethaia 21:67-78 (By permission of Taylor & Francis AS)

Note: This example from a Pleistocene embayment in what is now the outer coastal plain of North Carolina involved the replacement of a bottom community dominated by the slipper snail Crepidula with a more diverse community dominated by the clams Anadara and Ostrea. Patterns such as this one are common in shelly fossil beds deposited in low-energy environments and were once thought to be examples of

Source: Miller, William and J. R. Dunbar. 1988. "Community Replacement of a Pleistocene Crepidula Biostrome." Lethaia 21:67-78 (By permission of Taylor & Francis AS)

Note: This example from a Pleistocene embayment in what is now the outer coastal plain of North Carolina involved the replacement of a bottom community dominated by the slipper snail Crepidula with a more diverse community dominated by the clams Anadara and Ostrea. Patterns such as this one are common in shelly fossil beds deposited in low-energy environments and were once thought to be examples of ancient succession.

Community replacement—When interaction networks are disrupted and environmental tolerances of the component organisms are approached or exceeded, local ecosystems degrade and collapse. Collapse could be incremental when local extinction removes "hub" populations that have retinues of associated species depending on them for resources or habitat structure. Weakly interacting populations would disappear independently as the tolerance limits of different species are reached. A catastrophic collapse, eradicating all local populations in an ecosystem, could eliminate all of the organisms rapidly without regard for individual adaptations or interaction partnerships. Subsequently, a new assemblage of organisms could invade the area and establish a new ecosystem having a different composition, internal organization, and functional identity. Such transitions involve more than one local ecosystem and may take longer (perhaps on the order of10 to 10,000 years) to take place than succession and response. Directional eutrophication and in-filling of lakes is usually cited as an example of allogenic succession. Because a series of distinctively different ecosystems are involved, replacing one another as the environ-

ment changes from deep water to bog, this is really a form of community replacement. In paleontology, many well-preserved vertical transitions observed in localized assemblages of marine fossils are also examples of replacement, not succession (see Figure 1). Reorganization of plant assemblages paced by climate change over the last 10,000 years, when viewed at a particular locality, is another form of community replacement.

Regional transitions and turnover pulses— Regional ecosystems undergo processes that resemble succession and replacement but that are unique to the larger, more inclusive scale of such systems. This is the level of organization at which ecologic and species-level evolutionary processes intersect in many crucial ways. When a significant proportion of local ecosystems collapse, the regional system will be reorganized or replaced by a new system. The most important processes include interregional migrations, local and species-lineage extinctions, and speciation events. In terms of establishment of new regional systems, immigrants and newly evolved species may be swept into interaction networks in the early stages, with some species becoming the dominant players in the new regional economy; others, however, develop only minor roles, which may explain both abundant versus rare differences and the subsequent durable structure of such large systems. Regional transitions are largely forced by major changes in the surrounding environment, such as climate fluctuation in terrestrial settings and sea level changes in marine settings. Many coincident regional transitions take place during mass extinctions. Such patterns probably encompass 1,000 to 100,000 years and are as yet poorly understood. In the Devonian marine formations of New York state, groups of related fossil assemblages that probably record regional ecosystems, having durations of several million years, replace one another as a result of environmental changes that caused extinction and faunal turnover at the regional scale.

Larger transitions—Some paleontologists have claimed that local and regional ecosystem turnovers are examples of a kind of scaled-up succession, not unique processes involving

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