Ecological Succession

Ecological Succession Ecosystems are constantly changing in response to both natural and human disturbances. Examples: forest fires, strip mining, glacier recession, volcanic eruption, etc. It occurs when one community (animal and plant species) in an area gradually replaces another community over time. Each community has its own dominant plant and animal species and ecological succession results in certain species becoming less abundant and other species becoming more competitive and dominant over time.

Each successive (new) community is more favorable for new species Changes in stages until a climax community is established

Climax Community It is the final community that will exist in an ecosystem – usually consists of trees Ecological succession occurs in two forms: 1. Primary Succession 2. Secondary Succession

Composition of the community changes with time Past Community Future Community Present Community Some species in the past community were out-competed, and/or did not tolerate altered abiotic conditions Modifies such abiotic factors as: Light intensity Wind speed Air Temperature Soil composition Light quality Wind direction Soil water Humidity Changes allow new species to become established.

Primary Succession Occurs in an area where NO community had previously existing Bare rock present Receded glaciers Volcanic regions with eruptions of lava rock and ash First organisms to appear in these areas are called pioneer species (or colonizers) Climax Community Lichens, bryophytes And annual herbs Slower growing broadleaf species Grasses and Small shrubs Fast growing trees Bare Rock 100-200 years

Pioneer Species First organisms to live in a new community. Usually brought in by wind or animals

Primary Succession Begins very slowly because the physical environment must be changed to produce soil needed for the growth of plants Lichens (consisting of fungus and algae) and mosses are examples of pioneer species Lichen grows on rock and helps to break apart rock to form soil

Mount St. Helen 1980 – Washington State Another example of primary succession

Secondary Succession Takes place after a land clearance Fire, landslide, forest clearing, severe weather, human agriculture Soil is already there More rapid than primary Climax Community Open pioneer community (annual grasses) Mature woodland mainly oak Primary bare earth Grasses and low growing perennials Scrub: shrubs and small trees Young broad- leaved woodland 150+ 1-2 years 3-5 years 16-30 years 31-150

Secondary Succession Farmers field over a 20 year period

Secondary Succession After a forest fire

Gap Succession Example: forested area with a thick canopy has reduced light on soil When a tree falls sunlight fills forest floor New communities are formed