Traditionally ecologists have viewed succession as a progression of stages in which species come and go until a climax community arises. The climax community is thought to be a stable assemblage of species that experiences little change until an intense disturbance wipes out the community, sending it back to its initial stages again. As you will see later in this section, whether some communities reach a true stable end point is questionable. Let’s consider the mechanics of succession in more detail.

Succession is probably most easily observed after a catastrophic disturbance kills all the organisms in a community, leaving an environment devoid or nearly devoid of life. This type of change is known as primary succession. Glaciers, volcanic activity, and in some cases floods or landslides cause disturbance that initiates primary succession. Primary succession by its very nature can be slow because the early arrivals (known as pioneer or early successional species) must deal with extreme conditions. Many pioneer species deal with these conditions by employing certain *life history strategies or species interactions to their favor.

One of the best-known examples of primary succession is that seen in plant communities in the wake of the retreat of glaciers in Glacier Bay, Alaska (Figure 56.15A). Captain George Vancouver first recorded the location of glacial ice there in 1794, while exploring the west coast of North America. Over the last 200 years, the glaciers have retreated up the bay, scraping the landscape down to bare rock and leaving a series of moraines—gravel deposits dropped where the glacial front was stationary for a number of years. No human observer was present to observe changes over the entire 200-year period, but ecologists have inferred the temporal pattern of succession by studying the vegetation on moraines of different ages (Figure 56.15B). The youngest moraines, closest to the current glacial front, are populated with bacteria, fungi, and photosynthetic microorganisms that can support themselves on bare rock. Slightly older moraines farther from the glacial front are home to pioneer communities containing lichens, mosses, willows, and cottonwoods, which break down rocks and, when they die, decompose and contribute to the buildup of soil. Mosses and a few species of shallow-rooted shrubs in the genus Dryas eventually become established and contribute to soil building as they die and decompose. Still farther from the glacial front, successively older moraines have deeper soil layers that support shrubby willows and alder trees. Finally, a century after glacial retreat, a mature Sitka spruce forest dominates, fostering a diverse array of forest species.

Question

The oldest communities are located in the areas that have been exposed the longest since glacial retreat, such as the mouth of the bay, where succession has proceeded for 200 years. As the glaciers melt and retreat up the bay, the communities become younger and younger, with the youngest pioneer community closest to the glacial front.

Nitrogen is virtually absent from glacial moraines, so the plants that grow best on recently formed moraines at Glacier Bay are Dryas and alders, both of which have nitrogen-fixing bacteria in nodules on their roots (see Figure 35.6B). Nitrogen fixation by these plants improves the soil so that spruce trees can grow (see Figure 56.15B). Spruces then outcompete and displace the early colonists. If the local climate does not change dramatically, a climax community dominated by spruce trees is likely to persist for many centuries on old moraines at Glacier Bay.

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On Mount St. Helens, ecologists have had a unique opportunity to study primary succession from its inception. As described in the opening story, the alpine communities surrounding the lava dome experienced the greatest destruction; they were the first to be hit by an avalanche of mud and hot water and then pelted by hot, sterilizing pumice. This created the so-called Pumice Plain, an area completely lacking life, including any traces of organic matter. The first plant species to arrive, a year after the eruption, was the dwarf lupine (Lupinus lepidus). Dwarf lupines, like Dryas and alders, utilized the nitrogen produced by bacterial symbionts in root nodules to colonize the sterile environment of the Pumice Plain. They subsequently facilitated other species of plants by trapping seeds and detritus and increasing the overall nutrient content of the soil. By providing safe sites for seedlings to become established, the dwarf lupines greatly increased the rate of primary succession on Mount St. Helens. Because of the early inroads made by dwarf lupine, researchers documented roughly 20 plant species living on the Pumice Plain 20 years after the eruption.

A surprising discovery about primary succession on Mount St. Helens was the important role of animals in directing change. For example, scientists observed that newly formed and isolated ponds and lakes were colonized by amphibians at a faster rate than one might predict given the surrounding harsh conditions. Ecologists discovered that frogs and salamanders were using tunnels created by the northern pocket gopher (Thomomys talpoides) as refuges when they made their way from pond to pond across the dry landscape (Figure 56.16). Pocket gophers successfully survived the eruption by living in tunnels under the mud. They also benefitted by the expansion of their preferred habitat—grassy meadows—after the eruption. Pocket gophers facilitated plant succession through their burrowing activities, which brought to the surface organic matter, seeds, and fungal spores buried deep under the pumice.

Question

Pocket gophers most closely fit the ecosystem engineering species definition because they are able to create, modify, and/or maintain physical habitat for themselves and other species through their burrowing activities. They might also be considered a keystone species because their effect is large relative to their size and abundance. However, keystone species are thought to act mostly through food webs by creating trophic cascades.

The other type of succession, known as secondary succession, involves the reestablishment of a community when most, but not all, organisms have been destroyed. Secondary succession is often initiated by human activities (such as clear cutting) as well as by natural disasters (such as storms and fires). Secondary succession is more common and it progresses more rapidly than primary succession. For example, even though the eruption of Mount St. Helens completely destroyed some areas near the blast zone, there were large areas where organisms survived and secondary succession took place.

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Ecologists compared primary and secondary succession on Mount St. Helens by following small mammals in the community. As we mentioned in the chapter opening, different Mount St. Helens communities experienced different levels of disturbance from the eruption. Investigating Life: Rising from the Ashes describes research by Charles Crisafulli and colleagues at the USDA Forest Service, showing that the recovery of small mammal populations on Mount St. Helens varied significantly between primary and secondary successional habitats. The Pumice Plain (primary successional habitat completely devoid of life after the eruption) had the lowest species richness of small mammals, a pattern that persisted for at least 20 years. In contrast, small mammal populations recovered relatively quickly in the Blowdown Zone and the Tephra-fall Zone (secondary successional forest in which some life survived under downed trees and ash fall). In fact, within a few years, the number of small mammals in these two zones was similar to that in the undisturbed reference area 21 kilometers away. This comparison makes clear that the starting conditions characterizing secondary succession are critical in giving communities a “head start” in their recovery.