Chapter 19 conceptsSuccession occurs in a community when species replaceeach other over time.Succession can occur through different mechanisms.Succession does not always produce a single climaxcommunity.123
Defining succession1To predict how communities respond to environmental disturbances,we must understand how species composition changes over time.
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Observing succession1Direct observation of changes overtime is the clearest way to recordsuccession in a community.Example:In 1883 a volcanic eruption blew awaythree-quarters of the island of Krakatau.Remaining land was covered with volcanicash that obliterated all life.Researchers began observing the islandafter eruption; by 1886 they saw that 26plant species had colonized the island.Many early colonizing species were wind-and sea-dispersed. Later colonizers weredispersed by animals (e.g., birds and bats).
Observing succession1When direct observation of succession is not possible, scientists canuse methods of indirect observation, such as achronosequence.Chronosequence:a sequence of communities that exist over time ata given location.Example:Water level in Lake Michigan has fallen since the last glaciation; old sand dunes arefar from the shoreline and new dunes are along the shore edges.In the 1800s, Henry Cowles studied successionby examining a chronosequence of dunes.On younger dunes, he found early successionalspecies (e.g., grasses); on older dunes, he foundlater successional species (e.g., herbs, shrubs).On the oldest dunes, he found beech, oak,maple, and hemlock trees.
Observing succession1It is possible to look back in time by examining pollen preserved inlayers of lake and pond sediments.Flowering plants produce pollen grains with distinct sizes and shapesthat travel through the air and land on a lake surface.The pollen grains sink and become preserved in layers of sediment atthe bottom of the lake.Researchers can take a sample that penetrates through many layersof mud on the lake bottom.Carbon datingcan be used to identify the age of the pollen in eachlayer, and helps to determine the changes in plant speciescomposition around the lake over hundreds or thousands of year.
Primary succession1Primary succession:the development ofcommunities in habitats that are initially