Development and Succession Temporal Structure of Communities

Previously, we looked at how to describe changes in individual populations. The basic population growth models often presuppose that environmental conditions, as reflected in carrying capacity, are constant. Community changes occur in nature as a result of disturbances that alter individual populations and environmental conditions. The most commonplace of these are the changes of seasons. More catastrophic changes include floods, fires, extreme weather, or volcanic eruptions. The time scales of human disturbances can range from practically instantaneous (nuclear explosions) to decades or more (global warming or soil depletion by agriculture). These disturbances affect the entire ecosystem, which responds as a unit. Although potentially, we could describe these ecosystem-wide changes using population models for all the individual species, we can note some holistic patterns.

A new habitat can be caused by a large event, such as a forest fire or clearcutting of a forest, or by a small local change such as the falling of a tree or a pile of dung left by a bear. In each case, the new habitat will be colonized by pioneers who are especially good at invading. These are often the r-selected organisms. Eventually, they will be replaced by more efficient K-selected species. In between, a variety of species may come and go in abundance. The process of sequential population changes initiated by a disturbance is called succession. The actual sequence of communities is called a sere. Barring further disturbances, succession ends when ecosystems develop a stable condition called a climax. The climax community is ultimately a community that can succeed itself. A climax community may form within a season or it make take decades, as in the case of forests.

As an example, an abandoned farm field may be colonized by grasses, which inhibit the germination of trees. The grasses attract herbivores, which create openings for shrubs by intense grazing. The shrubs provide shade, which enables pine to germinate and eventually to dominate. However, when the cover becomes too dense, the pine seedlings will not grow, and hardwood trees gain an advantage. Eventually, a climax community is formed as a hardwood forest. Populations of birds and other animals change as the food supply changes.

Another example is algal succession in temperate-zone lakes. As the water warms in the spring, the first species that dominate may be the cyanobacter. Some cyanobacter fix nitrogen and so are limited only by phosphorus. Eventually, they deplete the available phosphorus, removing it from the water column as they die and settle to the bottom. Other algae, such as diatoms, may flourish. But these require silicon for their shells, and when that is depleted, they may make way for green algae.

There are several forces driving succession. A community may change its environment, making it more suitable for others that succeed it as the shrubs made way for the pines. Succeeding communities are often those that tolerate a lower level of resources, as the diatoms that followed the cyanobacter. Sometimes the succeeding community must overcome inhibition by the previous residents, as the shrubs were inhibited by the grasses. Newly exposed soils may experience a gradual drop in pH from the accumulating products of plant decay.

Overall, succession seems to be related to a balance of colonizing ability of some species vs. the competitive ability of others. A particular sequence may not be uniquely determined by environmental conditions. One could wind up with one of several communities, depending on contingent factors during succession. However, chance factors seem to operate most strongly in earlier stages of succession.

Odum (1987) has listed a number of trends identified with succession in the absence of further disturbances (Table 14.10). Gross primary productivity tends to form a peak during succession and then declines as a community approaches the climax. Succession occurs in a biological wastewater treatment plant whenever conditions are changed. Plant startup is a prime example of this. Primary productivity is negligible in this system,

TABLE 14.10 Trends Expected During Undisturbed (Autogenic) Succession

Energetics

Biomass (B) and organic detritus increase.

Gross productivity (P) increases in primary succession, little change in secondary. Respiration (R) increases. P/R ratio moves toward unity (balance). B/P ratio increases. Nutrient cycling

Element cycles increasingly closed. Turnover time and storage of essential elements increase. Cycling ratio (recycle/throughput) increases. Nutrient retention and conservation increase." Population and community structure Species composition changes. Species diversity peaks in middle or end of sere. Species evenness peaks in middle or end of sere. r-Selected organisms replaced by K-selected. Life cycles increase in length and complexity. Size of organism and/or propagule (seed, offspring, etc.) increases. Mutualistic symbiosis increases." Stability

Resistance increases." Resilience decreases." Overall strategy

Increasing efficiency of energy and nutrient utilization."

"Trend based on theoretical considerations, not yet validated in the field. Source: Based on Odum (1987).

but controlled energy input (organics in the wastewater) takes its place. This system is called heterotrophic succession. Respiration (per organism) peaks, then is reduced as substrate concentrations drop and respiration achieves balance with energy input.

As succession proceeds, nutrients are stored in biomass, and in soil in the case of primary succession. As these reservoirs build up, internal recycling increases, improving the efficiency of nutrient use. Species diversity may increase throughout the succession, although climax communities sometimes have less diversity than a peak. For example, relatively moist forests in the Great Lakes region showed a peak diversity in middle ages, whereas drier forests increase in diversity continuously.

The succession just described has a single direction of change. Some ecosystems exhibit cyclical changes even in the absence of external disturbance. The CdUm" heath in Scotland has a cycle of 20 to 30 years. Heather establishes itself on bare ground and matures in about 7 to 15 years. After 14 to 25 years, this perennial loses its vigor, and other lichens and mosses invade. They eventually die out, leaving bare ground, and the cycle repeats.

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