Primary succession: New space becomes available for occupation and is colonized by species that can cope with the extreme conditions an void of soil and organisms. The colonizers alter the environment to make it more livable for later successional species (new sand dunes, lava flows, areas exposed after glacial retreat). Succession: How communities come to be Secondary succession: A previously occupied space was cleared of vegetation by disturbance and is reoccupied. Soil and seed banks were left in place. Early colonizers must be able to cope high exposure to light, temperature and moisture change (burnt, plowed or clear-cut areas).

Many hundreds of years Primary Succession

Rock lichen facilitate the transformation of bedrock to soil.

Lupines: early successional species.

An example of primary succession: vegetation recovery on the mudflows of Mount St. Helens At 8:32 Sunday morning, May 18, 1980, Mount St. Helens erupted. The eruption latest 9 hours, but changed 200 square miles of forest into a lifeless, ash covered moonscape. Thousands of game animals died, 12 million salmon, and uncounted millions of birds and small mammals. 57 people were killed.

The largest landslide in recorded history swept down the mountain at speeds of 70 to 150 miles per hour and buried rivers under an average of 150 feet of debris. Some areas were covered by as much as 600 ft of debris.

Forest, before Same spot, after

Fireweed, seen just weeks after eruption, is an example of a “pioneer species”.

100 – 200 hundred years Secondary Succession

An example of secondary succession: The Yellowstone fire of The fire burnt 1.4 million acres (36% of the park) during a summer that was the driest in the Park's recorded history. $120 million was spent and 25,000 people participated in saving people and property. The fire itself was finally stopped by rain and snow in September.

a destructive event that removes all or a large part of the community. Disturbance: Fire Wind Water ParasitesHuman activities Overgrazing

Gap models of community dynamics: After disturbance Early successional stage Mid successional stage Late successional stage

Seed trees are left standing inside clear cut patches. Some silviculture methods rely on natural forest succession

A major question of the theory of succession: How is the succession of communities on the road to the “climax” community controlled?  Is succession an orderly, predictable process that selects the most appropriate species for every stage of succession and ends in a fully predestined climax community?  Is succession a chance-driven process that depends on what species are available and tolerate living there and ends in one of possibly many climax communities?

Is community composition “predestined”? No (Henry Allen Gleason) ( ) Yes (Frederic Clements) ( ) Community as “super-organism” Community assembled by chance and opportunity

Frederic Clements ( ) "Succession... must be regarded as the development or life-history of the climax formation.... Succession is preeminently a process the progress of which is expressed in certain initial and intermediate structures or stages, but is finally recorded in the structure of the climax formation.“ Plant Succession: An Analysis of the Development of Vegetation. Washington, D.C.: Carnegie Institution of Washington, 1916.

Henry Allen Gleason ( ) "The vegetation of an area is merely the resultant of two factors, the fluctuating and fortuitous immigration of plants and an equally fluctuating and variable environment. As a result, there is no inherent reason why any two areas of the earth's surface should bear precisely the same vegetation.... Every species of plant is a law unto itself, the distribution of which in space depends upon its individual peculiarities of migration and environmental requirements.... The species disappears from areas where the environment is not longer endurable. It grows in company with any other species of similar environmental requirements..", The Individualistic Concept of the Plant Association, Bulletin of the Torrey Botanical Club, 53 (1926), pp. 7-16,

Earliest successional stage Climax community Seres Clement’s succession model: Community 1: Community 2:

Some environmental gradient (altitude, soil moisture, soil fertility) Species abundance Gleason’s community concept: Community 1Community 2

The role of species interactions in succession: Facilitation model Earlier species alter the physical environment to pave the way for later species. Inhibition model Initial colonists hang on to the space they captured and exclude other colonists. Tolerance model Species neither facilitate not inhibit, but they simply get there at different times due to differences in life history.

Facilitation model Rock weathering and soil building species pave the way for species that require soil. Important in the early stages of primary succession. Lichen growing on lava Nitrogen fixing plants

Inhibition model Initial coral colonizers may hang on to their space until the next disturbance.

Tolerance model Early colonists: long-distance dispersal high seed production high germination rate fast growth Late arrival: high competitiveness slow growth large size

Earliest successional stage Climax community 1 Seres Invasions can alter the course of succession: Climax community 2 Land use changes Invaders

The sagebrush steppe (Great Basin Desert) perennial grasses & shrubs

Before 1900 Mack, Invasion by Bromus tectorum:

Exotic plants can interfere with the succession process: perennial grasses & shrubs Low disturbance regime overgrazing + Bromus seed Annual grasses fire

Summary: Communities are dynamic species associations. Species co-occur because they tolerate the same sort of environment, but… Species also alter the environment. Succession refers to the dynamics of species assembly on either new terrain (primary succession) or disturbed terrain (secondary succession). Primary succession takes much longer because soil needs to be formed. Invaders can alter successional events.