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Adaptations & Ecological Succession

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Adaptation through natural selection

An adaptation is an inherited trait that raises an organism’s survival and reproduction in its environment. Adaptations arise through natural selection: individuals vary, more offspring are produced than can survive, and those with favorable traits leave more offspring, so the favorable alleles become more common over generations. Adaptations can be structural (a cactus’s spines and water-storing stem), physiological (a mammal’s ability to concentrate urine to save water), or behavioral (bird migration). The greater a population’s genetic diversity, the more raw material selection has to work with.

Succession: how communities rebuild

Ecological succession is the predictable change in a community over time after a disturbance. Primary succession begins on lifeless ground with no soil — bare rock exposed by a retreating glacier, cooled lava, or a new sand dune. It is slow because soil must form first. Secondary succession begins where a disturbance (a fire, a flood, an abandoned farm field) removed the community but left the soil intact. Because the soil, seeds, and roots survive, secondary succession is much faster.

Pioneers, climax, and diversity

The first colonizers are pioneer species — hardy organisms like lichens and mosses in primary succession that can grow on bare rock and, by breaking it down, begin building soil. As soil deepens, grasses, then shrubs, then trees move in, each stage modifying conditions for the next. Over time the community may reach a relatively stable climax community. Through the sequence, species diversity generally rises from the pioneer stage toward the mature community, and the ecosystem accumulates more biomass and more complex food webs.

Primary vs. secondary succession
primary: starts with NO soil (bare rock) · secondary: starts with soil intact
The presence or absence of soil is the deciding test. Soil already present → secondary (fast). Bare rock, no soil → primary (slow).
Worked example

After a forest fire clears the trees but leaves the soil, seeds, and underground roots intact, ecologists track the recovery. Identify the type of succession and predict the sequence of dominant plants.

  1. 1.Check for soil: the fire left the soil, seeds, and roots — soil is intact, so this is secondary succession.
  2. 2.Because soil already exists, recovery is fast: surviving seeds and roots resprout quickly.
  3. 3.The typical sequence follows fast-growing, sun-loving plants first: grasses and weeds (early successional), then shrubs, then trees.
  4. 4.Diversity climbs as the structure rebuilds, eventually approaching a mature (climax) forest community.
Answer: This is secondary succession (soil intact, so recovery is rapid). The sequence runs grasses/weeds → shrubs → fast-growing trees → mature forest, with species diversity increasing along the way.
Checkpoint

A volcanic eruption covers a landscape in fresh, cooled lava with no soil. Which organisms are most likely to colonize first, and what type of succession is beginning?

Watch out

The single fastest way to tell primary from secondary succession is to ask: is there soil to start with? A retreating glacier or fresh lava leaves bare rock → primary. A fire, flood, or abandoned field leaves soil behind → secondary. Do not be fooled by how severe the disturbance looks; look for the soil.

Checkpoint

A desert kangaroo rat produces extremely concentrated urine and never needs to drink liquid water, obtaining all its water from metabolizing seeds. This trait is best described as:

On the exam

Know the three adaptation types by their keywords: structural = a body part, physiological = an internal process, behavioral = an action. And remember succession’s trend: diversity and biomass generally increase from pioneer stage toward the climax community.

Answer the 2 checkpoints as you read.

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