BIO226 : Community : Succession : Lesson

Glossary terms that are important in this lesson: Climax, continuum index, cyclic climax, facilitation, inhibition, monoclimax, pattern climax, primary succession, secondary succession, tolerance, transient climax, weed.

I. Succession

1. Introduction
   • Succession: sequence of changes in natural communities
   • Climax: ultimate association of species

2. Process of succession
   • New habitat rapidly colonized by invaders
   • Successional species change their own environment: shade, contribute detritus to the soil, modify soil moisture
   • Sequence of annuals, herbaceous perennials, shrubs, pines, hardwoods
   • Sere: a successional series of communities which is predictable
   • Primary succession: in newly formed habitats previously lacking plants
     • Glacial clay deposits
       • Deficiencies of nutrients, especially N; drying winds, cold
       • Mat-forming mosses and sedges, prostrate willows, shrubby willows, alder thickets (10-20 yrs), sitka spruce to spruce-hemlock forest (100 yrs)
     • Bogs: rooted aquatics at edge of pond, sedge mat, Sphagnum moss, shrubs, black spruce and larch, birch, maple and fir
   • Secondary succession: return of vegetation following a disturbance
     • Gaps in communities are rapidly filled; species with strong powers of dispersal are usually intolerant of competition from other species
     • Marine habitats in southern Australia: race between good colonizers (bryozoans and polychaetes) and strong competitors (tunicates and sponges)
     • Weeds: disturbance adapted species which colonize open patches quickly, mature early, elimated by superior competitors; frequent disturbances retain weeds in the system
     • Intertidal rocky shore (CA): limpet grazing depressed most species of algae but favored three species of rare and inferior competitors
   • Climax community
     • Ecological communities are open systems whose composition varies continuously over environmental gradients
     • Mature forest stands were arranged by species composition along a continuum index from dry (oak and aspen) to moist (sugar maple) sites
       • Individual stands could be either seral stages leading to an ultimate sugar maple climax or the end points of local seres determined by topographic or soil conditions

3. Causes of succession
   • Position of a species in a sere depends on its rate of invasion and its responses to changes
   • Facilitation
     • Early species improve habitat: contribute to nutrient and water levels, modify microenvironments at soil surface
     • Example: alders and black locust contribute nitrogen to soils; early marine colonists provide a substrate conducive for settling by other species
   • Inhibition
     • Competition for space, nutrients, and light; allelopathic chemicals
     • First arrivals take precedence
   • Tolerance
     • Early establishment depends on dispersal abilities and physical conditions
     • Later stages depend on relative competitive ability

4. Old-field succession
   • First 3-4 years
     • Rapid replacement of species dependent on individual life histories
     • Crab grass (annual) colonizes field in year of abandonment
     • Horseweed (winter annual) germinates in fall, rosette during winter, blooms in summer; needs full sunlight; decaying roots stunt other plants
     • Ragweed (summer annual) germinates in spring, flowers in late summer
     • Aster and broomsedge (biennials) germinate in spring and bloom in second year; asters shade their own seedlings, do not compete effectively for soil moisture

   • Toward climax
     • Progression of growth forms modifies conditions of light, temperature, moisture, and soil nutrients: grasses, shrubs, trees
     • Final biomass of climax limited by climate independently of events during succession
     • Community reaches climax when seedling and dominant species are the same
     • Time for succession is slowest on bare rock (1,000 years) and fastest on established soil
     • Communities are more diverse and complex at intermediate stages of succession
     • Community biomass is in steady-state at climax
     • More of community's store of nutrients are in organic form
     • Large proportion of productivity enters the detritus food chain
     • Tree roots protect soil from erosion

5. Species characteristics
   • Early successional species emphasize high dispersal ability and rapid growth
     • Many small seeds, often wind-dispersed, can remain dormant in the soil for many years
     • Rapid growth in full sunlight
     • Large biomass allocation to stems and leaves
   • Late successional species emphasize shade tolerance and competitive ability
     • Large seeds with nutrient store for initial growth
     • Large biomass allocation to roots and stems

6. Character of the climax
   • Monoclimax theory: one true climax of a region determined by climate
   • Pattern climax theory: regional pattern of open climax communities whose composition at any one locality depends on particular environmental conditions at that point
   • Relevant factors determining composition of climax community: soil, nutrients, moisture, slope, exposure, fire, grazing
     • Example: southeastern pine forests, chaparral, forest-prairie, African grasslands
   • Transient climax
     • Temporary or seasonal communities: spring pond, carcus, dung - a repeated and predictable pattern of change
   • Cyclic climax
     • Sequence of repeated and predictable changes resulting from disturbance: heath, grassy hummock, treefall, wave damage in marine systems
   • Concept includes cyclic patterns of changes, mosaic patterns of distribution, and alternative stable states: a dynamic, self-perpetuating system

When you have completed this lesson, go on to Review Questions