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the intertidal boulders were rapid: The secured small boulders had more species than the unsecured boulders within 16 months. Most ecological successions progress much more slowly.

Ecologists divide successions into two major types: primary succession and secondary succession. A primary succession begins on sites that lack living organisms. A secondary succession begins on sites where some organisms have survived the most recent disturbance. The patterns and causes of ecological succession are varied, but the species that colonize a site soon after the disturbance often alter environmental conditions so that they become favorable for other species.

A good example of a primary succession is the changes in the plant community that followed the retreat of a glacier in Glacier Bay, Alaska, over the last 200 years. The melting and retreating glacier left a series of moraines—gravel deposits formed where the glacial front was stationary for a number of years. No human observer was present to measure changes over the 200-year period, but ecologists have inferred the temporal pattern of succession by measuring plant communities on moraines of different ages. The youngest moraines, close to the current glacial front, are populated with bacteria, fungi, and photosynthetic microorganisms. Slightly older moraines farther from the glacial front have lichens, mosses, and a few species of shallow-rooted herbs. Still farther from the glacial front, successively older moraines have shrubby willows, alders, and spruces.

Youngest moraine

Slightly older moraine

Older moraine Oldest moraine

Youngest moraine

Slightly older moraine

Older moraine Oldest moraine

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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