Grasslands and Tundra

Temperate grasslands

Found in areas of moderate rainfall, typically between 25 and 75 cm per year.

Dry seasons sufficient to exclude most trees.

Frequent fires.

North American grasslands are dominated by “bunch grasses.”

The North American Great Plains can be divided into several prairie types. They differ primarily on the basis of the amount of rain and the time of the dry season.

Tall-grass prairie

Mixed-grass prairie

Short-grass prairie

Tundra ecosystems are found where cold, hard winds and/or permafrost prevent the growth of large woody plants. We recognize two types of tundra:

Arctic tundra

Alpine tundra

Arctic tundra have long winters of extreme cold. Permafrost is common.

Arctic tundra is dominated by lichens, mosses, sedges, and small woody plants.

Alpine tundra is found high in mountains at all latitudes. Conditions are variable. Permafrost is rare.

Are grassland ecosystems equilibrium systems? This is significant in determining whether they will return if stresses are removed.

Some have suggested that they may have “multiple stable states.”

Succession models

Population cycles are commonly seen in tundra habitats. They can be classified into three types:

1. 3-4 year cycles common in microtine rodents.

2. 9-10 year cycles as shown by snowshoe hares.

3. Longer cycles as seen in some larger tundra animals.

Lemming population cycle

Cycle of snowshoe hare and lynx.

Suggested Causes of Population Cycles

• Herbivory hypothesis

• Food supply-predation hypothesis

• Predation hypothesis

Human impact on grassland and tundra ecosystems has been severe.

Grazing impact of livestock is often due to selective herbivory.

Decreasers versus increasers

Invading species like cheatgrass (Bromius tectorum) has replaced native grasses over large areas.

Arctic National Wildlife Refuge

Oil in the ANWR