Primary succession and secondary succession differ in their initial stages
Ecologists recognize two types of succession that differ in their initial stages. The first type, primary succession, involves the colonization of habitats that are devoid of life (see Figure 17.5), either as a result of catastrophic disturbance, as we see on the Pumice Plain at Mount St.
Helens, or because they are newly created habitats, such as volcanic rock. As you can imagine, primary succession can be very slow because the first arrivals (known as pioneer or early successional species) typically face extremely inhospitable conditions. Even the most basic resources needed to fuel life, such as soil, nutrients, and water, may be lacking. The first colonizers, then, tend to be species that are capable of withstanding great physiological stress and transforming the habitat in ways that benefit their further growth and expansion (and that of other species, as we will see).The other type of succession, known as secondary succession, involves the reestablishment of a community in which most, but not all, of the organisms or organic constituents have been destroyed (see Figure 17.5). Agents of change that can create such conditions include fire, hurricanes, logging, and herbivory. Despite the catastrophic effect of the eruption on Mount St. Helens, there were many areas, such as the blowdown zone, where some organisms survived and secondary succession took place. As you might expect, the legacy of the preexisting species and their interactions with colonizing species can play a large role in the trajectory of secondary succession.
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