Hydrarch and Xerarch Succession: Ecological Change Stages

Hydrarch and xerarch are two types of ecological succession named by the amount of water at the start. Hydrarch begins in water, while xerarch begins in dry conditions like bare rock.

Both end in a forest or grassland suited to the regional climate, called the mesic climax. Comparing them shows how water availability controls the speed and species sequence of succession.

Key Concepts

Hydrarch succession starts in a pond or lake, also called hydrosere. Xerarch succession starts on dry rock or sand, called xerosere. Phytoplankton are tiny floating algae, the first producers in water. Lithosere is succession on rock, beginning with lichens. Mesophytes are plants needing moderate water and form the climax. Water depth is in metres, soil moisture often described qualitatively but can be measured as percentage by mass.

How It Works

1. In hydrarch, phytoplankton grow, die and settle, making the pond shallower.
2. Submerged plants, then floating and reed plants appear as depth decreases and sediment builds.
3. Sedges and grasses form marsh, then shrubs and finally trees establish on drier soil.
4. In xerarch, lichens secrete acids that slowly crack rock and trap dust.
5. Mosses, grasses, shrubs and then trees follow as thin soil deepens and retains more moisture.

Worked Example

Imagine a pond initially 4 metres deep. Sediment adds about 2 centimetres per year.

Step 1: After 50 years, sediment = 50 x 2 = 100 cm = 1 metre. Depth becomes 4 - 1 = 3 metres.

Step 2: After 100 years, sediment = 200 cm = 2 metres. Depth = 2 metres, suitable for rooted floating plants.

Step 3: After 150 years, sediment = 3 metres. Depth = 1 metre, reeds dominate.

Step 4: After 200 years, sediment = 4 metres. The pond fills and becomes marshy land. This simple rate illustrates hydrarch filling, though real ponds vary.

Common Misconceptions

• Hydrarch means dry start. Hydro means water, so it starts wet and becomes drier.
• Xerarch ends in desert. It ends in mesic forest if climate allows rain; xeric is only the starting condition.
• Both take equal time. Hydrarch can be faster where sediment input is high, while rock weathering in xerarch is very slow.

Real-World Applications

Lake management uses hydrarch knowledge to control eutrophication, where excess nutrients speed up filling and algal growth. Quarry restoration follows xerarch principles, using lichens and grasses to start soil formation. Wetland creation projects plant species in hydrarch order to establish stable ecosystems.

In summary, hydrarch succession fills water bodies through sediment stages, xerarch builds soil on dry rock from lichens upward, and both converge toward a mesic climax community.